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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. B7, PAGES 16,229-16,236, JULY 10, 2000
Strain
Zone
accumulation
at
latitude
across
the Eastern
California
Shear
36ø30'N
WeijunGan,• J. L. Svarc,J. C. Savage,
andW. H. Prescott
U.S. GeologicalSurvey,Menlo Park, California
Abstract. The motionof a linear arrayof monumentsextendingacrossthe EasternCalifornia
ShearZone (ECSZ) hasbeenmeasuredfrom 1994 to 1999 with the Global PositioningSystem.
The lineararrayis orientedN54øE,perpendicular
to the tangentto the localsmallcircledrawn
aboutthe Pacific-NorthAmerica pole of rotation,and the observedmotionacrossthe ECSZ is
approximatedby differentialrotationaboutthatpole. The observations
suggestuniform
deformation
withintheECSZ (strikeN23øW)(26 nstrainyr-• extension
normalto thezoneand39
nstrainyr-• simpleright-lateral
shearacross
it) withnosignificant
deformation
in thetwoblocks
(theSierraNevadamountainsandsouthernNevada)on eitherside. The deformationmay be
imposedby right-lateralslip at depthon the individualmajorfault systemswithinthe zoneif the
slipratesare:DeathValley-Furnace
Creekfault 3.2_+0.9
mmyr-•, HunterMountain-Panamint
Valleyfault 3.3_+1.6
mm yr-1,andOwensValley fault6.9_+1.6
mmyr-1. However,thisestimate
of the slip rate on the OwensValley fault is 3 timesgreaterthanthe geologicestimate.
1. Introduction
The Eastern California Shear Zone (ECSZ) [Dokka and
Travis, 1990a,b] is a -100-km-wide zone of deformation
trendingnorth-northwestinto Nevada from the east end of the
"big bend" in the San Andreasfault (Figure 1). North of the
Garlock fault the ECSZ spansthe Owens Valley-Little Lake,
Hunter Mountain-Panamint Valley, and Death Valley-Furnace
Creek-SouthernDeath Valley fault systems(Figure 2). The
1992 M=7.5 Landersearthquakeoccurrednear the southernend
of the ECSZ and the 1872 M=7.6 Owens Valley earthquake
occurred near its northern end (Figure 1). The shear zone
accommodates
-24%
of the Pacific-North
American
relative
plate motion (M.M. Miller et al., Refined kinematics of the
Eastern Californua
Shear Zone from GPS observations, 1983-
1998, submittedto Journal of Geophysical Research, 2000;
hereinafter referred to as Miller et al., submittedmanuscript).
As
is the case
for
other
zones
of distributed
deformation
[Savageet al., 1999a], it is not clear whetherthe deformation
in the zone is simply a manifestationof strain accumulation
due to slip at depthon closely spacedfaults within the zone or
is imposed by drag from a continuous distribution of
deformationin the viscousupper mantle [Bourne et al., 1998].
The former explanation implies that the strength of•the
lithosphereresides principally in the upper crust, whereasthe
latter explanationimplies that it residesin the upper mantle.
To study the distribution of deformationacrossthe ECSZ
the U.S. GeologicalSurveyin late 1994 installeda linear array
of geodeticmonuments(shearzone array in Figure 2) along an
azimuth of about N54øE extending from the Sierra Nevada
mountainsto near Beatty, Nevada. The array was surveyedin
1994, 1996, 1997, and 1999 using the Global Positioning
System (GPS). That array of monumentsjoined another
geodetic strain array (Yucca Mountain array in Figure 2)
centeredon Yucca Mountain,Nevada,whichhad beensurveyed
in 1993 and 1998 using GPS [Savageet al., 1999b]. Before
1995, codeless Ashtech LM-XII
GPS receivers were used.
Resolution of phase ambiguities was difficult in those
surveys. By 1995 the receivershad been upgradedto Ashtech
Z-12's, which receive full-wavelength data and allowed better
resolution of phase ambiguities. In the 1994 survey of the
shear zone array the individual monumentsin the array were
occupied for at least 6 hours in each survey; in the other
surveys the occupationswere with few exceptions for 6 or
more hourson each of two or more consecutivedays (see the
Web locationscited below for occupationhistories). The data
were reducedusing point positioning[Zurnbergeet al., 1997]
and GIPSY/OASIS-II software,release5 [Webband Zurnberge,
1995].
