Kentucky Geological Survey, University of Kentucky. Abstract. Over 1.5 kilometers of high-resolution, SH-wave refraction and reflection profiles have been ...
GEOPHYSICAL RESEARCH LETrERS, VOL. 20, NO. 15, PAGES 1615-1618, AUGUST 6, 1993
NEAR-SURFACE AS IMAGED
DEFORMATION BY HIGH
IN THE
RESOLUTION
NEW
MADRID
SH-WAVE
SEISMIC
SEISMIC
ZONE
METHODS
EdwardW. Woolcry, Ron L. Street,and ZhenmingWang Departmentof GeologicalSciences,Universityof Kentucky James B. Harris
KentuckyGeologicalSurvey,Universityof Kentucky Abstract. Over 1.5 kilometersof high-resolution, SH-wave refractionandreflectionprofileshavebeenperformedin the Madrid Bend areaof northwestern Tennessee,approximately 8 km north of Tiptonville, Tennessee,to establish the existence of neo-tectonic, near-surface deformation.
The
specificareawas chosenbecauseof the accessit providedto the Tiptonvilledome (a Holoceneflexure) and the trend of contemporary seismicitywithintheNew Madridseismiczone (NMSZ). We believedthe highestlikelihoodof near-surface deformationto be over these features. Partially reversed refraction/reflection walk-awaytestsandcommondepthpoint (CDP) seismicprofiling, usingthe non-traditionalSH-wave method, have traced antiformal flexure and numerous faults into
the
late
Eocene
and
Holocene
sediments
of
the
Tiptonvilledomeprovinceof the Lake Countyuplift (LCU). Vertical displacements exhibitedacrossthe faultsrangefrom approximately3 to 30 m. These structureshave been interpreted to be neo-tectonic features associatedwith tensionalstresses acrossthe Tiptonvilledome.
Introduction
Advantagesgainedfrom using SH-wave techniquesover the more traditionalcompressional (P) waves include:(1) soil/sedimentcolumn being sampled and not the water column;(2) lack of phasereversalsoutsidethe near offsets [Norminton, 1990]; and (3) higher resolutionof the lowvelocity SH-wave in the water-saturatedsoil/sediment column. SH-wavesare also generallysuperiorto vertically polarized(SV) shearwavesin thatinterferencefrom P-wave,
16 work encouraged us to expandthe studyintoareaswhere near-surface deformationwas thoughtmostlikely to occur. Approximately1 km westof the reconnaissance survey,line B-25 was shotover the axial high of the Tiptonvilledome [Crone and Brockman,1982] and zone of contemporary seismicitywithin the NMSZ, where it was believedthat the likelihood of near-surface deformation would be maxi•rfized.
Geographicand GeologicSetting
MadridBendliesinsidea meanderof theMississippi River at the extreme comers of southwesternKentucky and northwestern Tennessee (Figure1). Geologically,the areais situatedin theupperMississippiembayment wherePaleozoic bedrockis overlainby approximately 600 m of Cretaceous to Holocene,poorlyconsolidated, water-saturated sediment.This studyexaminedthe upperportionof the geologiccolumn, focusing ontheEocene-Holocene unconfonnity (approximate 40 m depth)thatseparates theJackson Formation(anEoceneaged, unconsolidated silty clay unit) from recent alluvial deposits, aswell ashigherintra-alluvialreflectingboundaries. The areaof interestbisectstheTiptonvilledomeprovince of the LCU, and crossesthe NMSZ in a perpendicular eastwest manner. Russ [1982] describedthe LCU as an active,
elongate deformation zoneof approximately 1,150km2 that 36.750
tectonic
deformation
of the Holocene
sediments.
89.500
89.250
t
I
i
I
/
NEW MADRII•.• tJ!Je•N,•
MADRID I :':i:::::::: •
_
I t
'?:3'.:";•;?" /• (g KENTUCKY - ...... h
Such
k•'•::•;••l TIPTONILLE ' •'/••••;•amoav•E
••::•::::::.•
deformationhas been maskedin previousNMSZ studies [Johnstonand Sherlock, 1992] that have used the more conventional P-wave methods.
