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Title
Author(s)
Citation
The transfer of slip between two en echelon strike-slip
faults : a case study from the 1992 Landers earthquake,
southern California.
Zachariasen, Judith.; Sieh, Kerry.
Zachariasen, J., & Sieh, K. (1995). The transfer of slip
between two en echelon strike-slip faults: a case study
from the 1992 Landers earthquake, southern California.
Journal of Geophysical Research, 100, 15281–15301.
Date
1995
URL
http://hdl.handle.net/10220/8475
Rights
© 1995 American Geophysical Union. This paper was
published in Journal of Geophysical Research and is
made available as an electronic reprint (preprint) with
permission of American Geophysical Union. The paper
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Systematic or multiple reproduction, distribution to
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JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 100, NO. B8, PAGES 15,281-15,301, AUGUST
10, 1995
The transfer of slip between two en echelon strike-slip faults:
A case study from the 1992 Landers earthquake,
southern
California
Judith Zachariasenand Kerry Sieh
Seismological
Laboratory,CaliforniaInstitute of Technology,Pasadena
Abstract. Detailed mappingof the fresh surficialrupturesin the right step betweenthe
HomesteadValley Fault and the Emerson Fault, two right-lateral en echelonfaults that
broke in the 1992 Landersearthquake,indicatesthat the transferof dextral slip between
the faults is accommodatedprimarilyby a seriesof obliquelystrikingright-lateralstrikeslip crossfaults. Right-lateral slip on the crossfaults and counterclockwise
rotation of the
interior blocksare sufficientto transfervirtually 100% of the dextral slip and to
accommodatethe extensionacrossthe jog. Asymmetryof the dextral slip curvesalong the
crossfaultsindicatesthat someof them may have been inducedto fail by slip on the
HomesteadValley Fault, while otherswere inducedto fail by slip on the Emerson Fault.
Comparisonof the magnitudeof slip in 1992 to bedrockand geomorphicoffsetssuggests
that the HomesteadValley Fault and severalsecondaryfaults have experiencedabout 100
nominalLanderseventsand that the stepoverstructureoriginatedat the sametime as the
HomesteadValley Fault, about 1 m.y. ago. Becausethe EmersonFault has an order of
magnitudegreatertotal offsetbut only slightlygreatersurficialslip in 1992,we conclude
that it is a significantlyolder fault. Repetition of 1992-1ikeeventsshouldeventuallylead to
a mergingof the HomesteadValley Fault with the Emerson.
Introduction
This paper focuseson the middle of thesethree stepovers,
which lies between the Homestead Valley and the Emerson
The fresh surficialrupturesof the Landers earthquakeof
June 28, 1992, caughtthe attention of earthquakegeologists
becauseof their complexity,length,and largeoffsets.About 85
km of strike-slipfaults,acrossa sparselypopulatedbut accessiblepart of the Mojave Desert of southernCalifornia, accompanied the M w 7.3 earthquake[Siehet al., 1993]. The superb
preservationof the rupturefor manyweeksfollowingthe event
and the complexconfigurationof faults provided a unique
opportunityto investigatethe natureof strike-slipfaultingand
Faultsandencompasses
anareaof some15km2.We examine
to examine
both
the behavior
of individual
faults
the geometryand slip functionsof thesetwo primaryfaultsand
of the secondaryfaults in the stepover.From these data we
considerthe kinematicsof the transferzone and speculateon
the dynamicrelationshipsof the ruptures.Finally,we compare
the rupture patternsin this earthquaketo the long-termpatterns revealedin the bedrockand geomorphology
to examine
the evolution of these faults and to speculateabout future
earthquakes.
and their
interactions.
The Landersearthquakewasgeneratedprimarilyby slip on
severaldistinctfaults (Figure 1). These faults are part of the
EasternCalifornia Shear Zone, an 80-km-wid½zone of rightlateral shear,composedof numerousnorthweststrikingfaults
that accommodateabout 15-20% of the relative plate motion
betweenthe North American and Pacificplates [Dokka and
Travis,1990b;Sauberet al., 1994] (Figure 1, inset).
The characteristic
en echelonstructureof strike-slipfaultsis
observedat all scalesalong the 1992 rupture. There are three
major en echelonstepovers:one betweenthe JohnsonValley
Fault and the HomesteadValley Fault, another between the
HomesteadValley Fault and the Emerson Fault, and a third
betweenthe EmersonFault and the Camp Rock Fault. All of
these are right stepping and hence dilatational, yet each
stepoveraccommodatesslip transfer between its bounding
strike-slip faults in a distinct manner. The southernmost
stepoveris discussed
by Spotilaand Sieh [1995].
Copyright1995 by the American GeophysicalUnion.
Paper number95JB00918.
0148-0227/95/95 JB-00918505.00
Field
Methods
To understandthe HomesteadValley-Emerson Fault transfer zone,we foundit necessary,
first,to map the freshfractures
in great detail. We mapped the surfacerupture traces onto
l:6,000-scale aerial photographs and 1:12,000-scaletopographicmaps.The aerial photographscoveredmostof the field
area except a narrow strip down the central section of the
stepoverzone. Precisionand detail in this strip is thus somewhat poorer than in the regionscoveredby photographs.Fortunately,the surficialruptureswere significantlyfewer in this
strip. On the photographs,we mapped every observedfault
trace as accuratelyas the scale of the photographsallowed.
The mappedtracesare thusexact,and not schematic,although
in placesthe groundcrackingwassodensethat not everycrack
couldbe recorded.We measuredoffsetsof magnitudesranging
from zero to severalmeterswhereverpossible.We determined
lateral and verticaldisplacementby matchingoffsetlinear features such as motorcycletracks, roads, and stream channels
and then measuringthe distancebetweenthem alongthe strike
of the fault. When goodpiercingpointswere not available,we
measuredvertical separation.We accordedan error to each
15,281
15,282
ZACHARIASEN
1116ø50
'
AND SIEH: SLIP TRANSFER BETWEEN FAULTS
1116ø30'
.Oe•1116ø40'
Figure 2
34o30 '
ß '.,,
N
I
10miles
I
10 kilometers
I
I
'• Landers-Kickapoo
'•'•1Fault
X,•
.•oCY2
Fautt
-,•em----•l
"•'•o
_ •'x•
f•x,x
•
•, epicenter
•x.,
••d• rupure •.••28
June
1992
' ••.
•
•Fe•/z
• • '--•
I
•r•
x•'•• •. :.•
I
Reaion
•f
I
Ei•u•e ]. •ap of the su•cia] mptu•cs of the ]992 •a•dc•s earthquake.Box outli•cs the Emc•so• •aultHomesteadValley •ault stopover,show• i• detail i• •i•u•c 2. ]•sct illustrates]ocafio• of •a•dc•s mptu•c
withi• southernCalifornia.•a•dc•s mptu•c topolo• aftc• •ieh et M. []993].
offsetmeasurementbasedon the rangeproducedby successive the surficialrupture throughoutthe field area. In alluvium,the
of the same feature and an estimate of the
rupture traces were diffuse but well-defined, having broken
possiblemisfit due to diffuseor nonlinearedgesof the offset through a young, fairly uniform surface.In bedrock,by confeature and distanceacrosswhich the feature was projected. trast, they often simply disappeared or dispersed into illWe ranked the general quality of most measurementsas ex- defined, relatively randomly oriented cracking without any
clearlydiscernibleoffset.The bedrockin this area is not mascellent, good,fair, or poor.
The pattern of en echelonfracturethat is seenon the scale sivebut rather is broken up into large roundedbouldersand
of the entire Landers rupture is also seen at larger scales. smallerrubble which tends to disperseand hide evidenceof
Secondarysplaysand Riedel shearsexistedon all scalesand fracture.Much of the energyof fracturewent into shiftingthe
were so mapped, though only those featuresthat seemedto bedrock rubble on the surfacerather than producingclear
play a role in the structureof the transferzone as a wholewill surficialfractures.Perhapsbelow the surface,where the bedbe discussedin detail in this paper. The rupture traceswere rock is more massive and coherent, the faults are better desometimessimple single strands and sometimescomprised fined.