The solutions are referred to the International
Terrestrial Reference Frame 1996 (ITRF96) [Sillard et al.,
1998] rotated so that North America is nominally fixed
[Savage et al., 2000, appendix]
determined
so as to minimize
(i.e., the rotation is
the velocities
at the fiducial
stations Algo (Ontario, Canada), Brmu (Bermuda), Drao
(British Columbia, Canada), Fair (Alaska), Nlib (Iowa), Pie l
(New Mexico), and Yell (Northwest Territories, Canada)).
Because it is not certain that all of the fiducial
stations are in
fact fixed with respect to North America, the velocities
relative to fixed North America assignedto the monumentsin
the array discussed here have a common (systematic)
uncertaintyof perhapsone or two millimeters per year. The
relative velocities of the monuments within the array,
however, are free of that systematicerror.
1OnleavefromtheInstitute
of Geology,
ChinaSeismological The overall accuracy in locating one geodetic monument
Bureau,Beijing.
with respectto anotherout to 400 km was expectedto be -4
mm (one standard deviation) in each horizontal component
Thispaperis notsubject
to U.S.copyright.Published
in 2000
[Zhang et al., 1997] for a daily solution. However, in this
by the AmericanGeophysicalUnion.
paper the error estimates, determined by scaling upward the
Papernumber2000JB900105.
formal errors from the solutionby a factor found appropriate
16,229
16,230
GAN ET AL.: EASTERNCALIFORNIA SHEARZO•
.......
:',i'-....... '...........
...........
..........
•...........
! ..........
occupiedin more than two surveysand thus are the only
monumentsfrom the Yucca Mountainarray shownin Figure
3.) In Figure 3 the displacementsfrom arbitrary positions
: ............
fixed in our reference frame have been resolved into N36øW
i i: "i'• i i 1872 [••
. ......
:
O :
'
•'
'
.......
:Y-,x •?•1857}
'
'
..........*6....
' ....
,x>-
••._
'"'•3
•
•
•
(direction of the tangentto the local small circle drawn about
the Pacific-North American pole of motion [DeMets et al.,
1990]) and N54øE components.A straightline, the slopeof
which represents the monument velocity relative to the
referenceframe,hasbeenfit to the observeddisplacements
at
each station. The data in Figure 3 are reasonablywell
explainedby the linear fits. Thus we assumethat the motion
of the monuments has been uniform in time.
On the basis of
that assumptionwe have usedthe adjustmentprogramQOCA
[Dong et al, 1998] (see also the Web site http://sideshow.jpl.
nasa.gov:80/~dong/qoca/)in the global mode to find the
velocity at eachmonument(Figure 2).
The velocity at monument Go05 is clearly anomalous
(Figure 2). That monumentis closeto the Coso geothermal
.....• ....
:...................... :......... • _• .... J....... •__
J...... t_-•q:- -',
Figure 1. Map showingthe principalhults in southern field and alsoto the epicenters(Figure2) of threemagnitude5
California and the location of the EasternCalifornia Shear earthquakeswhich occurredin 1996 and 1998 [Bhattacharyya
Zone (shaded). The locations of the monuments in the et al., 1999], within the interval of observation.R.W. King
geodeticarrayare shownby soliddots. Epicenters
of the (personalcommunication,2000), who has analyzed detailed
1857Fo• TQon,1872OwensValley,1952KernCounty,
and GPS surveysin the vicinity of the geothermalarea, doesnot
1992LanderseaChquakes
am shownby stars.
believe that the earthquakesnear Coso had any significant
effecton GPS stationsin our array. Wickset al. [1999] report
1992-1997subsidence
at therateof 30 mm yr-1centeredon
from the linear fits to displacement versus time plots of
individual stations [Savage et al., 2000, p. 3102], are
somewhatgreater (see error bars in Figure 3), particularly in
the early surveys. The additional uncertaintymay be due to
monument instability but more likely simply reflects the
uncertainty in locating monumentswith respect to the more
distant
fixed
interior
of
North
America.
