recommendation, or favoring by the United States Government or any agency thereof. ...... confusing results obtained by includjng seismic activity i n the Price area, much of ...... Hill, D. P., P. Mowinckel, and L. G. Peake (1975). Earthquakes,.
/DOE/ET/283923 78-28392.a.12
AN EVALUATION OF HYPOCENTER LOCATION S WITH APPLICATIO
Prepared for
DEPARTMENT OF ENERGY Division of
Geothermal Energy
DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency Thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.
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NOT ICE
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4
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DISCLAIMER
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i DISTRI3UTiON OF
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THIS DOCllMElT IS UNLIMITED
An E v a l u a t i o n of Hypocenter Location Techniques W i t h A p p l i c a t i o n s t o Southern Utah: Regional Earthquake D i s t r i b u t i o n s and S e i s m i c i t y of Geothermal Areas
by D. J . Wechsler and R. B. Smit:h
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ABSTRACT
Three techniques f o r the computation of earthquake hypocenter locations were compared t h r o u g h empirical r e s u l t s of synthetic t e s t cases u t i l i z i n g four different computer programs.
IO
approaches:
The three
( 1 ) the single event method, ( 2 ) the master event vethod,
and ( 3 ) the j o i n t hypocenter determination method , were analyzed w i t h
respect t o t h e i r application t o regional (epicenter-to-station distances from 10 t o 500 k m ) and local (epicentral distances from 0 t o 70 km) seismic network recording situations.
I t was f o u n d t h a t the
j o i n t hypocenter technique can significantly correct f o r inadequacies i n assumed velocity models by simultaneous computation of s t a t i o n
adjustments.
Earthquakes located using the j o i n t hypocenter technique
are located more precisely w i t h respect t o each other.
The j o i n t
epicenter location technique was then applied t o relocations of epicenters i n southern U t a h below 40"N l a t i t u d e w i t h the intent of resolving some spatial relationships of epicenter occurrence on t h i s
8
regional scale.
Earthquakes i n U t a h occur i n a diffuse zone 150 km t o
200 k m wide t h a t coincides w i t h the physiographic boundary of the
C o l o r a d o Plateau.
0
Swarm a c t i v i t y i s prevalent where the zone changes
from a north-south t o a southwest orientation, b u t except for a
c l u s t e r northwest of Cedar C i t y , the only major alignments of epicenters are north-south.
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On a local scale an objective was i n determining some
1
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relationships of epicenters t o two known geothermal resource areas: \
Cove F o r t a n d Roosevelt Hot Springs, Utah.
The differences i n the
character of these two areas as expressed by the earthquake occurrence
was emphasized by the j o i n t l y determined re1 ocations.
Areas of
possible active east-west and northeast faulting near Cove Fort were delineated, b u t i t was verified t h a t seismic a c t i v i t y i n the Milford area i s sparse.
Attention was given t o possible e f f e c t s of l a t o r a l
inhomogeneities i n the velocity model on earthquake locations. Evidence for substanti a1 1 ateral v a r i a t i o n s i n the vel oci t y model s used i n the locations was contributed from an analysis of P-wave residuals from broadside refraction d a t a .
Arrival times o f P-waves
were found t o be early near the Opal Mound fault., possibly due t o local siliceous cementation of a l l u v i u m .
Contacts between the
gneissic basement and the granitic pluton could account f o r some of the observed delays in P-wave a r r i v a l times.
Effects of l a t e r a l
velocity variations on earthquake locations were minimized by precise relocation using j o i n t hypocenter determination, thus a i d i n g i s defining seismically active areas.
Results of earthquake relocation
for the Cove Fort-Roosevelt Hot Springs KGRA's and adjacent areas
indicate t h a t methods of systematically and precisely locating earthquakes can better contribute t o understanding the seismicity and IO
velocity structure and the relationships of geothermal areas t o regional and local s t r e s s f i e l d s and geologic c h a r a c t e r i s t i c s .
0
1
TABLE OF CONTENTS Page
. . . . . . . . . . . . . . . . . . . . . . . . iv LIST OF TABLES. . . . . . . . . . . . . . . . . . . . . . . . . . v j i i ILLUSTRATIONS . . . . . . . . . .. . . . . . . . . . . . . . . ix ACKNOWLEDGEMENTS. . . . . . . . . . . . . . . . . . . . . . . . . x j i ABSTRACT..
CHAPTER
......................
1.
INTRODUCTION
2.
THEORETICAL CONSIDERATIONS AND NUMERICAL APPROACHES TO
............ Theory.. . . . . . . . . . . . . . . . . . . . . C o r r e l a t i o n o f R e s i d u a l s and Parameters. . . . . . Confidence Regions . . . . . . . . . . . . . . . . Determinants . . . . . . . . . . . . . . . . . . . Numerical Methods. . . . . . . . . . . . . . . . . COMPARISON OF HYPOCENTER LOCATION TECHNIQUES . . . .. The S i n g l e Event Method. . . . . . . . . . . . . . The Master Event Technique . . . . . . . . . . . . J o i n t Hypocenter D e t e r m i n a t i o n . . . . . . . . . . Comparison o f S p e c i f i c Programs. . . . . . . . . . P o i n t s ,of Comparison . . . . . . . . . . . . . . . S y n t h e t i c Data Generation. . . . . . . . . . . . . Regional Recording S i t u a t i o n . . . . . . . . . . . A p p l i c a t i o n s t o Southern Utah. . . . . . . . . . . P r e c i s i o n and Accuracy o f R e s u l t s . . . . . . . . . A n a l y s i s on a Local Scale. . . . . . . . . . . . . PATTERNS OF EARTHQUAKE DISTRIBUTION I N SOUTHERN UTAH . . Summary o f S t r u c t u r a l and T e c t o n i c C h a r a c t e r i s t i c s General P a t t e r n s o f Earthquake Occurrence. . . . . Procedures f o r A p p l i c a t i o n o f Method . . . . . . . I n t e r p r e t a t i o n s o f Revised Earthquake L o c a t i o n s . . THE HYPOCENTER LOCATION PROBLEM.
IQ
3.
0
61 4.
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1
5 5 11 14 17 17
21 21
22 24 29 29 31 32 37 48
55 69
69 71 75 80
TABLE OF CONTENTS (Cont.) Page
5.
EARTHQUAKE RELOCATIONS AND P-WAVE DELAY DATA FROM BROADSIDE REFRACTION RECORDS: AND COVE FORT AREAS
ROOSEVELT HOT SPRINGS
..
101
. . . . . 101
Earthquake A c t i v i t y i n Geothermal Areas. R e l o c a t i o n of Earthquakes i n the K o o s e v e l t Hot S p r i n g s and Cove Fort KGRA's I n t e r p r e t a t i o n o f R e l o c a t e d Epicenters V e l o c i t y Models and P-Wave Delays.
. . . . . .. .. .. .......105 109 . . . . . . . . 113
6.
REFERENCES
.........................
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0
vii
.127
LIST OF TABLES Table
Page
4.1
V e l o c i t y models f o r r e g i o n a l r e l o c a t i o n s :
5.1
V e l o c i t y models f o r l o c a l r e l o c a t i o n s : Roosevelt Hot S p r i n g s
s o u t h e r n Utah
..
...................
81
Cove F o r t
108
IL LU S TR AT I0 N S
Page
P1 a t e
I
Comparison o f Hypocenter L o c a t i o n Techniques.
. . . . . 123
Figure 2.1
Diagram i l l u s t r a t i n q d i f f e r e n c e i n magnitude o f changes
. . . . 13 events. . 33
i n h y p o c e n t r a l parameters w i t h s t a t i o n d i s t a n c e
3.1
Contour p l o t s o f l o c a t i o n e r r o r f o r s y n t h e t i c
3.2-6
Contour p l o t s o f l o c a t i o n e r r o r f o r s y n t h e t i c e v e n t s
. . . . . . . . . . . . . . . . . . . 39 Program HYPOELLIPSE: s t a t i o n s t i e f o r e 10-01-74 . . 40
i n s o u t h e r n Utah. 3.2 3.3 3.4 3.5 3.6
3.7
. . . . . 41 Program HYPOELLIPSE: s t a t i o n s a f t e r 10-01-74. . . 42 Program JHD77: s t a t i o n s a f t e r 10-01-74. . . . . . 43 Program SE77: s t a t i o n s a f t e r 10-01-74 . . . . . . 44 Program JHD77:
s t a t i o n s b e f o r e 10-01-74
Magnitudes and d r e c t i o n s o f q i s l o c a t i o n o f s y n t h e t i c events u s i n g two d i f f e r e n t r e f e r e n c e e v e n t s
IO
3.8
S t a t i s t i c a l r e s u t s f o r s y n t h e t i c e v e n t s o f F i g u r e 3.1.
3.9
Misplacement (kv) o f s y n t h e t i c e v e n t s o f F i q u r e 3.2
3.10
Misplacenent (km) o f s y n t h e t i c e v e n t s o f F i q u r e 3.3
3.11
Compari son o f two j o i n t hypocenter d e t e m i n a t i o n programs.
3.12
0
dr3
. . . . . . 46 50
. . 53 . . 54
. . . . . . . . . . . . . . . . . . . . . .57
I l l u s t r a t i o n o f a f a l s e v i n i v u n l i n t h e s o l u t i o n space f o r a p a r t i c u l a r event.
. . . . . . . . . . . . . . 61 *,
ILLUSTRATIONS (Cont .) Figure
3.13
Paqe D e v i a t i o n s o f 14 e s t i m a t e d e p i c e n t e r s f r o m t r u e
. . . . . . . . . . . . . 65 i n s o u t h e r n Utah. . . . . . 73
l o c a t i o n s o f s y n t h e t i c events
4.1
Map o f r e l o c a t e d e p i c e n t e r s
4.2
S i x areas o f r e g i o n a l e p i c e n t e r s t u d i e s i n s o u t h e r n Utah w i t h r e f e r e n c e events b e f o r e Oct. 1, 1974.
4.3
Areas o f r e g i o n a l e p i c e n t e r s t u d i e s w i t h r e f e r e n c e events a f t e r Oct. 1, 1974
4.4-9
. . . . . . . . . . . . . . 77
Regional maps o f r e l o c a t e d earthquakes , June 1962
. . . . . . . . . . . . . . . . . 82 R e g i o n I . . . . . . . . . . . . . . . . . . . . . 83 Region 11. . . . . . . . . . . . . . . . . . . . 84
t h r o u g h August 1978
4.4 4.5
,.
*,
4.8
. . . . . . . . . . . . . . . . . . . . 85 Region I V . . . . . . . . . . . . . . . . . . . . 86 Region V . . . . . . . . . . . . . . . . . . . . 87
4.9
Region VI.
4.6 4.7
Region I11
,.
I .
. . . . . . . . . . . . . . . . . . . 8% I.
...
4.10
Seven events o f magnitude g r e a t e r t h a n 3.0 (M2)
4.11
Histograms o f c u m u l a t i v e s e i s m i c energy r e l e a s e a l o n g t h e Colorado P l a t e a u
5.1
-
5.2 5.3a
5.3h
96
B a s i n and Range t r a n s i t i o n zone. 98
L o c a t i o n s o f s t a t i o n s f r o m q i c r o e a r t h q u a k e survey i n
. . . . . . 106 Relocated earthquakes i n t h e Cove F o r t - M i l f o r d area . 110 L o c a t i o n flap o f b r o a d s i d e r e f r a c t i o n a r r a y . . . . . . . 115 Residual p l o t f o r shot p o i n t 1. . . . . . . . . . . . 115 t h e R o o s e v e l t Hot S p r i n g s
10
. . . . 76
-
Cove F o r t area.
,.
X
1
ILLUSTRATIONS (Cont .) Page
Figure
IP
5.4a
Residual p l o t f o r shot p o i n t 3.
5.4b
Residual p l o t f o r s h o t p o i n t 5 .
5.5a
Residual p l o t f o r shot p o i n t 7.
5.5b
Residual p l o t f o r shot g o i n t 9 .
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xi
. . . . . . . . . . . 116 . . . . . . . . . . . . . 116 ,.
. . . . . . . . . . . . .117 . . . . . . . . . . . . 117
ACKNOWLEDGE ME NT S
Computer programs were made available with the assistance of John Dewey and Bruce Julian of the U.S.C.S.
and Bob Urhammer a t Berkeley.
This work was supported under the Department of Energy, Division of Geothermal Energy (DOE/DGE) contract number DE-AC07-78ET28392. Appreciation i s extended t o my committee members, R. B. Smith, 14. J . Arabasz, and R . L. B r u h n , f o r t h e i r assistance and cooperation. Many other students and faculty participated in helpful discussions on various aspects of the problem. I would also l i k e t o communicate my appreciation t o the persons
whose ccnpanionship in the past few years has proved invaluable, especially t o M. M. Schilly, M. Griscom, M. E. Conger, and A. G . Rappoport.
I wish t o express special gratitude f o r the s t a b i l i z i n g
support of my parents, whose example will always be a guiding influence.
CHAPTER 1 INTRODUCTION The a c c u r a t e l o c a t i o n o f earthquake hy oce t e r s has been t h e concern o f many s e i s m o l o g i s t s s i n c e G e i g e r f i r s t ; i n t r o d u c e d t h e l e a s t squares approach t o t h e problem i n 1912 (Lee and Lahr, 1975).
Most
computer a l g o r i t h m s p r e s e n t l y a v a i l a b l e a r e based upon s i m i l a r assumptions c o n c e r n i n g t h e f o r m u l a t i o n o f t h e f o r w a r d problem and t h e s t a t i s t i c s i n v o l v e d , and work q u i t e w e l l f o r t h e i d e a l r e c o r d i n g s i t u a t i o n w i t h simple f l a t - l a y e r e d v e l o c i t y Structure.
Problems,
however, e x i s t i n t h e common s i t u a t i o n s o f l a t e r a l i n h o m o g e n e i t i e s i n t h e v e l o c i t y s t r u c t u r e , unknown v e l o c i t y s t r u c t u r e , o r v a r i a b l e i n s t r u m e n t a l coverage r a n g i n g f r o m good t o poor o v e r s h o r t d i s t a n c e s . These problems a r e f r e q u e n t l y ignored. C o n s i d e r i n g t h e growing number o f nethods a v a i l a b l e t o t r e a t t h e hypocenter l o c a t i o n problem, a r e some more s u i t a b l e t o s p e c i f i c r e g i o n s t h a n o t h e r s ? A p r i m a r y o b j e c t i v e o f t h i s t h e s i s i s an e v a l u a t i o n o f t h r e e approaches t o t h e c o m p u t a t i o n o f e p i c e n t e r s :
(1)
t h e s i n g l e - e v e n t method, ( 2 ) t h e master-event method, and ( 3 ) j o i n t epicenter determination.
The f a c t t h a t a numerical s o l u t i o n i s
obtained i s not necessarily s u f f i c i e n t j u s t i f i c a t i o n f o r using a p a r t i c u l a r technique.
The r e a l q u e s t i o n i s , do t h e r e s u l t s add
s i g n i f i c a n t l y t o t h e u n d e r s t a n d i n g o f t h e problern? A p p l i c a t i o n t o earthquakes i n southern Utah i s o f c h i e f i n t e r e s t here, p a r t i c u l a r l y
2 t h e r e s o l u t i o n o f e p i c e n t e r s and t h e d e l i n e a t i o n o f e p i c e n t r a l p a t t e r n s t o examine s p a t i a l a s s o c i a t i o n s and r e l a t i o n s t o geothermal areas. Compared t o o t h e r r e g i o n s covered by t h e U r i i v e r s i t y o f Utah s e i s m i c network, r e l a t i v e l y l i t t l e work has been done i n s o u t h e r n Utah.
Because o f t h e sparse s t a t i o n d i s t r i b u t i o n i n t h i s area, w i t h
an average s t a t i o n - s p a c i n g o f g r e a t e r t h a n 100 km,the problems i n earthquake l o c a t i o n a r e compounded.
T h i s evphasizes t h e need f o r
optimum t e c h n i q u e s f o r r e l o c a t i n g e p i c e n t e r s . I t has been n o t e d (e.g.,
Ward, 1972) t h a t t h e r e may be a
c o r r e l a t i o n between s e i s m i c i t y and area o f geothermal p o t e n t i a l . workers (e.g.
Some
Smith, 1978; Anderson, 1978) have c o n s i d e r e d t h e
p o s s i b i l i t y t h a t t h e t r e n d s of e p i c e n t e r s and Q u a t e r n a r y d e f o r m a t i o n
1P
i n s o u t h e r n Utah, which seen t o show a change i n o r i e n t a t i o n a t about
38" N l a t i t u d e , have a t e c t o n i c s i g n i f i c a n c e t h a t may be r e l a t e d t o t h e o c c u r r e n c e o f s e v e r a l known geothermal r e s o u r c e areas.
But
because t h e q u a l i t y o f t h e s o l u t ons was r e l a t i v e l y unknown i t was d i f f i c u l t t o make any d e f i n i t i v e statements about p o s s i b l e t r e n d s observed.
L o c a l microearthquake surveys (e.g.,
Olson 1976), because
o f t h e i r l i m i t e d a r e a l coverage, have n o t c l a r i f i e d r e g i o n a l r e 1 a t i o n s h i ps.
0
O f supplementary c o n s i d e r a t i o n i n a c c u r a t e l y d e t e r m i n i n g t h e s e i s m i c i t y o f s o u t h e r n Utah i s t h e r e c e n t i n t e r e s t i n t h a t area f o r t h e s i t i n g o f U. S. s t r a t e g i c m i s s i l e systems and t h e development o f
1 arge-scal e commerci a1 power p l a n t s . F u r t h e r i n c e n t i v e f o r a t t e m p t i n g t h i s p r o j e c t was generated
io
3 d u r i n g t h e c o u r s e of r e v i s i n g earthquake l o c a t i o n s i n Utah f o r a 1962-1978 c a t a l o g (Arabasz
and McKee, 1979).
R e l o c a t i o n s were i n i t i a l l y computed w i t h HYP071, a program w r i t t e n b y Lee and L a h r (1975).
Subsequently, HYPOELLIPSE ( L a h r , 1979) was
adopted f o r t h e s y s t e m a t i c r e l o c a t i o n s .
The r e l a t i v e m e r i t s o f u s i n g
HYPOELLIPSE and d i s c u s s i o n s r e g a r d i n g t h e b e s t procedure f o r l o c a t i n g r e g i o n a l s e i s m i c e v e n t s suggested t h a t a more c a r e f u l a n a l y s i s o f t h e methods and problems s p e c i f i c t o earthquake l o c a t i o n i n Utah was i n order. The a n a l y s i s r e q u i r e d e x t e n s i v e t e s t i n g o f s e v e r a l computer programs i n o r d e r t o q u a n t i t a t i v e l y d e t e r m i n e t h e i r r e s p e c t i v e a p p l i c a b i l i t y t o t h e p a r t i c u l a r c h a r a c t e r i s t i c s o f earthquake r e c o r d i n g i n Utah.