•',,,'-/
LAKE COUNTY UPLIFT
converted P-wave to SV-wave, and converted SV-wave to P-
wave energy is effectively eliminated by reversing the polarityof the energysourceand stackingthe outputs. The objectiveof this studywas to use the advantages of SH-wave seismic techniquesto image and characterize
89.750
DOME
A reconnaissance SH-wave
refractionandreflectionsurvey,B-16, was shotover an area where faulting had been documented by the P-wave investigations of Zoback [1979] and Zoback et al. [1980]. This surveywas performedto test the abilitiesof the SHwavetechniquein the LCU area. Positiveresultsfrom the B-
36.120 . MIssissIPPI
I
RIVER •• 0
36.00 •,
Copyright1993by theAmericanGeophysical Union.
CEHIRAL IREHD OF
I tr
I
i 15 KM
I
-
Paper Number 93GL01658
Fig. 1. Locationmapshowingthe surveylinesin relationto the modemseismicity, as well asprominentgeographic and
0094- 8534/93/93GL-01658503.00
topographicfeatures(after Russ, 1982). 1615
1616
Wooleryet al.: ShallowStructurein New Madrid SeismicZone
hasitsmajoraxisaligned in a north-south orientation. He furtherstatedthat there is a strongcorrelationbetweenthe LCU andcontemporary seismicity(approximately 75 percent of activityfrom 1974 to 1978 was in the upwarpedarea). The Tiptonville dome is an asymmetrical,monoclinalridge that hasthe greatestamountof upwarpingand topographical relief on the LCU. Alongtheeasternedgeof theTiptonville dome lies the Reelfoot Lake basin, which is an area of
downwarped bedrock [Crone and Brockman, 1982]. Separatingthe dome andridge complexfrom the basinis the north-southtrendingReelfoot scarpthat Russ [1979; 1982] depictedas a complexmonoclinalfold, 3 to 9 m in height, with normalfaulting at its base. Previous Work
Deep, as well as surfacedeformationin the Madrid Bend
area has been recognizedin past seismic surveysand trenchingstudies.Shallowdeformation (faultingandfolding) in an exploratorytrench acrossthe Reelfoot scarp was described by Russ [1982]. He determined that this deformation hasthe samegeometry,andlies directlyabove the deep-seated faultingof the Late Cretaceous sediments and Paleozoicbedrock seen in vibroseisreflectionprofiles discussedby Hamilton and Zoback [1979], Zoback [1979], and Zoback et al. [1980].
point (CDP) seismicacquisitionconfigurations were set up alongthe shoulderof road bedswithin the studyarea. The B-16 (Figure 2) reconnaissance surveyconsistedof four walk-away SH-wave lines shot with 4.5 hz horizontal geophones and 6.1 m to 244 m offsets,and a short100 m 3fold CDP profile. The walk-away lines were used to determine target reflectors and the CDP acquisition parameters of; 6 m near-offset, 3 m groupandshotintervals, band pass filtering of 25/250 hz, and a 0.50 ms sample interval.
The receivers used for the CDP line were 4.5 hz
horizontalgeophones. The energysourcewasa woodenblock (coupled to a road surface with 25 cm nails) struck horizontallywith a 4.5 kg hammer. The 1.1 km B-25 (Figure 2) seismicsmweywas similarto the B-16 surveywith the exceptionsbeing; the walk-away lines and CDP profile used 30 hz horizontalreceiversto suppressgroundroll, the sampleinterval was increasedto 1.00 ms to obtain a deeper time section, and the CDP multiplicity was increasedto 6-fold. In an attempt to generatea higher frequencyenergy,the energy sourcewas changedto a pinnedsteelbeamstruckhorizontallywith a 4.5 kg hammer. All seismicdatawere processed on a microcomputer using commercialsoftware. A standardprocessingsequencewas usedfor CDP data;reformatting,sorting,first-an'ivalmutes, normalmoveoutcorrections(NMO), filtering/automatic gain control (AGC), and CDP stack. Data for line B-16 were
Approximately3.8 km to the westof the Reelfootscarp, andno AGC, whereasdata Zoback [1979] also interpretedfaultingof Cenozoicage. filteredwith a 10/50 hz bandpass This structure,shownin figure 2, wasusedfor the SH-wave for line B-25 werefiltered with a 20/60 hz bandpass and 100 reconnaissance test (B-16) in our investigation. Zoback ms AGC. There were no elevationstaticsappliedto either [1979]interpreted the Paleozoic-Cretaceous boundary offset set of data, since both areas have little or no relief. Because to be 24 m (?) and the offsetof the Cretaceous-Tertiary of its uniformity, as comparedto near-surfacehorizonsthat boundaryto be 11 m (?). The downthrowis to the west.