This
estimated
uncertaintydoes not include the systematicerror in fixing the
reference frame with respect to stable North America. An
independent estimate of the standard deviation of the
velocities for the Yucca Mountain array (abscissa>20 km in
Figure 4) basedon the assumptionthat there is no significant
relative motion between monumentsin that array is consistent
with the standarddeviations used here (J.C. Savage et al.,
Strain accumulation
near Yucca Mountain,
Nevada,
!993-
1998, submittedto Journal of GeophysicalResearch,2000).
In this paper we adopt the following conventions: All
uncertainties quoted in the text and tables are standard
deviations, but the error bars in the figures extend 2 standard
deviationson either side of the plotted points. In discussing
strain, extension is reckoned positive.
2.
the geothermal area at Coso; the subsidenceis reasonably
well modeledby the collapseof a sphericalsource(crossin
Figure 2) at a depth of 3.5 km.
Because the subsidence
probably affected Go05, we will exclude that monumentfrom
further discussionin this paper. MonumentsFork and V511
may also be affected by the deformation in the Coso
geothermal field, but we retain those monuments in this
discussion.
The velocitiesdeterminedfrom the QOCA adjustmentare
plotted in Figure 4 as functionsof the distancein the N54øE
direction
state line.
from
monument
The velocities
Mo93
on the California-Nevada
have been resolved into N54øE and
N36øW components,perpendicularand parallel, respectively,
to the tangentto the local small circle drawn about the PacificNorth American pole of rotation. The N54øE velocity
component(Figure 4b) indicateslittle systematicmotion in
that direction. The profile for the N36øW velocitycomponent
(Figure 4a) suggests shear acrossthe tangent to the small
circle, but that shearis confinedentirely to the monumentsin
California (negative abscissain Figure 4). The CaliforniaNevada state line seemsroughly to coincide with the eastern
edgeof the shearzone at this latitude.
Data
3. Strain Analysis
Detailed data (latitude and longitude of the monuments,
positionsof monumentsas a function of time, and velocities
Figure 4 suggeststhat deformationin California (negative
inferredfor the monuments)
and plotsfor the two arraysare abscissa in Figure 4) is relatively uniform, whereas
availableon the Web at http://quake.wr.usgs.gov:80/QUAKdeformation in Nevada (positive abscissain Figure 4) is
ES/geodetic/gps/Yucca.qoca/.
That Web site will be updated negligible. To examine this further we have solved for the
as new data become available.
uniform strain that best approximates the observed
The observedchangesin monumentpositionsin a reference deformationin California and Nevadaseparately.The QOCA
framenominallyfixed with respectto the interiorof the North adjustmentprogram outputsthe uniform strain rate and rigid
Americanplate are shownin Figure 3 as a functionof time for body motion most consistentwith the velocity solution for
thosemonuments
occupiedin morethantwo surveys.(Seethe any subsetof the adjustedarray. We have chosento examine
Web site referencedabove for plots of all data. Only two two subarrays,one composedof the monuments in California
monuments,l pdi and Shos,in the YuccaMountainarraywere and the other composed of the monuments in Nevada
GAN ET AL.' EASTERN CALIFORNIA
SHEAR ZONE
16,231
19W
iz
GPS Station
z
ß Yucca Mtn. Array
3187
,
.
\\
........
\\
.
.
ß
ß
>
10mm/¾r
,
.
.
30 Km
.
& Shear Zone Array
,
:
Fault
,
•/•,
Holoce
Quaternary
Figure 2. Map of the GPS arraysacrossthe EasternCalifornia ShearZone (solid triangles)and aroundYucca
Mountain (solid circles). The velocity relative to the fixed interior of North America for each monument is
shown by an arrow. The 95% confidenceellipse is shownat the tip of the arrow. The locationsof the towns
Beatty, Nevada, and Lone Pine, California, are shownas shadeddiamonds. The principal faults are shownby
sinuousblack lines, and the California-Nevadastateboundaryis shownby the diagonaldashedline acrossthe
figure. The diagonalhachuredline trendsN54øE, perpendicular
to the tangentto the local small circle drawn
about the Pacific-North American pole of rotation. The epicentersof the three magnitude5 earthquakesin
1996 and 1998 that occurred15 km eastof the Cosogeothermalfield are shownby the starsand the centerof
the sphericalsourceusedby Wickset al. [1999] to modelthe subsidence
in the CosoGeothermalareais shown
by the nearby cross.