This involved the generation o f synthetic data t o
s i m u l a t e e p i c e n t e r s recorded by seismic a r r a y s i n s o u t h e r n Utah, b o t h on a r e g i o n a l and a l o c a l i z e d s c a l e .
The t e s t i n g i n d i c a t e d t h e
l i m i t a t i o n s i n t h e accuracy t h a t can be a n t i c i p a t e d f o r l o c a t i o n s i n v a r i o u s areas o f t h e s t a t e r e l a t i v e t o t h e i n s t r u m e n t a l coverage.
It
a l s o enabled an e s t i m a t i o n o f t h e s e n s i t i v i t i e s o f p a r t i c u l a r l o c a t i o n t e c h n i q u e s t o i n s t a b i l i t i e s i n s o l u t i o n s i n t r o d u c e d by v a r i a b l e s t a t i o n coverage.
The u s e f u l n e s s o f t h e t e c h n i q u e s can now be shown
s p e c i f i c a l l y f o r s o u t h e r n Utah, b u t t h e s t u d y i n c o r p o r a t e d enough
0
g e n e r a l i t y t o enable e x t r a p o l a t i o n o f m a j o r c o n c l u s i o n s t o any r e g i o n a l o r l o c a l seismograph s t a t i o n s array. The b a s i c t h e o r y and methods o f earthquake l o c a t i o n t e c h n i q u e s a r e o u t l i n e d i n Chapter 2.
More d e t a i l e d and comprehensive
e x p l a n a t i o n s o f t h e nethods and programs a r e p r e s e n t e d i n Chapter 3,
4
the body of which concerns the r e s u l t s of t e s t i n g and comparison analysis.
I n Chapters 4 and 5 , the methods and conclusions discussed
i n Chapter 3 are applied t o specific regions i n U t a h , u s i n g relocation
of epicenters t o define more precisely any l i n e a r or clustering trends
Characteristics of earthquake occurrence are then
i n the d a t a .
discussed i n l i g h t of t h e i r structural and tectonic significance.
The
focus i s f i r s t on a regional scale (Chapter 4 ) , subsequently narrowing
t o specific l o c a l i t i e s around the Cove Fort-Roosevelt Hot Springs l o c a l i t i e s (Chapter 5 ) .
The issue i s assessment, of the value of
regional and local seismicity studies i n locating or studying potential geothermal areas. *
CHAPTER 2 THEORETICAL CONSIDERATIONS AND NUMERICAL APPROACHES TO THE HYPOCENTER LOCATION PROBLEM
Presented here i s a summary of the theoretical basis t h a t a l l o f the techniques t o be considered share i n common. A brief review of the fundamental aspects i s followed by a short section on the commonly employed numerical techniques used i n solving the problem. Implications of s t a t i s t i c a l r e s u l t s obtained by the l e a s t squares approach are discussed, as well as frequent misconceptions and possible problems related t o some b a s i c assumptions. Theory I n the earthquake location problem we are interested i n four
parameters:
the three spatial coordinates and the o r i g i n time.
These
a r e n o n l i n e a r functions o f a r r i v a l times o f various phases ( u s u a l l y P
a n d / o r S ) observed a t s t a t i o n s recording the event.
The arrival times
are dependent on the positions of the stations r e l a t i v e t o the event
as well as the velocity-depth relationships along the ray p a t h . Essentially, then, t h i s i s an inverse problem based upon several IP
simplifying assumptions about source mechanisms governing earthquakes. A point source of energy generates seismic waves t h a t a r r i v e a t the
recording s t a t i o n s ; these rays must somehow be traced back t o t h e i r source u s i n g information derived from t h e i r observed a r r i v a l times.
6 The m a j o r elements o f t h e f o l l o w i n g d i s c u s s i o n can be found i n b a s i c r e f e r e n c e s on s t a t i s t i c s o r s o l u t i o n o f p a r a m e t r i c e q u a t i o n s (e.g.,
Scheffe',
1959; Draper and Smith, 1966; Beck and Arnold, 1977).
My purpose i s t o c l a r i f y c e r t a i n assumptions and methods t h a t have
been i m p l i c i t l y o r e x p l i c i t l y a p p l i e d i n earthquake l o c a t i o n programs, o f t e n without proper explanation i f not incorrec:tly.
Also, c o q p a r i s o n
o f t h e programs and methods r e q u i r e s a t l e a s t a c u r s o r y u n d e r s t a n d i n g o f t h e t h e o r e t i c a l and s t a t i s t i c a l background used t o develop t h e c r i t e r i a by which t h e accuracy o f t h e r e s u l t i n g s o l u t i o n s o b t a i n e d i s judged. I n the f o l l o w i n g discussion, a l l variables i n c a p i t a l s refer t o m a t r i c e s , small l e t t e r s a r e v e c t o r s , and s u b s c r i p t e d v a r i a b l e s designate s c a l a r q u a n t i t i e s .
E s t i m a t o r s , whether v e c t o r o r s c a l a r ,
a r e designed by a h a t o v e r t h e v a r i a b l e . The problem as i t i s p r e s e n t l y approached b y s e i s m o l o g i s t s i s e s s e n t i a l l y an a n a l y s i s o f v a r i a n c e o r a l i n e a r r e g r e s s i o n on f o u r v a r i a b l e s ( B o l t and Freedman, 1968).
L e t x j , y j , z j , and t o j be t h e
s p a t i a l c o o r d i n a t e s and o r i g i n t i m e , r e s p e c t i v e l y , o f t h e j t h earthquake.
I f n i s t h e t o t a l number o f a r r i v a l t i m e s observed f o r
TI
earthquakes, t h e n t i s t h e ( n x 1 ) v e c t o r o f o b s e r v a t i o n s , y i s t h e (4m x 1 ) v e c t o r o f parameter changes (dx, dy, d r y d to), and A i s t h e
( n x 4m) q a t r i x r e f e r r e d t o as t h e system, c o n d i t i o n , o r c o e f f i c i e n t matrix.
The v e c t o r t i s expressed as a l i n e a r f u n c t i o n o f t h e
parameters; t h e n t h e hypocenter i s a d j u s t e d u n t i l t h e d i f f e r e n c e !Q
between t h e observed and c a l c u l a t e d a r r i v a l times, t h e r e s i d u a l r i j f o r t h e i t h s t a t i o n and j t h earthquake, i s minimized.
The l i n e a r i z e d
7
function can be expressed as a Taylor series expansion of the travel time about some i n i t i a l guess, and the estimate of the residual becomes A
r
= r + ij ij
a x jj x j
at + "Yj i j
a t ij
+ *Zj
'
ay3
neglecting higher order terms.
+
ati e t ;
J
j
+
e
ij
(2.1)
I n nratrix n o t a t i o n ,
I
r = A y + e.
(2.2)
The vector o f residuals, P, i s only a n estimate because we can never
know the true error i n the solution. j , where
7
The true error would be r = y
i s the vector of estimated parameters.
actually solving the systen! r^ = Ay (Conte and de Boor, 1972).
-
-
Instead we ar2
A i , or ? = Ar, since Ay = t
To simplify n o t a t i o n , however, i n
subsequent discussion the h a t over the r i s dropped. Ne seek t o minimize the q u a n t i t y 2 -1 Qj wijr ij,
-i4,
where 1 i s the number of observations for a particular earthquake j ( n = 1 X m) and Wij
station residual IO
are linear or non-linear weights a p p l i e d t o each
.
Various weighting procedures w i 1 1 be d i scussed i n
sections treating specific programs.
The m a t r i x A i n equation 2 . 3 has
been redefined t o include the weights \ti j
To retrieve t h e least
squares unbiased estimates of y t h a t w i l l minimize 2.3 the normal equations are a p p l ied ( ScheffeO, 1959) :
8
or
ATA y = AT r
,
then, (2.4)
The problem i s solved iteratively, w i t h the new estimates of y added
t o the previous hypocenter parameters u n t i l Q j i s reduced t o some specified m i n i m u m value.
Alternative methods f o r terminating the
iteration procedure are discussed
n Chapter 3.
There are several assumptions w h i c h a r e i m p l i c i t i n the above development: i ) t h e errors are add tive, i i ) E ( e ) = 0 , or the expectation o f the error vector e s zero, and i i i ) E (eTe) = V ( e ) = 2
6 I,
or the variance of the errors is constant and uncorrel a t e d (Beck
and Arnold, 1977).
I n other words
the errors are assumed t o be
independent and normally distributed as N 1968).
(O,a
2:) ( B o l t and Freedman,
I n general, these assumptions can p r o b a b l y b e considered v a l i d
f o r random errors due t o mistakes and inconsistency i n the t i m i n g and p i c k i n g of earthquake seismograms o r t o the misidentification of
phases.
However, f o r the residuals, which are estimates of the t o t a l
error, the assumptions may not.be justified.
It. has been p o i n t e d o u t
t h a t the t o t a l residual i s composed of random errors plus non-random
station and source terms, and t h a t unless these a r e estimated 10
separately the errors are not normal l y distributed (Bo1 t and Freedman , 1968). B u l and (1976) has reported t h a t certain characteristics of the
9
residuals for a localized array are known from experience.
lQ
Under
these circumstances the ordinary least squares procedure i s n o t recommended (Beck and Arnold, 1977; Draper and Smith, 1966), and application o f the maximum l i k e l i h o o d estimation involving minimization o f the sum of absolute errors rather t h a n squares of errors may be more appropriate ( B u l a n d , 1976). As previously mentioned, the residual gives a n estimate of the error i n the solution; therefore, careful analysis of the residuals may help t o confirm or invalidate the assumptions made (e.g., tests
for normal distribution). However, the magnitude of the residual does not always give an i n d i c a t i o n o f the magnitude o f the true error i n
the solution, as i s often t h o u g h t t o be the case.
( F o r a simple
example see Conte and de Boor, 1972, p. 143.) How well the residual approximates the true error i s dependent upon the "size1' of the system matrix and i t s inverse.
The "size" i s measured i n terms of some
matrix norm, w h i c h i s always greater t h a n or equal t o 1, and i s
denoted as
A measure. of the conditfon of R f s the condition
IIAllq
number,
W e already have t h a t A t = y,
Ae Then
=
e = r-
r.
A '
and similarly,
1 10
'Gs
and t a k i n g .norms (Ral ston and Rabi nowi t z , 1979) ,,
IIell A1 so ,
-- IIrll II All
II tll = IIA-lIl
*
(2.7) IIyll
D i v i d i n g ( 2 . 6 ) by ( 2 . 7 ) , a lower bound for the relative error i s
A similar derivation (Ralston and Rabinowitz, 1978) resu t s in an. uwer bound, so t h a t the relative error of the so1utiol.n can be expressed i n
terms o f the relative residual as
So i f C ( A ) i s close t o u n i t y , the relative error w i l l be essentially
equivalent t o the relative residual, and the system i s described as well-conditioned.
B u t i f the condition number i s large, the relative
residual places very l i t t l e constraint on the possible values o f the relative error (Conte and de Boor, 1972).
Furthermore, i t has been
shown ( B u l a n d , 1976) t h a t C ( A ) for the earthquake location problem can be very l a r g e q u i t e near t h e a r r a y s t a t i o n s , and i f t h e s t a t i o n
distribution i s poor, or i f only P arrivals are used, the condition number can b e large everywhere. a1 ways be i 1 1 -conditioned
T h a t i s , the system matrix A nay
.
The size of the condition number can also indicate how much o f the error i n the solution vector ( y ) i s contributed by error i n the d a t a vector ( t ) (Ralston and Rabinowitz, 1978).
However, i n most
practical cases, i t i s very d i f f i c u l t t o compute A - 1 exactly, so C ( A ) may be quite inaccurate; i n f a c t , i t i s seldom computed.
The
11
discussion here was t o emphasize the inadequacy of the use of t h e residual as a direct estimate of the error i n the solution. Correlation of Residuals and Parameters
I n general there are 4m parameters (recalling t h a t rn i s the number of earthquakes) estimated by n observations. have only n
- 4m degrees
The residuals
of freedom; thus they are not independent.
I t can be shown t h a t the correlations i n the residuals depend entirely
on the elements of the A matrix (Draper and S m i t h , 1966, p. 93-94). Correlations of coefficients of A are of course correlations o f parameters.
I t i s known from empirical results t h a t origin time and
d e p t h generally exhibit a h i g h correlation coefficient.
This may be
seen by examining the elements of the system matrix i n more detail. The u i j are the coefficients of the u n k n o w n parameters, or the p a r t i a l derivatives o f the travel times w i t h respect t o each parameter, and
can be written as:
7
Q
(2.10)
Since the least squares procedure for earthquake l o c a t i o n as described above i s ori g i nal l y attributed t o Geiger, these are c o m o n l y
12
referred t o as Geiger's coefficients.
\ k i t i n g them i n t h i s f a s h i o n
gives some insight as t o the r e l a t i v e significance o f the earthquake parameters i n the s t a b i l i t y o f the problem and t h e i r differing degrees of resolution i n the ultimate solution.
All of t h e coefficients
include the reciprocal of the slope of the travel-time curve ( l / V a p p ) . Vapp i s the apparent velocity a t the particular distance Aij, ( i t h
recording s t a t i o n for j t h hypocenter) and can e i t h e r be read d i r e c t l y from travel time tables o r calculated from the velocity model.
It is
i n t u i t i v e l y reasonable t o suppose, and f u r t h e r verified from geometrical considerations, t h a t the change i n h i j w i t h respect t o a
10
change i n depth i s small when compared t o changes with respect t o x and y ( 1 ongi tude and 1 a t i tude, usual l y )
as
.
A i j f o r the nearest s t a t i o n increases.
T h i s becomes more pronounced
Figure 2.1 i l l u s t r a t e s
t h i s e f f e c t , and also shows the beneficial e f f e c t of even one d i s t a n t
s t a t i o n t o enhance the azimuthal control.
For depth, the difference
i n magnitude of t h e partial derivatives f o r each s t a t i o n becomes
dependent only on the change i t s e l f .
The derivatives for depth d o not
have the extra azimuthal effect at separate stations, which h e l p s to
assure differing parameter changes i n x and y.
A t a particular
distance from the array, which i s a function of the array geometry w i t h respect t o the event, the s t a t i o n residuals a r e no longer
affected independently,by a change i n focal depth.
(This can a l s o
occur i f the a r r a y has a poor distribution of s t a t i o n s w i t h respect t o distance, even i f the event i s within the a r r a y . )
io
F o r an event a t or
outside t h i s distance, any adjustment i n depth can be d i r e c t l y compensated f o r a t a l l stations by an adjustment i n the o r i g i n time,
:u
I
4
DETECTIWG STATION
I
Figure 2.1. Diagram i l l u s t r a t i n g d i f f e r e n c e i n magnitude o f changes i n hypocentral parameters w i t h s t a t i o n distance. (1) change o f epicenter. ( 2 ) change o f focal depth ( s t a t i o n s projected t o l i n e containing hypocenter). Changes i n focal depth parameter w i l l not be as w e l l resolved.
14
since the corresponding change i n distance i s e s s e n t i a l l y equal f o r a l l s t a t i o n s and i s therefore no longer a factor. I f there are no stations w i t h i n a distance of 1 t o 2 times the
focal depth, t h e e n t i r e column of the system matrix associated with the depth parameter may approach zero, which will make the solution unstable i f not unsolvable (reducing the rank o f A by one).
In
a d d i t i o n , since the change i n the residuals i s r e l a t i v e l y insensitive 8
t o depth ( a z j ) , the control o f the step-size f o r t h i s parameter i s not we1 1 -regulated and the depth may fluctuate w i Idly, seeming t o converge t o some meaningless and obviously erroneous values.
The wrong value
f o r depth then has an adverse a f f e c t on the epicenter location.
For
these reasons, some programs include a convention t o f i x the focal depths a t some predetermined value and reduce the dimension of the condition m a t r i x A.
Focal depths a r e commonly fixed a t sone
reasonable average value f o r a particular l o c a l i t y i f the distance from the event t o the nearest s t a t i o n i s greater t h a n 1 t o 2 times the
assumed focal depth o r i f the epicentral distance i s greater t h a n the array diameter.
Confidence Regions The confidence e l l i p s o i d commonly computed f o r earthquake location solutions i s another s t a t i s t i c a l r e s u l t t h a t i s often incorrectly interpreted.
Flinn (1965) i s usually credited w i t h being
the f i r s t t o introduce the concept t o seismologists concerned w i t h the I
location of earthquakes.
1 0
subsequently incorporated the techniques i n t o t h e i r programs, d i d n o t
Flinn, and presumably most of those who have
15 I
misinterpret the meaning of the confidence regions, b u t the lack of
IP
adequate explanations l e f t much room f o r l a t e r misconception. The development of the equation f o r the j o i n t confidence region
as used by Flinn and other workers can be f o u n d i n references on probability and s t a t i s t i c s in problem solving (e.g. Draper and S m i t h , 1966; Beck and Arnold, 1977; Scheffe-, 1959).
The j o i n t 100 (1
-
a)%
confidence region f o r a l l parameters i s expressed as A T T 2 (Y-Y) A A ( Y - ~ ) = ps F ( p , n - p , l - a )
,
(2.11)
where p i s the number of parameters ( p = 4m) or elements o f y , n i s the number of observations, and s2 i s the estimate o f e r r o r variance. This estimate i s obtained from equation 2 . 3 as s2 =
Qj
,
(2.12)
0
and i s also referred t o as the mean squared e r r o r or s t a n d a r d e r r o r
(Draper and S m i t h , 1966).
F ( p , n-p, 1-a) i s the (1-a) percentile
p o i n t of the F distribution found from tables of t h e F or distributions.
x2
Note t h a t (ATA)/sZ i s t h e inverse o f t h e parameter
covariance matrix, and from t h i s the standard e r r o r f o r each parameter i s P i i l / 2 , where P = s2 ( A T A ) - l (Flinn, 1965; Draper and S m i t h , 1966). Confidence e l l i p s o i d s can be computed f o r the hypocenter ( t h r e e spatial parameters), b u t are often computed just for the epicenter (two location parameters). final i t e r a t i o n .
Usual l y they a r e calculated d u r i n g the
Sometimes an extra i t e r a t i o n i s performed t o o b t a i n
the s t a t i s t i c s , since the elements of the computation a r e dependent on the magnitude o f the l a s t movement i n parameter 'space.
16
The problem w i t h presentation of the confidence e l l i p s o i d s has been pointed o u t by Evernden (1969a),and i s summarized here.
I f the
probability t h a t the t r u e hypocenter will be w i t h i n the confidence region i s X, then the correct interpretation of the 100% confidence region around the estimated hypocenter i s as follows.
I n the
frequency interpretation, in 100 estimates of hypocenters, 100( X ) of
the computed confidence e l l i p s o i d s will cover the corresponding t r u e hypocenter.
This does not say anything about t h e probable location
e r r o r of the t r u e hypocenter. e l l i p s o i d does
not indicate
I n other words, w h a t the confidence
i s how f a r o f f the estimated solution i s
l i k e l y t o be for a particular event.
I t does n o t place e r r o r bounds
on the t r u e hypocenter, which theoretically could l i e anywhere w i t h i n the confidence region or anywhere outside of i t .