We measuredoffsetsacrossthe wider and more complex
wide and complex zones of anastomosingand intersecting
strands.The surficial material through which the faults cut zones in various ways. If the zone comprisedseveralwellplayeda significantrole in definingthe locationand nature of defined strands, each of which offset the same feature, we
measurements
ZACHARIASEN
AND SIEH: SLIP TRANSFER
measuredeach strand separatelyand accordedit an error
estimation.
We then added these measurements
to determine
the total offset acrossthe zone. We calculated the error by
taking the squareroot of the sum of the squaresof the individual errors. In some complexzones,separatemeasurement
of eachstrandwastoo complicated.In suchcaseswe projected
alongthe offsetlinear feature acrossthe whole zone and measuredthe cumulativeoffset.In someplaceswe could measure
only one or two of severalstrandsin a complexzone. In that
case, we recorded the offset but did not include the measurementsin the determinationof slipalongthe fault or cumulative
slip acrossthe transferzonebecausethey representminimum,
not total, slip.
Observations
Fault Geometry
The transfer zone is composedof two primary bounding
faults,the HomesteadValley Fault on the southwestside and
the Emerson Fault on the northeast. These faults overlap
about5 km alongstrikeand are separatedby a 2-km-wideright
step.These subparallelen echelonright-lateralfaults,which
strike approximatelyN35øW, are linked by about five rightlateral crossfaults that cut diagonallyacrossthe rectangular
area betweenthe main faults.Figure 2 is a simplifiedmap of
the faults, including topographyand representativelateral
(Figure2a) and vertical(Figure 2b) offsets.
The HomesteadValley Fault, which cutsthroughthin alluvium in this area, has a very clear,well-definedrupture trace.
The fault traceis contiguousthroughoutits lengthand is nearly
rectilinear,with deviationsof no more than a couple of hundred meters. These small bends and kinks do not appear to
correlatewith the intersectionsof secondaryfaultsor anyother
obviousstructures.The largestjog in the Homestead Valley
BETWEEN
FAULTS
15,283
cuts down through the hills to the south and breaks out into
alluvium at its southern end. This fault has a similar rightlateral componentto that of the northern fault, but the fault
here is vertical to steeplywest dippingand has a normal sense
of dip-slipmotion. The westernsideis downdroppedabout 10
cm relativeto the hills.Why this splaychangesfrom an oblique
thrust to an oblique normal fault halfway along its length is
unclear.This behavioris probablynot related to local irregularitiesin the HomesteadValley Fault. It doesimply, however,
a changefrom near-surfaceshorteningto extensionacrossthe
Homestead Valley Fault. This, in turn, may imply counterclockwiserotation of the block east of the HomesteadValley
Fault about a pole near the changefrom reverseto normal
faulting. Conceivably,this behaviormay also be explainedby
tilting of the block betweenthe Western Splayand the Homestead Valley Fault.
The Emerson Fault is approximatelyparallel to the HomesteadValley Fault where it cutsthroughalluvium at the northern end of the stepover(Figure 2). It divergesto a more
easterlystrike where it leavesthe alluvium and runs along the
eastern flank of the bedrock
hills. The southeastern
reaches of
the EmersonFault did not break duringthe 1992 earthquake,
but its continuation
to the southeast
of the transfer
zone is
clearly expressedin both the geomorphologyand the bedrock
[Dibblee, 1967a]. The mapped trace of the Emerson Fault
continuessome20-25 km southeastward
beyondthe 1992 rupture.
The secondaryfaults that link the Emerson and Homestead
Valley Faults strike approximatelynorth-south (Figure 2).
They divide the intervening stepoverregion into a series of
parallelogram-shaped
blocks.The largestand best definedof
these cross faults
is the northernmost.
This Northern
Cross
Fault intersectsthe Homestead Valley Fault about 1.5 km
from the Homestead Valley Fault's northwesternterminus in
Fault, near its northwest end, is, however, associatedwith a an extensiveand complicatedtriangular zone of subsidiary
large outcroppingof bedrock.The dip-slipcomponentof rup- crackingand faulting (Figures2 and 3b). In the southernpart
ture on the Homestead Valley Fault is predominantlywest- of the intersectionzone, most of the splaysconstitutea series
of short cracks,oriented subparallelto the crossfault. These
side-up.
Secondarycrackingand splayingis commonin the vicinityof die off rapidlywith distancefrom the HomesteadValley Fault.
the HomesteadValley Fault. Most of thesecracksand splays The northern end of the intersectionhas longer, more wellare on the east sideof the fault, within the stepoverarea. The defined subsidiaryfaults, which cut at very high anglesto the
slipon thesevariesconsiderably,
from purelytensionalcracks HomesteadValley Fault right where they meet that fault and
to shear fractures with several tens of centimeters of lateral
sweeparound in an arcuateshapeas they approachthe cross
slip. Most shear fracturesare right-lateral,but a few have fault. Minor crackingin the area of the intersectionis pervaleft-lateral offset.In some areas there are severalsubparallel sive, and in placesthe alluvial surfacepresentsa shattered
splaystogetherin a groupthat alternatein senseof slip.Many appearance.The HomesteadValley Fault doesnot appearto
secondaryfractures are oriented subparallel to the larger have been deformed by movement on the Northern Cross
transferfaults,but we havefound splaysat virtuallyeveryangle Fault in 1992, and there is no obviouswarping, cracking or
to these faults.
faulting west of the Homestead Valley Fault at the intersecOne of the largestsplaysof the HomesteadValley Fault, tion.
The Northern
Cross Fault intersects the Emerson Fault at
which we call the Western Splay (WS), is west of the main
fault, at the southernend of the transferzone (Figures2 and the northernend of the map area (Figures2 and 3c). Right at
3a). The splaycutsalongthe westernbaseof a low ridge of the point of intersectionis a smallhill, the top of which is an
hills. The HomesteadValley Fault cutsalong the easternside uplifted alluvial fan surface.The hill may well have resulted
of thisridge.In 1992,the ridgerosealongboth the Homestead from numerous1992-1ikestrainepisodes,at the intersectionof
Valley Fault and Western Splay.Substantialright-lateral slip the two faults, but its small dimensionsimply no regional
The EmersonFault passesalongthe easternedge
also occurred along both traces. The northern half of the significance.
WesternSplayis a right-lateralthrust fault that dips 25ø-50ø of the hill as two strandsthat bound a small graben.As at its
northeastward,toward the Homestead Valley Fault. Dextral southern terminus with the Homestead Valley Fault, the
slip rangesfrom 30 to 40 cm, and dip slip rangesbetween10 Northern Cross Fault does not cut or deform the Emerson
and 20 cm. To the south, about halfway along the ridge, the Fault at their intersection.The importanceof this observation
in a
thrustfault ends,and a new splayappears.This structureis up in constrainingthe kinematicsof the stepoveris discussed
in the hills rather than at the edge of the alluvium,though it subsequentsection.
15,284
ZACHARIASEN AND SIEH: SLIP TRANSFER BETWEEN FAULTS
LEGEND
•
.-
: '
•
:5'
?/.:. -..,.....•
..,
ß
.; ...:...
ß
...
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II
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......
5 . ...:':'
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' .
preexisting
fault
trace
(did
notrupture
in1992)
II
siteofoffsetmeasurement
righHateral
offset,cm
contour
interval
= 100ft (eastern
third,200ft)
'•Oo'
20_2q .........
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fault
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(reverse)
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'•
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trace
(vertical
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':;.
I
•-.. •"1
' 1MILE
,' ':..."'
'...............
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•'i•i•."'• :i":'""',.••'
Figure2. Mapof theEmerson
Fault-Homestead
ValleyFaultstepover
region,
showing
traceof surficial
ruptures
fromthe1992event.Alsoshown
arethetraces
of faultsthatdidnotrupture
in 1992butthatare
evident
in thebedrock
and/orgeomorphology.
