Jan 10, 1999 - while in the second, fluids accumulate in a wide dam- aged zone of high .... Calif., 1995. Bakun W. H., and A. G. Lindh, The Parkfield, California,.
JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 104, NO. B1, PAGES 1131-1150, JANUARY
10, 1999
High-resolution electromagnetic imaging of the San Andreas
fault
in Central
California
Martyn Unsworth GeophysicsProgram, University of Washington, Seattle
Gary Egbert College of Oceanic and Atmospheric Sciences,Oregon State University, Corvallis
John Booker GeophysicsProgram, University of Washington, Seattle
Abstract. Although there is increasingevidencethat fluids may play a significant role in the earthquake rupture process,direct observationof fluids in active fault zonesremains difficult. Sincethe presenceof an electricallyconductingfluid, such as saline pore water, strongly influencesthe overall conductivity of crustal rocks, electrical and electromagneticmethods offer great potential for overcomingthis difficulty. Here we presentand compareresultsfrom high-resolutionmagnetotelluric
(MT) profilesacrosstwo segments of the San AndreasFault (SAF) whichexhibit very different patterns of seismicity:Parkfield, which has regular small earthquakes and creep events, and in the Carrizo Plain, where the fault is seismicallyquiescent and apparently locked. In both surveys, electric fields were sampled continuously, with 100 m long dipoles laid end-to-end acrossthe fault. From 100 to 0.1 Hz the
data from both profilesare consistent with a two-dimensional (2-D) fault-parallel resistivitymodel.When both transverseelectricand magnetic(TE and TM) mode data are includedin the interpretation,narrow (-0300-600m wide) zonesof low resistivity extending to depths of 2-4 km in the core of the fault are required at
both locations. However,at Parkfieldthe conductance(conductivitythickness product)of the anomalousregionis an orderof magnitudelargerthan at Carrizo Plain, suggestingmuch higher concentrationsof fluids for the more seismically active Parkfield segment. We also image structural differencesbetween the two segments. At Carrizo Plain, resistive, presumably crystalline, rocks are present on both sidesof the fault at depths below 3-4 kin. In particular, we clearly image resistivebasementextending•10 km or more east of the SAF, beneath the Elkhorn Hills and Temblor Range. At Parkfield the situation is quite different with a resistiveblock of Salinian granite west of the fault and an electrically conductive, presumably fluid rich Franciscan complex to the east. It is possiblethat these structural differencescontrol the difference in mechanical behavior of the fault, either directly by affectingfault strength or indirectly by controllingfluid supply. 1. Introduction
heat flow anomalies[Brune et al., 1969; Lachenbruch,
Severallines of evidencesuggestthat high-pressure 1980],implythat the SanAndreasFault (SAF) is inWillcocket al. [1990]esfluids in fault zonesmay play a critical role in control- herentlyweak. Subsequently, tablished that oceanic transform faults are also weak.
ling the rupture dynamicsof earthquakes. First sugWhile it is possiblethat the intrinsic weaknessof strikegestedby HubbertandRubey[1959],this conceptgained slip faults resultsfrom the presenceof materials in the support fromtheobservation by Zob•cket al. [1987] fault zone with a very low coefficientof friction, it is that in situ stressindicators,and the lack of significant difficult to reconcilethis explanationwith laboratoryderived estimates of typical rock friction coefficients Copyright 1999 by the American GeophysicalUnion. Paper number 98JB01755.
0148-0227/99/98JB-1755509.00
[Byerice,1978].As a result,lowintrinsicfault strengths have most often beenexplainedin terms of high pressurefluids in the fault zone [e.g., Sleepand Blanpied, 1992;Byeflee,1990, 1993;Rice, 1992]. This interpreta1131
1132
UNSWORTH
ET AL.-
SAN ANDREAS
tion is supported further by geologicalmapping of exhumed faults, which providesdirect evidence that fluids were once present in zonesof active strike-slip deforma-
MT
STUDY
vey MT data collected in the Parkfield area, Eberhart-
Phillips and Michael [1993]inferreda significantresistivity contrast acrossthe San Andreas Fault, with conductive material east of the fault. Unpublished USGS
tion [e.g.,Sibsonet al., 1988; Chesteret al., 1993]. Zobacket al. [1987]demonstratedthat simplemod- data (W. D. Stanley, personalcommunication,1994) els of overpressuredfluids in the crust are inconsistent suggestedthat situation at Carrizo Plain was reversed, with
in situ stress observations.
To avoid unobserved
hydrofracturing, fluid pressureswould have to be limited by the minimum principal stress. To overcomethis, and related difficulties, more sophisticatedand detailed theoretical models for the dynamical behavior for fluids in mature active fault zones have been proposed
with an unexpectedresistivebody eastof the San Andreas Fault beneath the Temblor Range. Mackie et al.
[1997]imageda similar pattern of resistivityvariations
along two additional profiles acrossthe fault within the Carrizo Plain. These surveysuseddata collected at sites severalto tens of kilometers apart. Although they have [Sleepand Blanpied,1992; Byeflee,1990, 1993; Rice, imaged large-scalecontrastsin crustal resistivity across 1992]. Although differingin many important details, the fault and have sometimesprovided indirect evidence these models all suggest a rather complicated picture for enhanced conductivity within the fault zone, samin which the distribution, connectivity, and pressureof pling was not dense enough to provide a detailed view fluids may vary dramatically in space and time, in a of the actual fault zone. manner which is closely linked to the earthquake cycle In this paper we describe the results of two highon individual fault segments. resolution electromagnetic surveys acrossdistinct segTo distinguish between theoretical models such as ments of the San Andreas Fault where the earthquake these, it is necessaryto map the location of fault zone cycle is very different: at Carrizo Plain and at Middle fluids in detail. While deep drilling such as that pro- Mountain, Parkfield (Figure 1). In these surveyswe posed by Hickman et al. [1994] will ultimately be sampled the electric fields on the surface with a continneededto determine the physical state within the cores uous series of dipoles laid end to to end. This style of of active faults, remote sensingtechniquescapable of continuousMT profiling gives a high spatial data dendetecting in situ fluids to a depth of at least 10 km sity and minimizes the possibility of spatial aliasing of
are clearly useful. Eberhart-Phillipset al. [1995]reviewed the resolution that might be obtained by seismic, electrical, and electromagnetictechniques. Since the presenceof an electrically conductingfluid, such as saline pore water, strongly influencesthe overall electrical conductivity of crustal rocks, electrical and electromagnetic methods offer great potential for detecting fluids at depth within the fault zone. Electromagnetic exploration methods can be divided into those us-
II
42 ø l:• 0
100 200 km
300
N
ing natural sourcessuchas magnetotellurics (MT) and those that require the generation of artificial electromagnetic signals. Electromagneticsignalspropagate as lossfree wavesin the atmospherebut diffuse as highly attenuated wavesin the ground which can generally be considereda good conductor. This attenuation is char-
38 ø
acterized by a skindepth5 = (2/luwrr) •/•', whereco is the frequency of the signal, /t is the magnetic permeability, and rr is the conductivity of the earth. To explore deep structure requires large skin depths and usually low-frequencysignals. Such signalsare difficult to generate artificially, but for most frequenciesbelow 100 Hz, natural sourcesignalsare often strong. MT is thus generally used for crustal scalesounding. In recent years, MT profileshave crossedthe San An-
34 ø
124 ø
120 ø
116 ø
dreasFault at a numberof locations.Park et al. [1991] collectedwidely spacedlong-periodsites from the Pacific Ocean to Nevada, with one profile crossingthe San Andreas Fault closeto Parkfield. This study suggested that the Great Valley was bounded by electrically conductive
suture
zones but
that
the San Andreas
Fault
San Andreas Fault-locked segments
San AndreasFault-creepingsegments Other major faults( onlyshownin California)
Figure 1. Map showingSan Andreas Fault and the was not a significantly conductivefeature in the lower locations(soliddots) of continuous MT profilesat Carcrust and upper mantle. Based on U.S. GeologicalSur- rizo Plain (C) and Parkfield(P)
UNSWORTH
ET AL'
SAN ANDREAS
MT STUDY
1133
near-surface static shiftsinto deeperstructure[Torres- site to permit remote referenceprocessingof the time Vetdin and Bostick,1992]. The method allowsus to seriesdata [Gambleet al., 1979]. The remotesite for obtain high-resolutionimagesof electricalconductivity the Carrizo Plain profile was located -•5 km from the variations within and near the fault zone to depths of around 4-6 km. Experimental details are provided in section 2. We then present and interpret results from modeling and inversion of the MT data for each pro-
southwestend of the profile; at Parkfield the remote referencewas --•15 km from the profile. Data were recorded at each location overnight so that 500 to 1500 m of profile were covered per day.