The procedures f o r
o b t a i n i n g a probable e r r o r estimate are di'fferent, and involve
computing solutions t o a number of t e s t cases and subjecting the r e s u l t s t o methods of normal bivariate s t a t i s t i c a l analysis (Evernden, 1969a). The value of the confidence ellipsoids lies i n the fact that
t h e i r orientation and s i z e are dependent on t h e i r position w i t h respect t o the stations used t o record the events ( F l i n n , 1965). Therefore they can give a q u a l i t a t i v e indication of the capability of the seismic array i n resolving hypocenters i n a particular region. The orientation and elongation of e l l i p s e s can a l s o show, f o r a specific area, whether trends seen i n the epicenters could be due t o a preferential dispersion of the solutions which i s solely the r e s u l t of
0
da
the a r r a y ' s i n a b i l i t y t o locate them precisely.
17
Determi nants Another result t h a t i s obtained during the solution of the earthquake locations, and one t h a t can give a n estimate of the relative c a p a b i l i t y of the seismic array w i t h respect t o a given 0
region, i s the determinant o f the matrix of normal equations, ATA. This matrix i s essentially the inverse of the covariance matrix for
I
the parameters and hence the magnitude of i t s determinant can give an
io I
i n d i c a t i o n of how well the variance of the parameters i s b e i n g
restrained by the station distribution.
I n general, when the
determinant i s small, the variance and covariance terms of ATA w i l l be large ( F 1 i n n , 1965). The user could then contra1 the amount of variance allowable i n the parameters o f the solution by documentation of an acceptable range of values for the determinant.
Numerical Methods The purpose of this section i s t o p o i n t o u t some of the differences i n the numerical techniques employed i n each program t o solve the normal equations and t o assess these differences i n terms o f possible effects on the solutions.
I t i s beyond the scope of this
thesis t o present a comprehensive analysis of the subtle numerical aspects; therefore, the f o l l o w i n g i s a brief discussion of some characteristics t h a t may be important considerations for users of the IO
programs. Each of the programs studied employs some variation of Gaussian e l i m i n a t i o n w i t h back substitution t o o b t a i n the solution.
Dewey's
(1978) programs (JHD77 and SE77) use the classical form of Gaussian
18 reduction for a l l b u t the last iteration.
I n GHYP1, Urhammer ( B o l t ,
Okubo, and Urhammer, 1978) uses the Crout factorization; in
HYPOELLIPSE (Lahr, 1979) and HYPO71 (Lee and Lahr, 1975) the authors
use the Doolittle factorization.
The l a t t e r two methods are
vari.ations of the compact Gauss elimination technique, and are special
cases of the decomposition of the A matrix into u n i t lower ( L ) a n d upper ( U ) t r i a n g u l a r matrices and a diagonal ( 0 ) matrix (Ralston and Rabi nowi t z , 1979) :
(2.13)
A = LDU.
The Doolittle and Crout methods (like the abbreviated form of Gauss elimination) reduce roundoff errors by eliminating the need for storing intermediate resul t s t h r o u g h the accumul a t i on of i nner products and the use of double precision arithmetic.
The Crout
algorithm has additional storage advantage over the Doolittle algorithm, and they are essentially equivalent i n stability characteristics, so t h a t f o r many cases the Crout method i s selected (Ral s t o n and Rabi nowi tz, 1 9 7 8 ) .
The above numerical techniques are regarded as direct methods ( a s opposed t o iterative methods such as Jacobi and Gauss-Seidel ) , b u t they are often used iteratively, a s i s the case here. methods i s generally preferred f o r small (order
systems.
For 1 arge (order
>
2 ) indicating a large directional b i a s i n the a b i l i t y of the a r r a y t o place a n event a t t h a t point, the direction o f displacement generally corresponds t o the orientation of the major semi-axis.
For synthetic
d a t a , larger confidence el 1 ipses a1 so correspond, t o 1 arger deviations
IP
of the estimate from the true solution. T h u s a useful result i s t h a t f o r a specific region w i t h less
s t a t i o n coverage solutions may show an a r t i f i c a l l i n e a r trend or clumping.
The possibility of t h i s occurring can be anticipated by
studying the ratios and orientations of the semi-axes of the confidence e l l i p s e s .
The success of predicting such a phenomenon i s
lessened by the f a c t t h a t , for noisy d a t a , the r e s u l t s are v a l i d only i f the e f f e c t s of poor s t a t i o n d i s t r i b u t i o n are greater t h a n the
IO
noise, as was observed previously in Figure 3.8. complication e x i s t s when using the JED technique.
A further
It was stated t h a t
ELLIPSE (H Y P o E L L I P s E) SCALE
FOR V E C T O R S
* -
1
5
K
M 1
\ \ \
\ \ \
1
46
'37' 19'
Figure 3.9. Misplacement (km) of synthetic events of Figure 3 . 2 . True locations are g r i d points; tips o f arrows dre computed locations. Dashed 1 ines are major axes of standard error el 1 ipses
.
0
Irs
54 C O N F I D E N C E ELLiPSE
SCALE
FOR
VECTORS
5 KM
--=vi
4
I
?
- 40'
't
-4
-
-4-
t
IO 3;
j-37'
Figure 3.10. Misplacement (km) o f synthetic events of Figure 3 . 3 . True locations are g r i d p o i n t s ; t i p s of arrows are computed locations. Dashed 1ines are major axes o f confidence el 1 ipses.
0
Grs
55 t h e o r i e n t a t i o n o f t h e d e v i a t i o n s from t h e t r u e l o c a t i o n were dependent i n p a r t on t h e c h o i c e o f r e f e r e n c e e v e n t .
Confidence
e l l i p s e s a r e o f course t h e same r e g a r d l e s s o f t h e r e f e r e n c e event. F i g u r e 3.7 shows some o f t h e e f f e c t s o f t h e r e f e r e n c e e v e n t on t h e d i s p l a c e m e n t and t h e r e l a t i o n s h i p s t o t h e o r i e n t a t i o n s o f t h e c o n f i d e n c e e l 1 ipses
.
A n a l y s i s on a Local S c a l e Seismic event l o c a t i o n on t h e s c a l e o f a m i c r o e a r t h q u a k e survey i s d i f f e r e n t i n some r e s p e c t s f r o m l o c a t i o n w i t h a r e g i o n a l network o f permanent seismograph s t a t i o n s .
The area under c o n s i d e r a t i o n i s
u s u a l l y covered b y more s t a t i o n s ( u s u a l l y more t h a n 10) , and e v e n t s generally l i e w i t h i n t h e array. o r 70 k i l o m e t e r s .
S t a t i o n d i s t a n c e s ranqe t o about 60
T h i s means t h a t t h e p o s s i b i l i t y o f r e s o l v i n g t h e
f o c a l depths e x i s t s i n many cases.
I n a l o c a l survey, sources o f
random n o i s e such as a r e c o n t r i b u t e d by d i f f e r e n t t y p e s o f s t a t i o n s and d i f f e r e n t a n a l y s t s p i c k i n g and t i m i n g a r e p r o b a b l y reduced. The h y p o c e n t e r programs were r u n on a number o f t e s t cases u s i n g s t a t i o n s i n a m i c r o e a r t h q u a k e survey s e t up i n t h e R o o s e v e l t Hot S p r i n g s and Cove F o r t areas d u r i n g t h e summers o f 1974 and 1975 (Olson, 1976).
The s y n t h e t i c d a t a were generated i n a manner s i m i l a r
t o t h a t f o r t h e regional t e s t data. The f i r s t o b j e c t i v e was t o d e t e r m i n e t h e d i f f e r e n c e s between t h e two j o i n t l o c a t i o n programs.
Some a s p e c t s o f Dewey's program JHD77
(Dewey, 1978) have a1 r e a d y been d i s c u s s e d under t h e r e g i o n a l
!O
I ~
d i s c u s s i o n , where i t was used as a j o i n t e p i c e n t e r d e t e r m i n a t i o n (JED)
I
program.
The other program, G H Y P l (Bolt e t a1
. , 1978) was
written
by R. Urhammer t o invert on earthquake locations, s t a t i o n adjustments, and P and S velocities f o r a simple model composed of a single
half-space.
Because of the half-space model i t i s applicable only t o
a local network w i t h event-to-station distances not exceeding a b o u t sixty k i 1 ometers. A variety of different t e s t runs were performed t o experiment
w i t h a number of variables such as s t a t i o n d i s t r i b u t i o n , number of P and S a r r i v a l s , s e n s i t i v i t y t o the depth parameter, and i n i t i a l
s t a r t i n g location and origin time.
Figure 3.11 shows the r e s u l t s of a
t e s t r u n on noisy d a t a (standard deviation of noise = 0.10 s e c ) .
For
t h i s case the d a t a were generated f o r d i f f e r e n t depths, b u t i n the
solutions the depths were fixed a t one intermediate depth (7.0 kilometers).
Stations used were d i f f e r e n t for each event; the number
of recordings was intended t o be typical f o r actual events located a t similar positions.
True locations are a t the center of each p l o t ; the
points represent the deviations of the JHD epicenters from the t r u e positions.
IQ
B o t h programs failed t o locate accurately one event t h a t was
purposely created w i t h few arrival times and a t a position a t the edge of the recording array.
G H Y P l vislocated two other events t h a t were
also lacking close ( w i t h i n 20 k m ) stations.
However, f o r the most
p a r t the r e s u l t s for the two programs were comparable i n accuracy. J H D 7 7 i s the more v e r s a t i l e and general i n format; G H Y P l works as well
for a situation in which l i t t l e or nothing i s known a b o u t the velocity model.
c
0
SYNTHETIC E V E N T S JOINT HYPOCENTER P R O G R A M S
0
JHD77
GHYPI
Figure 3.11. Coniparison o f two j o i n t hypocenter determination progranis. N ne synthetic events; random noise added w i t h standard d e v i a t i o n o f 0.10 sec. Depths v a r i e d from 0 t o 1 5 km, but were f i x e d i n computations a t 7 kni. Points i n d i c a t e positions o f computed l o c a t on r e l a t i v e t o t r u e l o c a t i o n s p l o t t e d a t t h e center o f each square.
58 T h i s i n t r o d u c e s t h e q u e s t i o n o f how s e n s i t i v e t h e computed l o c a t i o n s a r e t o v a r y i n g t h e v e l o c i t y model.
A t f i r s t i t seerned
r a t h e r s u r p r i s i n g t h a t GHYPl had comparable r e s u l t s t o JHD77 s i n c e t h e v e l o c i t y model was known and used i n JHD77 whereas an average v e l o c i t y , p u r p o s e l y computed t o be s l i g h t l y t o o h i g h , was used i n
GHYP1.
B u t t h e r e s u l t s emphasize one advantage i n u s i n g a j o i n t
hypocenter d e t e r m i n a t i o n ( J H D ) method t h a t s i m u l t a n e o u s l y computes s t a t i o n adjustments.
To a l a r g e degree, any i n a d e q u a c i e s o r e r r o r s i n
t h e v e l o c i t y model a r e i n c o r p o r a t e d i n t o t h e s t a t i o n adjustments.
In
b o t h s i n g l e e v e n t programs considered, HYPOELLIPSE and SE77, t h e o p t i o n e x i s t s t o e s s e n t i a l l y improve upon known v e l o c i t y models b y a d d i n g a p p r o p r i a t e s t a t i o n d e l ays, o r adjustments.
The problem s t i 11
remains c o n c e r n i n g how t o compute t h e s e a d j u s t m e n t s i n i t i a l l y . There a r e some f e a t u r e s i n t h e program GHYPl t h a t a r e h e l p f u l i n d i a g n o s i n g some c h a r a c t e r i s t i c s o f hypocenter s o l u t i o n s , a l t h o u g h t h e y may n o t be necessary f o r r o u t i n e l o c a t i o n work.
These a d d i t i o n a l
f e a t u r e s p r o v i d e d some v a l u a b l e i n f o r m a t i o n d u r i n g t h e t e s t runs.
One
f e a t u r e was t h e p r i n t o u t o f t h e parameter c o r r e l a t i o n m a t r i x f o r each event.
IP
F o r reasons d i s c u s s e d i n Chapter 2, t h e d e p t h and o r i g i n t i m e
I
parameters were h i g h l y c o r r e l a t e d f o r a1 1 e v e n t s ( c o r r e l a t i o n c o e f f i c i e n t s g r e a t e r t h a n 0.80).
C l o s e r s c r u t i n y o f t h e degree o f
c o r r e l a t i o n w i t h respect t o various s t a t i o n d i s t r i b u t i o n s revealed t h a t as d i s t a n c e s f r o m event t o s t a t i o n decreased t h e c o r r e l a t i o n c o e f f i c i e n t between d e p t h and o r i g i n t i m e i n c r e a s e d .
Similar
o b s e r v a t i o n s were made f o r g r e a t e r a z i m u t h a l c o n t r o l , a l s o o f t e n
0
kd
accompanied by an i n c r e a s e i n t h e c o r r e l a t i o n c o e f f i c i e n t .
Initially
59 t h i s seems c o n t r a r y t o what m i g h t be expected, b u t i t can be q u a l i t a t i v e l y explained by n o t i n g t h a t , f o r a very poor d i s t r i b u t i o n
o f s t a t i o n s i n e i t h e r d i s t a n c e o r azimuth, c o n t r o l o v e r t h e f o c a l d e p t h parameter i s e s s e n t i a l l y l o s t , i.e., measured q u a n t i t y .
i t i s n o t c o r r e l a t e d t o any
As s t a t i o n d i s t r i b u t i o n improves, t h e program
r e g a i n s some c o n t r o l o v e r t h e d e p t h and t r i e s t o r e s o l v e it; t h e n i t i s more h i g h l y c o r r e l a t e d t o t h e o r i g i n t i m e parameter.
A second aspect o f t h e program G H Y P l t h a t c o n t r i b u t e s some a d d i t i o n a l i n f o r m a t i o n i s t h e method used b y Urhammer ( B o l t e t al.,
1978) t o t e r m i n a t e t h e i t e r a t i o n procedure.
It i s s i m i l a r t o
HYPOELLIPSE i n t h a t i t i s based upon t h e s t e p s i z e i n parameter space. I n t h e t e s t r u n s t h e convergence c r i t e r i o n was r a r e l y s a t i s i f i e d i f t h e f o c a l d e p t h c a l c u l a t i o n was i n c l u d e d , so t h e program was m o d i f i e d t o include only t h e epicentral coordinates i n t h i s p a r t i c u l a r cornputation.
Using a convergence c r i t e r i o n i n s t e a d o f s i m p l y
s p e c i f y i n g t h e number o f i t e r a t i o n s , as Dewey (1978) does, g i v e s t h e u s e r more c o n t r o l o v e r t h e u n i f o r m i t y o f t h e r e s u l t s .
O f course a
s i m i l a r e f f e c t can be accomplished b y e x p e r i m e n t i n g w i t h t h e number o f i t e r a t i o n s and s t u d y i n g t h e o u t p u t t o see i f t h e r e s u l t s s a t i s f y some
/o
c r i t e r i o n , b u t i t i s l e s s time-consuming t o l e t t h e computer do t h i s . The d i s a d v a n t a g e i s t h a t e v e r y one o f t h e group o f e v e n t s must s a t i s f y t h e s p e c i f i e d v a l u e f o r convergence i n GHYP1, o t h e r w i s e t h e program
i8
w i l l stop s h o r t o f c o m p l e t i n g t h e s t a t i s t i c a l a n a l y s i s . D u r i n g t h e i n i t i a l t e s t s o f JHD77 t h e number o f i t e r a t i o n s was s e t t o o l o w ( 3 i t e r a t i o n s ) , and an i n t e r e s t i n g r e s u l t was noted which
0
Grs
c o n t r i b u t e d some i n s i g h t i n t o t h e s t a b i l i t y o f s o l u t i o n s and t h e
60
practice of f i x i n g depths.
Figure 3.12 shows the position of a
particular event w i t h respect t o the recording a r r a y geometry. Theoretical travel times f o r t h i s synthetic event were computed f o r a depth o f 15 km, b u t i n the program the depth was s e t f i r s t a t 7 kv, then a t 15 km.
The epicentral solution f o r the correct depth was not
as good as i t was f o r the incorrect depth a t 7 km.
Seven more runs
were then carried out with depths fixed a t several values, and the r e s u l t s are seen in Figures 3.12b and 3 . 1 2 ~ . I t can be seen t h a t a m i n i m u m i s reached a t a fixed depth of 7 km f o r the epicentral
mislocation and f o r the standard error.
The explanation I propose f o r
this i s t h a t f o r each particular combination of hypocenter, velocity
model, and s t a t i o n distribution the p a t h t h r o u g h parameter space i s not smooth.
This i s an example of a f a l s e m i n i m u m i n the solution
space t h a t could have been a v o i d e d i f the program was allowed more i t e r a t i o n s (subsequently verified i n further t e s t runs).
From t e s t s
such as t h i s w i t h poor d a t a where i n s t a b i l i t i e s are l i k e l y t o occur, the conclusion was t h a t a t l e a s t f i v e t o six i t e r a t i o n s are necessary when r u n n i n g JHD77, a l t h o u g h w i t h b e t t e r d a t a the solutions d i d n o t change significantly a f t e r the t h i r d or f o u r t h i t e r a t i o n .
This
problem would not occur i n p r o g r a m such as GHYPl and HYPOELLIPSE, because testing the s i z e of the previous step i n parameter space would have forced the computations t o continue f o r a t le'ast one more
0
iteration. The preceding discussion i s applicable t o s i t u a t i o n s where conditions are such t h a t the d a t a are almost s u f f i c i e n t t o resolve the
0
63
depth.
I n most cases when the focal depth i s fixed there i s no
61
Figure 3.12. I l l u s t r a t i o n of a false minimum i n the solution space for a particular event located w i t h three iterations of JH077.a)Location o f event, velocity section, and some possible ray paths t o stations. The true focal depth was 15 km. Several different depths were fixed i n computations; after three iterations the errors i n solutions were as shown i n b ) and c ) . The preferred depth was 7 km. S i x iterations resulted i n smaller errors and a preference f o r the true focal depth of 15 km.
62
, ERROR IN
16.0.
LONGITUOE
u a:
4
412.0,
I/
u. 0
/
/
E R R O R IN L A T ! T U 0E
II)
I-
/ I
8.0.
I
/
3
i ;
z 2
/
4.0.
e -I
/
I 1
1-
-- --
I
/
-1
\
&--
3.0
1 0
6
5.0
7.0 FIXED
9.0 11.0 OEPT-
13.0
15.0
17.0
63 significant change i n the epicentral location f o r d i f f e r e n t values of the fixed depth.
This indicates t h a t the underlying assumption f o r
f i x i n g the depth i s valid; t h a t i s , the fixed parameter no longer has
any influence on the solution because i t s changes a r e negligible
/o
compared t o the other parameters. Further t e s t s were conducted u s i n g JHD t o determine whether the e r r o r i n depth, when included as a parameter i n the solution ( n o t f i x e d ) , was more dependent on the distance t o the closest s t a t i o n , the s i z e of the largest g a p between event-to-station azimuths, o r the distance between the event under consideration and the reference event.