MonumentMo93, locatedon the stateline, is assignedto the
Nevada subarray. The principal strain rates and the rotation
rates found for those two subarraysare shown in Table 1.
From independentGeodolitemeasurements
of the 30-km-wide
Owens Valley trilateration network, which extends between
monumentsCerr and Ovro (Figure 2), Savage and Lisowski
[1995] found that the principal strain rates for the western
shearacrossvertical planesparallel to the tangentto the local
small circle drawn about the North American-Pacific pole of
rotation. In a coordinate system with the 2 axis directed
the 2-st.d.level with the strainrate for the Californiasubarray
in Table 1, the Owens Valley strain rates do nevertheless
suggestthat strain accumulationin the western part of the
California subarray(Owens Valley) may be higher than the
parallel to the tangent to the local small circle. That is, the
motion across the shear zone is approximated by rotation
aboutthe Pacific-NorthAmerican pole of rotationwith angular
velocity increasingwith distancefrom the pole.
An interpretation of the California subarray strain rates
somewhatmore consistentwith the simple shearis suggested
by a resolutionof the strain field in a coordinatesystemwith
N36øW parallel to that tangent the strainratesare
6.7-+6.5nstrainyr'1, t}12=-40.7-+4.4
nstrainyr'1, and•22=
18.2_+7.2
nstrain
yr'1. In thiscaset}•tis notsignificant,
t}22
is only marginallysignificant,and t}12differs significantly
portionof theECSZ(OwensValley)were82_+15
nstrainyr'1 fromthe rotationrate(0 (Table 1) onlyin sign(i.e.,
N73ø_+3øW
and-39_+15nstrainyr'1 NI7ø_+3øE,extension Thus, to a reasonable approximation the deformation field
reckoned positive. Although that strain rate is consistentat
representssimple (t}t2=-(0) shear acrossvertical planes
averagerate over the entire subarray(Table 1).
As suggestedby Figure 4 the strain rates for the
California subarray(Table 1) can be approximatedby simple
16,232
GANETAL.:EASTERN
CALIFORNIA
SHEAR
ZONE
.......•
GOO,•
700
......•
..........
•
3187-
600
•
••
Fork
500
E
E
400
•-• .........• ....... • T•s,
+
s,ovj
7
_
T19sT.
.........._•
Stov
..................
.•
'•' ..........
4
•...............
•
_ P166•'
•
...........
•
X137_- X137•
............
•..................
• ..........
....... • ..............
• .........•u•r
300
_
L
200
--
M137
-
................
......................
,,.
..................
_-
200
Oerr
i
i
13dd
Hold
100
•..........
•(•
G165
•
;•
_
.z.
Hold
5.................
-•..........................
• ...............
• -
100
F23x
•...............
• ...............
• ......... •
-
...............................
'•
•
5
.•.
......
........
.•.
.............................
• •uo• Mo93•
1
_
_
............................
• ..............................
•
1994
,,,•1•,,,I
1995
....
0-
I,,,•1•1,,,,
1997
1998
1996
• Bm8•
1999
2000
1994
,,,•1,1,,1•,•,1
1995
1996
Time, Yr
200
....
I ....
I ....
I ....
....
1997
1998
1999
2000
Time, Yr
I''''1''''1''''
_
_
_
Ovro-
E
E
150
•
-
Ovro-
-
•
_
-• ................
3...............
'-•...........
-•Jctn-!..................................
•........................
'•...............
•Jctn!
_
E
ß 100
--•P16•
._
_
_
,Sho
to 5O
z
-
•
--
I 3: IShog
I
!
I
--
_
_
1pdi-
o
1993
'-r,,,
I,,,
1994
• I, •,,
1995
I,,,,
1996
I,,,,
1997
I,,,,
1998
I,•
1999
in
2000
Time,Yr
x,,,I,,•l•l•,,I
1993
1994
....