file. As we shall show, there are significantdifferences Time seriesdata were analyzedusingthe robust proin geoelectricstructure betweenthe lockedCarrizo seg- cessing codeof Egbert[1997](a multiple-stationextenment and the Parkfield segmentwhich has regularsmall sionof the methodof EgbertandBooker[1986])to comearthquakes and creep events. In section 5 we consider pute estimatesof the MT impedancetensorfor frequenpossibleimplications of the different conductivity struc- ciesbetween100 and 0.005 Hz. In general,data quality tures for fault dynamics. was very good, except in the frequencyrange 1-0.1 Hz. In this "dead band" a combinationof low MT signal strength,noisein the magneticchannelsdue to ground 2. Data Acquisition and Time Series
Analysis Data were collected using two Electromagnetic Instruments Incorporated MT-1 systems. The first sys-
motion, and coherent cultural noise makes collection of high-quality MT data difficult. For these frequencies the remote reference estimates proved essential to re-
move downward bias in impedancemagnitudesdue to excessivenoise in magnetic field components. Multiple
tem recorded10 channelsof data, includingthe horizontal magneticfields at the centerof each array, and electric fields for five along-profileand three across-profile dipoles. The layout is illustrated and contrasted with the conventionalMT layout in Figure 2. Along the profile, dipoles were laid end-to-end, so that a continuous profile of electric. fields was obtained across the fault. At Carrizo Plain, 100 m dipoles were used close to the fault, with 300 m dipoles at the ends of the profile. At Parkfield, 100 m dipoles were used throughout the survey. The secondinstrument was used to simultaneously
field profiles. The source of this noise remains unclear. Reprocessingwith the robust multiple-station approach reducedthe effectsof this coherentnoisesomewhat, but estimates at these frequencies still show more scatter for some arrays and generally have larger error bars. However, these effects are confined to relatively nar-
collect standard five-channel MT data at a permanent
row bands. Comparisonto resultsfor neighboringfre-
CONTINUOUS
MAGNETOTELLURIC
station processing usingthe approachof Egbert[1997] detected low levelsof coherentnoisein narrow frequency bands near 4, 2, and 8 s. Curiously, noise in these same bands was found for both the Carrizo
Plain
and Park-
PROFILING
MT profile
CONVENTIONAL
MAGNETOTELLURIC
SURVEY
Electric field dipole
Magneticfield sensors
Figure 2. (top) Layoutof electricdipolesandmagneticsensors usedfor electromagnetic array profiling,(bottom) comparedto conventional MT surveyconfiguration.The eight dipolesdisplayedfor the continuousMT profile are recordedsimultaneouslyin a singlearray. Additional arrays were laid out end-to-endto allow continuousalong-profilesamplingof the electricfields. Closeto the fault, electricfield dipoleswere 100 m in length;farther away,they were 300 m.
1134
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SAN ANDREAS
MT
STUDY
quencies, where no coherent noise was detected showed soa [1986]showFranciscanat a depth of 3 km beneath that the impedances were not significantly biased by the TemblorRange), but there is little direct evidence this noise. for its presence. Previous MT surveys in the Carrizo At Parkfield the remote site failed after the first two Pla.in detected resistive basement at depths lessthan 3 arrays had been completed. It was thus not possi- km beneath the Temblor Range [Mackie et al., 1997; ble to compute standard remote reference estimates for W.D. Stanley,personalcommunication,1994.] most of this profile. To reduce bias in the dead band In the absenceof microseismicity,the location of the impedances,we again used the multivariate approachof San Andreas Fault at depth is uncertain. Recent geo-
Egbert[1997].This schemeautomaticallycomputeses- morphicstudiesby Arrowsmith[199,5]suggested that a timates of signal-to-noise ratio for each channel, allow- vertical fault to a depth of severalkilometers is required ing unbiased estimates of impedancesto be computed to explain uplift of the Elkhorn Escarpment. Seismicrefrom an eigenvectordecompositionof the spectral cross flection and well log data to the southwestof the San
productmatrix [Park and Chave,1984]. Althoughde- Andreaswereinterpretedby Davis et al. [1988]to show velopedfor multiple-station processing,it can be shown that the Morales thrust shallowswith depth and the rethis scheme should also work well with the 10sulting horizontal detachment may have offset the San channel MT profiling data consideredhere. In effect, Andreas 5-14 km to the east at depths of 5-7 km. the replicated electric field componentscan be used for the remote. As a check, results obtained by this ap- 3.1. DimensionaliCy and Distortion Analysis proach were compared to robust remote reference reAfter the time series analysis described above, the suits for the first two arr•tys, where a remote reference dimensionality of the MT data was investigatedusing was available. In general, no consistentbiasesare obthe tensor decomposition techniqueof Chaveand Smith served in this comparison, so we believe the results for [1994]. Frequency-independent strikeswere computed the full Parkfield profile are unbiased, even in the dead that
for four frequencybandsfor eacharray ( i.e., the five MT sitescollectedeachday). The misfit betweenthe
band.
3.
Carrizo
impedance predicted by the distortion model and the
Plain
measured impedance was computed for strike angles
The SanAndreasFault at CarrizoPlain (Figure1) is from 0ø to 900 in terms of a root mean square(rms) crossed by numerous offset creeks which clearly define
the right-lateral offset of the fault. This segmenthas been locked since the 1857 Fort Tejon earthquake, and seismicity has been minimal since the advent of mod-
misfit. The strike anglesthat minimized the rms misfit for each array are shownin Figure 3. In the high-
frequencyband(100-10Hz) the bestfittingstrikeis 80 east of the surfacetrace, but a significantscatter occurs
ern instrumentation[Hill et al., 1990].The Cretaceous east of the surfacetrace. In the middle-frequencybands Salinian block lies to the west of the fault with granitic (10-1 Hz and 1-0.1Hz) thereis little variationbetween and metamorphic crystalline basement overlain by 1-4 km of predominantly nonmarine Tertiary sedimentary
arrays, and strikes are within 50 of the fault trace. The
lowestfrequencyband (0.1-0.01Hz) showsa consistent
rocks [Dibblee,1973a,b; Page, 1981]. Severalauthors strike 250 west of the fault strike. Figure 4 showshow well the decompositionmodel fits the impedance data Andreas is not part of the Salinian block [Ross,1984; for site 30 at Carrizo Plain. In the three high-frequency are fit to within Irwin, 1990]. The so-calledBarter Ridge Sliceappears bands(100 to 0.1 Hz) the impedances to have an origin similar to the crystalline rocks of the arms normalized misfit in the range 1-5. At lower frehave suggestedthat the crustal block west of the San
San Gabriel
Mountains
quencies,in the 0.1-0.01 Hz band, the two-dimensional
to the south.