Plots of the e r r o r i n focal depth versus each of these
influences separately and i n combination revealed no simple empirical 0
relationship for a single cause o r combination of these influences. Tests were also conducted t o determine the s e n s i t i v i t y of the hypocenter location and s t a t i o n adjustments t o varying numbers of
Q
s t a t i o n s , a z i m u t h a l control, and t o c h a r a c t e r i s t i c s of the reference event.
Details of the t e s t s and r e s u l t s are not essential t o this
d i scussi on , b u t several concl usi ons were reached which are a p p l i cab1 e
t o a l l of the programs used.
Conclusions regarding the reference
event apply equally t o the master event and JHD techniques. The standard e r r o r ( i n JHD77) and the RMS residual ( i n HYPOELLIPSE) can decrease with decreasing station density and poorer
distribution of s t a t i o n s w i t h respect t o distance and a z i m u t h .
This
can be accompanied by no difference i n the location of the event, indicating t h a t the distribution was good enough with fewer stations and rernoving some of the stations was equivalent t o removing random
64 noise.
It can a l s o o c c u r i f t h e d i s t r i b u t i o n becomes so poor t h a t a
number o f d i f f e r e n t l o c a t i o n s w i l l s a t i s f y t h e l e a s t squares procedure e q u a l l y we1 1
.
S e l e c t i n g t h e wrong f o c a l d e p t h f o r t h e r e f e r e n c e event appears t o e f f e c t t h e accuracy o f t h e o t h e r l o c a t i o n s v e r y l i t t l e , b u t s t a t i o n a d j u s t m e n t s may absorb t h e e r r o r i n t r o d u c e d i n t o t h e t r a v e l - t i m e computations, d i f f e r i n g by s e v e r a l t e n t h s o f a second i n sowe cases. S e t t i n g t h e wrong o r i g i n t i m e f o r t h e r e f e r e n c e event w i l l b i a s a l l o r i g i n t i m e s and s t a t i o n a d j u s t m e n t s by s i m i l a r amounts.
Setting the
wrong l o c a t i o n f o r t h e r e f e r e n c e event w i l l b i a s a l l o t h e r l o c a t i o n s and s t a t i o n a d j u s t m e n t s by s i m i l a r amounts; t h e d i r e c t i o n o f t h e m i s l o c a t i o n s w i l l a l s o be s i m i l a r .
Stations not recording the
r e f e r e n c e e v e n t do n o t show s i g n i f i c a n t l y l a r g e r magnitude s t a t i o n a d j u s t m e n t s o r r e s i d u a l s f o r t h e JHD t e c h n i q u e , b u t a r e v a r i a b l e f o r t h e master event technique.
As was p r e v i o u s l y mentioned, r e s u l t s f o r Dewey's JHD77 and Urhammer's G H Y P l j o i n t l o c a t i o n programs a r e comparable.
JHD77 i s
more general and more a p p l i c a b l e t o s i t u a t i o n s where t h e v e l o c i t y model i s n o t c o m p l e t e l y unknown.
F o r t h e s e reasons i t was decided
t h a t f u r t h e r work would be done o n l y w i t h JHD77 as r e p r e s e n t a t i v e o f t h e j o i n t hypocenter d e t e r m i n a t i o n (JHD) method. The n e x t s t e p was t o compare t h r e e t e c h n i q u e s : event, and s i n g l e event methods.
F i g u r e s 3.13a,
t h e JHD, n a s t e r
b , c y and d show t h e
d e v i a t i o n s o f 14 e s t i m a t e d e p i c e n t e r s from t h e known s y n t h e t i c event
io
l o c a t i o n s a t t h e c e n t e r o f each p l o t . was generated w i t h o u t any n o i s e added.
The d a t a f o r t h i s comparison A l l o f the stations recording
65
a)
JOINT DETERMINATION
b) M A S T E R E V E N T
1 8
c ) SINGLE EVENT SE77 *
0.
d ) SINGLE E V E N T HYPOELLIPSE A
0.5
KM
Figure 3.13. Deviations of 14 estimated epicenters from true locations of synthetic events. No noise added t o arrival times, b u t events i n different locations used different subsets of stations and different numbers of S-wave arrivals. Circle of diameter 1.25 km i s for reference.
66 r e a l events i n t h e r e g i o n o f t h e s y n t h e t i c event l o c a t i o n were used, Note t h a t
b u t S - a r r i v a l s were computed o n l y f o r t h e n e a r e s t s t a t i o n .
t h e r e i s a d i f f e r e n c e i n s c a l e between t h e s e p l o t s and t h o s e i n F i g u r e 3.11.
T h i s i n d i c a t e s some o f t h e d i f f e r e n c e t h a t , r e g a r d l e s s o f t h e
l o c a t i o n method used, can be expected when randoq n o i s e i s added t o t h e d a t a . . I n F i g u r e 3.11 m i s l o c a t i o n s were on t h e o r d e r o f 0.5 t o 1.5 k i l o m e t e r s , whereas i n F i g u r e 3.13a t h e m i s l o c a t i o n s were a l l w i t h i n
0.25 k i l o m e t e r s . The r e s u l t s i n F i g u r e 3.13 show c l e a r l y t h a t JHD and master event t e c h n i q u e s a r e b e t t e r t h a n t h e two s i n g l e event programs a t l o c a t i n g most events c o r r e c t l y w i t h r e s p e c t t o t h e r e f e r e n c e event. s c a t t e r i s reduced i n F i g u r e s 3.13a and 3.13b,
The
and t h e r e a r e more
events c l o s e r t o t h e c o r r e c t l o c a t i o n s a t t h e c e n t e r .
For t h e
s y n t h e t i c cases t h e l o c a t i o n o f t h e r e f e r e n c e e v e n t i s known p e r f e c t l y , so t h e p l o t s show t h e a b s o l u t e accuracy of each s y n t h e t i c event. Comparison of F i g u r e s 3.13a and 3.13b r e v e a l s a f e a t u r e o f t h e master event t e c h n i q u e t h a t can l e a d t o a p o o r e r l o c a t i o n t h a n w i t h a s i n g l e event method.
T h i s can o c c u r i f t h e s t a t i o n adjustrnents
computed r e l a t i v e t o t h e master event a r e i n c o r r e c t l y a p p l i e d t o another event, i.e.,
t h e r e f e r e n c e event i s n o t a d e q u a t e l y
r e p r e s e n t a t i v e o f a l l events i t was used t o h e l p l o c a t e .
The events
which f a l l o u t s i d e t h e c i r c l e o f r a d i u s 0.25 km i n F i g u r e 3.13b a r e p r o b a b l y i n t h i s c a t e g o r y , and a l l o f them a r e events f a r t h e s t f r o m t h e r e f e r e n c e event o r approaching t h e edges o f t h e a r r a y coverage. T h i s l e a d s t o one o f t h e s i g n i f i c a n t advantages o f t h e JHD
67 technique.
Because t h e r e f e r e n c e event i n JHD i s i n t r o d u c e d t o
c o n t r i b u t e a d d i t i o n a l s t a b i l i t y t o t h e e q u a t i o n s o f c o n d i t i o n and n o t o n l y t o compute s t a t i o n adjustments, t h e e f f e c t d i s c u s s e d i n t h e p r e v i o u s paragraph l e s s pronounced.
Adjustments a r e computed r e l a t i v e
t o a group o f up t o f i f t e e n events, and no s t a t i o n s h o u l d have an I
a d j u s t m e n t t h a t i s unreasonable f o r some o f t h e events.
I f these
a d j u s t m e n t s a r e t h e n used t o r e l o c a t e more e v e n t s t h e r e i s a b e t t e r chance t h a t t h e y w i l l be r e p r e s e n t a t i v e o f t h e whole r e g i o n i n q u e s t i o n , r a t h e r t h a n j u s t a small area s u r r o u n d i n g t h e one n a s t e r event. The f a c t t h a t JHD must t a k e i n t o account a l l s t a t i o n - e v e n t r e s i d u a l s i n computing s t a t i o n a d j u s t m e n t s i s u s u a l l y advantageous, as j u s t discussed.
But i f one o r more e v e n t s a r e c o n s i d e r a b l y d i f f e r e n t
f r o m t h e r e s t o f t h e group t h e i r i n f l u e n c e can reduce t h e e f f e c t i v e n e s s o f the technique.
This was observed i n one case where
t h e whole group o f JHD-located e v e n t s was w i t h i n a s m a l l e r r a d i u s o f t h e t r u e l o c a t i o n s t h a n t h e same group l o c a t e d w i t h t h e master e v e n t technique.
However, t h e r e were a few e v e n t s t h a t l o c a t e d b e t t e r w i t h
t h e master event method.
Thus, f o r JHD, t h e group o f events as a
whole w i l l have b e t t e r l o c a t i o n s t h a n w i t h s i n g l e e v e n t o r master event methods, b u t t h e accuracy o f t h e b e s t e v e n t s may s u f f e r s l i g h t l y t o compensate f o r t h e changes i n s t a t i o n a d j u s t m e n t s due t o improveqents i n t h e l o c a t i o n s o f t h e p o o r e s t events. I
A summary of t h e m a j o r comparisons and c o n c l u s i o n s d i s c u s s e d i n t h i s chapter
was
compiled i n t o P l a t e 1.
S p e c i f i c problems and
s i t u a t i o n s a r e 1 i s t e d , and t h e a p p l i c a b i l i t y a n d / o r l i m i t a t i o n s o f
68 each program s t u d i e d a r e presented f o r a l i s t o f s p e c i f i c problems and situations.
I n f o r m a t i o n d e r i v e d f r o m fly e x p e r i e n c e w i t h t h e methods
( a p p l i c a t i o n s f r o m Chapters 4 and 5 ) was a l s o i n c o r p o r a t e d i n t o P l a t e I .
l3 I
l
CHAPTER 4
PATTERNS OF EARTHQUAKE DISTRIBUTION IN SOUTHERN UTAH
The primary r e s u l t of t e s t s on the various epicenter location techniques was t h a t , for detailed analyses of earthquake occurrence, the j o i n t epicenter determination (JED) method provides b e t t e r solutions f o r r e l a t i v e earthquake locations.
I n t h i s chapter
interpretations of earthquake patterns are presented for relocations of epicenters in southern Utah u s i n g the JED technique.
Throughout
the following discussions the question will be considered:
How does
the relocation o f epicenters contribute t o the understanding of earthquake occurrence, and are the patterns tectonically s i g n i f i c a n t ? Summary of Structural and Tectonic Characteristics The t r a n s i t i o n between two major physiographic provinces, the Basin-and-Range and t h e Colorado P l a t e a u , i s expressed i n
south-central Utah as a 150 km t o 200 km -wide zone o f changing crustal and probably upper-mantle properties.
This zone may have
represented a structural and depositional t r a n s i t i o n zone between cratonic and marginal crust since the end of the Precambrian (Stewart, 1 9 7 8 ) ; hence, the coincidence of many structural features has a long and complex history.
A summary of a l l of the geologic and tectonic
characteri s t i c s and possi bl e model s t o explain t h e i r occurrence woud IP
be t o o extensive f o r the purposes o f t h i s study, and can already be
70 found elsewhere (GSA Memoir 152).
However, i t i s a p p r o p r i a t e t o
b r i e f l y r e v i e w some o f t h e g e o l o g i c a l and g e o p h y s i c a l o b s e r v a t i o n s t h a t may have a d i r e c t r e l a t i o n s h i p w i t h c o n c e n t r a t i o n s o f earthquakes i n s o u t h e r n Utah. Eaton e t a l .
(1978) compared r e g i o n a l , l o n g wave-length Bouguer
g r a v i t y d a t a o f t h e w s t e r n U.S.
t o o t h e r g e o l o g i c a l and geophysical
i n f o r m a t i o n , i n c l u d i n g s e i s m i c i t y , heat f l o w , c r u s t a l t h i c k n e s s , s u r f a c e topography, and Cenozoic v o l c a n i s m and f a u l t i n g .
They found
evidence f o r a broad c r u s t a l upwarp expressed b y a r e g i o n a l g r a v i t y l o w (-90 mgals) i n t h e Great Basin.
The g r a v i t y l o w c o i n c i d e s w i t h
r e l a t i v e l y h i g h heat f l o w (>1.25 HFU) and l o w P, 7.6 km/s (Smith, 1978).
v e l o c i t i e s o f 7.5 t o
T h i s anomalous r e g i o n i s bounded on t h e e a s t
and west b y a decrease i n t h e age o f f a u l t i n g and i n c r e a s e i n s e i s m i c activity. Cenozoic f a u l t i n g i s c d n s i d e r e d t o be contemporaneous w i t h bimodal b a s a l t - r h y o l i t e volcanism a l o n g much o f t h e w e s t e r n m a r g i n of t h e Colorado P l a t e a u ( S t e w a r t , 1978; C h r i s t i a n s e n and McKee, 1978). S i l i c i c volcanism, t h e c o m p o s i t i o n o f which i s i n d i c a t i v e o f m e l t i n g o f c r u s t a l m a t e r i a l (Nash and Hansel
, 1975;
Lachenbruch and Sass,
1978), c o u l d have an o r i g i n i n c r u s t a l p a r t i a l m e l t zones.
The
presence o f p a r t i a l m e l t zones i n t h e c r u s t i s suggested b y t h e abnormally h i g h heat f l o w ( B l a c k w e l l
,
1978; Lachenbruch and Sass,
1978) and c r u s t a l l o w v e l o c i t y l a y e r s ( S m i t h e t al.,
1975).
' T h e r e a r e o t h e r mechanisms t h a t can e x p l a i n t h e presence of c r u s t a l low v e l o c i t y l a y e r s .
The presence o f l i s t r i c f a u l t i n g , w i t h
f a u l t planes t h a t r o t a t e t o almost h o r i z o n t a l a t depths of 3 t o 7 km,
71 i s i n d i c a t e d by r e f l e c t i o n d a t a i n t h e o v e r t h r u s t b e l t f r o m Nyoming t o s o u t h e r n Utah.
These f a u l t planes may converge t o a s i n g l e detachment
h o r i z o n t h a t may be t h e l o c u s o f h i g h c o n c e n t r a t i o n s o f pore f l u i d s . One model f o r t h e l i s t r i c f a u l t i n g and a s s o c i a t e d d&ollement has been proposed b y D a v i s and Coney (1979).
zones
They r e l a t e t h e presence
o f a sedimentary c o v e r deformed b y "growth f a u l t s " t o t h e e x t e n s i o n a l t e c t o n i c s and c o n c u r r e n t f o r m a t i o n o f metamorphic c o r e complexes. Prograde metamorphism c o u l d t h e n p r o v i d e a source f o r excess f l u i d s . c
A d d i t i o n a l l y , t h e decollement s u r f a c e c o u l d be g e n e t i c a l l y r e l a t e d t o r e g i o n a l u n c o n f o r m i t i e s o r t o p r e v i o u s planes o f S e v i e r o r Laramide t h r u s t i n g ( D a v i s and Coney, 1979).
The c h r o n o l o g i c a l sequence of
events has n o t been r e s o l v e d , however.
A number o f mechanisms have been suggested t o account f o r a l l o r some o f t h e phenomena observed i n t h e r e g i o n o f t h e Great B a s i n Colorado P l a t e a u t r a n s i t i o n .
-
Some p r e f e r an a c t i v e s p r e a d i n g
mechanism, such as m a n t l e u p w e l l i n g (e.g.
Eaton e t a l . ,
1978).
C h r i s t i a n s e n and McKee (1978) have summarized e v i d e n c e 1endi ng support t o a model t h a t e x p l a i n s t h e p r o p e r t i e s o f t h e r e g i o n as t h e p a s s i v e b e h a v i o r o f i n h e r e n t l y d i f f e r e n t c r u s t a l r e g i o n s responding t o \
s t r e s s e s i n t r o d u c e d by p l a t e movements.
Another t h e o r y summarized and
favored by S t e w a r t (1978) i s t h e presence o f back-arc s p r e a d i n g and
io
a s s o c i a t e d m a n t l e u p w e l l i n g above a s u b d u c t i o n zone. General P a t t e r n s o f Earthquake Occurrence The c h a r a c t e r o f e p i c e n t r a l t r e n d s and p a t t e r n s i n U t a h and t h e i r
IO
r e l a t i o n s h i p s t o t e c t o n i c s and s t r u c t u r e has been d i s c u s s e d i n s e v e r a l
72 r e c e n t pub1 i c a t i o n s (e.g.
IQ
, Smith, 1979; Smith, 1978; Anderson, 1978).
The p r e s e n t s t u d y e l a b o r a t e s on some o f t h e i d e a s suggested by t h e s e authors, b u t t h e emphasis i s on assessing t h e c o n t r i b u t i o n o f a c c u r a t e and p r e c i s e earthquake l o c a t i o n t o t h e i n t e r p r e t a t i o n s . I n examining t h e v e r y general t r e n d of Utah s e i s m i c i t y , i t has been observed ( S m i t h and Sbar, 1974) t h a t earthquakes o c c u r i n a r a t h e r d i f f u s e zone t h a t t r e n d s n o r t h - s o u t h a l o n g a p p r o x i m a t e l y 112" W longitude.
A t a p p r o x i m a t e l y 38" 30' N l a t i t u d e t h e t r e n d of
earthquakes changes t o a more southwest o r i e n t a t i o n (Smith, 1979). F i g u r e 4.1 i s a map o f Utah showing t h e r e l o c a t e d earthquakes i n t h i s zone o f s e i s m i c a c t i v i t y and t h e i r s p a t i a l r e l a t i o n s h i p s t o r e g i o n a l Bouguer g r a v i t y c o n t o u r s (Cook e t a1
., 1975)
and a 1 i n e marking t h e
e a s t e r n e x t e n t o f t h r u s t f a u l t i n g o f t h e S e v i e r orogeny a c c o r d i n g t o H i n t z e (1975).
The e a s t e r n margin o f t h e g r a v i t y l o w d i s c u s s e d b y Eaton e t a l . (1978) c o i n c i d e s w i t h t h e zone o f earthquakes ( F i g u r e 4.1)
i n Utah,
and a l s o w i t h an i n c r e a s e i n c r u s t a l t h i c k n e s s , as determined from r e f r a c t i o n p r o f i l i n g (Smith, 1979).
The c h a r a c t e r i s t i c s o f t h i s
t r a n s i t i o n f r o m t h e Great B a s i n t o t h e Colorado P l a t e a u have been s t u d i e d e x t e n s i v e l y , and sumaries can be found i n many papers (e.g. Anderson, 1978; Smith, 1978; Best and Hamblin, 1978).
Faulting i n
s o u t h - c e n t r a l Utah (Anderson, 1978; B e s t and Hambl i n , 1978) and f o c a l mechanism s o l u t i o n s complied by Smith and L i n d h (1979) suggest t h a t t h e area i s s u b j e c t t o east-west extension. Some general o b s e r v a t i o n s can be made i n comparing t h e r e l o c a t e d
IO
c3
earthquakes i n F i g u r e 4.1 t o p r e v i o u s c o m p i l a t i o n s o f e p i c e n t e r s .