Abbreviations
are:HVF, Homestead
ValleyFault;EF,
Emerson
Fault;
NCF,Northern
Cross
Fault;
WS,Western
Splay;
ES1,Eastern
Splay
1;ES2,Eastern
Splay
2;
ES3,EasternSplay3; ES4,EasternSplay4. Boxesdelineate
specific
areasof interest,
whichareshown
in
detailin Figure3. (a) Includes
representative
right-lateral
offsets
from1992.(b) Includes
representative
vertical offsets from 1992.
The other crossfaults to the south are lesswell-defined and
field area, about 6 km from the northern terminus of the
lesssignificantthan the Northern CrossFault. One and a half
Homestead
ValleyFault(Figures2 and3e). Nearitsinterseckilometers south of the intersection of the Northern Cross tion with the HomesteadValley Fault, this structurecurves
Fault andthe HomesteadValleyFault,a discontinuous
linear aroundthe southernandeasternmarginsof a smallhill. The
zoneof minorcrackingandfaultingstrikesnorthwardfromthe factthatthefaulttracehugsthesideof thehill suggests
to us
Homestead
ValleyFault(Figures2 and3d). The largestob- thatits locationis controlled
by the presence
of the hill. Farservedlateral offsetis 24 cm, and most offsetsare lessthan 5
ther north, the fault cuts into the alluvium and assumesthe
cm.Thesurficial
discontinuity
of thiszoneof cracking
suggestsnorth-south strike of the other crossfaults. This fault trace is
thatthe faultzonemaybe discontinuous
at depthaswell.
intermittent also but can be followed into the bedrock hills of
Another,slightlymoreprominentsecondary
fault,ES1,in- the stepoverregion.In the hills, this crossfault has a small
tersectsthe HomesteadValleyFault at the southernendof the
rightstep(to ES2), aroundthe southernandeasternsidesof a
ZACHARIASEN AND SIEH: SLIP TRANSFER BETWEEN FAULTS
15,285
..
LEGEND
• ße*e* (did
fault
trace
(vertical
or
normal)
not
rupture
in1992)
faulttrace(reverse)
ß
ßß .
'' ..:: .•;:. 'O
'\ß
.:...
/' '...:.
..
ß
•
.
..
'"::"
:':::..:i-:
'"'"
".,
site of offset measurement
45 es',.
O
:.
II
..
preexisting
fault
trace
open,
eastsideup
II
44
closed,westsideup
4+_2
verticaloffset,cm
contourinterval
= 100ft (easternthird,200ft)
53__.5
0vertic'ia•..:,.
'30_+½i5.
ß
ß
....
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.--.'ß
10+2
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[:':.38:'+7esu
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'... "e. ß ".•.
"-.
60__.10
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.':....•,..... ß........
35__.3
.
e"
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...
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........:..........
:.:.....:•."'., i.•,
ß.' '"': "I '-....::':.:'"..
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10+2
""'""
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:.
Figure 2. (continued)
3768-foot-high
peak.ES2intersects
theEmerson
Faultat the havior.The lateral offseton the HomesteadValley Fault near
southernterminusof the 1992ruptureof the contiguous
part
of the EmersonFault. Discontinuouscrackingalongthe EmersonFault occursfarthersouthof here,e.g.,on ES4, but it is
thesouthernendof thefieldareaisapproximately
3 m, though
it variesfrom 2 to 3.5 m (Figure 4a). This is the average
endsat theintersection
withthislargesplay,ES2.Virtuallyall
section,
thereisanabruptdecrease
byabout1 m in theamount
surficial
slipalongmostof the Homestead
ValleyFaultasit
to the south[Siehet al., 1993].The lateralslipdies
relatively
minor.Unlikethe Homestead
ValleyFault,which continues
continues
withsignificant
offsetwellpasttheintersection
with off toward the northern terminus at the rate of 0.2 mm/m up to
the lastlargesplay,the 1992ruptureof the EmersonFault the intersectionwith the Northern Cross Fault. At this inter-
ValleyFault. The concomitant
thelateralslipontheEmerson
Faultistransferred
to theES2 of slip on the Homestead
northward
increase
in slipontheNorthern
Cross
Fault
crossfault. Two smallfaultsfarther south,ES3 and ES4, splay abrupt
offtheEmerson
Fault.They,however,
appearto dieoutin the is about 1-1.5 m. From this we infer that about half the slip
alluviumand do not intersectthe HomesteadValley Fault.
fromtheHomestead
ValleyFaultwasshuntedontotheNorthern CrossFault. North of this intersection,slip continuesto
Slip DistributionAlongthe Faults
decrease
alongtheHomestead
ValleyFaultbutovera shorter
(0.5mm/m),towarditsnorthwestern
terminus,
about
Thesliponeachof themapped
faultsisextremely
variable distance
1.5
km
from
the
intersection.
This
rate
of
slip
decrease
suggests
alongthe lengthof the fault.Despitethe short-wavelength
rock.Thisis
variability,
however,
thereisfairlyregularlong-wavelength
be- strainsof about2.5 x 10-4 in adjacentcrystalline
15,286
ZACHARIASEN AND SIEH: SLIP TRANSFER BETWEEN FAULTS
al
ß
..
:
:
.,
:.
I
500
rn
I
..
200 rn
el
N
Figure3. Detailsof theLanders
surface
rupture.
(a) Detailof theWestern
Splay(WS),a secondary
fault
outside
(west)
of themainHomestead
Valley-Emerson
Faultstepover.
Thissplay
hasoblique
right-iateral
sense
of slip.Thenorthern
partdipsnortheast
andhasa reverse
component,
andthesouthern
halfdips
southwest
andhasa normal
component
ofslip.(b)Detailoftheintersection
between
theHomestead
Valley
FaultandtheNorthernCrossFault.Theareaof denseshattering
occurs
in alluvium.
Thefaulttraceisless
diffuse
andmoredifficult
to seewhereit traverses
bedrock.
(c)Detailoftheintersection
between
theEmerson
FaultandtheNorthern
Cross
Fault.Boththehillandthesmall
playaareprobably
duetoverylocalfault
geometries
anddonotreflect
broaddeformations
withinthetransfer
zone.(d) Detailof thesporadic
but
linearseries
ofsmall
cracks
cutting
through
thecenter
ofthetransfer
zone,
between
thelarger
Northern
Cross
FaultandES1/ES2
cross
faults.
Thisseries
ofcracks
mayrepresent
animmature
cross
fault.Thelargest
offset
observed
along
thistrendis24-cm
right-lateral
and15-cm
vertical
(west
sideup).(e)DetailofSplitHilland
theintersection
between
ES1andtheHomestead
ValleyFault.TheHomestead
ValleyFaultright-laterally
displaces
thetwosmall
hillsinthecenter
byabout
300m.Fromitsintersection
withtheHomestead
Valley
Fault,ES1curves
around
thesouthern
endofthesouthern
hillandoffsets
a smalldikeabout10m right
laterally.
Thefaultin thesouthwest
corneristhetailendof theWestern
Splay.
ZACHARIASEN AND SIEH: SLIP TRANSFER BETWEEN FAULTS
15,287
EF
35O
3OO
25O
2OO
150
100
5O
0
200
ES2
150r
lO0
NCF
ES3
so
•
o
.
ES1
lOO
so
50
35O
3OO
25O
2OO
150
100
5O
0
WS
100
•50
.....
I
0
1(JO0
I
2000
I
3000
40•00
50'00
6(JO0
DISTANCE FROM NORTHERN TERMINUS OF HOMESTEAD VALLEY FAULT (m)
Figure 4a. Lateral-slipdistributionalongall the major faultsof the stepoverregion.All valuesare rightlateral.Eachcurveis placedin a horizontalpositionthat reflectsthat fault'sspatialrelationshipto the other
faults. Only measurementsof offsetacrossthe entire zone, either measuredacrossall the strandsof the zone
or the sum of measurementson each of the strands of the zone, are included here.
approximatelythe maximumstrainthat can be storedin granite withoutfailure [Handin, 1966;Jaegerand Cook, 1979;Brace,
1964]. We did not observeoff-fault fracturing around the
northernreachesof the HomesteadValley Fault. The bedrock
that outcropseast of the fault, however, could easily have
hidden a myriad of sub-millimeter-sizedfractures.