East of the San Andreas Fault, Cretaceousto Tertiary marine sedimentary units are exposed in the Temblor Range, and thrusting and folding indicatesfault-normal
(2-D) distortedmodel doesnot fit well, with rms misfits from 10-50. This poor fit indicatesa 3-D regional structure and is consistentwith the changeof strike at
compression[Dibblee,1973a,b;Page, 1981; Ryder and thesefrequencies.Fits to the distorted two-dimensional Thomson, 1986]. Extensivemappingandwelllogdata in model for the other siteswere similar. Figure 5 shows the oil fields of the eastern Temblor Range have shown the impedance skew for all sites. Above 0.1 Hz most that up to 7.5 km of sedimentary units underlies the skewsare lessthan 0.1, againsuggestinga 2-D geoelecElk
Hills
some 15 km east of the San Andreas
Fault
tric structure. Belowthis frequency,skewbecomes large indicating a 3-D geoelectricstructureis beingsensed. have shown up to 12 km of sedimentary rocks in the The small skewvaluesand uniform strike at frequenGreat Valley [Colburn and Mooney, 1986]. However, ciesbetween100 and 0.1 Hz suggeststhat the MT data the character and depth of basement between the oil are consistent with a two-dimensional (2-D) interpretafieldsand the SanAndreasFault (i.e., beneaththe Tem- tion, in a fault normal and parallel coordinate frame. blor Range)remain uncertain. It is assumedthat the Data from 0.1 to 0.01 Hz should be included only with wedge of Franciscan formation that abuts the fault to caution, particularly if interpreted in the same coorthe north is alsopresenthere( e.g., Ryderand Thom- dinate frame. The changeof strike estimatedfor this
[Fishburn,1990]. Farther east, seismicrefractiondata
UNSWORTH
35 ø 08'
ET AL.'
SAN ANDREAS
100- 10 Hz
STUDY
1135
10-1 Hz
Mean strike =8deg ' Site30
MT
'?:•,::.X,
Mean strike =5.6 deg
ß
35 ø 06'
35 ø 04'
35 ø 08' o.1 - O.Ol Hz
Mean strike =-25deg
35 ø 06'
35 ø 04'
119ø 40'
0
119ø 38'
1
2
119ø 36'
3
4
119ø 40'
119ø 38'
119 ø 36'
5 km
Figure 3. Map of continuousMT profile at Carrizo Plain with arrows for each array showing best fitting 2-D strike direction in four frequency ranges. Except for the lowest-frequencyband the geoelectricstrike is consistentlywithin 5øof the surfacefault trace.
last band
and the increased
misfit
to the 2-D distorted
modelsuggestthat theremaybesomecontamination by three-dimensional (3-D)effects at long periods,where
1.0 -
ImpedanceSkew for Carrizo Plain profile
the MT data sample a larger volume. In particular, the
deepstrike(25øWofthesurface trace)mayreflect the 0.8 Carrizo
•
.E
lOO
lO
0.4
.__-••o o
0.6
Plain site 30
Vv' X/"vy '
0.1 0.01
0.2
I I I IllIll 100
10
1.
0.1
0.01
Frequency(Hz)
0.0
Figure 4. Fit of data to decomposition modelfor site 30 at Carrizo Plain. The solid curve indicates the minimum misfit that occurs as strike is rotated from 0 ø to 900 . An ideal minimum misfit would be 1. The dashed curve indicates the maximum misfit. When minimum and maximum misfit are similar in value there is lim-
ited control on strike, when different the strike is well constrained.
100
10
1.
0.1
0.01
Frequency (Hz)
Figure 5. Impedance skewfor the all siteson the Carrizo Plain profile.
1136
UNSWORTH
I
!
!
ET AL-
SAN ANDREAS
MT STUDY
,
mmmmmmmmmmmlmmm
mmlmmmmmm
ILl
N
N
N
N
N
UNSWORTH
ET AL.:
SAN ANDREAS
MT
STUDY
1137
changein directionof the SanAndreasto the southeast of the Big Bend or the surfacegeologicstrike of the Cuyamavalley and relatedfaults to the south [Page, 1981].Alternatively,theseindicationsof 3-D complications could reflect the changein direction of the coast-
of the highly conductive ocean on MT measurements
Booker[1991]was appliedin the followingsteps: 1. HorizontallyaverageTM apparentresistivity(PTM) and phase((I>TM)data, and invert for a 1-D layered Earth model. At each frequencyan averagepT• and (I>T• was computed for all sites on the line. This pro-
surveysby Mackie et al. [1997]andW. D. Stanley(personalcommunication,1994). Given the way in which
cedure allowed the inversion in step 2 to convergemore rapidly than if started from a half-space.
scheme alone.
made onshore[ e.g., Ranganayakiand Madden, 1980: Mackie et al., 1988]. The TM mode data are fit well,
to a normalized rms misfit of 2.1. This correspondsto misfits of individual data of m 10 percent for apparent line in southern California. resistivities, and 50 for phases. Note that ideally misfits The resultingapparentresistivityand phasedata in would be equal to the standard errors in the data i.e., a fault parallel( i.e. unrotated)coordinatesystemare the rms misfit would be 1. However, the TE response shown in Plate la. The horizontal ticks denote the cenof this model does not fit the data below a frequency of ters of the electric field dipoles,and frequencyis shown i Hz. 3. A two-dimensionM inversion was started from the on the vertical scale. Because of the skin depth effect noted above, this pseudo-section givesa senseof how model obtained in step 2 and required the TE apparent the structure varies with depth. TE mode apparent re- resistivity(PT•) and phase((I>T•)to be satisfiedin adsistivity and phasecorrespondto electriccurrentsflow- dition to the TM mode data. The RRI algorithm was ing alongthe fault, whilethe TM modedata correspond unable to significantlylower the misfit for the combined data set because it was trapped in a local minimum of to electric currents flowingnormal to the fault. the data misfit function. Using trial-and-error forward 3.2. Modeling and Inversion modeling to improve the fit to the TE responseswithThe apparent resistivity and phase data were con- out significantly altering the TM response,we found verted into a spatially smoothelectricalresistivitymodel that both TE and TM mode data could be fit reasonusing a combinationof inversionand forward model- ably well by the addition of the large conductor east ing. The rapid relaxationinversion(RRI) of Smithand of the profile that was imaged in previousregional MT
the RRI algorithm searchesmodel spaceand the lack of data on the east end of profile, it is not surprisingthat this better fitting model was not found by the inversion 4.
A final inversion was started from this hybrid
2. Using this model as a starting point, begin a 2- model, resulting in a combined rms misfit of 2.6 for D inversionto fit the TM data (pT• and •T•). The both modes. The model and its responseare shown in resultingmodel and computedresponseare shownin Plate lc . The verticalstril•esin PT•,are due to the Plate lb. Note that the actual model used extended 300 km to the east and west and included the Pacific Ocean
application of frequency-independent static shift coefficients that were computed by the inversion and disas a fixed feature, to allow for the significant effects played in Figure 6a. Allowing the inversionto estimate
2 -
c
a
o
•
1
._•
o-
•
E• -'•
I
6 ••,
-- --,
I
,I
TEApparent resistivity
h
TEphase
J•
TMApparent resistivity
••'
O[
I
I -4 km
I -2 km
,
(b)
I •i•
TM phase
I• •'.
I , 0 km
I
I 2 km
•
I 4 km
Distance from surface trace (km)
Figure 6. (a) Estimatedstaticshiftcoefficients for the CarrizoPlain data. (b) Misfit between data and predictedresponseof model in Plate lc as a function of distancefrom the surfacetrace.
1138
UNSWORTH
ET AL.:
SAN ANDREAS
static shifts effectivelydownweightsthe resistivitydata, making the interpretation lesssusceptibleto distortion by galvanic effects and is equivalent to the technique of manually "correcting" for static shift. Note that both modes are generally fit well down to a frequency of 0.1 Hz. Below this frequency the predicted phases are higher than the observed.This may reflect the fact that
a 2-D fault-oriented
model is invalid
at these fre-
quencies,as implied by the tensor decompositionand skew values.