Mini-SOSIE reflectionprofilesdiscussed by Sextonand Jones[1986]andSexton[1988]alsoestablished thatfaulting extended from the Paleozoic bedrock to at least the Eocene
sediment,with more recentdeformationpossiblebut not conclusive.
have been complicated by erosional and depositional processes, the groundsurfacealsoservedas the datum. This datum was also chosenbecausethe weatheredzone (which is most often used)in unconsolidatedsedimentenvironmentsis
ill-defined or non-existent. Geomorphic (i.e., channel deposits)and depositional(i.e., facies tracts)processesin these environments can also create dramatic lateral variations
AcquisitionProcedures
in subsurface materials over small distances, therefore
Thedatawererecorded usinga 15-bit,12-channel, floating pointseismograph. SH-wavewalk-away andcommon depth
-- 36.450
requiringlateral varying correctionsto the seismicdata in order to prevent the changesin velocity from introducing apparent structure(i.e., faults) into the seismic section. Lateral correctionsto the data in this study,however,were unnecessarysince the velocity above the Eocene-Holocene boundary increased gradually (east to west) from approximately183 rn/sto 205 m/s acrossthe studyarea. No radicalchangein velocityover a smallpart of the studyarea was observed. Results
•. ........... !............ I•,N.l•. T•qT-2 [_/•__
The reflectionfrom the top of the JacksonFormation,an Eocene-Holocene unconformity,was recognizedthroughout both survey areas (Figure 3). The reflector displayed waveletswith a dominantfrequencyof approximately 40 hz anda shear-wave stackingvelocityof approximately 190m/s.
Thisis equivalent to a verticalresolution of 1.2 m usingthe one-quarter wavelength criterion[Sheriff,1980]. 89•
89.450
Fig.2. Detailedsitemapshowing thelocation of B-16and B-25 su•eys.
The reconnaissancesurvey, B-16, consisted of four refractionprofilesderivedfrom walk-awaytests,and a short 100 m 3-fold CDP line. The test-2refractionplot, shownin
Woolery etal.:Shallow Structure inNewMadrid Seismic Zone Trace
6
1617
TRACE
12
16
EAST
0.1
Eocene-Holocene
Unconformity
T I
0.5
M
EOCENE-
E 0.3-
T
Eocene Unit
HOLOCENE UNCONFORMITY
(s)
I M
Diffraction
E
(s)
Fig. 5. Portion of the B-16 reconnaissance, CDP survey. Trace distanceis approximately1.5 m.
1.0
reflectors at about trace 48.
The noticeable westward
thinningof theunitbetween350 msand390 msfromtrace 47 andthe apparentsediment thickening on the downthrow are other structuralindicators. The sedimentthickeningalso
1.5
Fig. 3. Raw datasetbeforeCDP processing stepswere applied.A 20/60Hz bandpass filterand100msAGChave beenappliedto thedata. Tracespacing is 3 m.
suggeststhe faulting to be contemporaneous with the sedimentation (i.e., synsedimentation feature). Based on conventionalX2-T2 analysis[Robinsonand Coruh, 1988], the reflectionat 350 ms in the CDP section
corresponds to the top of the JacksonFormation,which wouldimply at leastLate Eocenetectonicactivityin this area. The faultingseenin theCDP profilealsocorresponds
figure 4,iscoincidental withaportion oftheZoback [1979] profile thatexhibits faulting. Thefirstan'ivals fromthe to that shownon the walk-awayrefractionplots.Moreover, deepest interface clearly suggest faulting, withthedownthrow
the faultingseenon the refractionandCDP sections agree both spatially and geometrically with the deeper faulting calculated tobeapproximately 3 m. Thishorizon correlates verywelltothetopoftheJackson Formation, about 40m describedby Zoback [1979] usingconventionalP-wave
to the west. The vertical displacementcomponentwas
belowground surface.Thespread at thissitewasnot reverseddue to a brokenextensioncable.