The
BOUGUER G R A V I T Y C O N T O U R I N T E R V A L 10 M C A L S
(COOK A N D O T M E R S , l 9 7 S )
/
QUATERNARY FAULT (ANDERSON A N D MlLLER,l979)
,/
THRUST FAULTING EASTERN E X T E N T
i
(HlNTZE,1975)
i
f
+39"
+
38
I
I
37
113"
,
11 2 '
110"
Figure 4.1. Map o f relocated epicenters i n southern Utah. original earthquake data s e t i s within boxed out.line. 4
37" 109" I!
111"
Area o f
74 r e l o c a t e d earthquakes e x h i b i t e d a more l i n e a r c h a r a c t e r p a r a l l e l i n g t h e m a j o r Wasatch and J o e ' s V a l l e y f a u l t zones, which w i l l be d i s c u s s e d i n more d e t a i l p r e s s e n t l y .
C l u s t e r s o f e p i c e n t e r s were
observed t o t i g h t e n - u p a f t e r b e i n g r e l o c a t e d , c o v e r i n g l e s s area on P r e c i s e l o c a t i o n s o f earthquakes can t h u s b e t t e r d e f i n e
t h e map.
t h e i r r e l a t i o n s h i p s t o regional f a u l t patterns, although t h e a d d i t i o n o f f o c a l depth c o n t r o l i s probably e s s e n t i a l i n r e s o l v i n g t h e relationship with possible l i s t r i c faulting. A c t i v i t y i s a l s o c o n c e n t r a t e d south o f t h e E l s i n o r e f a u l t , a t a p p r o x i m a t e l y 38" 1 5 ' N l a t i t u d e .
T h i s c l u s t e r corresponds s p a t i a l l y
t o a r e g i o n a l g r a v i t y l o w o f about 20
t o 30 mgals.
The o r i e n t a t i o n
o f t h e earthquake zone i n Utah changes i n t h i s same area f r o m a n o r t h - s o u t h t o a southwest d i r e c t i o n .
P o s s i b l e east-west t r e n d s i n
t h e r e g i o n a l g r a v i t y c o n t o u r s ( F i g u r e 4.1) Eaton e t a l .
(1978).
have been r e c o g n i z e d by
They suggest t h a t m a j o r east-west e x t e n s i o n
requires transverse structural features p a r a l l e l t o the d i r e c t i o n o f extension.
Attempts t o r e c o g n i z e m a j o r east-west t r e n d s i n t h e
s e i s m i c i t y have been l a r g e l y q u a l i t a t i v e .
Precise locations o f
earthquakes r e l a t i v e t o each o t h e r would have d e l i n e a t e d any m a j o r trends.
B u t t h e s p a t i a l a s s o c i a t i o n o f geothermal areas, T e r t i a r y and
Q u a t e r n a r y volcanism, and e p i c e n t e r c l u s t e r s i n t h i s area o f Utah i s b e t t e r r e s o l v e d by t h e r e l o c a t i o n s . There a r e o t h e r p o s s i b l e causal mechanisms t o e x p l a i n p a t t e r n s o f earthquake occurrence, which have o n l y an i n d i r e c t r e l a t i o n s h i p t o any t e c t o n i c framework.
Decreases i n w a t e r t a b l e s o r a q u i f e r s due t o
abnormal demands c l o s e t o popul a t i o n c e n t e r s o r h e a v i l y i r r i g a t e d
75
regions could produce local subsidence and d i f f e r e n t i a l compaction t h a t m i g h t r e s u l t i n small, shallow earthquakes.
Any abnormal
increase or decrease in the water t a b l e could change the e f f e c t i v e s t r e s s in a particular region by changing the pore pressure (Gretener, 1978).
The b r i t t l e or ductile behavior of the rock units may also be
affected by a change i n pore pressure (Brace, 1 9 7 2 ) .
Either of these
mechanisms could conceivably be sufficient t o produce shallow earthquakes, or a t l e a s t t o localize s t r e s s release t o enhance the occurrence of events i n c l u s t e r s . Procedures f o r Application of Method For these analyses catalogs and maps i n southern U t a h from 1962 t h r o u g h 1978 (Arabasz and McKee, 1979) were studied t o delineate areas
of i n t e r e s t , including areas where ( i ) changes i n directions of
regional trends were apparent, ( i i ) possible correlations w i t h general geologic or structural c h a r a c t e r i s t i c s were observed (e.g., f a u l t i n g , recent vol cani sm) , ( i i i ) concentrations of earthquakes i n 1 i near trends, zones, o r c l u s t e r s existed.
C o n s i d e r a t i o n was also given t o
the position o f areas with respect t o the s t a t i o n d i s t r i b u t i o n , f o r reasons discussed in Chapter 3 . W i t h i n each of the six regions delineated i n Figures 4.2 and 4.3,
a number of events were chosen from the catalog of U t a h earthquakes (Arabasz and McKee, 1979) as representative reference events f o r t h a t particular area.
The considerations i n e f f e c t f o r the choice of
reference events, i n order of r e l a t i v e importance, were number of phases recorded f o r the event, a z i m u t h a l g a p i n s t a t i o n d i s t r i b u t i o n ,
76 T
I
REFERENCE STATIONS
I
EVENTS
B E F O R E 10-01-74
+
+
P
Q%. , 1 +
3
Z 8 37 114
IO
iGrs
I I
I13
+
'40
+
-3 9
+
k38
0
I I
112
1
I
111
I i
110
; 37
109
Figure 4.2. S i x areas o f regional epicenter studies i n southern Utah. Circles represent reference events located w i t h the j o i n t hypocenter determination program JHD77. Roman numerals a r e region numbers r e f e r r e c t o in t e x t .
77 I I
'ds
I
REFERENCE STATIONS
+
t
1
EVENTS
AFTER
10-01-70
T I
+ O0
0
+
I +
I
+
b40
+
t39
+
I
1
38
Figure 4.3. Areas same as Figure 4 . 2 , w i t h d i f f e r e n t reference events for period a f t e r October 1, 1974.
78
magnitude and RMS residual.
Reference events had t o be chosen f o r the
two time blocks discussed i n Chapter 3:
before and a f t e r October 1,
1974. IO
The method originally employed f o r the relocation of t h i s d a t a
was an a d a p t a t i o n of the single-event program HYPOELLIPSE ( L a h r , 1979) This technique and i t s limited
to compute s t a t i o n adjustnents.
The problem i s t h a t
application was discussed b r i e f l y i n Chapter 3 .
the single-event locations f o r the reference events must be used.
The
residuals are not treated as random variables (Dewey, 1971b) d u r i n g location of the reference events; therefore, the final station adjustments are based on reference event locations t h a t do not have the benefit of siwltaneous travel-time adjustnents.
I t was f o u n d ,
however, t h a t i n some cases the r e l a t i v e locations of groups could be irllproved u s i n g t h i s technique.
The basis f o r assessing the
improvement was the observation of reduction i n the s c a t t e r o f events, accentuating c l u s t e r s and trends i n the d a t a . Using the procedure suggested by Dewey (1978) and outlined and tested i n Chapter 3 , the earthquakes i n the areas of i n t e r e s t were relocated u s i n g the JED programs.
The f i r s t step was t o locate the
reference events and compute s t a t i o n adjustments using the program JHD77 (Dewey, 1975).
A t o t a l of twelve d i f f e r e n t s e t s of station
adjustments were obtained i n t h i s manner:
two f o r each o f the s i x
regions representing the two time blocks studied.
These s t a t i o n
adjustments were then applied t o other events i n the same region using the single event prograrn, SE77 (Dewey, 1978).
S t a r t i n g positions f o r
b o t h techniques were the o r i g i n a l catal ogued s i n g l e-event
79
(HYPOELLIPSE) locations.
However, one data s e t was also located using
the center o f the group o f earthquakes as the s t a r t i n g position for the JED events w i t h i n t h a t region.
The solutions f o r b o t h s t a r t i n g
locations were e s s e n t i a l l y identical, d i f f e r i n g by l e s s t h a n one kilometer.
A l i s t i n g of a l l relocated earthquakes appears i n Appendix
C.
Maps of relocated earthquakes depict the f i n a l epicenter locations u s i n g the JED technique.
Comparison of the two s e t s of
final locations, JED and adjusted HYPOELLIPSE, resulted i n conclusions t h a t were consistent w i t h expectations based on theory and on the
previous t e s t cases.
The JED solutions showed considerably l e s s
s c a t t e r t h a n those determined by the single-event HYPOELLIPSE program w i t h s t a t i o n adjustments. I n i t i a l l y i t was t h o u g h t t h a t i t m i g h t be possible t o p a r t i a l l y
adjust f o r mislocations due t o the recording s t a t i o n distribution by f i r s t creating a s e t of t e s t events for each region, computing s t a t i o n
adjustments w i t h JED, and a p p l y i n g these adjustments t o the reference events.
This was attempted f o r a l l of the regions f o r the stations i n
the d a t a set before October 1974.
I t was found t h a t the adjustments
computed for the t e s t cases were, a t most, an order o f magnitude smaller t h a n the final adjustments computed f o r the reference events (hundredths compared t o tenths of seconds).
Any e f f e c t s on subsequent
station adjustments o r locations by the a d d i t i o n o f these preliminary adjustments were e s s e n t i a l l y indiscernable.
I t was concluded t h a t ,
beyond the q u a l i t a t i v e knowledge of the varying location capability across the a r r a y obtained from the t e s t cases, the e f f e c t s o f poor
s t a t i o n d i s t r i b u t i o n cannot be significantly corrected by any method. For earthquakes in southern Utah relocated with the regional sei smic array two velocity model s were used.
Sei smograph s t a t i o n s
were divided into two groups, one comprised of s t a t i o n s located within the eastern Great Basin and the other including s t a t i o n s in the Viddle Rocky Mountains and Colorado P1 ateau (Richi ns , 8979).
A1 1 s t a t i o n s
west of 111' W longitude, except f o r the s t a t i o n GCA, were placed in
the Great Basin group, f o r which the velocity model was based on a refraction profile of Keller e t a l . , 1975.
the Wasatch Front model.
I t will be referred t o as
The second velocity model, referred t o as
the Colorado Plateau model, was from a reversed p r o f i l e discussed by Roller (1965).
Velocities and depths f o r these two models are given
in Table 4.1. Interpretations of Revised Earthquake Locations l
The general epicentral patterns were evident before the
0
relocations were done, and have n o t been greatly altered by relocation of the epicenters using the JED technique.
However, focusing on
specific regions of i n t e r e s t on a local scale emphasizes some more d e f i n i t e results.
The maps of Figures 4.4 t h r o u g h 4.9 outline the
same regions as in Figures 4.2 and 4.3, b u t include a l l of the relocated earthquakes i n each region occurring from July 1962 t h r o u g h October 1978.
The following discussion examines these regions in more
detail , concentrating on spatial and temporal grouping such as clustering of epicenters in swarms or along coherent trends.
0
Gs
As was
stated previously, recognition o r del ineation o f grouped epicenters i s
V e l o c i t y models f o r r e g i o n a l r e l o c a t i o n s :
Table 4.1.
southern Utah.
WASATCH FRONT MODEL P4 A V E VEL (ffin/s)
-
DEPTH TO TOP (Km)
COLORADO PLATEAU MODEL
LAYER THICKNESS (ffin)
P-WAVE VEL (Km/s)
DEPTH TO TOP (Km)
LAYER THICKNESS (Km)
.00
1.4
3.0
.00
1.5
5.9.
1.40
14.1
6.2
1.50
26.0
6.4
15.50
9.9
6.8
27.50
12.5
7.4
25.40
-
7.8
40.00
-
3.4
82
:o
Figures 4.4, 4.5, 4.6, 4.7, 4.8, 4i9. Regional maps of relocated earthquakes, June 1962 through August 1978. -
EXPLANATION
Earthquake epicenter Town
/’ / / (
.. .
Quaternary f a u l t
Lake or reservoir
I - >
*-*
,
-__--./
Swamp :Quaternary b a s a l t 7
:Tertiary vol cani cs or intrusives :Tertiary t o Quaternary alluvium, colluvium :Precambrian t o Tertiary formations
0
15'
1 1 l"30'
Figure 4.4.
15'
Region I .
84
4:
39’31
Figure 4.5.
Region 11.
85
5
0
IO
2 0 KM
4 5'
38030"'
I
Figure 4.6. Region 111.
86
45
38 30
4 5'
Figure 4.7.
1 1 2 30'
Region
IV.
!O
t 0
.OO*E 1 1
#OE
b
e
+ e
89 enhanced by t h e use o f t h e JED technique.
Additionally, detection o f
a r e g i o n a l b i a s i n t h e l o c a t i o n s , as i n d i c a t e d b y s y s t e m a t i c changes i n t h e r e l o c a t i o n s u s i n g JED, i s i m p o r t a n t i n i n t e r p r e t i n g p o s s i b l e r e g i o n a l v a r i a t i o n s i n t h e v e l o c i t y model.
I P
The maps i n F i g u r e s 4.4 t h r o u g h 4.9
show o n l y gross
c h a r a c t e r i s t i c s o f geology and s t r u c t u r e t h a t appear t o have some relationship t o the seismicity. be t h e same on each map.
Features i n c l u d e d , t h e r e f o r e , may n o t
I n t h e f o l l o w i n g d i s c u s s i o n s Regions I
t h r o u g h V I r e f e r t o t h e areas o u t l i n e d i n F i g u r e s 4.2
and 4.3.
Region I. F i g u r e 4.4 shows t h e area n o r t h w e s t o f t h e San Rafael Swell
, an
east o
area o f Lararnide u p l i f t .
t h e San Pete V a l l e y .
I t i n c l u d e s t h e Wasatch P l a t e a u
I n t h e n o r t h e a s t p o r t i o n o f t h e map a
number o f earthquakes o c c u r w i t h i n t h e J o e ' s V a l l e y graben o r f a u l t zone.
These f a u l t s a r e g e o l o g i c a l l y young , e x h i b i t i n g displacement as
r e c e n t y as 11,000 y e a r s ago (Anderson and H i l l e r , 1979).
Recent
m i c r o e a r t h q u a k e surveys i n t h e same r e g i o n (W. Arabasz, o r a l corn.) have shown a c t i v i t y c o n c e n t r a t e d i n g e n e r a l l y t h e same area, which suggests t h a t t h e r e g i o n a l JED l o c a t i o n s a r e more a c c u r a t e t h a n t h e single-event locations.
B u t t h e most n o t i c e a b l e f e a t u r e on t h i s map
i s the d i f f u s e character o f the epicenter d i s t r i b u t i o n .
Except f o r
t h e n o r t h e r n p a r t o f Joe's V a l l e y , and a few e p i c e n t e r s p o s s i b l y associated w i t h f a u l t i n g i n t h e c e n t r a l p o r t i o n o f t h e region, t h e r e appears t o be no obvious c o r r e l a t i o n w i t h any s u r f i c i a l geology o r structure.
IO
! O
Earthquakes i n t h e southern p o r t i o n o f t h i s area :Jere
'63
systematically relocated approximately 5 t o 10 km east of t h e i r previous locations.
This suggests t h a t there i s a bias i n the
travel-times t o stations from these events.
This could e i t h e r be due
t o a l a t e r a l velocity inhomogeneity, o r departure from the simple flat-layered velocity model, or i t could r e f l e c t a constant e r r o r i n the focal depth (fixed a t 7.0 k m ) f o r t h i s g r o u p of events.
Since the
station d i s t r i b u t i o n i n t h i s area i s f a i r l y good, I favor the former p o s s i b i l i t y because an e r r o r i n focal depth would r e s u l t i n systematic mislocations only i f the azimuthal and distance control were quite poor. Region 11.
I n Figure 4.5 a zone of rather diffuse seifmicity
represents the southern continuation of the Wasatch f a u l t .
The
valleys east of the southern portion of the Wasatch range were areas f o r which possible correlations between water-level changes and
seismicity could be made.
A t the northern end olf the Juab Valley, i n
the Goshen and Utah Valleys, c l u s t e r s of epicenters a r e composed i n p a r t o f events o c c u r r i n g t h r o u g h o u t t h e r e c o r d i n g p e r i o d (1962-1978);
however, there are also several swarms ( 3 events) d u r i n g particular years ( 1973, 1976, 1977, 1978).
Water-1 eve1 change maps have been
compiled annually f o r the U t a h and Goshen Valleys, and d u r i n g these years document fluctuations of -3 t o +5 f e e t west of S a n t a q u i n ( U .S.Geol
. Survey,
Cooperative Investigations Reports 13, 15, 16, 18).
Records i n the Utah and Goshen Valleys are particularly complete, showing changes f o r water-table aquifers and f o r various depths i n the 1 0
artesian aquifer.
However, changes i n water levels i n other years,
91 w i t h no swarm a c t i v i t y , a r e n o t s i g n i f i c a n t l y d i f f e r e n t .
The same i s
t r u e f o r p o s s i b l e s p a t i a l and temporal a s s o c i a t i on w i t h water-1 eve1 changes n o t e d f o r t h e small c l u s t e r o f events i n t h e Juab V a l l e y west o f t h e Chicken Creek R e s e r v o i r .
Lack o f complete a r e a l coverage o f
t h e w a t e r - l e v e l r e c o r d s p r e c l udes d i r e c t c o r r e l a t i o n w i t h s e i s m i c i t y , I
b u t t h e s p a t i a l c o i n c i d e n c e should n o t be i g n o r e d i n a t t r i b u t i n g earthquake occurrence e n t i r e l y t o t e c t o n i c causal mechanisms.
Focal
d e p t h i n f o r m a t i o n would a i d i n s e p a r a t i o n o f t h e s e e v e n t s f r o m t e c t o n i c - r e l a t e d earthquakes.
IP
Region 111. F i g u r e 4.6
The d i f f u s e earthquake o c c u r r e n c e i n t h e area i n
i s even more pronounced i n t h e n o r t h e r n p a r t t h a n i t was i n
Region 11.
The prominent f a u l t zone i n c l u d e s t h e n o r t h e r n e x t e n s i o n
o f t h e S e v i e r f a u l t , w i t h segments t h a t have been a c t i v e w i t h i n t h e last 3
M Y (Anderson, 1978).
The E l s i n o r e f a u l t passes t h r o u g h t h e
town o f R i c h f i e l d , and t h e area was t h e s i t e o f l a r g e earthquakes i n 1901 (ML 6.5) and 1921 (ML 6.0)
(Arabasz and Smith, 1979).
South o f t h e t o w n o f S e v i e r i s a p r o m i n e n t c l u s t e r o f e p i c e n t e r s
t h a t i n c l u d e s swarms d u r i n g October 1967 and i n l a t e June t h r o u g h August 1978.