Slip on the EmersonFault in this area is at least as variable
over short distancesas that on the HomesteadValley Fault,
but alsodisplaysa regularlong-wavelength
pattern(Figure4a).
slip distributionalongthis fault is markedlyasymmetric.Near
the intersectionwith the HomesteadValley Fault, the slip
increasesabruptlynorthwardfrom zero to the maximumof 1.6
m within 500 m. It then tapersoff graduallyto the north. There
is a secondspikeof about 1.4 m right at the intersectionof the
NorthernCrossFault and the EmersonFault. The nextlargest
splay, ES2, has about 1.5 m of right-lateral offset near its
intersectionwith the EmersonFault. Dextral slip on this fault
and its continuation,ES1, falls off rapidly as the faults cut
It increases northward from zero to about 3 m on the flank of
throughthe hills, decliningto about 10-20 cm in the alluvium
the stepover.Slip on the EmersonFault continuesto increase at the southernend of the zone. The asymmetryof the lateralnorthward, however,beyond the transfer zone [Sieh et al., slip magnitudealong most of the splayfaults we believe is
1993].
significantand is discussedlater.
On all the crossfaults, the predominantsenseof slip is
The verticalslip on all the faultsis quite variableand shows
right-lateral.The Northern CrossFault has 1-1.5 m of right- little systematic
behavior,although,like the lateral-slipdistrilateraloffsetalongmostof its length(Figures2a and 4a). The bution, it is somewhatasymmetric(Figure 4b). Becausethis
15,288
ZACHARIASEN
1
8O
AND SIEH: SLIP TRANSFER
BETWEEN
FAULTS
EF
= 60
I
e 40
20
....
ES4
ES2
=40
oI-
-100
e 20
"-
-20
ES3
20
--r
J-20
NCF
ES1
40 I-
40
e 20
0
•
""
0
-20
-20•
HVF
2O[
o
-2o
= -4o
•: -60
-1 O0
-120
4O
20 e
o
I
0
1000
I
2000
I
3000
I
4000
I
5000
I
6000
DISTANCEFROM NORTHERNTERMINUSOF HOMESTEADVALLEYFAULT(m)
Figure 4b. Vertical slipdistributionalongall the major faultsof the stepoverregion.Positivevaluesindicate
east-side-up;negativevaluesindicatewest-side-up.
transferzone is a right-steppingjog betweentwo right-lateral Cumulative Slip
faults, one would expect extensionalstructuresbetween the
Wc calculatedthe cumulativeright-lateral slip acrossthe
faults. We do see vertical slip on the main faults that is conzone and in the general direction of the primary faults by
sistentwith the extensionaldevelopmentof a basin.In general,
the senseof slip on the HomesteadValley Fault is downto the summingthe componentsof slip on all the faults in the zone
along a N35øW strike and plotting the total as a function of
east, and that on the Emerson Fault is down to the west. The
effect therefore is coseismicsubsidenceof the hills by an av- distancefrom the northernterminusof the HomesteadValley
erage of about 30 cm with respectto the alluviumoutsidethe Fault (Figure 6). Althoughcicaflythe cumulativeslip is quite
transfer zone. Vertical slip on the crossfaults is lesspredict- variable along strike, over all, the slope of the curve of slip
able: on the Northern Cross Fault, ES2, and the northern versusdistanceis approximatelyzero, with average slip of
reachesof ES2, the westsideis predominantlydownrelativeto about 3 m. There is no significantdecreaseof slip throughthe
the east side. Figure 5 is a block diagram that illustratesthis. transfer zone. Although this calculationdoes not addressthe
later, Figure
Rather than opposingsensesof vertical slip on the bounding issueof dynamicrupture,whichwill be discussed
secondaryfaults,we seea westwarddownstepping
setof faults. 6 showsthat virtually all the surfaceslip on the Homestead
ZACHARIASEN
AND SIEH: SLIP TRANSFER
BETWEEN
FAULTS
15,289
Figure 5. Block diagram showinga simplifiedconfigurationof faults in the Homestead Valley-Emerson
Fault stepover.Becausethe crossfaults have the samesenseof lateral slip as the major boundaryfaults,the
structureis a "strike-slipduplex" [Woodcockand Fischer,1986].
Valley Fault was effectivelytransferred acrossthe observed
structuresin the stepoverto the EmersonFault. Thus, in this
case,a 2-km stepbetweentwo en echelonstrike-slipfaults did
not presentan impedimentto ruptureor to the transferof slip
from one fault to the other. Given the lack of significantaf-
terslip at the several locationsmonitored for severalweeks
after the earthquake[Sylvester,
1993], we infer that all of the
slip was transferredacrossthe zone coseismically.
Long-Term Offset
How doesthe long-termhistoryof slip on the faults of the
Homestead Valley-Emerson transfer zone comparewith the
EF
ES4
slip in 19927To make this comparison,we searchedfor geoNCF
ES2
ES3
morphic and bedrock expressionof previousfaulting in the
HVF
ES1
area. The Emersonand HomesteadValley Faultsand manyof
WS
the secondarystructuresshowevidenceof prior activity.Many
•, 500
have geomorphicindicationsof 100-200 times the 1992 slip.
• 400
We found that the total offset acrossthe Homestead Valley
o
Fault is approximately300 m. The best evidenceof this occurs
•
300
at the southwestend of the field area (Figures2 and 3e and
2oo
Plate 1). There the fault bisectsand offsetsa smallbedrockhill
._•
(Figure 7). We call the two resultantknobsSplit Hill. Their
._g 100
geomorphicoffset is approximately320 _+70 m, basedupon
the assumptionthat the crestsof the hills can be used as
o
E
piercingpoints(C-C' in Plate 1).
o
1000
2000
3000
4000
5000
6000
This geomorphicoffsetequalsthe bedrockoffset.Dibblee's
Distance from northern terminus of Homestead Valley fault (m)
[1967b]l:62,500-scalemap first suggested
a bedrockoffsetof
Figure 6. Sum of the dextraloffsetacrossthe entire stepover
about 300 m. Our mappingat a scaleof 1:6,000showsseveral
zone, measuredalong a strike of N35øW, approximatelythe
strike of the Homestead Valley Fault. The total offset was bedrock units that have been cut and offsetby the movement
calculatedby adding the componentof lateral offset in the of the HomesteadValley Fault. PiercingpointsA-A' and B-B'
directionof strike for each of the faults composingthe zone. in Plate 1 indicatea right-lateraloffsetof 320 _+40 m and 250
The extent of each of the faults is shownat the top of the _+ 40 m, respectively.B-B' must be considereda minimum
value for the total offsetbecausethe contact near point B is
figure.
15,290
ZACHARIASEN
AND SIEH: SLIP TRANSFER BETWEEN FAULTS
::
"':•
.•,•"i::.•.•:;.•
.......................
'.::.::::..:':.':'::•:'-:
:::::::::::::::::::::::::::::::::::::::::::::::::::::
•::;i •.•
Figure 7. Profileof Split Hill. View is towardthe northeast.The locationof the HomesteadValley Fault is
indicatedby the white arrows.The fault passesbetweenthe hills,behindthe left hill and in front of the right
hill as viewed from this direction.The two hills are clearly right-laterallydisplacedalong the Homestead
Valley Fault.
obliqueto the fault zone,and diffuseslipoccursacrossthe fault
zone there. Total slip couldbe as much as 60 m greater than
have been raised to its presentheight by 200-400 Landers
events.The actual number of sucheventsmay well be signifithe 250-m value.
cantly less than 200-400, becausethe assumptionthat the
Landers-style
uplift is not
Since the magnitudesof the geomorphicand the bedrock ridgewascreatedsolelyby coseismic
offsetsof Split Hill are indistinguishable,
we concludethat the strong.We know,for example,that Split Hill, just to the south,
HomesteadValley Fault was first activatedsubsequentto the existedas a landform prior to activationof the Homestead
creationof the presenttopographyin this part of the Mojave Valley Fault.
Desert. A hundred nominal Landers earthquakescould acThe geomorphicevidencealongthe EmersonFault is consistentwith the other faults. The northernmostmajor east
count for the total dextralslip.