The
normalized
MT
STUDY
are small(Figure6a), indicatingthat strongdistortion of the whole profile is unlikely. Furthermore,this type of distortion has lesseffecton the phase,which we have emphasizedin our inversion. The 3-D effectscould also
resultfrom conductivestructures( e.g., C4) with limited continuity along strike. In this case the TM responsewould be virtually unaltered, but pT•. could be artificially loweredby electricchargesdevelopingat the
endsof the conductor[Wannamakeret al., 1984].A 2-
rms misfits for each site
D interpretation emphasizingpT•. could thus lead to an overestimate of conductancefor the 3-D feature. Note, however, that this discussionpresumesthe existenceof the conductivestructure. It is the magnitude of the conductance, not the presence or absence of the feafrequencies (0.1-01Hz) andprobablyreflectsa combina- ture which might be called into question by possible tion of local distortion and the changeof strike direction unresolved3-D effects. Given the consistentgeoelectric strike, the small skews, and the essential similarity of at these frequencies. A similar model was obtained by the inversion se- the TM and TE+TM models, we believe that a 2-D inquence:(1) Fit TE data startingfrom a 10 f2 m half- terpretation of this dataset is reasonable. However, it space;and (2) from this model,fit both TE and TM should be borne in mind that quantitative estimates of mode data. This gave a good fit to the TE data by conductancefor somefeaturesmight be biasedto some placing the conductive structure at depth east of the small extent by 3-D end effects. profile but did not fit the small-scalestructure required by the TM data at high frequencies.While somedetails 3.3. Interpretation in the geometry vary, both the TM data and TE+TM Our interpretation of the Carrizo Plain resistivity
are shownin Figure 6b. Misfit levelsare relatively uniform across the profile, except for two adjacent sites 2 km east of the fault trace. This poor fit to the TE data is almost all due to the very poor fit for the lowest
data modelshavethe followingconductive(C) and re- model is summarized in Plate 2a. Note that faults can sistive(R) featuresin common: changethe electricalresistivityof the groundin at least C1, 5-10 f2 m conductive layer west of the surface trace of San Andreas Fault; C2, 3-7 f2 m conductive zone beneath the surface trace in the upper km; C3, 7-20 f2 m conductive layer at the east end of the profile;
C4, Zone of slightly enhancedconductivitybeneath the fault trace; R1, 50-100 f2 m resistive,westwarddipping layer; R2, 300-1000 f2 m resistivebasementbelow 2 km east of fault; R3, resistive basement below depth of 3-4 km west of San Andreas
Fault.
The differences between the TM
two distinct ways. They can produce a zone of low resistivity by trapping groundwateruphill of a fault and
alsoby forming a zone of low resistivityin the gouge and breccia at the core of the fault. 3.3.1.
Structure
west
of San
Andreas
Fault.
The shallow conductivity structure is well-correlated with geologicmappingand well log data from the area.
West of the fault, Vedder[1970]mapped the Wells Ranch syncline, consistingof Caliente formation sed-
and TE+TM
mod-
els can be understood by consideringthe physicsof 2-D MT. Thin vertical conductors are essentially invisible to TM mode electric currents. Positive and negative electric chargesdevelop on opposite sides of the conductor at depth, but the effect of these chargeslargely
imentary units [Plate 2a]. This corresponds to conductive unit C1, and the westwarddipping resistivity contours between
3 and 5 km west of the SAF corre-
late well to the dip of the easternlimb of the syncline. At a depth of •4 km the resistivityincreasessignificantly (R3), presumablycorresponding to crystalline
cancels out at a location on the surface. However, TE
basement of the Salinian
mode electric currents flowing along the conductorare readily detectable on the surface, even if the body is very narrow. Thus the Carrizo Plain TM data are sensitive to the contrast in resistivity acrossthe fault but do not require a significantfault zone conductorbelow 2 km. In contrast, the TE data are more sensitive to
boundary inferred from seismicreflection by Davis et
block.
The location of this
al. [1988]agreeswell with the resistivitymodel. Davis et al. [1988]suggestthis contactis a backthrust fault (F4) associated with the deeperMoralesthrust, whose inferred 3.3.2.
location Shallow
is shown in Plate 2a. structure
east
of San
Andreas
the presenceof the narrowconductor(C4) in the fault Fault. Immediately to the east of the surfacetrace is zone, which extends in the combinedTE+TM model to a wedgeof low resistivity(C2) with its westernbounda depth of 6 kin. It is important to consider the possibility of 3-D effects. One possiblesourceof 3-D effectsis the shallow, but very conductivebasin partly occupiedby Soda Lake at Carrizo Plain. This could systematically distort the
ary directly beneath the surface trace. Note that the
surfacetrace liesjust east of the changein elevationassociatedwith the Dragonsbackpressureridge. Mapping and well logscompiledby Vedder[1970]indicatethat C2 is the Bitterwater Creek shale, a clay rich sequence electricfieldsso that large static shift coefficientswould with low electricalresistivity.Trappingof groundwater be required to fit pT•.. However,estimatedstatic shifts uphill (east)of the fault may alsocontributeto the low
UNSWORTH
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MT
STUDY
1139
resistivity.Arrowsmith[1995]founda numberof fault- who collected MT data at a site east of the Temblor controlledspringsin this area, consistentwith the idea Range. Thus the resistive unit is most likely only a that fault can act as a barrier to groundwaterflowing horizontally limited sliver of crystalline rock. Clearly, from the northeast
in the near surface.
Farther
to the
it does not extend into the oil fields east of the Temblor
east, R1 correlateswith the Santa Margarita formation,
Range, where wells have penetrated up to 7.5 km in sed-
described by Vedder[1970]andDibblee[1973a,b]to be a
imentaryrocksof the Great Valley sequence[Fishburn, 19901. While Zobacket al. [1987]showedthat the San Andreasis inherentlyweak, Castilloand Hickman[1995]
sedimentaryunit containinggranite bouldersup to 12 ft in diameter, in addition to gneissand other clasts. This may account for the relatively high resistivity of this
unit, and the boundaries(inferredto be thrust faults) suggestedthat within 5 km of the San Andreas Fault correlate well with the spatial extent of the central shallow resistive zone. Near the eastern end of the profile, C3 is interpreted as a sandy-clayrich faciesof the Monterey shale which also has a low resistivity. Note that there are two distinct zones of low resistivity, and that both of theselie east of, and topographicallyabove,the
the Carrizo segmentis strong. Our data support this suggestionsince it implies there is crystalline, presumably mechanicallystrong material on both sidesof the fault below 2 km depth. The material to the west could be Salinian granite or, alternatively, crystalline basement of the Barret Ridge Slice. The material to the thrustfaults(F2 andF3) mappedby Vedder[1970]and east is of undetermined origin. It could be a fault slice Ryderand Thompson[1986].Concentrations of ground- of Salinian granite or part of the Barret Ridge Slice, water trapped by thesefaults, may contribute to the low which is believed to originate in the crystalline basement that underlies the Mojave desert and San Gabriel resistivitiesimaged along the MT profile. 3.3.3.
Fault.
Resistive
basement
east
of San
Andreas
Mountains[Ross,1984;Irwin, 1990].
The most striking feature east of the fault is
3.3.4.
Fault
zone
conductor.
In
contrast
to
theshallowdepth(•0 2 km) to resistive basement (> 300 Parkfield (see Unsworthet al. [1997a]and also secf2 m). Seismicreflectionand MT data at Parkfield tion 4) the SAF at this locationis a relativelysubtle [Unsworthet al., 1997a]showedthat the top of the featureoftheresistivity model.Although theMT data Salinian granite correspondedto the 100 f2 m contour. are consistentwith a modest reduction of resistivity in This suggestsa crystalline basementat a depth of 2 the fault zone, this feature is not particularly striking. km beneath the Elkhorn Hills and Temblor Range east In the upper kilometer the fault appearsto act as a barof the San Andreas. However, this interpretation is at rier to groundwater flow, as a zone of low resistivity is variancewith currently acceptedtectonic models of this foundeast (on the uphill side)of the fault. Belowthis area [Page, 1981] in which Great Valley sedimentary depth a zone of reduced resistivity extends to depths units and the Franciscan formation extend westward to of at least 4 km. This feature could represent a weak the San Andreas
Fault.