1995
1996
1997
I•l•l•n
1998
1999
2000
Time,Yr
Figure3. Plotsof displacement
referred
to anarbitrary
originasa function
of timefor all monuments
in the
geodetic
arraysmeasured
in morethantwo surveys.The displacements
havebeenresolved
intoN36øWand
N54øE
components.
Theerrorbarsrepresent
twostandard
deviations
oneither
sideoftheplotted
point.
the 2 axis parallel to the N23øW trend of the ECSZ. In that
coordinatesystemthe strain rates are s//= 24.9+6.5 nstrain
.!
the extension
rate •2 parallelto thetrendof theshearzoneis
negligible. Our preferredinterpretation
of deformationin the
yr-], t}[2=-39.2+4.4nstrainyr-], andt•j2= 0.0_+7.2
nstrain ECSZ at this latitude is simple shearacrossthe trend of the
yr-•. Noticethattheshear
ratet•[2 differssignificantly
from zone plus extensionperpendicularto it.
the rotationrate to (Table 1) only in sign(i.e., t•/'2=-to
) and
The deformation
of the Nevadasubarray(Table 1) is at best
GAN ET AL.: EASTERN CALIFORNIA SHEAR ZONE
20
16,233
Earthmodelin which the relativemotionbetweentwo rigid
(a)
plates is accommodated by uniform deformation in the
parallel-sidedzone that separatesthem [Savageet al., 1995,
appendix]. Thatis, thecurvature
of theEarthis neglected
and
15
southern Nevada
E
E
•
o
and the Sierra
Nevada
mountains
are
represented
by rigid blocksseparated
by a parallel-sided
zone
(ECSZ) subjectto uniformdeformation
(Figure5b). We have
shownabovethatthe strainratein theNevadasubarray
to the
10
eastof the shearzone is not significant,and the SierraNevada
blockto thewestof the shearzoneis generallytreatedasrigid
[e.g.,Hearn and Humphreys,1998]. Thusapproximating
the
SierraNevadablockand the southernNevadablockas rigid
z
5
plates is reasonable. The strain within the ECSZ (California
subarray)
conforms
to therequirements
(•/2=-r0 and t}22=0)
•
•: •--
0
ß .o• .....
-' $0
X
i
*
-1 O0
7111
'
,....
-$0
ß
for a zoneof deformation
separating
two rigid plates[Savage
et al., 1995, appendix](noticethat in this referencethe sign
I I I •-
, i•/
ß
!
....
0
conventionfor r0 is oppositethat employedhere). Thus the
!
lOO
lO
ECSZ canbe interpreted[Savageet al., 1995,appendix]as a
zoneof uniformdeformationseparating
rigid plates(southern
Nevada and the Sierra Nevada mountains). Notice that in this
flat-Earthmodel,thereis no rotationof theSierraNevadaplate
relativeto southern
Nevada. Specifically,
thereis no apparent
rotation of the Sierra Nevada block relative to fixed interior
North America that would permit a fan-like openingof the
Basin and Range province to accommodatemore east-west
spreadingin the north than in the south [Bogen and
Schweickert, 1985]. Nor was such a rotation found in the
preferred representation (model M2) of deformation in the
sameareaby Hearn and Humphreys[1998].
4. Fault
Model
Here we attemptto explain the deformationobservedacross
-lO
N54øE Distance from Mog3, km
Figure 4. Profiles of the (a) N36øW and (b) N54øE
componentsof velocity as a funcition of distanceN54øE from
monumentMo93 on the California-Nevadaboundary. The
the ECSZ as beinggeneratedby strikeslip at depthon the
threeprincipalfault systems
(the DeathValley-Furnace
Creek,
the HunterMountain-Panamint
Valley, andthe OwensValleyLittle Lake) that crossthe geodeticarray(fault crossings
are
indicatedby vertical dashedlines in Figure 4).