Severalother faults also have geomorphicevidenceof pre- flowing stream channel south of the Northern CrossFaultviousactivity,thoughin mostplacesthe magnitudeof the total EmersonFault intersectionshowsabout 260 _+50 m of rightslip is not recoverable.The Northern CrossFault crossesa lateral separationacrossthe Emerson Fault. Given that the
northwestflowing stream channel and appearsto have right- 1992right-lateraloffsetin thisarea is about1.5 m, we calculate
laterally offset it about 200 ___70 m. Given that this fault about 170 _+30 similareventscouldhaveproducedthe offset.
All the faults that we can constrainquantitativelyshowof
experiencedabout 1.2 m of offsetat this locationin 1992,the
total offsetcouldhave accruedduringabout 166 ___
60 nominal the order of 100-200 nominal Landers events.This implies
Landers events.This is approximatelythe same number of that the transfer structure we observed in 1992 has existed for
eventsas have occurredon the HomesteadValley Fault.
100-200 eventsand hasnot actedasan impedimentto rupture
The ridge betweenthe Western Splay and the Homestead duringthat period.The onlyexceptionto the 100-eventrule is
Valley Fault rose 10-20 cm duringthe 1992 earthquake.If we in the bedrockoffsetalong the EmersonFault on which both
assumethe ridge was createdby coseismicuplift like that in 1992 and cumulativeslip are greatest.Bedrockoffsetsalong
1992, and we dividethe 1992uplift into the total heightof the thisfault are of the order of 3-5 km. Dibblee[1964]mappeda
ridge, which is about 40 m, we calculatethat the ridge could quartzmonzoniteporphyriticdike offsetsome3.5-5 km along
ZACHARIASEN
AND SIEH: SLIP TRANSFER
BETWEEN
FAULTS
15,291
3000'
N
200 rn
contour
interval
= 40 ft
Legend
Veryfinegrainedwhitecluartzite;
Variant
ofbrecciated
calc-silicate
with
tan
locallyshowsgood bedding
dolomiticmarbleclasts;locallyshowsgood bedding
Biotitequartzmonzonite;porphyritic
intrusivewithK feldsparphenocrysts
Mediumgrainedleucocraticdike
withquartzand K feldspar
Green saccharoidal calc-silicate
Coarse plutonicrockwith dark
green amphibolelathsand whitefeldspar;,
possiblya contactrockbetweenbiotitequartz
withdiopside
Brecciatedvariationof green calc-silicate,
often contactmetamorphosed
monzonite
and calc-silicate
Fine grained, well-foliatedgneiss
Plate 1. Geologicaloutcropmap of the Split Hill regionon the HomesteadValley Fault The 1992 rupture
is in pink. The dark regionsoutlined in solid lines are outcrops.The lighter colorsoutlined in dashedlines
inficatethe extent of the unit extrapolatedfrom the outcropdistribution.A-A', B-B', and C-C' are piercing
pointsfor estimatingcumulativelateral slipin thisarea.The Split Hills are offsetgeomorphically(C-C') about
300 m. The plutonicand metamorphicrocksunderlyingthe hill displayan equivalentoffset(A-A' and B-B')
15,292
the Emerson
ZACHARIASEN
Fault
about
15 km north
AND SIEH: SLIP TRANSFER BETWEEN FAULTS
of this transfer
zone.
F. Gomez and K. Sieh (unpublishedmapping,1992) confirma
likelyvalueof about4.6 km, baseduponcorrelationof plutonic
and metamorphiccontactsacrossthe fault, also about 15 km
north of the stepover.The bedrockoffsetalongthe Emerson
Fault, therefore, is about an order of magnitudehigher than
that alongthe HomesteadValley and associatedstructures.
Discussion
Transfer Zone Geometry
It is widelyheld that a right step,or jog, betweentwo rightlateral strike-slipfaults producesextensionin the regionbetween the two faults. Such extensionaljogs and pull-apart
basins have been observed, in studies of both individual earth-
quakerupturesandlong-termstructuralfeatures[e.g.,Crowell,
1974; Tchalenkoand Ambraseys,1970; Clark, 1972; Clayton,
1966;Sharp,1975;Woodcockand Fischer,1986;AydinandNur,
1982;Brownand Sibson,1987;Hemptonand Neher, 1986;Sims
and Ito, 1990].Stepovershavealsobeentheoreticallymodeled
[Rodgers,
1980;SegallandPollard,1980]and observedin sandbox and claycakeexperiments[Chinnery,1966; Tchalenko,
1970;Naylor et al., 1986].Any extensionaljog in brittle materials requiresthe developmentof secondarystructuresto accommodatethe extensionand to transferthe lateral slip from
one primary fault to the other. To provide these structural
services,variouscombinationsof normal faults and strike-slip
structureshavebeenpostulatedand observed(Figure 8).
Laboratory and theoretical models and field work have
found evidencefor the developmentof normal faults cutting
through the stepoverat high anglesto the primary faults, as
sketchedin Figure 8. Clayton[1966] has mappeda stepover
with such a configuration,in which two right-steppingrightlateral strandsof the Hope Fault in New Zealand are joined by
a seriesof curvingnormalfaultsthat departfrom the bounding
faults at anglesfrom 10ø to 70ø (Figure Be). Crowell's[1974]
model of a pull-apartbasinis more complex,with oblique-slip
faults at the ends of the boundingfaults and a few reverse
faultsin the interior of the stepoveraccompanying
the normal
faults (Figure 8b). The right stepoverat Mesquite Lake betweenthe right-lateralImperial and BrawleyFaults(Figure8f
[afterJohnsonand Hadley,1976])showsoblique-slipand normal faults developingto accommodateextension.
In other cases,both observedand modeled,strike-slipfaults
develop as significantsecondarystructuresresponsiblefor
transferringthe slipthroughan extensional
jog.Rodgers[1980]
modeled the developmentof a pull-apart basinbetweentwo
lateral strike-slipfaults.His elasticdislocationmodel analyzes
the developmentof secondarystructuresin responseto motion
along the two masterfaults. In his models,two small depressions,occupiedby normal faults,developnear the endsof the
masterfaults,while strike-slipfaults occupythe center of the
pull-apart basin (Figure 8c). Basin depth is 10-15% of the
lateral offseton the master faults.Sibson[1986] developeda
model of a meshwithin the stepovercomposedof extensional
fractureslinkedby shortlateral-slipfaultsof both sensesof slip
(Figure 8d).
An example of an extensionaljog with secondarylateral
faultsis foundin the Dasht-eBayaz,Iran, earthquake(Figure
8g [from TchalenkoandAmbraseys,1970]). Woodcockand Fischer[1986] describethe stepoverconfigurationthere as an
"extensionalduplex,"in which secondaryen echelonobliqueslip faults successively
peel off from the primary faults to ac-
commodateextensionandto transferlateral slipasthe primary
faultslengthenand the zone maturesover time. Finally, some
strike-slipfaults are linked by other strike-slipfaultswith the
oppositesenseof slip. In the Imperial Valley in southernCalifornia, the right-lateralBrawleySeismicZone and Superstition Hills Fault are separatedby a right step. In the 1987
SuperstitionHills earthquake,right-lateralfaulting on the SuperstitionHills Fault was precededby rupture on a seriesof
left-lateralcrossfaultsorientedat right anglesto the two flanking dextralfault zones(Figure 8h [after Hudnut et al., 1989;
Sharpet al., 1989]).
Despite the wide range of geometriesfrom previousobservations,models,and experiments,none appliesexactlyto the
HomesteadValley-EmersonFault case.The HomesteadValley Fault-Emerson Fault stepoverdoes not conform to the
modelof a pull-apartbasindepictedin Figures8a, 8b, 8e, or 8f.