These rock units would be ex-
pected to have a lower resistivity than the 200-3000 f2
fault zone conductor that has been smeared out by the inversion, as observed in synthetic MT model studies
m observed,althoughPark et al. [1991]reportedresis- by Eberhart-Phillipset al. [1995]. However,the fault tivities greater than 500 f2 m in the Franciscanforma- zoneconductance (conductivitytimesfault zonewidth) tion, possiblydue to greenstonesor metabasalts. Significantly, Graff [1962]reportsthat the Western Gulf Oil Vishnu No. i well (locatedsome6 km alongstrike from our profile on the northeast side of the San An-
at Carrizo Plain is of the order of only 20 S. This is an order of magnitude less than the 250 S fault zone conductanceimaged at Parkfield which Unsworthet al.
dreas)bottomedat 2114 m in granite. This resistive
concentration.
unit has also been detected by previous MT surveys
[1997a]interpretedas due to an anomalouslyhigh fluid What
is the maximum
fault zone conductance
con-
that crossed the San Andreas Fault at several locations
sistent with the data? To explore this question, the within Carrizo Plain [Mackie et al., 1997; W.D. Stan- best fitting model was modifiedby decreasingresistivley, personalcommunication,1994]. Thus the resistive ity within the fault zone in a variety of ways. Figure feature is neither a local structure confined to a sin7 showsthe observedTE mode data and the responses of selectedmodels at site 30, which was typical of sites gle profile, nor an artifact of the data acquisitionand processingreported here. within 2 km of the surfacetrace. Adding a 100m wide 2 How far east does the resistive
structure
extend?
It
certainly extends to the eastern end of the profile, and beyond this point our data provide minimal control but are still sensitive to some structure at depth. During the inversion processdescribedabove, it was found that to fit the essentiallyfiat pT• responses,lower resistivity material was required somewhereto the east of the profile and that the resistive basement block could be terminated
•6
km east of the SAF
and not shown in
12m fault zoneconductor( i.e., 50 S conductance) lowers pT•,significantlybelow 0.5 Hz. Increasingthe width
to 300 m (150 S conductance) producesan evenmore unacceptablefit. Alternatively, increasingthe resistivity of the fault betweeni and 3 km to 10 i2 m raisespT•, so that the data are not fit between 5 and 0.2 Hz. Note
that the TM data for the samemodelsshownin Figure 7 are much less sensitive
to variations
in fault zone con-
ductivity. Thus the fault zone conductanceis at least Plate 2a). This wasconfirmedby Mackieet al. [1997], partially constrained by the TE data. However, as we
1140
UNSWORTH
ET AL'
SAN ANDREAS
MT
STUDY
Carrizo Plain - Dipole 30 - TM mode
Carrizo Plain - Dipole 30 - TE mode
i
60
m
50
•
40
g
30
60
-•
20
20
TI[
I I I Il'"l
100
, , ,,,,,d 10
, , ,,,,,,I 1
, 0.1
frequency(Hz)
I
100
I ' l l,,,,I
, i ,,,,,,I
, , ,,,,,,!
10 1 Frequency(hz)
0.1
,
Best fittingmodelfrom Plate lc Add 2 ohm m, 100 m fault zone
......
Add 2 ohm m, 300 m fault zone
Remove conductive fault 1-3 km depth range
Figure 7. TE and TM mode responsesand measureddata at dipole 30 for a set of models incl•udinga more conductivefault zone. Note that the TM mode data are virtually insensitiveto the fault zone conductance, while the TE data constrain this quantity. have discussedin section 3.2, the possibility of 3-D end effects adds some uncertainty to quantitative estimates
the San Andreas
Fault.
The contact
between C2 and
R1 might be a thrust fault (F1) that offsetsthe San
of fault
zone conductance. Andreas Fault to the west at around 1500 m depth. Farther to the east, F2 and F3 are thrust faults 3.3.5. Faulting pattern. Shallowcrustal deformation in the vicinity of the MT profile is dominated mappedat the surfaceby Vedder[1970]and Ryder and by a seriesof westward dipping thrust faults, denoted Thomson[1986]. The locationsof thesefaults within as F1-F4 in Plate 2a. In our interpretation the main the upper 2 km of sedimentary rocks are strongly corvertical trace of the San Andreas Fault is offset laterrelated with variationsin the resistivitymodel [Plate ally to the west by 200-300 m at a depth of around 500 2a]. These faults may also penetrate the crystalline m. Basedon geomorphicmapping,Arrowsmith[1995] basement and possibly intersect the San Andreas at proposeda modelfor the uplift of the Dragonsbackpres- depth, but the MT data offer no supporting evidence sure ridge in the Elkhorn Hills, in which a thrust fault for this possibility. Indeed, our model does not exhibit intersectedand beheadedthe originally vertical SAF at any significantvertical offsetsin depth to the resistive around 500 m depth. The inferred location of F1 cor- basementbeneath this part of the profile.
relates well with the near-surface zone of low resistivity
Can our data constrain
the location
of the San An-
in our modelin Plate 2a. Arrowsmith[1995]estimated dreas Fault at depth? The location of the Morales that an offset of around 200 m was needed to produce thrust inferredby Davis et al. [1988]is shownin Plate the necessaryuplift. Again, this is in good agreement with the modeled location of the deeper fault zone conductor in Plate 2a. Note that fault F4 (inferredfrom the electromagneticallymapped locationof the resistive
2a. This resistivity model shows no evidence for con-
ductivesedimentaryunits belowthis depth. Certainly, a horizontally limited feature at this depth is near the limits of what we might resolvewith our 9 km long basementcontact)mightbe a continuation of F1 across profile. The longerprofile of long-periodMT measure-
Carrizo Plain
(a)
Elkhom Plain
Dragonsback escarpment
SAF
,
0km
i I
ID 2 km
F1
F2
F3
zl
!
I
I I I I I
I I
Tm
\
I I I I
4 km
II ii ii II II
Kg R3
??
• Morales Th'rust• .
"• 6 km
R2
Slack
Middle
Canyon
(b)
Little
Mountain
Cholame Creek
SAF
GHT
0 km 'For
c] 2 km
t
Kg
\
R1
300
100
30
10
3
I
Resistivity (ohm m)
Pla•e 2. Interpretationof resistivitymodels. (a) Carrizo Plain with geologicunits labeled by Vedder [1970]: Kg, Salinian granite; Tbw, Bitterwater Creek Shale;Tms, Monterey shale; Tsm, SantaMargaritaformation;DZ, damagedzone.Seetext for discussion of faultsF1-F4. Middle Mountain, Parkfieldwith geologyeastof the San AndreasFault taken from Sims [1990] Tcr, Tertiary Coast Range units; Tgv, Great Valley sequence;Kjf, Franciscanformation. GHT, Gold Hill thrust Fault. Dots denote earthquake hypocentersfrom the Berkeley seismicnetwork. Models shown with no vertical exaggeration.