I n t h i s area t h e Marysvale v o l c a n i c s , composed o f
Oligocene t o Miocene b a s a l t f l o w s , b r e c c i a s , and ash f l o w t u f f s , s t r a d d l e t h e boundary between t h e Basin-and-Range p r o v i n c e s (Steven e t a1
., 1978).
and Colorado P l a t e a u
E x t e n s i v e Basi n-and-Range normal
f a u l t i n g i n t h i s area p r o b a b l y began about 21 MY ago, conternporaneous w i t h a change i n t h e o v e r a l l c o m p o s i t i o n o f t h e f l a r y s v a l e v o l c a n i c s
i0
i u
from intermediate t o s i l i c i c - a l k a l i c .
About 2 1
-
17 MY ago, e r u p t i o n s
92 o f ash f l o w t u f f accompanied f o r m a t i o n o f t h e Mount Belknap and Red H i l l s Caldera ( S t e v e n e t a l , 1978).
The l a t t e r c o i n c i d e s w i t h t h e
p r e s e n t o c c u r r e n c e o f t h e e p i c e n t e r swarm.
An east-west t r e n d i n t h e
g r a v i t y c o n t o u r s i n t h i s r e g i o n ( s e e F i g u r e 4.3
IP
and Eaton e t al.,
1978) may be i n d i c a t i v e o f a causal mechanism on a l a r g e r s c a l e t h a t would account f o r t h e s p a t i a l a s s o c i a t i o n o f h o t s p r i n g s , T e r t i a r y v o l c a n i c s , and e p i c e n t e r swarms. A l s o n o t a b l e i n t h i s r e g i o n i s t h e v e r y sparse s e i s m i c a c t i v i t y a l o n g t h e S e v i e r f a u l t zone near t h e town o f Monroe, t h e s i t e of t h e
10
Monroe geothermal area. Region I V . . v o l c a n i c system.
The Cove F o r t area was a l s o p a r t o f t h e Marysvale Earthquakes shown i n F i g u r e 4.7 e x h i b i t a c l u s t e r i n g
i n t h e Three Creeks c a l d e r a area, s i t u a t e d i n t h e headwater r e g i o n o f C l e a r Creek.
T h i s was t h e source o f t h e O l i g o c e n e Three Creeks Tuff
member o f t h e B u l l i o n Canyon V o l c a n i c s (Steven e t a1
., 1978).
E p i c e n t e r s a r e a l s o c o n c e n t r a t e d i n mapped areas o f m a f i c b a s a l t flows o f P l e i s t o c e n e t o Holocene a g e n e a r Cove F o r t .
However, s t u d i e s of
Holocene f a u l t i n g ( C l a r k , 1977) i n t h e area d e l i n e a t e d t h e most a c t i v e f a u l t s , and i n general t h e r e l o c a t e d e p i c e n t e r s do n o t c o i n c i d e w i t h these.
E r u p t i o n s i n t h i s area were p r o b a b l y a l o n g n o r t h - s o u t h
f r a c t u r e systems; t h i s and l o c a l f o c a l mechanism s o l u t i o n s i n d i c a t e t h a t t h e area i s e x t e n s i v e l y f r a c t u r e d and a c o r r e l a t i o n may e x i s t w i t h some unmapped fault’.
There i s a l s o t h e c o n s i d e r a t i o n o f
i n t e r s e c t i o n o f f a u l t planes w i t h t h e s u r f a c e .
I n t h e case o f b a s a l t ,
f a u l t planes p r o b a b l y change o r i e n t a t i o n t o c o n f o r n w i t h t h e
93
drs
preferential vertical j o i n t i n g .
A t depth the d i p of f a u l t planes i n
the Cove Fort area probably decreases t o the regional a l t i t u d e of 55"
-
70" (Clark, 1977), and earthquakes occurring on lower portions of
the f a u l t s would have epicenters on the surface projected o f f the fault trace. Also notable i s the lack of seismicity a l o n g the Opal Mound f a u l t bounding the Mineral Mountains on the west, i n the Roosevelt Hot
IQ
S p r i ngs geothermal area.
T h u s , three geothermal areas , Monroe , Cove
Fort, and Roosevelt, close t o each other and presurnably i n the same general tectonic regime, have very d i f f e r e n t expressions i n terms of the regional seismicity.
The Cove F o r t and Roosevelt KGRA's w i l l be
discussed i n more detail i n Chapter 5 . Region V.
The general f a u l t trends and diffuse seismicity
discussed f o r Region I11 are continued i n the southern Tushar Mountains and the Beaver Valley, Figure 4.8.
Several c l u s t e r s are
observed : one northwest of C i rcl evi 11 e , p r o b a b l y associated w i t h f r a c t u r e s w i t h i n t h e g r a b e n i n t h e southern T u s h a r M o u n t a i n s , a n d one
a t the southern end of the Tertiary g r a n i t i c intrusive o f the Mineral Mountains.
Near Beaver a c l u s t e r of a c t i v i t y i s concentrated i n the
valley and drainage system o f S o u t h Creek. This general area i n Utah i s where the trend of earthquake occurrence changes t o a southwest orientation. exhibit any systematic relocation.
JED locations d i d not
I n p a r t i c u l a r , events d i d not
a1 i gn themsel ves a1 ong any east-west trends.
Region VI.
This area (Figure 4.9) i s dominated by the Hurricane,
94 Paragonah, and Rush Lake f a u l t zones.
Anderson and Mehnert (1979) , i n
a r e c e n t s t u d y o f s t r a t i g r a p h i c and s t r u c t u r a l r e l a t i o n s h i p s , suggest t h a t t h e H u r r i c a n e f a u l t i s t o o young t o be t h e p h y s i o g r a p h i c boundary between t h e Colorado P l a t e a u and t h e Basin-and-Range area.
provinces i n t h i s
Based on t h e s e i s m i c i t y , which appears t o be more p r e v a l e n t
west o f t h e f a u l t zone ( a l t h o u g h t h e d a t a a r e r a t h e r sparse) and t h e f a c t t h a t t h e r e has been no evidence f o r Holocene s u r f a c e f a u l t i n g on t h e H u r r i c a n e f a u l t ( u n l i k e t h e Wasatch f a u l t ) , Anderson and Mehnert p l a c e t h e p r e s e n t boundary west o f t h e H u r r i c a n e f a u l t . S p a t i a l and temporal c o r r e l a t i o n s can be p o s t u l a t e d f o r t h e occurrence o f e p i c e n t e r s and w a t e r - l e v e l changes i n t h e Cedar V a l l e y . I n 1971 seven earthquakes o f magnitude 3 ( M L )
O r
g r e a t e r , f o l l o w e d by
hundreds o f a f t e r s h o c k s d e t e c t e d by p o r t a b l e s t a t i o n s i n s t a l l e d a f t e r t h e l a r g e r shocks, o c c u r r e d n o r t h w e s t o f Cedar City.
Ground-fissuring
had been observed p r i o r t o t h e events (Arabasz and Smith, 1979) , and problems w i t h subsidence combined w i t h a n a l y s i s o f water-1 eve1 change maps suggest a s p a t i a l r e l a t i o n s h i p a t l e a s t .
But t h e r e l a t i o n s h i p
may be a precusory d i l a t a n c y e f f e c t r a t h e r t h a n any causal mechanism (Arabasz and Smith, 1979).
More swarm a c t i v i t y i n November o f 1977
and t h e 2-3 months f o l l o w i n g can a l s o be c o r r e l a t e d s p a t i a l l y w i t h areas o f l a r g e w a t e r - l e v e l f l u c t u a t i o n s , f r o m -3 t o -6 f e e t i n e a r l y 1977 t o +3 t o +5 f e e t i n l a t e 1978 (U.S.Geo1.
Survey, C o o p e r a t i v e
I n v e s t i g a t i o n s Reports 16, 18). The d i s t r i b u t i o n o f t h e 1971 e p i c e n t e r s i n two d i s t i n c t zones, 10
km a p a r t , r a i s e d q u e s t i o n s about t h e accuracy of t h e l o c a t i o n s .
The
a f t e r s h o c k s l o c a t e d w i t h t h e p o r t a b l e s t a t i o n s were about 3 t o 5 kn
95 east of the northern group of epicenters; t h i s and other information led t o a preliminary conclusion t h a t a l l o f the epicenters were actually located i n the vicinity of the northern g r o u p (Arabasz and S m i t h , 1979).
The stations used in the regional locations varied
s l i g h t l y f o r each event, and the possible mislocations were attributed t o some sort o f systematic s t a t i o n bias.
Figure 4.10 shows the
HYPOELLIPSE single event and the JED locations f o r the seven events of magnitude greater than 3.0 ML, w i t h arrows drawn from the single-event
t o the JED location.
The four southern events moved 1-2 km north, b u t
remained c l e a r l y separated i n t o two zones.
Furthermore, l a t e r
a c t i v i t y (1978) was concentrated i n the v i c i n i t y of the southern group.
For these reasons i t was concluded t h a t there were l i k e l y two
d i s t i n c t zones of a c t i v i t y d u r i n g the 1971 swarms.
Comparisons of the si ngle-event HYPOELLIPSE 1 ocations, the
HYPOELLIPSE relocations w i t h station adjustments, and the JED relocations reveals evidence t h a t the dispersed character of the epicenters i s not due only t o "smearing" o f the locations as a function of the s t a t i o n d i s t r i b u t i o n ; i n general, there i s no
0
consistent correlation between the direction of novement of the relocations and the orientations of the major axes of confidence' ellipses.
IO
This factor cannot be ruled out, however.
From Figures 3.9
and 3.10 i t i s evident that the principal e f f e c t of the s t a t i o n
distribution i s t o s h i f t computed locations i n east-west directions.
0
u
The direction of the s h i f t will vary depending on which s t a t i o n s actually record a specific event, b u t i s reasonable t o assume t h a t the
I
IQ
Figure 4.10. Seven events o f magnitude g r e a t e r than 3.0 (ML) occurring i n November 1977 in the Cedar City area. Arrows point from singleevent locations t o j o i n t l y determined locations (open c i r c l e s ) . Closed c i r c l e i s reference event used in JHD77. Heavy dashed l i n e s indicate mapped positions o f Quaternary f a u l t s . L i g h t dashed line approximates valley margins.
97
general tendency i s t o mislocate epicenters in directions perpendicular t o the predominant structural trends of the region. I
The
magnitude of t h i s mislocation i s dependent on the computed variances, b u t i s probably no more t h a n 10 km f o r any single event, and usually
less t h a n 5 km. These general r e s u l t s are sufficient t o indicate t h a t the diffuse earthquake occurrence i n southern U t a h i s r e a l .
A graphical
v i sua1 i z a t i o n of the nature of earthquake occurrence i s depicted i n
Figure 4.11.
Duration magnitudes were used i n the relation ( R . B.
S m i t h , oral comm., 1978)
l o g E ( e r g s ) = 9.4 + 2.1 ML
-
0.02M~~
t o generate histograms of cumulative seismic energy release a t 10 km intervals across the t r a n s i t i o n zone.
The dashed l i n e in Figure 4.11
corresponds approximately t o the l i n e delineating the east extent of thrust faulting i n Figure 4.1, as well as the approximate physiographic boundary between the Colorado P1 ateau and the Great Basin. Computations were carried o u t f o r a d a t a s e t l a r g e r t h a n t h a t delineated i n Figure 4.1. 37' N t o 40" N.
Open bars represent a l l d a t a from l a t i t u d e s
Closed (diagonally patterned) bars include d a t a only
to 39" N l a t i t u d e .
This was done i n order t o eliminate the possibly
confusing r e s u l t s obtained by includjng seismic a c t i v i t y i n the Price area, much of which i s t h o u g h t t o be mining-related.
This diagram was
generated u s i n g a combination of relocated earthquakes and previous single-event locations, b u t a t t h i s scale and due t o the averaging e f f e c t o f 10 km increments over a l l of southern Utah, the e f f e c t of
1
E 5
U
c v)
.r
I
5 P
m
n
98
99
relocations i s negligible.
F i g u r e 4.11 emphasizes t h e p o i n t t h a t v o s t
o f t h e energy i s r e l e a s e d o v e r a broad 150 km t o 200 km wide zone between t h e B a s i n and Range and Colorado Plateau.
P r e v i o u s workers
(Anderson, 1978; Smith and Sbar , 1974) have p o s t u l a t e d a t e c t o n i c mechanism f o r t h e loss i n coherency o f t h e I n t e r m o u n t a i n Seismic B e l t . -
They suggest t h a t zones o f prominent s e i s m i c a c t i v i t y may d e f i n e i n t r a p l a t e o r s u b p l a t e margins.
E p i c e n t e r s i n s o u t h e r n Utah seem t o
d i v e r g e i n t o two zones: one f o l l o w i n g t h e boundary o f t h e Colorado P l a t e a u P r o v i n c e and one c o n t i n u i n g on i n an east-west d i r e c t i o n i n t o s o u t h e r n Nevada.
I f t h e t r a n s i t i o n between t h e Basin-and-Range and
t h e Colorado P l a t e a u i s a c t u a l l y r e p r e s e n t e d by a l o w e r c r u s t
-
upper
m a n t l e upwarp, as suggested by r e f r a c t i o n , g r a v i t y , and P,-delaY (Eaton e t a1
., 1978;
data
Smith, 1978; Anderson, 1978), t h e n t h e d i f f e r i n g
response o f upper c r u s t a l m a t e r i a l t o t h i s upwarp c o u l d account f o r t h e broad zone o f a c t i v i t y .
The l a c k o f depth c o n t r o l i n a r e g i o n a l
s t u d y such as t h i s p r e v e n t s much s p e c u l a t i o n about t h e o c c u r r e n c e of hypocenters a l o n g l i s t r i c
-
normal f a u l t planes o r zones of
decol lement, a1 though t h i s c o u l d c e r t a i n l y make t h e e p i c e n t e r s appear randomly s c a t t e r e d on t h e s u r f a c e . Occurrence o f swarms ( F i g u r e s 4.6 -4.8)
i n t h e Cove F o r t and
S e v i e r areas and southeast o f t h e M i n e r a l Mountains, which cannot be e a s i l y a t t r i b u t e d t o changes i n groundwater l e v e l s o r o t h e r mechanisms, may be c h a r a c t e r i s t i c o f t h e change i n t h e r e g i o n a l t e c t o n i c s e t t i n g a t approximately t h i s l a t i t u d e .
I f f a i l u r e along
t h r u s t planes d u r i n g t h e S e v i e r orogeny was enhanced by t h e presence o f abnormal p o r e pressures, i t m i g h t be reasonable t o assume t h a t
100 localized pockets of anomalous pore pressure s t i l l e x i s t i n the c r u s t . Alternatively, excess pore f l u i d s could also be present due t o metamorphic a c t i v i t y (Davis and Coney, 1979) discussed previously. These could be concentrated i n t h i s region due t o the change i n the lower crustal structure.
Some combination o f these factors coupled
w i t h the possiible mantle-upwarp would account f o r the broad
coi nci dence of geothermal regions , general 1y diffuse sei smi c i t y , and
swarm a c t i v i t y i n 1 oca1 ized areas.
CHAPTER 5 EARTHQUAKE RELOCATIONS AND P-WAVE DELAY DATA FROM BROADSIDE REFRACTION RECORDS:
ROOSEVELT HOT SPRINGS
AND COVE FORT AREAS
The r e s u l t s of j o i n t hypocenter determination (JHD) o f 1 ocal (epicenter-station distances u p t o 70 k m ) earthquakes i n the Roosevel t Hot S p r i n g s and Cove Fort KGRA’s are discussed i n t h i s chapter. Interpretations of relocated 1 ocal and regional earthquakes are then combined t o provide a synthesis of earthquake d a t a w i t h i n f o r m a t i o n a b o u t the velocity structure i n the Milford and Cove F o r t areas.
Flew
information includes an interpretation of local P-delays from a
broadside refraction survey.
The purpose i s t o present the seismic
d a t a and also t o assess the possible value of regional and local
earthquake monitoring i n geothermal areas i n l i g h t of the previous d i scussi on of speci a1 ized earthquake 1 ocation techniques.
Earthquake Activity i n Geothermal Areas A number of studies have been carried o u t i n the past several
years on the seismicity of geothermal areas (e.g., Ward and Bjornsson, 1971; Hamilton and Muffler, 1972; H i l l e t a l . , 1.975; Combs and Hadley,
1977).
Ward ( 1 9 7 2 ) documented known occurrences of microearthquakes
i n geothermal areas, and a d d i t i o n a l l y proposed several uses f o r
accurately located microearthquakes.
Among them: ( i ) location of
102 a c t i v e f a u l t s t h a t may a c t as c o n d u i t s f o r h o t w a t e r , ( i i ) d e l i n e a t i o n o f t h e f racture-domi n a t e d r e s e r v o i r , ( iii ) c o r r e l a t i o n s o f earthquakes w i t h aspects o f t h e t e c t o n i c s e t t i n g , t o a i d i n u n d e r s t a n d i n g t h e o c c u r r e n c e of t h e geothermal system, and ( i v ) r e c o r d i n g o f s e i s m i c a c t i v i t y b e f o r e and a f t e r f l u i d w i t h d r a w a l .
Other i n v e s t i g a t o r s (e.g.
S t e e p l e s and P i t t , 1976) have s t u d i e d geothermal areas i n which t h e l a c k o f s e i s m i c i t y was i n t e r p r e t e d as s i g n i f i c a n t .
They suggested
t h a t absence of seismic a c t i v i t y , combined w i t h t h e o b s e r v a t i o n of o t h e r phenomena such as h i g h heat f l o w and delayed o r a t t e n u a t e d a r r i v a l s o f P and S waves, m i g h t be i n d i c a t i v e o f a d u c t i l e response
t o s t r e s s , due t o i n c r e a s e d temperatures. S e i smi c a l l y a c t i v e geothermal areas may be c h a r a c t e r i zed by c o n t i n u o u s o r s w a r v - l i k e a c t i v i t y t h a t has been e x p l a i n e d by a number o f mechanisms (Ward, 1972; Nard and Jacobs, 1971). (1)
Among t h e s e are:
H i g h p o r e pressures changing e f f e c t i v e s t r e s s e s a l o n g t e c t o n i c p l a n e s o f weakness ( f a u l t s )
.
P r i n c i p a l stresses
r e l a t e d t o p o t e n t i a l planes o f f a i l u r e a r e reduced b y increased p o r e pressure.
Dependi ng upon t h e o r i g i n a l val ue
o f t h e d e v i a t o r i c s t r e s s , a b n o r m a l l y h i g h f l u i d pressures may cause shear f a i l u r e , s h i f t i n g t h e M o h r ' s c i r c l e i n t o
t h e r e g i o n o f i n s t a b i 1i t y .
(2)
Chemical c o r r o s i o n o f f r a c t u r e s .
Weakening o f p r e - e x i s t i n g
f r a c t u r e s may be caused b y i n c r e a s e d s o l u b i l i t y o f m i n e r a l s (e.g.,
1979).
q u a r t z ) i n s o l u t i o n s under h i g h p r e s s u r e ( G r e t e n e r , T h i s process, however, i s dependent a1 so on
d i f f e r e n t i a l s t r e s s e s and on t e m p e r a t u r e c o n d i t i o n s .
103
‘ 0
( 3 ) Weakened rock units due t o hydrothermal a l t e r a t i o n .
a complex relationship.