In these cases,the two boundinglateral faults are joined at
obliqueto right anglesby normalfaults,and subsidence
of the
plugin the middle accommodates
the extension.Althoughthe
interior of the Emerson-HomesteadValley stepoveris downdropped an averageof about 30 cm relative to the ground
outsidethe boundingfaults,aspredictedfor an extensional
jog
[e.g.,Rodgers,1980;SegallandPollard,1980],we do not seethe
extensionalstepover structure dominated by normal faults
(Figure 9a). Rather, the secondarystructuresare a seriesof
obliqueright-lateralstrike-slipfaults that strike 300-40ø from
the primaryfaultsinto the interior (Figure 9b). Althoughthey
possess
a normalcomponentof slip,they are primarilylateralslip faults. For example, on the Northern Cross Fault, the
averagelateral slip of over I m accompaniesan averagevertical displacementof 10-20 cm. Furthermore, lateral slip
across these
faults
is in the same sense as that
across the
primaryfaults,that is, dextral.Thusthe secondaryand primary
faults together have created a right-lateral strike-slipfaultbounded
rhombohedral
block
or blocks
that
constitute
the
interior of the transferzone (Figure 5).
It is clear that the structural configurationof Figure 9b
cannotbe maintainedwithout changeover time. The normalfault-boundedblockof Figure9a could,theoretically,continue
to exist without significant distortion through innumerable
earthquakecycles,simplydroppingfarther and farther down
with each event. However, the behavior of the right-lateral
strike-slip-bounded
block of Figure 9b is constrainedby the
slipon its boundingfaults,and thusthe blockmustnecessarily
be distorted
over successive events.
That all of the faultsare dextral-slipfaultsrequirescounterclockwiserotationof the interiorblock(s).Without suchrotation, right-lateral strike-slip motion along the cross faults
would producepronounceddeformationof the masterfaults.
Geomorphicand bedrockfeaturesdescribedearlier indicate
that severalof thesefaults,includingthe Northern CrossFault,
have been active through many rupture eventsbut have not
deformedthe principalboundingfaults. If the interior block
hadslipped200m or soby pure translationalongthe Northern
Cross Fault, it should have deflected the trace of the Home-
steadValley Fault and/orthe EmersonFault by 200 m. We do
not observesuchdeflections.Pure translationalslip couldoccur on the crossfaultswithoutaffectingthe primaryfaultsonly
if the slipvectorfor the crossfault were parallel to the strike
of the boundingfaults.However,when we combinea N35øW
slip vector azimuth with the offset data along the Northern
CrossFault, we find that the plungeof the slip vector must
changealong strike from about 20ø in the southto 0ø in the
ZACHARIASENAND SIEH: SLIP TRANSFERBETWEENFAULTS
Models
I
•
Braidedright-slipzone
•
--
t •,•
t
-
'
__•_ •---"•"•.
15,293
Oblique-
• slipfaults
Pull-Apart,,,,,..
,,)", \
',
;.o.;
a
intersection
-• - • ¾alus
Breccias b
c
d
e
o
ß
ß
g
h
Figure
8. Examples
ofmodeled
andobserved
dilational
stepovers.
(a)Modelofa pull-apart
basin
created
bytworight-stepping
right-lateral
faults.
Theareabetween
thefaults
drops
down
along
themajor
faults
and
along
intervening
normal
faults
thatdevelop
inresponse
totheextensional
stresses
created
bythejog.(b)
Idealized
model
ofa pull-apart
basin,
withnormal
andoblique-slip
faultsdeveloping
assecondary
structures
to accommodate
extension.
After Crowell[1974].(c) Sketchof an extensional
jog basedon an elastic
dislocation
model
ofoverlapping
faults
where
theoverlap
istwicetheseparation.
Sketch
shows
thelocation
ofright-lateral
secondary
faults
thatmight
develop.
Thepossible
conjugate
left-lateral
faults
arenotshown.
Theregions
labeled
"N"arezones
ofpossible
normal
faulting.
AfterRodgers
[1980].
(d)Sketch
oflinked
tensile
cracks
andstrike-slip
faults
withinanextensional
jog.Normal
faults,
withticksondownthrown
side,
bound
thejog.FromSibson
[1986].
(e)Mapofobserved
structures
ina rightstepoftheright-lateral
Hope
Faultin NewZealand.
AfterClayton
[1966].(f) Mapof theright-lateral
ImperialFault-Brawley
FaultZone
extensional
jog.Themainsecondary
structures
arenormal
faults
witha dextral
component
ofsliponsome.
AfterJohnson
andHadley
[1976].
(g)Mapofrupture
traces
fromthe1968Dasht-e
Bayaz,
Iran,earthquake.
Theleft-stepping
traces
ofthesinistral
faultarelinked
byanother
left-lateral
fault,anda series
of small
fractures
ofunknown
slipoccupy
theintervening
area.AfterTchalenko
andAmbraseys
[1970].
(h)Mapofthe
faultsin theregion
of the1987Superstition
Hillsearthquake.
Therightstepbetween
theright-lateral
Superstition
HillsFault(SHF)andBrawley
Seismic
Zoneistraversed
byleft-lateral
cross
faults
such
asthe
ElmoreRanchFault(ERF). AfterHudnutet al. [1989].
15,294
ZACHARIASEN
AND SIEH: SLIP TRANSFER BETWEEN FAULTS
north and that the dip of the crossfault at the northern end
mustbe shallowerthan the topographicslope,whichis clearly
impossible(seethe appendix).So the observedslipconstraints
and dip of the faultsindicatethat the slipvectoris not in fact
parallelto the primaryfaults.Thereforethere musthavebeen
some counterclockwise
rotation
of the blocks.
If the interior blocksare so rotating, they shouldproduce
compression
and extensionat the NW-SE and NE-SW corners
of the block, respectively.There is no incontrovertibleevidenceof suchcompressionand extension,exceptpossiblyat
the ridgebetweenthe WesternSplayand the HomesteadValley Fault. The north to southtransitionfrom reverse-obliqueto
normal-obliqueslip on the Western Splaymay be causedby
the counterclockwise
rotation
of one of the blocks east of the
HomesteadValley Fault.
Evolution of the Stepover Geometry
Continuedslip along thesepreexistingstructuresand continued counterclockwise
rotation
of the blocks over time will
alter and distort this stepoverzone from its presentconfiguration. Long-term counterclockwise
rotation shouldcontribute
to narrowingandlengtheningof the stepoverzone(Figure 10).
The crossfaults shouldrotate toward the primary faults,their
strikesbecomingless oblique. The HomesteadValley Fault
Figure 9. (a) Simplifiedsketchof an idealizedright-lateral and the Emerson Fault shoulddraw closertogether and ultistepbetweentwo dextralfaultslinkedby normalfaultsat right
mately coincide.This would be consistentwith modeled and
angles. (b) Simplified sketch of the Homestead ValleyEmersonFault stepover.The two primaryright-lateralstrike- observedbehavior of extensionaljogs, where discontinuous
slip faults are linked by a seriesof smallerobliquelystriking and steppeden echelonfault strandsmerge and becomeconright-lateralstrike-slipcrossfaults. One crossfault (NCF) is tinuouswith increasingoffset [e.g., Wesnousky,1988; Wilcox,
continuousacrossthe whole stepover,while the others (the 1973;Segalland Pollard, 1980].
We expect to see the individual faults composingthe
EasternSplays)are discontinuous.
Figure 10. Model of the stepoverevolvingovertime. Counterclockwise
rotationand dextralslipon primary
and secondaryfaults causesthe stepoverzone to lengthenand becomenarrower.The boundingfaults are
deflectedinto the zone and approachcoincidence.
ZACHARIASEN
AND SIEH: SLIP TRANSFER
stepoverevolve over time as well. We may see signsof this
evolutionalready.The Northern CrossFault, which is the only
throughgoingcrossfault and which carries the most slip, is
currentlythe primarytransferstructure.Its large offsetand its
continuityindicatethat it is probablythe most mature of the
observedcrossfaults. It has experiencedapproximatelythe
samenumber of eventsas the HomesteadValley Fault and is
likely as old as that fault. By contrast, the Eastern Splays,
thoughwe have no quantitativemeasureof their age, appear
significantlylesswell developed.They are discontinuous,
with
lessslip than the Northern CrossFault. We proposethat these
small
faults
are immature
versions
of the
Northern
Cross
Fault. As the transfer zone evolves,we might expectto see
increasingcoherenceof these faults, perhaps the linking of
ES1 and ES2 into one throughgoingfault like the Northern
CrossFault. The intermittentzone of faulting thoughthe center of the zone might then developinto a full-fledgedfault,
and/or other new, similar faults might initiate within the
stepover.Thus in the EmersonFault-HomesteadValley Fault
stepover,we may be seeingthe incipientstagesof the strikeslip duplex structure suggestedby Woodcockand Fischer
[1986].