1142
UNSWORTH
ET AL.' SAN ANDREAS
MT STUDY
ments describedby Mackie et al. [1997]also fails to spond to the San Andreas Fault offset to the east at image conductivestructure below the inferredMorales depth. thrust, suggestingthat if the Morales thrust extends this far east as a subhorizontal detachment, it is most
4. Middle Mountain, Parkfield
likely underlain by crystalline basement.However,it is possiblethat sedimentsthrust to 8 km depth might be The seismic behavior of the Parkfield segment difonly weakly conductiveand that the conductivesurface fers significantlyfrom the Carrizo segment.Here there layer (C1 and C2) maskswhat wouldthen be a rela- is extensive microseismicity,with the shallowestevents tively subtle feature in the MT data. There is alsono occurring in repeating clustersat depths of •3 km. Foclear evidence in either the resistivity model of Plate cal mechanisms,and the repeat sequence,suggestthat 2a or the model of Mackie et al. [1997]for an east- seismicityis fluid driven [Nadeauet al. 1995; Johnson ward offset in the San Andreas Fault at 8 km depth, and McEvilly, 1995]. CharacteristicM ..•6 eventshave as suggested by Davis et al. [1988].However,evenif a ruptured this segmenton averageevery 22 years since horizontal detachment is invisible to MT, the San An1857 [Bakun and Lindh, 1985]. In contrastto Carrizo dreas Fault at depth might be imagedelectrically. The Plain the location of the fault at depth is seismically midcrustal conductor detected east of Carrizo Plain by well-definedand is closeto vertical to a depth of 8 km. deepsoundingdata of Biasi et al. [1990]might corre- Previous studies of the Parkfield segment using both
36 ø 00'
100-
10 Hz
10-1
Mean strike = 0 deg
Hz
Mean strike = 2.4 deg
===============
35 ø 58'
36 ø 00'
I -0.1
0.1 - 0.01
Hz
Mean strike = 5 deg
Hz
Mean strike = -11.7 deg
35 ø 58'
::::::::::::::::::: ....
120ø33'
120ø30'
0
1
2
120ø33'
120ø30'
3 km
Figure 8. Map of continuousMT profileat Parkfield,with arrowsshowingbest fitting 2-D strike direction for each array in four frequencyranges.
UNSWORTH
ET AL.'
SAN ANDREAS
direct current (DC) electricalmethods[Park and Fitterman,1990]and MT [Eberhart-Phillips and Michael, 1.0 1993]suggestthat electricalresistivitiesare low in and near this fault segment. The MT survey also revealed a
broad(20+ km) widezoneof lowresistivityin the upper 5 km of the crust northeast
of the SAF.
MT STUDY
1143
ImpedanceSkew for Parkfieldprofile
0.8
This zone was
correlated with a pronouncedseismiclow-velocity zone
[Eberhart-Phillipsand Michael, 1993]. Our profileat
0.6
Parkfield is only 4 km in length but is sampleddensely enoughto reveal significantlymore detailed images of
shallowfault zoneresistivitystructurethan havebeen 0.4 available previously. An initial interpretation of these
data wasgivenby Unsworthet al. [1997a],who showed that the fault zonewascoincidentwith a relativelynar- 0.2 row (500 m wide) verticalzoneof low resistivity,which was attributed to the presenceof significantamounts of fluid. Here we provide a more detailed description of 0.0 the inversion results and explore the range of possible l llllll I llllllll I llllllll I lllllul I llllllll I IIIIIIII modelsthat are compatible with the data. We will show 100 10 1. 0.1 0.01 in particular that the TE data imply that the zone of low resistivity extends to a depth of 2-4 km. Frequency (Hz)
4.1. Dimensionality
and Distortion
Analysis
Tensor decomposition was applied to the impedance data, following the procedure describedin section 3.1, and results are presentedin Figure 8. The decomposition model fits the data quite well in each of the four frequency bands. Although there is some scatter in the directions at shorter periods, the estimates of geoelectric strike for arrays west of the SAF are generally consistent and within SAF
the strike
+50 of the surface trace.
is less well-constrained
East of the
as the TE
and
Figure 10. Impedance skew for all sites on the Parkfield profile
4.2. Modeling and Inversion
The data were inverted using a sequencesimilar to that described for the Carrizo Plain data. Inversion of
the TM data produces the model shown in Plate 3b. This model fits the TM data well, but does not come
closeto fitting the TE data. To adequatelyfit both modes, the TM model was modified to improve the fit to the TE data. We found that a conductivelayer of 1 9 m, 1 km thick fixed in the model at a depth of 2 site (site 21) is shownin Figure 9. The minimumrms km, and located 4 km to the east of the SAF solved misfit can be seen to be acceptable at all frequencies. this problem. This closelymatchesthe conductivezone The impedance skewsshownin Figure 10 are almost all east of the SAF mapped in previousMT surveysof this below 0.2. Thus for the bulk of the profile the decompo- area[Eberhart-Phillips andMichael,1993].This model sition and skew analysissuggestthat a 2-D interpreta- was used as a starting model for a joint TE+TM invertion will be reasonableover the whole frequency band. sion usingRRI. The model and responseare shownin The TE and TM apparent resistivity and phase in this Plate 3c. Note that in the TE+TM model the fit to the fault-oriented co-ordinate system are shown in Plate 3a TM data has been improved,particularlyin (I)TMeast in pseudo section format. TM responsesare relatively similar, in contrast to the west where they are very different. The fit of the decomposition model to the impedance data for a typical
of the fault tracedueto the additionof a resistivezone,
R2. Again, the TM data image the contrastin resistivity acrossthe fault at depth and alsothe top of the large fault zone conductor. However,the bulk of the fault zone conductor,C2, is invisibleto TM data and is only imagedby the TE mode. The verticalstripesin the computedPTEresult from applicationof static shift coefficients (Figure 11a) to the responses. Static shifts arelargerfor Parkfieldthan for CarrizoPlain, probably
Parkfield site 21
100 10
1
0.1 0.01
in part dueto topographic effects.Figure11(b)shows 100
10
1.
0.1
0.01
that the rms misfit is uniform acrossthe profile,with all sitesfit comparablywell. As discussedfor the Carrizo Plain data, interpretaFigure 9. Fit of data to decompositionmodel for site Frequency(Hz)
20 at Parkfield.
tion of TE mode data must be undertaken with care
1144
UNSWORTH ET AL.. SAN ANDREAS MT STUDY
SAF
SAF
ß
mmmmmmmmmmmmmm
•
II1! IIIII!lt
milli
I11110
•m•
Cl SAF
R1 11
IIIIIIIIIIII
66 Hz
I
-1
km
-2
km
C2
3 km
' 'R1
IIII
i
Apparent resistivity
.
.
0.004 Hz!
'• tlHIIIIIItlIIII
IIII
IIH
66 Hz
!
Itlltllilllllll
i.l•.1tlitlllll
I
'• '
.--
Phase 0.004 Hz 'q'Iltllllltllllllllllllllllllll
66 Hz
I
alii
I
IllIll
Ilti
IIit111ttlt111
I
I
!
Apparent resistivity 0.004 Hz
,
'"'11II111 Itl I t111111111111 " II I
.,,
-.
66 Hz
I
11 Itlt
III111111111I•"•
lilt
I Ill I
III
I11111111111 I
.
111t
'rM
Phase 0.004 Hz
I
I
I
I
-
-2 -1 0 I kmfromsurfacetrace
-2 -1 0 I kmfromsurfacetrace
(a) DATA
-2 -1 0 1 kmfromsurfacetrace
(b)TM only
(c)TE + TM
rms misfit = 3.8
rms misfit = 1.7
Resistivity / Apparentresistivity (ohm m) 300
15
100
10
45
I
65
Phase(degrees)
Plate 3. (a) Datapseudo-sections for the Parkfield MT profileand(b) modelandresponse obtained byinverting theTM data.(c)Modelandresponses obtained byinverting andmodeling bothTE andTM modedata.Models andtopography areplotted-with novertical exaggeration.
UNSWORTH
ET AL.:
SAN ANDREAS
MT
STUDY
1145
A [
[
I
[
[
[
Apparent resistivity • TM Apparent resistivity (b) -2 km
-1 km
0 km
1 km
Distance from surface trace (km)
Figure 11. (a) EstimatedTE modestaticshiftcoefficients for Parkfielddata. (b) Misfit between data and predicted responseof model Plate 3c as a function of distance from the surface trace. as this mode can be more susceptibleto contamination by 3-D effects than the TM mode. Since the inclusion of the TE mode data significantly effects the inversion results for Parkfield, the possibility of 3-D effects needs careful
consideration
here.