We have
representedthe fault systemsby simple dislocationsin an
elastichalf-space[Okada, 1985] and determinedthe Burgers
vector for each dislocationsuchthat the observedvelocity
error bars represent two standarddeviations on either side of
the plotted point. The continuous solid lines indicate the
velocitespredictedby the dislocationmodel of Table 2. The
field is best explained. The faults are represented
by
monumentsClai, B191,Mile, 67tj and Whme (Figure 2) from
(e.g., slip on the Hunter Mountain fault is the same as on the
Panamint Valley fault). Notice that no allowance has been
dislocationsegmentsas shownin Figure 5a. The Owens
Valley-Little Lake fault systemhas been represented
by a
dashedvertical lines indicatethe locationof profile crossings
single
screw
dislocation
and
the
Death
Valley-Furnace
Creekof the principal faults.
Southern
DeathValleyandHunterMountain-Panamint
Valley
fault systems
haveeachbeenrepresented
by segmented
screw
dislocations.All dislocations
areplacedat a depthof 15 km
marginally significant [Savageet al., 1999b]. Wernickeet al. (the locking depth), and the slip on each fault systemis
to be constantalongthe lengthof the system
[1998] have argued that independent GPS surveys of constrained
1991 to 1998
indicate
a N65øW
strain rate of 50_+8 nstrain
yr-1. However,thedataavailable
to Wernicke
et al. [1998] made for normal slip on the Death Valley-FurnaceCreekSouthern
DeathValleyandHunterMountain-Panamint
Valley
used here, and we are confident that the strain rate in the fault systems
noron theSierrafrontal faultsystem,although
were much less completethan the U.S. GeologicalSurvey data
Nevada subarrayis very low (Table 1). The observedabsence
of significantstrain accumulationin the Nevada subarrayis of
some importanceas the proposedU.S. high-level radioactive
waste disposalrepositoryis located close to monumentMile
(Figure 2).
The observed deformation of the entire geodetic array
(CaliforniaplusNevadasubarrays)
canbe represented
by a flat-
suchslip probablyoccursthere; data on vertical deformation
wouldbe requiredto effectivelyconstrainsuchnormalslip.
The bestfit valuesfor slip rateson the variousfault systems
are shownin Table 2. With the importantexceptionof the 2
mmyr-1geologic
sliprateestimate
fortheOwens
Valleyfault
[Beanland and Clark, 1994], the slip ratesin Table 2 are
withinthe rangeof previousgeologicandgeodeticestimates
16,234
GAN ET AL.: EASTERN CALIFORNIA SHEAR ZONE
Table 1.
Uniform Principal Strain and Rotation Rate Approximationsto
Observed
Deformation
in theSubarrays
Subarray
California
Nevada
t}l,
nstrain
yr-l
53.6_+6.9 N77ø_+ 3W
9.9_+5.4 N60ø_+13øE
t•2,
nstrain
yr-l
0),
nrad.yr-l
-28.7e5.5 N13ø_+ 3øE
-11.5_+7.4 N30ø_+13øE
37.1_+4.1
6.0_+4.4
Quoted uncertaintiesare standarddeviations
(see Table 1 of Hearn and Humphreys [1998] for a recent
compilation of slip rates) and reproduce the observed
deformation satisfactorily (continuous lines in Figure 4).
Moreover, the slip rate estimatesin Table 2 are completely
changingthe locking depth to 10 or 20 km does not change
the slip rates significantly and gives almost as good a fit to
the observationsas the 15 km locking depth employed.
The discrepancybetween the geologicestimateof slip rate
consistent with similar estimates of Bennett et al. [1997],
(2_+1mm yr-1 [Beanlandand Clark, 1994])on the Owens
Valleyfaultandthe estimate
(6.9_+1.6
mm yr-l ) in Table2
McClusky et al. [1999], and Miller et al. (submitted
manuscript,2000) basedon independentGPS measurements.
However, becausethe velocity profiles acrossthe shearzone
are relatively smooth, our dislocation models for the
individual faults are not closely defined. For example,
calls into questionthe validity of representingdeformationin
the ECSZ solely by slip at depth on the three principal fault
systemsthere. Although Beanland and Clark [1994] admit
some uncertaintyin the geologic estimateof the slip rate on
the Owens Valley fault, their argumentsfor a low slip rate are
convincing.For example,the 6.9_+1.6
mm yr-1 slip rate
proposedhere would require an earthquakesimilar to the 1872
Owens Valley event roughly every 1000 years, whereas the
geologicevidencesuggestsonly three suchearthquakesin the
last 10,000 to 20,000 years [Beanland and Clark, 1994].