The surfaceconfigurationof faultingleadsusto speculateon
the three-dimensionalstructureof the stepover.We havebeen
discussing
the stepoverregion as one or more rhombohedral
blocks,boundedby vertical, right-lateral-slipfaults. Is this, in
fact, a justifiableassumption?
To what degreedoesthe strikeslip duplex configurationof faulting that we have mapped at
the surfacerepresentthe three dimensionalstructureof the
stepover?Does the surface geometry extend relatively unchangedthroughoutthe brittle crust?Or is it replacedby other
geometriesat depth, suchas a single,continuouscurvedfault
cuttingdiagonallyacrossthe stepover?
Though we cannot answerthese questionsconclusively,direct observationsand inferencescan help illuminate the issue.
For the surfacegeometryto continueto depth relativelyunchangedrequiresthat the dipson the faultsbe approximately
vertical. Measurementsof dips at the surfaceand aftershock
locationsare our only direct indicatorsof fault dip. In the
shallow subsurface,the dips of the Homestead Valley and
Emerson Faults mustbe vertical or nearly so,becausethe 1992
fault planes, where exposed,are steeply inclined. Unfortunately, these dips cannotbe extrapolatedwith confidenceto
depthsof severalkilometers.
cross sections of aftershock
FAULTS
15,295
in aftershocklocations,and we cannotpreciselydeterminethe
locationsand dips of the faults at depth from aftershockdata
alone. Thus we cannot argue conclusivelyfrom aftershocklocationsthat the duplexmaintainsits surfacegeometryat depth
or that it does not. On the other hand, the relativelygreater
scatterleadsus to believe that there probablyremainssome
complexityat depth and the surfacefaults do not merge into a
simpleplanar fault at depth.
It is tempting to use measurementsof the vertical deformation within the stepoverto try to ascertainthe shapeof the
stepoverat depth.Whereasvolumetricconstraintsinitiallysuggesta shallowstepoverstructure,they do not precludecontinuation of the duplex to deeper levels.A simple massbalance
calculation,usingonly the observedsurficialslip, indicatesa
mass deficit in the interior.
If the surface structures continued
to depth, then 3 m of right-lateral slip on the Homestead
Valley Fault and the Emerson Fault shouldproduce a mass
deficiencyin the stepoverof 3 m times the width of the zone
(about2 km) timesthe thickness
of the fault block(say15 km),
or 0.09 km3. The observed
30-cmdowndropping
aloneaccountsfor a volume of 30 cm times the width of the zone (2
km) timesthe lengthof the zone (about5 km), or about0.003
km3, only3% of the requiredmass.However,thisadmittedly
Structure at Depth
Vertical
BETWEEN
distribution
in several
placesalongthe 1992Landersruptureshowthat aftershocksof
the earthquakedelineatea near-verticalplane that extendsto
depthsof 10-15 km [Haukssonet al., 1993]. The clearestexamplesof this occuron segmentssuchas the JohnsonValley
Fault and the Emerson Fault north of the transfer zone, where
there is a single-well-definedfault trace at the surface.However, many of the crosssections,especiallyin areas like the
Emerson Fault-Homestead Valley Fault stepoverwhich have
a complicatedsurfacegeometry,showa cloud of aftershocks
that do not clearly define planar fault structures.Haukssonet
al. [1993] have interpreted the aftershockcloud as evidence
that the Emerson Fault-Homestead Valley Fault stepover
comprisesa broad band of right-lateral shear rather than a
singlefault at depth. Such a broad shear band could be the
continuationof the strike-slipduplexthat we seeat the surface.
However,the diffuseaftershockpatternis in part due to errors
crude calculationis too simplisticand does not account for
possiblenarrowingof the zone that we postulatedwaslikely to
occur with the counterclockwise
rotation
of the interior
block.
How much narrowingwould be required to accountfor the
other 97% of the volume? The entire volume of 0.09 km 3 could
fit in a block5 km longby 15 km deepby 1.2 m thick. Thus the
amount of narrowingrequired to accommodatethe missing
massis a mere 1.2 m, an amount impossibleto resolvein the
field. Even over the courseof 100 earthquakesof this nature,
we would seeonly 120 m of narrowing.Given the irregularities
of the fault, it seemshighlypossiblethat 120 m of narrowing
couldhave occurredacrossthis stepover.Thus we find that we
cannotuse the verticalcomponentof slip in 1992 to quantify
the depth to the baseof the duplexstructure.
Seismological
studiesof the Landersearthquakeare inconclusiveregardingthe continuityof surficialstructuresto depth.
Coheeand Beroza [1994] used near-sourcedisplacementrecordingsto model the Landersrupture and found no disruption in the propagationof rupture throughthis stepover.They
suggestthat rupture occurred as if on a throughgoingfault.
Studyingshearwavestrapped in the Landersfault zone,Li et
al. [1994]found evidencefor a fault discontinuityat the Johnson Valley-Homestead Valley Fault stepoverbut none at the
Homestead Valley-Emerson Fault stepover.However, using
strong motion, teleseismic,geodetic and geologicdata from
the Landersearthquake,Wald and Heaton [1994] find that at
both of the major stepovers,the JohnsonValley-Homestead
Valley and the Homestead Valley-Emerson, there is a clear
slowingdownof the rupturefront asit navigatesthe jump. This
rupture retardationmay indicatea fault discontinuityat depth.
We cannot know, of course,if sucha discontinuousfault geometry is identicalto the surficialgeometry.
Nor do comparisons
with other stepovershelpushere. Some
studies [e.g., Barka and Kadinsky-Cade,1988; Harris et al.,
1991] have concludedthat stepoverslessthan 1 km wide are
likely to merge, while others [e.g.,Bakun et al., 1980;Rymer,
1989]havesuggested
that fault stepoversof evena few 100 m
width can continueto depth. While the completetransfer of
slip throughthis stepovermight seemto argue for continuity
acrossthe zone, there is no real need to have a throughgoing
15,296
ZACHARIASEN
AND SIEH: SLIP TRANSFER
fault in order to transferslip acrossa stepof only 2 km. Using
a two-dimensionalfinite differencemodel of rupture propagation through a fault step that continuesto depth,Harris and
Day [1993]find that earthquakerupturescanpropagateacross
stepsas wide as 5 km without any linking crossstructures.
Finally, Sibson[1986] finds that while an extensionaljog can
impederupture on timescalesof seconds,a smalljog is neverthelessunlikelyto preventrupture on quasi-statictimescales.
If the surficialgeometrydoesnot continueto depth and the
principal faults merge, we suspectthat it is the Homestead
Valley Fault that bends at depth to join the EmersonFault,
rather than the converse.As mentionedin the precedingsection, the EmersonFault hasaboutan order of magnitudemore
total slip than the HomesteadValley. We believethis implies
a substantiallyshorter lifetime for the Homestead Valley
Fault. If this is correct, then the Emerson Fault has acted
alone, without the Homestead Valley Fault during most its
existence.We have no reasonto believe,then, that the dip of
the EmersonFault in the stepoverwould deviatefrom its dip
elsewherealong strike.
A more reasonablehypothesisis that the youngerof the two
faults, the HomesteadValley, would have formed with a contorted, nonplanarshapenear its intersectionwith the older,
throughgoingEmerson Fault. It is beyond the scopeof this
principallydescriptivepaper, however,to create continuummechanicalmodelsthat might quantifysuchshapes.Thus, in
lieu of more definitivedata, we argue only for a nearlyvertical
EmersonFault. The subsurfaceshapeof the HomesteadValley Fault is not well-constrained.