There
are several
lines of
evidencethat support a 2-D interpretation of this segment of the SAF. First, the higher-frequencydata which we emphasize in the inversion fit a distorted 2-D model quite well, with a nearly fault-parallel geoelectricstrike.
see how sensitivethe data were to the depth extent of the fault zoneconductor.Figure 12 showsthe responses at site 21 of models derived from the TE+TM
model
shownin Plate 3c. The responseat this site is observed
at sites16 to 25 and is clearlynot a single-siteanomaly. In models 24 and 23 the conductor is terminated
at shal-
lower depths, resulting in a minimum in •T•. at around 1 Hz and a rapidly rising pT•. which are not observed in the data. This is particularly true when the conduc-
Impedanceskewsare small (< 0.2) at all frequencies. tive zoneis truncatedat only 1 km depth (model24). Thus there
is no evidence
in the data
itself for 3-D ef-
Model 23, which extends the fault zone conductor to 2
fects. Furthermore, additionalMT profiledata were km depth, gives a comparable fit to the model with a collected
on Middle
Mountain
in late
1997.
Profiles
to
the northwest and southeast of the 1994 line interpreted
conductorextendingto 3 km. Extending the fault zone to a depth of 10 km in the presenceof the eastern con-
hereshowsimilar resistivityand phasedata [Unsworth ductor (model 25) produces•. that is too high and et al., 1997b].Verticalmagneticfield data canprovidea p•, that are too low. Thus the TE data indicate that useful indication of the connectivity of conductorsalong
the strong conductor extends to a depth of 2-4 km but does not continueto significantlygreater depth. Note survey but were collected in the 1997 survey. The in- that termination at 2 km is possible, but the best fit duction vectorsfrom this surveyare generallynormal to to this data occurswith --•3 km depth extent. As with the fault zone, again consistentwith a 2-D geoelectric the modelsconsideredin Figure 7, the TM responsesof structure with fault-parallel strike. Furthermore, ver- these modelsare essentiallyindistinguishable. tical field data from a fault-normal profile some 3 km The requirement for the moderately resistive zone northwest of the 1994 profile closelymatch the vertical (R2) east of the SAF was examinedby replacingthe fields predicted by the 2-D model of Plate 3c down to resistor with a 10 S2m zone. This altered •M by • 50 strike.
0.2 Hz.
Vertical
field data were not collected
All of these observation
in the 1994
are consistent
with
a
2-D interpretation, and none provide direct evidencefor 3-D complications. Thus, while we cannot completely dismissthe possibility that 3-D effects may bias quantitative estimates of fault zone conductance, we believe that a 2-D interpretation incorporating TE phase data
east of of the SAF and indicates our data are sensitive to
this zone. If observedat just one MT site, this anomaly might be disregardedas a single-siteanomaly. However, in the continuousMT profiling data presentedin this paper, we see such features in multiple sites. This redundancyin spatial samplingallowssmall featuresin is not unreasonable in this case. the data to be interpreted with more confidence that Having given substantialevidencethat the fault zone that obtained with widely spacedsites. is relatively 2-D, we will now demonstrate that a conOur preferredmodel consistsof a conductingwedge ductor within the fault zone is required to fit the TE (C2) that pinchesout at around2-4 km, with a weak mode data. Further forward models were computed to conductive root present below that depth. However,
1146
UNSWORTH
ET AL.- SAN ANDREAS MT STUDY
lO
1
ml iii 100
illill
iii
illill iii 1.
illill iii 0.1
Frequency(Hz) ,
Model 21
Fault zone conductive to - 3 km inversion result of Plate 3c
Models 23-25 producedby editing model 21 ....
,
ß
Model 25
Fault zone conductive
Model 23
Fault zone conductiveto only 2 km
Model 24
Fault zone conductiveto only 1 km
Model25
to 10 km
Model21
ß
10
3.5
km 3
I1
3.5
km
5 10
7
km
Model 23
I1
km
3.5
km
Model 24
lO
lO
7
10
3.5
km 10
I1
10
10
10
7
km
7
km
Figure 12. TE responsesat dipole 21 for a set of modelswith the fault zoneconductorterminat-
ing at depthsof i km, 2 km, 3 km, and 10 km. Theseresultsdemonstrate that if the geoelectric structureis 2-D, then the low-resistivity zonebeneaththe surfacefault traceextendsto a depth of 2-4 km. Resistivitymodelsare shownwith no verticalexaggeration.
UNSWORTH
ET AL.'
SAN ANDREAS
MT
STUDY
1147
The two models shown in Plate 3 could be considthe presenceof the deeperroot below 4 km is not constrainedby the MT data, as shownby resolutionanal- ered end-members of a spectrum of models consistent ysisusingsyntheticMT data [Eberhart-Phillips et al., with the TM data. Depending on how much weight 1995]. Low resistivities below4-5 km in our modelat is given to the TE data, the fault zone conductor will depth in the fault zone couldbe an artifact causedby have varying conductance. If the TE data are considered unreliable, then it will be restricted to the upper vertical smoothingof the regularizedinversion.
4.3. Interpretation
of the Middle Mountain
Model
A detailed interpretation of the near-surfaceportion of the model has been givenpreviouslyby Unsworthet al. [1997a].This showedthat the fault zonewascharacterized by a positive flower structure with a strong central conductor,interpreted as a fluid saturated zone
kilometer. However, if the TE data are deemed reliable and can be interpreted in a 2-D context, a fault zone conductor extending to a depth of 2-4 km is required by the data. Note once again that a conductor
terminating as shallow as 2 km on Middle Mountain is a possibility. Both models in Plate 3 have related hydrogeologicalinterpretations. In the first, fluid flows to the surface along the interface between the Franciscan, while in the second, fluids accumulate in a wide damaged zone of high porosity produced by fracturing of
of breccia at the core of fault. In this paper we have reexamined the prominent fault zone conductor more carefully and shown that the TE data are consistent country rock. The earthquake hypocentersin Plate 2b appear to lie with a conductive fault zone that pinches out over the on the west side of the fault zone conductor. However, depthrange2-4 km (assuming that the fault zoneis suf-
ficiently2-D to allow interpretationof the TE mode). whentheseeventswererelocated[Ellsworth,1996],they Given the nonuniquenessinherent in the inversion of magnetotelluricdata, variantson this model may also reproducethe data. Clearly, alternate conductivitydistributions may be possibleeast of the profile, given the lack of data in this area. The following major features
were found to lie some 200 m east and 400 m deeper
tendingeast of the profile. Thesefeaturescan be con-
more resistive fault
than the locations(from the BerkeleySeismicStation) shown in Plate
2b. Thus
it is unclear
whether
the mi-
croearthquakesare in the center or on the edge of C2. In either casethey are located within a conductive, preare requiredby the TM mode data: (R1) a resistive sumably high fluid content fault zone, which stands in blockwestof SAF and (C1) a conductivezoneeastex- contrast to the seismically inactive and comparatively zone at Carrizo
Plain.