Dixon et al. [2000] suggestthat the current high slip rate on
the Owens Valley fault inferred from GPS might be explained
by postseismicrelaxation following the 1872 earthquake.
The slip rate in the later part of the earthquakecycle would be
expectedto be much less, so that the slip rate averagedover
theentireinterseismic
intervalcouldapproach
2 mmyr-l.
5. Discussion
Hearn and Humphreys [1998] modeled the deformation
acrossthe Eastern California Shear Zone using geologic slip
ratesandthe sparsegeodeticdatathenavailable. They divided
the region into fault blocks using the major fault systemsas
boundariesand then, subject to constraintsimposedby the
geodeticand geologicobservations,determinedthe slip rates
on those faults which minimized the elastic energy
accumulation. Their model M2 represents the best
approximation to block motion consistent with the data.
BecauseHearn and Humphreys[ 1998] model the block motion
(i.e., motion averagedover many earthquakecycles) whereas
we observethe interseismicdeformation,direct comparisonof
of their model with our observations is not possible.
However, the observedmotions of the rigid blocks on either
sideof the ECSZ are generallyconsistentwith model M2. For
example,their model M2 predicts12.7_+1.5mm yr-•
Table 2. Slip Rates Estimatedfrom GPS Measurements
Figure 5. Two modelsof deformationin the ECSZ. (a)
Deformation attributedto slip at depth on individual faults.
for FaultSystems
withintheEastern
CaliforniaShearZone
Fault System
R.-L. Slip Rate,
mmyr-1
The tracesof the dislocationslip planesused to representthe
three major fault systemsare shownby the heavy straight
lines. (b)
Deformation attributed to uniform strain
accumulation
(heavycrossedarrowswith principalstrainrates
in nanostrainper year) in a parallel-sided
shearzone(shaded)
with no strain accumulation in the blocks on either side of it.
Death Valley-FurnaceCr.-S. Death Valley
Hunter Mountain-PanamintValley
Owens Valley-Little Lake
Quoted uncertaintiesare standarddeviations
3.2_+0.9
3.3_+1.6
6.9_+1.6
GAN ET AL.: EASTERN CALIFORNIA SHEAR ZONE
N50ø_+5øW
for the velocityof theSierraNevadablockand2.5
mmyr-1N87øWfor thevelocityof thesouthern
Nevadaplate,
whereasour observedvelocity on the Sierra Nevada plate
(monument
3187)is 13.9_+1.4
mmyr-1 N39ø_+6øW
andthe
average
velocityfor thestations
in theNevadasubarray
is 3.0
mmyr-1 N71øW. In distributing
slipwithintheECSZmodel
M2 requires
a 1.5-2.3mmyr-l right-lateral
sliprateonthe
16,235
representedin the thin viscous sheet model of deformation
[England and Jackson, 1989], where the entire lithosphereis
modeled by a thin viscouslayer. The elastic strain imposed
upon the uppercrustis eventuallyrelieved by ruptureon faults
so that in the long term the upper crust deforms by block
motion.
Our observationsacrossthe ECSZ suggestthat deformation
OwensValley-Little Lake fault system(in good agreement is relatively uniform across the shear zone, not clearly
withthegeologic
estimate
of Beanland
andClark[1994])and concentratedupon the principal faults in the zone. This would
a 5-6 mmyr-• right-lateralslip rate on the DeathValleyFumaceCreek-SouthemDeathValley fault system,whereaswe
infer(Table2) 6.9_+1.6
mmyr-• ontheformerfaultsystem
and
3.2_+0.9
mmyr-• onthelatter.Theagreement
is betterfor the
HunterMountain-Panamint
fault system(modelM2 predicts2-
appear to favor the hypothesis that the deformation is
imposed by a shear zone at depth in the upper mantle.
However, if reasonable locking depths are assigned to the
principalfaults within the ECSZ and slip ratesfor thosefaults
selected to be as consistent
with the observed deformation
as
possible,the deformationat the surfaceof the shear zone is
In summaryour observations
are consistent
with the overall relatively uniform (Figure 4). However, the slip rate assigned
pattemof deformation
predicted
by Hearn and Humphreys to the Owens Valley fault in this model is not in agreement
[1998]but differ in the detaileddistribution
of slipinsidethe with the secularslip rate inferred from geology.