In additionto thewalk-away test,a 3-foldCDP linewas shotover the area. Althoughof marginalquality,the processed section (Figure 5) wasinterpreted toindicate a near verticalfaultinvolving thereflections between 350 msand 450 ms in the vicinityof trace48. This interpretation is basedon the abrupttermination of the strongreflection occurring at420msattrace47,thechange in dipofthe350 msreflector alongtheprofileattrace47,andtheoffsetin the
TRACE #11 22 E oW..1 •. 334455 1.0 ..
• --
Tiptonvilledomeandthezoneof contemporary seismicity. Refractiondata derivedfrom walk-away testsdefinedup to
fourvelocitylayersanda half-space (2 m to 97 m in depth), with discontinuities found on test lines 1 and 2 at the extreme
easternend of the survey. As expectedin this type of environment, the shear-wave velocitiesarevery low; ranging fromapproximately 110m/sat thenear-surface to 580m/sat approximately105 m depth. The 6-foldCDP line alongB-25 (Figure6) exhibitshigh-
numbers 54 and403), offsetreflectors (i.e.,numbers 335 and 286), and diffractionpatterns(i.e., number438). The
T 0.6 I-' I
SurveylineB-25consisted of elevenwalk-away testsand a 1,172 m 6-fold CDP line acrossthe bedrockhigh of the
anglefaultsat CDP numbers 54, 286, 335,403, and438. Theseinterpretations are basedon the loss of seismic coherency and the abrupttermination of reflectors (i.e.,
_
0.8
T0'51•
techniques.
I •
reflectors at about 0.3 s and 0.7 s representthe Eocene-
._
6o• M/S
-
Holoceneunconformity anda lowerunit withinthe Eocene,
respectively. Thesereflectors exhibita subtleantiformal
E
structure whichagreeswiththecrossing of thebedrock axial
1.0'
highof theTiptonville dome.Theprofilecrosses Croneand
0.2 f 00•
1
12
Brockman's[1982] locationof the axis betweenCDP nos. 400 and410. Anotherimportantresultfrom the profileis
82••S •
thattheimaged faultsappear to extendwellabovethetopof
6
0
GEOPHONE LOCATION
(SPACING= 6.1M)
(a)
(b)
the JacksonFormation,into the Holocene alluvium. This
suggests thestructural evolution of theTiptonville domehas continued into Holocene time. Conclusions
Fig.4. (a)SH-wave composite oftest-2 walk-away. Trace separation is 6.1m. (b)First-arrival plotfromthetest-2 walk-away.
The SH-waveseismicsurveys revealantiformalflextire,as well as numerousnear-verticalnormalfaultsalongtheflanks
54 E
....
I
286
_.
...............
•
ß......... ".......
:'"
.....
I
335
'
403
I
438
I
-'-
•
•,-
I
•--
•
--
,.-•t•.•-•
•
..-,•.
.... 0.5 T
I
..
M E
(s)
.
...r•
1.0
.•
.•.. ,•,
Fig. 6. 6-fold CDP profile froln the B-25 seismicsmwey. Trace distanceis approximately1.5 m.
andacross thecrestof theTiptonvilledome.Thefaultingin theareahasapparently beenactivethroughout thegeologic
past,asevidenced bythesynsedimentary faultinterpreted in thereconnaissance profiles.Mostimportant, however, is the
GeologicalSurveyOpen-FileReport 79-1688, 1-8, 25 pls., 1979. Johnston,A.C., and Shedlock, K.M., Overview of
researchin the New MadridSeismicZone,Seismological
Research Letters, 63, 193-208, 1992. atpresent. Thismaybeevidenced bytheinterpretation of' Norminton, E.J., Seismic model studies of the overburden deformationwell above the Eocene-Holoceneunconformity, bedrock reflection: P-wave and S-wave, 60th Ann. Internat.
suggestion thatstructural deformation maywellbeoccun-ing andpossibly evento thesurface nearCDP no.438 of the Tiptonville domesurvey.The datashownin thisstudyare alsothe first evidenceof near-surfacedeformationimagedby
seismictechniques in the NMSZ, and may alleviatethe concernsof Johnstonand Shedlock[1992] as to the abilities
Mtg., Soc. Expl. Geophys.,ExpandedAbstracts,376-379, 1990.