The depth extent of the fluid saturated zone implied sidered a minimum structure model that is required by by the model of Plate 2b agreeswell with the model the TM data. However, as we have shown, 3-D effects appear to be small. Thus the combinedTE and TM of Andersonet al. [1987]basedon the mappingof exmode data can be used to add a resistive zone east of humed faults. In both, the zone of breccia terminates the fault (R2) anda verticalconductor in the fault zone around 5 km depth. The MT image is also consistent with recentseismictomographydata which showsa 500 (C•). A revisedinterpretation, basedon comparisonto the m wide seismiclow-velocity zone extending to a depth
geological map of Sims[1990],is shownin Plate 2b on of around4 km beneathMiddleMountain(C. Thurber, the same scale as the Carrizo Plain interpretation. The personalcommunication,1997). The presenceof fluid unit R1 is interpreted as being granite of the Salinian lowersboth electrical resistivity and the seismicvelocity block. The fault that bounds the east side of C2 is and is a natural explanation for the structures imaged clearlythe Gold Hill Thrust Fault. ResistorR2 is coin- by both datasets. cident with the anticline between the Parkfield syncline and the Gold Hill Thrust Fault. Is the relatively high reDiscussion and Conclusions
sistivityof this featurereasonablegiventhe high salinity of fluids that are widespreadin the Franciscanforma-
On both MT profiles we have imaged narrow fault
tion [Eberhart-Phillips and Michael,1993]? Unsworth zone conductors extending to depths of 2-4 km and et ai. [i997a] usedArchie'slaw to estimatethe poros- sharp variations of resistivity gcrossthe fault contact. ity requiredto accountfor the low resistivityin the core of the fault zone. At a depth of 3 km, a porosity in the range of--•10-30% was requiredwithin the fault zone to accountfor the low resistivity on the basisof fluids alone.Assumingthe sameporefluid salinity,porosities in R2 would be in the range 1-10%, where upper and lowerlimits correspondto exponentsof 2 and 1, respectively,in Archie'slaw. Sims[1990]showsthat the area
The high-density electric field samplinghas also allowed us to resolve many details of near-surface resistivity
structure,alongwith variationsin depth to the resistive basement. Although there are broad similarities in the two resistivity models of Plates 2a and 2b, there are also
somevery significantdifferences. Theseinclude(1) the
contrast of resistivity acrossthe fault; at Carrizo this is small with 1000+ i2 m material on each side, and at denotedas (R2) in our interpretationhas Franciscan Parkfield there is a west to east changefrom 1000 to 30 formation at its core at a depth of .-• 2 km. Given the i2 m; (2) the magnitudeof the low resistivityanomaly heterogeneityof the Franciscanformationthe inferred in the fault zone; at 3 km depth fault zone conductance at Carrizo Plain is -,•20 S, while at Parkfield our analyporosity is not unreasonable.
1148
UNSWORTH
ET AL.:
SAN
sis suggestsa conductanceof approximately 200 S; and
ANDREAS
MT
STUDY
The possibilitythat seismicityis controlledby fluids
(3) the width of the damagedzoneat the coreof the in the fault has been proposedby many people. The fault (as inferredfrom resistivitycontrasts)is lessthan correlationof fluid flow through the fault with abun300 m at Carrizo Plain and around 600 m at Parkfield. dant seismicityand creep was initially suggestedby These segmentsalso have very different patterns of Irwin and Barnes [1975]. They proposedthat these seismicity.Even though we are not imagingfault structure to the depth of the M • 6 eventsat Parkfield or the larger events that may periodically rupture the Carrizo segment,our resistivity data are samplingdepthswhere shallow seismicity is present at Parkfield but absent at Carrizo Plain. How might the differencesobservedin the resistivity sections be related to the differencesin seismicity?
fluids originated in metamorphic reactions within the Franciscanformation and that fault creepoccurswhere Franciscanrocksabutted the fault and provideda ready supply of fluids. While we have consideredonly two locationson the San Andreas Fault system,our findings are consistentwith this hypothesis. The MT
data for Parkfield
and Carrizo
Plain do not
directly constrainstructure at depths where the large First, could difference(1), the contrastin structure earthquakesthat periodicallyrupture this segmentnuacrossthe fault, contribute to the observedpattern of cleate. If the hypothesisof Irwin and Barnes[1975]is
seismicity? Our results show that at Carrizo Plain, correct and fluids enter at depths below 10 km and then resistive, presumably crystalline, rocks are present on flow to the surface,the high concentrationof fluids in both sidesof the fault. This is supportedby the results the near-surface at Parkfield may be indicative of an of Castillo and Hickraan [1995],who usedstressanaly- ample fluid supply at depth. Alternatively, the fluids sis to showthat the Carrizo segmentis locally strong, may be entering the fault at shallowdepths and be unin contrast to the general weaknessof the fault docu- related to the characteristicearthquakes. The resistor
mented by Zobacket al. [1987]. While the top of the
eastof the SAF (R2) doesnot necessarily representa
resistive zone is deeper on the west side of the fault than on the east, these mechanicallystrong blocksmay give the fault zone increased strength, compared to other parts of the San Andreas Fault. At Parkfield the situation is quite different with a resistive block of Salinian granite west of the fault and an electrically conductive, presumablyfluid rich, and possiblymechanicallyweaker Franciscancomplex to the east. Weak materials east of the fault could limit the strength of the fault and result in a cycle of more frequent, smaller earthquakesthan is
barrier to fluid flow, rather just a zone of lower porosity with the Franciscan formation.
At Carrizo Plain
the absenceof fluids in the near-surfacemay indicate a relatively dry fault zone at depth. In reality, a combination of factors is probably responsible for the differences in seismic behavior of the
two fault segments.We have shownthat there are major differencesin geoelectricstructurebetweenthe Parkfield and Carrizo segments.Our results are consistent with previoussurveysof both fault segments,but close observed at Carrizo Plain. to the fault, higher resolution of shallow structure is Alternatively, it could be difference2, the properties facilitated by densespatial sampling. Subject to the of the fault zone, specificallythe fluid distribution, that absenceof 3-D effects, our interpretation of the data are controlling seismicity. At Middle Mountain, Park- is that the San AndreasFault at Parkfield has a highfield, a number of lines of evidencesuggestthat both porosity core, while at Carrizo Plain the fault is essenshallow clusters of microearthquakes, and the charac- tially dry. Significantly,microearthquakesoccur in the teristic deeper M • 6 events are fluid driven. Johnson shallow fault zone at Parkfield but not at Carrizo Plain. and McEvily [1995]suggestedthat earthquakeclusters Given careful analysis,we believe that MT exploration at Parkfield had focal mechanisms consistent with fluid can provide valuable new constraints on the physical cycling. At the depth of the characteristicM .• 6 events conditions within active fault zones.
a zone of low Vp and high Vp/V• consistentwith the Acknowledgments. Fieldwork was made possible by presenceof excessfluid has been reported [Michelini Nong and McEvilly, 1991]. In contrastto Middle Mountain, WangFei,YangYangjing,WeerachaiSiripunvaraporn, Carrizo Plain is characterized by an almost complete absenceof seismicity. In this paper we have shownthat
the conductance of the shallow(0-4 km) fault zoneat
Wu, Xinghua Pu, Zhiyi Zhang, Yuval Zudman, and Arnell Dionosio. The staff of the BLM in Bakersfield,notably JohnaCochran,Judi Weaserand Larry Vredenburgh, are thanked .for administrative and logistic support. The Nature Conservancyis thanked for letting us stay at the
Middle Mountain could be at least an order of magnitude greater than on the quiescentCarrizo Plain seg- PaintedRockRanchon CarrizoPlain, andTonyandGeorge were most helpful hosts. Many private landowners at Carment. This difference may be partly explained by fluid
chemistry( i.e., the high salinity of 30,000 ppm chloride at Middle Mountain would produce a lower elec-
trical resistivitythan a lowersalinityat CarrizoPlain).
rizo Plain are thanked for accessto their land, as are Kevin
Kester
and Jack Varian
at Parkfield.
Tom Bur-
dette is thankedfor help with permitting. This work was supportedby NSF grant EAR-93-16160and U.S. Geological SurveyNational Earthquakehazardreductionprogram grant 1434-94-6-2423. The developmentof the magnetotelluric inversioncode was supportedby U.S. Department of
However, even with a high salinity of 30,000 ppm at Parkfield a porosity of .-• 10% was requiredto explain the low resistivity. Thus we believe our data suggest Energy grant DE-FG06-92-ER14231. Developmentof data higher porositiesin the San Andreas Fault at Parkfield processingcode was supported by U.S. Department of Enthan
at Carrizo
Plain.
ergy grant DE-FG06-92-ER14277.
The authors acknowl-
UNSWORTH
ET AL.:
SAN ANDREAS
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(unswort h@geophys. washingto n.edu) G.D. Egbert, College of Oceanic and Atmospheric Sciences,Oregon State University, Corvallis, OR 97331-5503.
(egbert@oce. orst.edu). (ReceivedMay 23, 1997; revisedFebruary13, 1998; acceptedMay 14, 1998.)