ECSZ where our observations suggest deformation is
3 mmyr-• right-lateral
slipwhereas
weinfer3.3_+1.6
mmyr-•).
concentrated more to the west side of the shear zone (Owens
6. Conclusions
Valley-LittleLakefaultsystem)andmodelM2 indicates
that
the deformationis concentratedon the east side (Death ValleyFumaceCreek-SouthemDeath Valley fault system).
Distributed shear such as we have observed in the ECSZ is
To a first approximationdeformationacrossthe ECSZ can
be attributed to distributed uniform simple shear across
vertical planesparallel to the tangentto the local small circle
drawn about the North America-Pacific pole of rotation.
However, the observations imply a marginally significant
generally
associated
withductileflow. However,because
the
uppercrustitselfis thought
to be brittle,ductileflow thereis
rated22=18.2-+7.2
nstrain
yr-1parallel
to theshear
not a viable explanation of the observed distributed extension
direction(N36øW) that is not explainedby the model. A better
deformation. Rather, the deformation must be attributed to
elasticdeformationin the upper crustthat occursin response solution is to resolve the strain rates into a coordinate system
with the 2 axis parallel to the trend of the ECSZ (N23øW). In
to deformation in the lower crust and upper mantle. Two
that
coordinatesystemthe deformationcorrespondsto simple
explanations
are availablefor the observed
concentration
of
strainaccumulation
alongplateboundaries.The explanations right-lateralsheare•2
-39.2_+4.4nstrainyr-• acrossa
are basedon different assessments
of the relative strengthsof
vertical plane striking parallel to the shear zone plus an
the uppercrustandthe uppermantle.
The conventional model (Figure 5a) assumesthat the
extension,df•= 24.9_+6.5
nstrainyr-• perpendicular
to the
trend of the zone, with no significantextensiond2'• parallel
strengthof the lithosphereresidesprincipallyin the upper to the zone. This correspondsto the flat-Earth deformation
crust;thatis, the uppercrustactsas a stressguide. A shear zone model of Savage et al. [1995, appendix] in which the
zone at the surfaceis then interpretedas the consequence
of Sierra Nevada block is translatingboth parallel to the Eastern
elastic strain accumulationdue to slip at depth on a fault
California ShearZone and away from it relative to the southern
Nevada block (Figure 5b). In that model the motion is
accommodated by uniform extension and shear within the
[Savage and Burford, 1973]. For example,considera
transform
boundary.In the absence
of a plateboundary
fault,
the contactingplates would both be uniformly strained. ECSZ, and the Sierra block does not rotate to accommodate a
However,wherea transformfault existsat the plateboundary, fan-like opening of the Basin and Range province. The
the relativemotionof the platesis accommodated
by slip at deformation can also be attributed to slip at depth on the
depthonthefault,withonlytheuppermost
10to 20 kmof the principalfault systemswithin the ECSZ (Figures4 and 5a and
fault locked. This flaw (a slippingfault at depth)causesthe Table 2). However,suchan explanationrequiresa slip rate on
surfacestrainaccumulation
to concentrate
alongthe fault with the Owens Valley fault at least 3 times greater than that
negligible
strainaccumulation
beyond
a horizontal
distance
of inferred from geology [Beanland and Clark, 1994], a
morethanaboutsix lockingdepthsfrom the fault; hencethe discrepancy which calls into question the validity of the
appearance
of a shearzoneat thesurface.
Alternatively,
onemayargue[England
andJackson,
1989;
model.
Acknowledgment.We thankDananDongfor hisassistance
in using
Bourne et al., 1998] that the strength of the lithosphere his adjustmentprogramQOCA.
resides
primarilyin theuppermantle.Theuppercrustis then
regarded
asa badlyfractured
layerthatpassively
followsthe References
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Geological Survey, MS/977, 345 Middlefield Rd, Menlo Park, CA
94025. ([email protected])
(Received October 18, 1999; revised March 9, 2000;
acceptedMarch 24, 2000)