This i s
Byerlee and Brace (1968) found t h a t
some a1 t e r a t i on mi neral s ( i .e. cal c i t e and serpentine) , favor release of s t r e s s by s t a b l e sliding rather t h a n stick-sl ip.
Chlorite and micaceous a1 t e r a t i o n products were
found t o have varied in responses t o applied s t r e s s .
In
some cases these alteration products favored s t i c k - s l i p . B u t in other cases the response was stable s l i d i n g , thus
favoring deformation without b r i t t l e f a i l u r e . Other l o c a l i t i e s may have l i t t l e or no seismicity d i r e c t l y associated with known geothermal reservoirs (e.g. , Hamilton and Muffler, 1972; Steeples and P i t t , 1976). Possible causes of t h i s are: (1) High temperatures favoring f a i l u r e by s t a b l e sliding rather t h a n s t i c k - s l i p (Brace, 1 9 7 2 ) .
Rocks in higher temperature
regimes may not store e l a s t i c s t r a i n energy t o be released by b r i t t l e f a i l u r e , deforming instead by creep mechanisms such as crystal dislocation o r diffusional processes (Weertman and Weertman, 1 9 7 5 ) ; (2)
Empirical evidence suggests t h a t porous materials (e.g. t u f f ) favor f a i l u r e by stable sliding (Byerlee and Brace, 1968) ;
( 3 ) Hydrothermal alteration t o minerals favoring stable sliding (see ,444, above) ; (4)
jb
Lack of adequate recording s p a t i a l l y or temporally.
104
Hssumi ng f o r the present t h a t a possible geothermal system h a s a1 ready been
dentified, w h a t i s the use of microearthquake monitor ng
t o delineate the relationships between earthquake occurrence and the geothermal resource?
I f the geothermal reservoir i s a
fracture-dominated system, the chemistry of geothermal brines may -
reduce permeability and porosity by cementing fractures.
I t can be
postulated t h a t some sort of continuous fracturing must be occurring
t o maintain an open reservoir.
Microearthquakes could be used t o help
delineate the f a u l t and associated fracture zones or the areal extent of the permeable reservoir.
Alternatively, lack of b r i t t l e f a i l u r e
may also delineate the extent of a hot region t h a t has a ductile response t o s t r e s s . Given t h a t the occurrence of geothermal areas i s related t o tectonic processes such a s are typical a l o n g plate margins, i n regions of recent volcanic a c t i v i t y , o r related t o major f a u l t systems,
acccurate delineation of microearthquake a c t i v i t y coupled w i t h regional seismicity studies could aid i n the interpretation of spatial relationships of geothermal systems w i t h major geologic a n d / o r structural trends.
I n other words, seismic a c t i v i t y i n geothermal
areas i s probably a t l e a s t p a r t i a l l y the r e s u l t of aspects of the geothermal environment, such as presence of excess pore f l u i d s contributing t o the release of pre-existing tectonic stresses. Finally, earthquakes can be used as sources o f energy for studying crustal properties related t o geothermal systems from: travel-time delays, ( i i ) v p / V ,
( i ) P and S
r a t i o s leading to determination o f
105 P o i s s o n ' s r a t i o and ( i i i ) amp1 i t u d e i n f o r m a t i o n f o r a t t e n u a t i o n studies. I t i s e v i d e n t t h a t any c o n c l u s i o n s drawn f r o m t h e above
r e l a t i o n s h i p s depend upon t h e p r e c i s i o n and accuracy o f t h e earthquake
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locations.
It has a l r e a d y been demonstrated t h a t t h e p r e c i s i o n o f
earthquake l o c a t i o n s , i.e.,
t h e i r l o c a t i o n s r e l a t i v e t o each o t h e r , i s
s u b s t a n t i a l l y improved b y t h e use o f j o i n t l o c a t i o n techniques.
In
a d d i t i o n , a b s o l u t e accuracy can a l s o be improved t h r o u g h t h e use o f a good r e f e r e n c e event.
I f o t h e r i n v e s t i g a t i o n s tire b e i n g c a r r i e d o u t
s i m u l t a n e o u s l y , as would p r o b a b l y be t h e case i n a geothermal area, a r t i f i c i a l sources (e.g.,
e x p l o s i o n s ) c o u l d be used as t h e c a l i b r a t i o n
events. R e l o c a t i o n o f Earthquakes i n t h e Roosevelt H o t S p r i n g s and Cove F o r t
KGRA's
Microearthquakes i n t h e Roosevelt and Cove F o r t geothermal areas were o r i g i n a l l y l o c a t e d by Olson (1976) u s i n g t h e s i n g l e - e v e n t method. The procedures I used f o r r e l o c a t i o n o f t h e s e events were s i m i l a r t o t h o s e used f o r t h e r e g i o n a l r e l o c a t i o n s d i s c u s s e d i n Chapter 4, and r e f e r e n c e events were chosen u s i n g s i m i l a r c r i t e r i a .
I n Olson's
(1976) s o l u t i o n s many o f t h e f o c a l depths had t o be f i x e d , e s p e c i a l l y i n t h e Roosevelt Hot S p r i n g s area.
Thus an a d d i t i o n a l s p e c i f i c a t i o n
f o r t h e r e f e r e n c e events was t h a t t h e s t a t i o n s r e c o r d i n g t h e r e f e r e n c e event had t o be s u f f i c i e n t t o r e s o l v e t h e f o c a l depth i n O l s o n ' s solutions.
IO
Grs
F i g u r e 5.1 i s a map o f t h e 1974-75 microearthquake s t a t i o n a r r a y
I
A
KANOSH
m +O.'O
38'+S
F 0
0
0
A
A
38' 15
a 0
5
0
I
*
10
.20
113"OO'
BEAVER
SYNTHETIC EVENTS
STAI'IONS
I
I
I
F i g u r e 5.1.
30 K M
I i*
0
I
I
1 12 " 3 0'
Locations o f s t a t i o n s from microearthquake s u r v e y i n the
Roosevelt Hot S p r i n g s
-
Cove F o r t a r e a (Olson 1976). Values a s s i g n e d t o g r i d p o i n t s are magnitudes (km) o f m i s l o c a t i o n of s y n t h e t i c e v e n t s l o c a t e d u s i n g j o i n t h y p o c e n t e r d e t e r m i n a t i o n . Contour i n t e r v a l i s 0.05 km.
107 (Olson, 1 9 7 6 ) .
Sixteen synthetic events were generated a t each grid
point in Figure 5.1, using representative subsets of the array.
This
means t h a t each synthetic event had arrival times a t s t a t i o n s t h a t normally recorded eathquakes near t h a t particular g r i d point. Contours of mislocation of these synthetic events are similar t o those produced f o r the regional t e s t cases in Chapter 3 (Figures 3.2 t o 3.6).
Figure 5.1 thus gives a generalized indic:ation of the varying
location capability of a local microearthquake survey, specifically the portable network deployed by Olson (1976). Two velocity models were used f o r these relocations
Stations
located i n the Milford Valley used a model derived from a reversed refraction profile across the Mineral Mountains (Gertson, 1979) Another model was applied t o stations in and east of the Mineral Mountains.
The l a t t e r model was a combination of Gertsonls (1979)
information and the velocity model for the Colorado Plateau described in Chapter 4.
Table 5.1 gives the velocity and depth information f o r
the two model s. Even with the good station distribution o f the 1974-75 survey (average station-spacing o f 10 k m ) about 50% of the events were not recorded by a s u f f i c i e n t number of s t a t i o n s t o resolve the focal depths, particularly i n the Milford region. reference events were computed twice:
Epicenters for the
once with depths fixed a t
Olson's (1976) hypocenters and once with depths unconstrained i n the final two iterations. IO
Nhen depths were included the systems of
equations were generally more unstable.
I n almost every case when the
solution included. the computation of the depth parameter, the
T a b l e 5.1.
V e l o c i t y models f o r l o c a l r e l o c a t i o n s :
Cove F o r t
Roosevelt H o t S p r i n g s
MINERAL MOUNTAINS MODEL
MILFORD VALLEY MODEL P- WP,V E VEL (Km/s)
-
DEPTH LAY EF? TO TOP (hi) THICKNESS (Km)
P-WAVE VEL (Kni/s)
DEPTH TO TOP ( K m )
LAYER THICKNESS (Knl)
1.8
0.0
0.3
2.9
0.0
0.2
4.0
0.3
1.2
3.2
0.2
0.3
6.7
1.5
23.9
4.0
0.5
1.1
7.4
25.4
-
5.7
1.6
12.7
6.4
14.3
11.7
7.4
26.0
-
109
' 0
JHD-computed f o c a l depths were 1-3 km s h a l l o w e r t h a n O l s o n ' s s o l u t i o n s , computed w i t h HYPO71 (Lee and Lahr, 1.975). r e s u l t was n o t e d f o r s y n t h e t i c data; i.e.,
A similar
t h e r e l a t e d program
HYPOELLIPSE c o n s i s t e n t l y l o c a t e d events 1-2 km deeper t h a n t h e t r u e I t i s b e l i e v e d , b u t has n o t been v e r i f i e d , t h a t
solutions.
HYPOELLIPSE may have a c o n s i s t e n t e r r o r i n f o c a l d e p t h computations. S i n c e no o t h e r m a j o r d i f f e r e n c e s were n o t e d i n t h e f o c a l depths computed b y HYPO71 and JHD77, and because o f t h e i n a b i l i t y t o s o l v e f o r 50% o f t h e f o c a l depths, o n l y e p i c e n t r a l computations were c a r r i e d o u t f o r t h e r e m a i n i n g ( n o n - r e f e r e n c e ) events.
The f o c a l depths were
f i x e d a t t h o s e computed by HYPO71 and t a b u l a t e d b y Olson (1976).
They
a r e p r o b a b l y c o r r e c t t o w i t h i n two k i l o m e t e r s , w i t h t h e e r r o r b i a s e d towards t h e deeper values.
No f u r t h e r e x a m i n a t i o n o f depth was made
i n t h e p r e s e n t study, b u t f r o m Olson (1976) i t appears t h a t f o c a l depths a r e v a r i a b l e t h r o u g h o u t t h e Roosevelt Hot: S p r i n g s area, o c c u r r i n g m o s t l y w i t h i n t h e range o f a 0-12 km. as 14-15 km.
A few may be as deep
I n t h e Cove F o r t area t h e m a j o r i t y o f events occur a t
f o c a l depths l e s s t h a n 8 km. I n t e r p r e t a t i o n o f Relocated Epicenters Comparison o f O l s o n ' s (1976) e p i c e n t e r s w i t h t h e l o c a t i o n s computed u s i n g t h e c o m b i n a t i o n JED program and s t a t i o n adjustments i n t h e accompanyi ng s i n g l e event program showed s i g n i f i c a n t decreases in t h e s i z e s o f areas covered by swarm a c t i v i t y f o r t h e JED events. F i g u r e 5.2
shows t h e r e l o c a t e d earthquakes r e l a t i v e t o c o n c e n t r a t i o n s
o f r e g i o n a l events l o c a t e d w i t h t h e U n i v e r s i t y o f Utah permanent
4 s . t
\
t\
3P.f
rn
MILFO~IO
115.415
Figure 5 . 2 . Relocated earthquakes i n the Cove Fort - Milford area.Arrows point from former single-event locations of O l m n (1976) t o j o i n t l y determined locations (closed c i r c l e s ) . Heavy 1ines are Quaternary f a u l t s . Irregular outlines a r e major concentrations o f earthquakes located with the permanent regional seismograph s t a t i o n s from 1962-1978.
111 network.
Arrows a r e drawn f r o m t h e former HYPO71 l o c a t i o n s t o t h e
e p i c e n t e r s computed by t h e JED program.
No s y s t e m a t i c s h i f t i n
r e g i o n a l e p i c e n t e r s i s obvious, b u t n o r t h o f Y i l f o r d t h e d i r e c t i o n s and magnitudes o f some o f t h e l a r g e s h i f t s i n e p i c e n t e r s (5-19 km) can be a t t r i b u t e d i n p a r t t o t h e s t a t i o n d i s t r i b u t i o n .
Semi-major axes o f
c o n f i d e n c e e l 1 i p s e s a r e g e n e r a l l y o r i e n t e d n o r t h w e s t i n t h i s area, suggest ing p o o r e r r e s o l u t i on in t h a t d i r e c t i on. Some l a r g e s h i f t s i n l o c a t i o n s ( g r e a t e r t h a n t e n k i l o m e t e r s ) were observed f o r events w i t h poor s t a t i o n coverage ( f o u r r e c o r d i n g Out o f an o r i g i n a l 163 events,
s t a t i o n s w i t h no S-phase i n f o r m a t i o n ) .
about 10% were e l i m i n a t e d f r o m t h e r e l o c a t i o n procedure because t h e y were r e c o r d e d b y o n l y 3 s t a t i o n s .
F i f t e e n percent o f t h e remaining
e p i c e n t e r s showed t h e s h i f t s o f g r e a t e r t h a n 10 km.
Such l a r g e
d i f f e r e n c e s i n two s o l u t i o n s f o r t h e same e v e n t emphasizes t h e f a c t t h a t even i n a good microearthquake survey many e v e n t s a r e n o t r e c o r d e d by enough s t a t i o n s t o p r o v i d e adequate c o n t r o l f o r t h e i r
1o c a t ion. Comparison of t h e e p i c e n t e r s i n F i g u r e 5.2 w i t h t h e c l u s t e r s of r e g i o n a l e p i c e n t e r s ( a l s o i n F i g u r e s 4.6, similarities i n t h e i r distributions.
4.7 arid 4.8)
shows some
About f i v e k i l o m e t e r s e a s t o f
Cove F o r t i s a c l u s t e r o f events t h a t was l o c a t e d w i t h t h e r e g i o n a l stations.
Almost a l l o f t h i s a c t i v i t y o c c u r r e d i n swarms t h r o u g h o u t
t h e f i r s t s i x months o f 1977.
A number o f t h e earthquakes south and
e a s t o f Cove F o r t o c c u r r e d about t h i s same t i m e (May t o mid-June,
!O
IGrS
1977). The e l o n g a t e d c l u s t e r of events e a s t o f Cove F o r t i n O l s o n ' s
112 1974-75 d a t a s e t occurs between t h e two r e g i o n a l l y - l o c a t e d c l u s t e r s . The a c t i v i t y i s m o s t l y s h a l l o w ( < 5 km), and 01:jon (1976) p o s t u l a t e d t h a t t h e earthquakes o c c u r r e d a l o n g a n o r t h - t r e n d i n g f a u l t .
However,
t h e c l u s t e r o f r e l o c a t e d e p i c e n t e r s shows a d e f i n i t e east-west e l o n g a t i o n , s u g g e s t i n g an e a s t - t r e n d i n g o r i e n t a t i o n f o r t h e a c t i v e earthquake zone.
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Several f a u l t s s t r i k i n g east-west a l o n g Cove Creek
have been mapped by Moore and Samberg (1979).
Most o f t h e s e a r e
concealed b y a l l u v i u m , and some appear t o be t r u n c a t i n g , o t h e r s i n t e r s e c t e d by, n o r t h - s o u t h f a u l t s t h a t have mapped d i s p l a c e m e n t s i n t h e T e r t i a r y v o l c a n i c s (Moore and Samberg, 1979:).
Thus a t l e a s t some
o f t h e east-west f a u l t s a r e a p p a r e n t l y younger t h a n l o c a l Miocene t u f f deposits.
Based on t h i s i n f o r m a t i o n , i t i s p o s t u l a t e d t h a t an a c t i v e
east-west f r a c t u r e zone may o c c u r a l o n g Cove Creek, p o s s i b l y e x t e n d i n g west beneath t h e a l l u v i u m o f t h e Cove F o r t graben. Regional s t a t i o n d i s t r i b u t i o n s i n c e 1977 has been f a i r l y good i n t h e Cove F o r t
-
km of Cove F o r t
M i l f o r d area, i n c l u d i n g f i v e s t a t i o n s w i t h i n about 60 However, t h e s e f i v e s t a t i o n s (CFU , MNU , MSU , PUU and
RHU) were n o t o p e r a t i n g u n t i l 1977 ( R i c h i n s , 1979); hence, none o f t h e earthquakes d e t e c t e d by 01 s o n ' s (1976) microearthquake a r r a y were d e t e c t e d b y t h e r e g i o n a l network. C o n s i d e r i n g t h e d i f f e r e n t s t a t i o n d i s t r i b u t i o n s and t h e d i f f e r e n t r e f e r e n c e events used f o r t h e r e g i o n a l and l o c a l earthquakes, i t i s p r o b a b l e t h a t t h e r e i s a c l o s e r s p a t i a l a s s o c i a t i o n o f t h e 1974-75 microearthquakes and t h e post-1977 earthquakes t h a n i s obvious from F i g u r e 5.2. Earthquakes n o r t h o f Cove F o r t d e f i n e a n o r t h e a s t - s o u t h w e s t
cus
113 trend, more Dronounced in the relocations.
Th-is swarm coincides w i t h
a northeast trending zone of regionally located earthquakes. Based upon t h i s study of relocated epicenters i n the Roosevelt
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Hot S p r i n g s and Cove F o r t areas, i t i s apparent t h a t the more precise locations of earthquakes resulted i n f i n e r delineation of seismically active zones.
10
I n the Milford Valley there are probably several
f a u l t s , the presence of which has been detected by local g r a v i t y
investigations (Crebs, 1976).
The occurrence o f scattered earthquakes
suggests that these may be more active a t present t h a n the more prominent f a u l t s bounding the Mineral Mountains horst (e.g. , the Opal Mound f a u l t ) .
B u t sparse earthquake a c t i v i t y appears t o be I
c h a r a c t e r i s t i c of the Roosevelt Hot S p r i n g s area.
I n contrast, the
Cove F o r t area shows earthquakes occurring on f a u l t s of two d i s t i n c t orientations:
one prominent east-west and several northeast-striking
zones, which may r e f l e c t active faulting n o t mapped on the surface. ( r e f e r t o Figures 5.2 and 4.7). Velocity Models and P-Wave Delays A common probl em in determi ni ng microearthquake 1 ocati ons i s
inadequate knowledge of the velocity model.
I n t h i s investigation of
earthquakes i n the Roosevelt and Cove F o r t KGRA's a l l relocations were done w i t h the two velocity models i n Table 5.1, predominately derived from the refraction profiles of Gertson (1979).
B u t Gertsonls
profiles and local gravity studies (Carter and Cook, 1978; Crebs, 1976) indicate t h a t the f l a t - l a y e r a p p r o x i m a t i o n i s not a good model IO
f o r detailed investigations o f t h i s complex area.
114 F u r t h e r evidence s u g g e s t i n g l a t e r a l inhomogeneity i n t h e v e l o c i t y models e x i s t s i n t h e f o r m of P-wave a r r i v a l - t i m e d e l a y s measured from refraction blasts.
D u r i n g t h e 1977 r e f r a c t i o n experiment seven
p o r t a b l e s t a t i o n s were arranged b r o a d s i d e t o t h e r e f r a c t i o n 1 i n e (Gertson, 1978).