Robinson,E.S., andCoruth,C., BasicExploration Geophysics, 562 pp., JohnWiley and Sons,New York, 1988.
of seismicmethods for definingnear-surface featuresin this
Russ,D.P., LateHolocenefaultingandearthquake recurrence
region. The deformationis believed to be tectonicin origin, and
in the Reelfoot Lake area, northwesternTennessee,Bull. Geol. Soc. Am., 90, 1013-1018, 1979.
nottheresultof geomorphic processes or othersoft-sediment Russ,D.P., Style and significanceof surfacedeformationin deformationfeatures,sincethe discontinuities and antiformal the vicinityof New Madrid,Missouri,in htvestigations of flexure extendacrossseveralhorizons. Althoughbelievedto theNew Madrid,Missouri,Earthquake Region,editedby F.A. McKeown and L.C. Pakiser, U.S. Geol. Stow. Prof. be tectonicin origin,the faultsare mostlikely secondary tectonic features associated with the tensional stress across
thecrestof theTiptonvilledome. Theydo notappearto be
primaryfeatures directlyassociated with localseismicity, sincefocal mechanismsolutions[Stauder,1982] suggesta predominandycompressivestressregimein this seginentof the NMSZ.
The resultsof this investigationalsodemonstrate that SHwave seismicexplorationis a viable techniquefor imaging shallow structural features in unconsolidated, water-saturated, sediment
environments
similar
to
that
found
in
the
Mississippiembayment.
Acknowledgements.We would like to thank the Kentucky GeologicalSurvey,especiallyDr. JohnKiefer, for supportof this project, and Mike Lynch of the Kentucky Division of Disaster and Emergency Servicesfor financial assistance. The field equipment used in study was provided by Universityof Kentucky ResearchGrant 7E-8E35-33. References
Paper1236-1,95-114, 1982. Sexton,J.L., andJonesP.B, Evidencefor recurrentfaulting in the New Madrid seismiczone froln Mini-SOSIE highresolutionreflectiondata, Geophysics,51, 1760-1788, 1986.
Sexton,J.L., Seismicreflectionexpressionof reactivatedstructuresin the New Madrid rift complex, Seismological ResearchLetters,59, 141-150, 1988. Stauder,W., Present-dayseismicityand identificationof active faults in the New Madrid Seismic Zone, in
Investigationsof the New Madrid, Missouri,Earthquake Region,editedby F.A. McKeown and L.C. Pakiser,U.S. Geol. Surv. Prof. Paper 1236-1,21-30, 1982. Zoback, M.D., Recurrentfaulting in the vicinity of Reelfoot Lake, northwesternTennessee:Bull. Geol. Soc. Am., 90, 1019-1024, 1979. Zoback, M.D., Hamilton, R.M., Crone, A.J., Russ, D.P., McKeown, F.A., and Brockman, S.R., Recurrentintraplate tectonism in the New Madrid seismic zone, Science, 209, 971-976, 1980.
Crone, A.J., and Brockman,S.R., Configurationand deformation
of the Paleozoic bedrock surface in the New
Madrid seismiczone, in Investigationsof the New Madrid, Missouri, EarthquakeRegion, edited by F.A. McKeown andL.C. Parker,U.S. GeologicalSurveyProfessionalPaper 1236-1, 115-135, 1982. Hamilton, R.M., and Zoback, M.D., Seismic reflection
profilesin the northernMississippiembayment,U.S.
R. Street,Z. Wang, E. Woolery, Departmentof Geological Sciences,Universityof Kentucky,Lexington,KY 40506. J. Harris, Kentucky Geological Survey, University of Kentucky,Lexington,KY 40506. (ReceivedMay 6, 1993; revised June 3, 1993;
acceptedJune 14, 1993.)