F i v e seismograph s t a t i o n s f r o m t h e permenanat
network a l s o r e c o r d e d t h e b l a s t s . l o c a t i o n s and s h o t p o i n t s .
F i g u r e 5.3a ,shows t h e s t a t i o n
B l a s t s i z e s ranged f r o m 1000 t o 3000
pounds and produced general l y c l e a r , though n o t a1 ways impul s i ve, P-wave a r r i v a l s .
These were t i m e d t o t h e n e a r e s t
0.02
A
seconds.
t o t a l o f 13 S-wave a r r i v a l s were a l s o t i m e d b u t were n o t s u f f i c i e n t t o pursue i n v e s t i g a t i o n s of V p / V s
r a t i o s o r S-wave a t t e n u a t i o n .
Three
b l a s t s were r e c o r d e d f r o m shot p o i n t 1, two f r o m s h o t p o i n t 9 , and one b l a s t f r o m s h o t p o i n t s 3, 5, and 7. F i f t e e n microearthquakes were a l s o d e t e c t e d d u r i n g t h e 2-week period.
I n i t i a l l y t h e earthquakes were t o be used t o supplement t h e
b l a s t i n f o r m a t i o n , b u t i t was determined t h a t t h e e r r o r i n v o l v e d i n t h e i r l o c a t i o n would c o n t r i b u t e more t o t h e r e s i d u a l s t h a n t h e d e v i a t i o n s f r o m t h e v e l o c i t y models. T r a v e l - t i m e r e s i d u a l s f o r P-wave a r r i v a l s a t each s t a t i o n were computed based upon t h e f l a t l a y e r v e l o c i t y models ( T a b l e 5.1.). These r e s i d u a l s a r e p l o t t e d f o r each s h o t p o i n t i n F i g u r e s 5.3b, and 5.5.
5.4,
The shot p o i n t l o c a t i o n s a r e i n t h e c e n t e r o f each p l o t ;
each g r i d p o i n t i s t h e mean o f a l l P-wave r e s i d u a l s a t t h a t p a r t i c u l a r azimuth and d i s t a n c e .
P o s i t i v e elements i n d i c a t e p o s i t i v e d e l a y s
( 1 a t e a r r i v a l s ) ; negat ve Val ues correspond t o e a r l y a r r i Val s.
A s y s t e m a t i c exam n a t i o n of t h e r e s i d u a l s from each s h o t p o i n t
aCFU
0 ~
~
0
D
SHOTPCINT
STATICN
/'FAULT
Figure 5 . 3 . a ) Location map of broadside refraction array and f i v e permanent seismograph stations relative t o shotpaints of 1977 refraction line (Gertson , 1979). b ) 3-0 plots of travel-time residuals (sec) w i t h distance (kin) from shotpoint- 1.
116
Figure 5.4. a ) and b) 3-0 plots o f travel-time residuals (sec) w i t h distance (km) from shotpoints 3 and 5, respectively.
18
Figure 5.5. a) and b ) 3-D plots of travel-time residuals (sec) w i t h distance (km) from shotpoints 7 and 9, respectively.
0 I
118 r e v e a l s t h a t most can be a t t r i b u t e d t o v e l o c i t y v a r i a t i o n s i n t h e upper t h r e e km of c r u s t below sources and r e c e i v e r s .
SEP, and WEL used t h e M i l f o r d V a l l e y model.
S t a t i o n s FLT,
A l l a r r i v a l s a t these
s t a t i o n s were l a t e , w i t h p a r t i c u l a r l y l a r g e d e l a y s (0.8 t o 1.4 sec) r e c o r d e d f o r shot p o i n t s 1 and.3.
These d e l a y s a r e observed because
t h e model used was an a p p r o x i m a t i o n o f t h e v e l o c i t y s e c t i o n on t h e e a s t e r n margin o f t h e v a l l e y , whereas t h e d e p t h o f t h e a l l u v i u m i n c r e a s e s f r o m about 0.3 m t o 1.0 km i n t h e c e n t e r o f t h e v a l l e y (Gertson, 1978).
S t a t i o n s r e c o r d i n g b l a s t s f r o m s h o t p o i n t s 1 and 3
a t d i s t a n c e s of over 20 km g e n e r a l l y show small ( l e s s t h a n 0.3 sec) p o s i t i v e and n e g a t i v e r e s i d u a l s .
A r r i v a l s a t t h i s distance are
a
p r o b a b l y e q u i v a l e n t t o pg, t h a t i s , r e f r a c t e d a r r i v a l s from t h e Precambri an c r y s t a l 1 ine basement, p r o b a b l y banded gnei ss mapped i n t h e M i n e r a l Mountains area by N i e l s o n e t a l .
(1978),,
Gneiss may t y p i c a l l y
have v e l o c i t i e s o f about 4.5 t o 6.0 km/s (Press!, 1966). 4.0
The v a l u e o f
km/s p r o b a b l y r e p r e s e n t s an adequate average v e l o c i t y a l o n g t h e
ray path.
R e l a t i v e t o t h e v e l o c i t y model t h e basement v e l o c i t i e s may
be h i g h e r , b u t l o w e r v e l o c i t i e s would be i n e f f e c t i n t h e t h i c k e r alluvium.
F a u l t i n g w i t h i n t h e M i l f o r d V a l l e y and p o s s i b l e v e l o c i t y
a n i s o t r o p y i n t h e g n e i s s c o u l d account f o r o t h e r small v a r i a t i o n s (up t o 0.2 sec) i n t h e r e s i d u a l s . A r r i v a l s from shot p o i n t 5 a r e a l l e a r l y ( F i g u r e 5.4b), e s p e c i a l l y a t t h e c l o s e s t s t a t i o n , RHU.
Ray p a t h s a r e a p p r o x i m a t e l y
p a r a l l e l t o t h e M i n e r a l Range f r o n t , and t h e p r o x i m i t y t o t h e Opal Mound f a u l t suggests t h a t t h e y may be t r a v e l i n g t h r o u g h cemented
.
alluvium (Parry e t a1 , 1 9 7 7 ) , that may account f o r the higher vel oci t i es. Residuals from the blast a t SP7 are generally small (absolute values l e s s than 0.3 sec) and can be a t t r i b u t e d t o l a t e r a l variations such a s the presence of faulting, hydrothermal a l t e r a t i o n , and d i p p i n g layers.
Residuals a t S t a t i o n s FLT and SEP i s a g a i n positive, probably
due t o inadequate modeling of the thickness of ,aluvial f i l l under these stations.
The negative RHU residual can /be attributed t o rays
traveling t h r o u g h more granitic material t h a n wtas modeled for t h i s station. Except a r r i v a l s a t stations PUU and MSU i n the Cove Fort graben, rays from shot point 9 a l l traveled t h r o u g h the Mineral Mountains. Large positive delays a t PUU and MSU can probably be attributed t o the presence of alluvial f i l l and Pennsylvanian t o Permian sediments and Tertiary ash flow t u f f s . were a l s o l a t e .
to
WAV.
J
I
Arrivals a t a l l other s t a t i o n s except WAV
P-wave arrivals from SP9 were impulsive, including
B u t one arrival a t WAV had a reversed sen'se of motion (down)
while other a r r i v a l s were a l l normal ( u p ) .
This could indicate t h a t
the f i r s t a r r i v a l was mis-identified because a l l a r r i v a l s from an
8
explosive source should have the same direction of f i r s t motion. .
0
The
p o l a r i t y was n o t reversed a t s t a t i o n WAVY because a r r i v a l s from other
shots on the same day were consistent. A teleseismic P-delay study was recently completed by the USGS
for the Roosevelt and Cove F o r t region (Robinson and Iyer, 1 9 7 9 ) . Their main result was the detection o f an elongated zone (parallel t o the Mineral Mountains range) of low velocity material extending from 5
120 I
t o 35 km below t h e Roosevelt Hot Springs.
The r e g i o n ws i n t e r p r e t e d
as h a v i n g a maximum v e l o c i t y decrease o f 5 t o 7'%. T h i s c o u l d be a s s o c i a t e d w i t h a d e n s i t y decrease o f about 0.15
g/cm3.
A g r a v i t y low
o f 15-20 mgals ( C a r t e r and Cook, 1978) corresponds t o t h i s general r e g i o n (Robinson and I y e r , 1979). The p o s i t i v e P-delays f r o m SP9 may i n d i c a t l e t h a t r a y s t r a v e l l e d t h r o u g h t h e upper p a r t of t h e same l o w v e l o c i t y r e g i o n d e t e c t e d b y t h e t e l e s e i s m i c r a y paths.
Robinson and I y e r (1979) a t t r i b u t e d t h e l o w
v e l o c i t i e s t o a pipe o f p a r t i a l l y molten material. (0.5
-
1.2 sec i n t h e r e f r a c t i o n data; 0.1
-
0.3
But t h e residuals
sec i n t h e
t e l e s e i s m i c d a t a ) c o u l d be a s s o c i a t e d w i t h t h e f r a c t u r e d n a t u r e and p o s s i b l y f l u i d - f i l l e d p o r o s i t y o f t h e western edge o f t h e M i n e r a l Mountains p l u t o n .
They c o u l d a l s o be due t o a c o m p o s i t i o n a l change
f r o m g r a n i t e t o gneiss.
Mean d e n s i t i e s f o r t h e s e r o c k s i n t h e M i n e r a l
Mountains area a r e 2.59 gm/cc and 2.69 gm/cc, Cook, 1978).
T h i s d e n s i t y c o n t r a s t o f 0.10
r e s p e c t i v e l y ( C a r t e r and
gm/cc may be s u f f i c i e n t t o
produce t h e observed v e l o c i t y decrease. Evidence f r o m t h e P-delays o f t h e b r o a d s i d e a r r a y t h u s suggest s e v e r a l inadequacies i n t h e s i m p l e f l a t - l a y e r e d models used t o l o c a t e i n t h e microearthquake l o c a t i o n .
Inadequate m o d e l i n g o f t h e depth o f
a l l u v i u m a t some s t a t i o n s , h i g h v e l o c i t y zones a s s o c i a t e d w i t h cementation and hydrothermal a l t e r a t i o n , l o w v e l o c i t y zones a s s o c i a t e d w i t h f r a c t u r e systems and l i t h o l o g i c c o n t r a s t s between g r a n i t e and gneiss, a1 1 c o n t r i b u t e t o t h e c o m p l e x i t y o f t h e v e l o c i t y model.
IO
10
The q u e s t i o n becomes how much d i f f e r e n c e l a t e r a l v e l o c i t y
121 v a r i a t i o n s make on t h e hypocenter l o c a t i o n s .
Based upon t e s t s u s i n g
O l s o n ' s (1976) f l a t - l a y e r e d model f o r a l l s t a t i o n s and t h e two models determined f r o m r e f r a c t i o n , f o r s i n g l e - e v e n t l o c a t i o n s d i s c r e p a n c i e s i n t h e two s o l u t i o n s range f r o m 1 t o 15 km, w i t h an average d i f f e r e n c e o f 5 km.
However, as was p r e v i o u s l y demonstrated f o r t h e t e s t cases
i n Chapter 3, t h e JHD ( o r JED) t e c h n i q u e m i n i m i z e s t h i s d i f f i c u l t y t h r o u g h s y s t e m a t i c adjustment o f t h e v e l o c i t y models t h r o u g h t h e s t a t i o n corrections. Based on i n t e r p r e t a t i o n o f e p i c e n t e r r e l o c a t i o n s u s i n g t h e j o i n t hypocenter d e t e r m i n a t i o n method and on a p p l i c a t i o n t o r e g i o n a l imp1 i c a t i o n s of s e i s m i c a c t i v i t y i n geothermal /
Comparison o f Hypocenter L o c a t i o n Techniques and Programs (Cont. )
. m
e Event
s /
-/PROBLEMS
_____
Travel Tiitie Bias Included i n Variance Ca 1culations
I
HYPOELL.
HYPOCENTER LOCATION METHODS Master E v e r t
SE77M
SEllS
L -
--
Disadvantage: 1) I n c o r r e c t weights applied, l e a d i n g t o i n c o r r e c t s o l u t i o n s ; 2 ) adverse e f f e c t on s i z e and eccentricity o f error ellipses
I n c l u s i o n o f Events Not Subject t o S i i i i i l a r Biases
-Joi n t JHD77
Deteriiti na t i o n
I
GHYP1 --
Advantage: C o r r e c t i o n o f t r a v e l - t i m e bias; n o t i n c l u d e d i n variance terins Disadvantage: May a f f e c t e n t i r e group o f l o c a t i o n s
_ _ I _ -
_ _ _ _ _ I _ _ _ _ -
I -
Results t o r Speci f i c Programs : 1) Teriiiinatiott o f Iteration
2) Statistical Iksults
Size o f l a s t step i n pa rariieter space
RMS I SE ) .
SE Eilipsk; Means o f s t a t i o n residuals; Progression o f paratiieter a d j us tiitents
I
I
I
I I
I I
Humber o f i t e r a t i o n s
I
I
I
I 1 I
I Standard e r r o r (SE).
Confidence e l l i p s e .
Station I
adjustiiients w i t h variances; iteans o f s t a t i o n r e s i d u a l s ; o p t i o n a l : progression o f parairteter adjus titierits
I I
I
Nuniber o f iterations; convergence based on s i z e o f l a s t step i n ;?:.ra;;ietei- space
c
€
PLATE I .
APPLICATIOS, 4 B L E M S
Coinparison o f hypocenter Location Techniques and Programs (Cont.)
Single
I
-HYPOELL.
__________._
3 ) !Jei ght ing
IIYPOCENTER LOCATION METHODS Master Event
Evenk 1
SE77S
---I,
I
SE77M
I
J o i n t Determination JtlD77
i
GHYPl
I
Forin o f Form o f J e f f r e y ' s weighting. No distance weighting. User deterwines i n i t i a l weights and variances Jeffrey's weighting ; I others I
-t-
I
I I
Pearson Type V I 1
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Arabasz, W. J . and R. B. Smith (1979). The Nove!mber 1971 earthquake swarms near Cedar City, Utah, i n Earthquake S t u d i e s i n Utah, 1850 t o 1978. Arabasz. Smith. and R i c h i n s . eds.. 423-432. Beck, J. V. and K. 3. A r n o l d (1977). Parameter E s t i m a t i o n i n E n g i n e e r i n g and Science, W i l e y and Sons, Inc., New York. Best, M. G. and W. K. Hamblin (1978). O r i g i n o f t h e n o r t h e r n B a s i n and Range p r o v i n c e : i m p l i c a t i o n s f r o m t h e geology o f i t s e a s t e r n boundary, i n Cenozoic T e c t o n i c s and Regional Geophysics of t h e Western C o r d i l l e r a , Mem. Geol. SOC. Am. -152, . 313-340. B l a c k w e l l , D. D. (1978). Heat f l o w and energy l o s s i n t h e western U n i t e d S t a t e s , i n Cenozoic T e c t o n i c s and Regional Geophysics o f t h e Western C o r d i l l e r a , Mem. Geol. SOC. Am. .-152, 175-208. B o l t , B. A. (1970). Earthquake l o c a t i o n f o r small networks u s i n g t h e 60, 6 , g e n e r a l i z e d i n v e r s e method, B u l l . Seism. Solc. Am. 1823-1828. B o l t , B. A,, and H. Freedman (1968). Group a n a l y s i s o f v a r i a n c e f o r earthquake l o c a t i o n and magnitude, N a t u r e -217, 47-49. B o l t , B. A., P. Okubo, and R. A. Urhammer (1978). Optimum s t a t i o n d i s t r i b u t i o n and d e t e r m i n a t i o n .of hypocenters f o r s m a l l networks, Report f o r Corps o f Engineers, U.S. A r m y Engineer Waterways, Univ. o f C a l i f o r n i a , Berkeley. Bolt,
B. A. , P. Okubo, and R. A. Urhammer (1978). program f o r j o i n t hypocenter l o c a t i o n s .
GHYP1, a computer
Brace, W. F. (1972). L a b o r a t o r y s t u d i e s o f s t i c k - d i p and t h e i r a p p l i c a t i o n t o earthquakes, i n Forerunners o f S t r o n E a r t h uakes, Savarensky and R i k i t a k e , eds., T e c t o n o p h y s i ~ 2 0 0 .
128 Buland, R. (1976). The mechanics o f l o c a t i n g earthquakes, B u l l . Seis. SOC. Am. 66, 1, 173-187. B y e r l e e , J. D. and W. F. Brace (1968). S t i c k - s l i p , s t a b l e s l i d i n g , and earthquakes e f f e c t o f r o c k t y p e , pressure, s t r a i n r a t e , and s t i f f n e s s , J. Geophys. Res. 73, 18, 6031-6037.
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C a r t e r , 3 . A. and K. L. Cook (1978). Regional g i r a v i t y and aeromagnetic surveys of t h e M i n e r a l Mountains and v i c i n i t y , M i l l a r d 77-11, DOE/DGE g r a n t and Beaver Counties, Utah, T o p i c a l rep. -EY-76-S-07-1601, Univ. of Utah, 178 p. C h r i s t i a n s e n , R. L. and E. H. McKee (1978). L a t e Cenozoic v o l c a n i c and t e c t o n i c e v o l u t i o n o f t h e Great B a s i n and Columbia i n t e r m o n t a n e r e g i o n s , i n Cenozoic T e c t o n i c s and Regional 152 Geophysics o f t h e Western C o r d i l l e r a , Mem. Geol. SOC. Am. -' 283-312. C l a r k , E. E. (1977). L a t e Cenozoic and t e c t o n i c a c t i v i t y a l o n g t h e e a s t e r n m a r g i n o f t h e Great Basin, i n t h e p r o x i m i t y o f Cove F o r t , Utah, Brigham Young U n i v e r s i t y Geology S t u dies, 24, p t . 1, 87-114. Cook, K. L. , J . R. Montgomery, J. T. Smith, and E. F. Gray (1975). Simple Bouguer G r a v i t y Map of Utah, Univ. olf Utah. Combs, J . and D. Hadley (1977). Microearthquake i n v e s t i g a t i o n of t h e Mesa geothermal anomaly, Geophysics 42, p. 17.
iQ
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'0
Dewey, 3. W. (1971b). S t a t i s t i c a l i n v e s t i g a t i o n o f t h e method of j o i n t d e t e r m i n a t i o n , of hypocenters, Trans. Amer. Geophys. Union 52, 283-284. Dewey, J . W. (1978). documentation.
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Kaminski, W. and G. Muller (1978). pl o t t ing program. Q
GM, t r a v e l - t i m e c a l c u l a t i o n and
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K e l l e r , G. R.,
1093-1098. Lachenbruch, A. H. and 3 , H. S a s s ( 1 9 7 8 ) . Models of e x t e n d i n g l i t h o sphere and h e a t flow i n the Basin and Range p r o v i n c e , i n Cenozoic
130 T e c t o n i c s and Regional Geophysics o f t h e Mestern C o r d i l l e r a , Mern. Geol SOC. Am. 152, 209 250-
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