Comparison With Seismological and Geodetic
Inversions
We have observedthat throughthe stepover,slip decreases
northward on the Homestead Valley Fault and increases
northward on the Emerson Fault. Figure 6 showsthat the
cumulativesurficiallateral slip on all the faults remainsrelatively constantacrossthe stepoverat a value of --•3 m.
WaMand Heaton [1994],Coheeand Beroza[1994],andHudnut et al. [1994]usedobservations
similarto thoseof Siehet al.
[1993]andPonti [1992],smoothedoverwavelengthsof several
kilometers, as constraintsin the inversionsof seismological
and/or geodeticdata for fault slip and rupture history.Within
a factor of 2 or 3, their best fitting inversionsreproducethe
observationsof surficialslip within the stepoverregion. The
surficial data are superior to the modeled slip values at the
surface,becausethey are directly observedand becausethey
provide finer resolutionof structuraldetail. The inversions,
however,provideimportantcluesto the behaviorof the faults
at depth,whichcannotbe deducedfrom our measurements
of
BETWEEN
FAULTS
Perhapsthe discrepancyis due to the narrow aperture of the
geologicmeasurements.We cannot see off-fault and subsurfacedeformation.Alternatively,the geodeticand seismological
modelsmay suffer from nonuniqueness.
In either case, this
discrepancysuggeststhat strict extrapolationof surficialgeologicaloffsetsto depthand the assumptionof no largeoff-fault
deformationmay be inappropriate.
Nevertheless,geologicalobservationsdo place important
constraintson the analysisof fault behavior.In the following,
we discussthe possiblerole of geologicdata in analysisof
earthquakerupture dynamics.
Sequenceof Rupture Through the Duplex
What we know of the dynamicruptureof the Landersearthquake comeslargely from seismologicalobservations.Inversionsof seismological
data showunequivocallythat the Landersearthquakewasproducedby nearlyunilateralrupturefrom
southto northwest.The detailsof the progressionof the rupture are, however,very difficultto resolvefrom the seismological data, alone, and remain controversialand ambiguous.
(Note, for example, the significantdifferencesbetween the
ruptureprogressions
modeledby Coheeand Beroza[1994]and
Waldand Heaton [1994].In large measure,the ambiguitiesin
the seismological
recordsarisebecauseonly the longer-period
(2-15 s) componentsof the recordscan be utilized in the
inversions.Theseperiodscontainlittle informationbearingon
the detailsof the rupturehistoryoverfault lengthsshorterthan
severalkilometers.Unfortunately,the two major stepovers
have dimensionsof only a few kilometers. At this scale of
seismologicalambiguity,then, it would be useful to have informationbearingon the detailsof the rupture history.
The seismological
inversions,plus the nature of slipwe see
in the field, lead us to speculatethat we may be able to see
patternsof dynamicrupture in the staticslip signatureleft on
the groundsurface.We have noted abovethat one aspectof
the surfaceruptureswithin the stepoveris the asymmetryof
their dextral-slipfunctions.Figure 4a showsthat alongmostof
its length,slip on the Northern CrossFault diminishesaway
from the HomesteadValley Fault. Slip on the EasternSplays
decreasespredominantlyaway from the Emerson Fault. We
think these asymmetriesmay bear upon the issueof rupture
dynamicsacrossthis stepover.
We suggestthat the slip asymmetrymay indicatethe direction in whichrupturepropagatedalongthe crossfaults.That is,
the slipis highestnear the site of ruptureinitiation and lowest
near its termination.
We believe that the Northern
Cross Fault
failed becauseof stresses
inducedby failure of the Homestead
Valley Fault and that the Eastern Splaysfailed becauseof
stresses
inducedby failure of the EmersonFault. Calculations
of stresses
at the endsof cracksin elasticmediashowthat edge
surficial offsets.
dislocations
produceincreasesin shearstresses
in the region
All of the best fitting inversionsproduce two broad-scale surrounding
the cracktip [e.g.,Chinnery,1963,1966;Jaegerand
features that are relevant to understandingthe stepoverbe- Cook, 1979;Segalland Pollard,1980;Rodgers,1980;Steinet al.,
tween the Homestead Valley and Emerson Faults. First, all 1992; Harris and Simpson,1992]. These shear stressesare
revealthat the patchof veryhighsurficialslip(3-6 m) northof greatestat the cracktip and diminishawayfrom it. If a favorthe stepover,on the EmersonFault, doesnot extendbelow a ably oriented secondcrack, located near the tip of the first
depth of about 8 km. Second,all the inversionsproduce a crack,hasuniform strengthalongstrikeand is then inducedto
patchofvery highdextralslip(3-8 m) about10 km in diameter fail, slip on the crackwill initiate at the point nearestthe first
on the HomesteadValley Fault, near the southernedgeof the crack tip and propagateaway (S. Ward, written communicastepoverand 5-10 km beneath the surface.
tion, 1994). Furthermore,the magnitudeof slip on that crack
The slip in this region as determinedby surficialoffsetsis shoulddiminishawayfrom the tip of the first crack(S. Ward,
half to one third the slip determined by seismologicaland written communication,1994). Admittedly, calculationsfor
geodeticmethodsthat samplea wider windowof deformation. cracksin an isotropicelasticmedium do not perfectlymimic
ZACHARIASENAND SIEH:SLIPTRANSFERBETWEENFAULTS
15,297
ß
ß
ß
ß
ß
ß
ß
ß
**
ß
Figure
11. Proposed
sequence
ofruptures,
based
upon
ourobservations
oftheasymmetry
ofsliponthe
cross
faultsandin thecontext
of WaldandHeaton's
[1994]dynamic
rupture
model.
Lightgreylinesarefault
traces
thathave
notyetruptured.
Solid
lines
indicate
traces
intheprocess
ofrupturing.
Dashed
lines
aretraces
thathavealreadyruptured
andmayor maynotstillbe slipping.
very
thosein the faultedEarth,duringthe propagation
of a major s after initiationof the earthquake,rupturepropagated
alongtheHomestead
ValleyFaultto the
earthquake
rupture.
Theydo,however,
suggest
thattheasym- rapidlynorthward
edgeof the stepover
(Figure11a).At that point,
metryofsliponthefaultsofthestepover
mayreveal
boththe southern
in therupture-front
velocity
from
source
of thestresses
thatproduced
failureof thesefaultsand therewasanabruptdecrease
the directionin whichslippropagated
alongthem.
Let us assume,then, that asymmetryin slip distribution
correlates
withpropagation
direction.
Figure11 is our speculative reconstruction
of rupturepropagationthroughthe
about4 to 1.4km/sat thestepover
(Figures11b-llf). Between
12and16s,ruptureoccurred
principally
withintheregionof
the stepover.
Theirdatado not allowthemto resolvethe
detailedsequence
of faultrupturewithinthe stepover.
However,
we
propose
that
their
slow
(1.4
km/s)
propagation
of
the
stepover,
baseduponour observations
of andassumptions
frontnorthward
through
thestepover
mayreflectthe
abouttheasymmetry
of surface
slip,in thebroader
context
of rupture
rupturesequence
we deduce
fromthe slipasymmethe seismological
andgeodetic
inversions
[WaldandHeaton, complex
1994;CoheeandBeroza,1994].According
to the modelof tries.
In theearlypartof this4-s-long
period,rupturepropagated
WaldandHeaton[1994],duringtheperiodfromabout10to 12
15,298
ZACHARIASEN
AND SIEH: SLIP TRANSFER BETWEEN FAULTS
to the northern end of the HomesteadValley Fault (Figure
lib). As it nearedthe end of the fault, slip decreaseddramatically on the HomesteadValley Fault (Figure 4a), while rupture velocitymay alsohave decreased.The shearstresses
producednear the end of the HomesteadValley Fault by this slip
function
then caused south-to-north
failure
of the Northern
Table 1. Long-Term OffsetsAlong SeveralFaults in the
Stepover,Determined From Offset Bedrock and/or
GeomorphicFeaturesand Comparedto the 1992
Offset in the Same Locations
1992
Geomorphic
Fault
Offset
Offset
CrossFault (Figure 11c).Shearstresses
producedby failure of
the Northern Cross Fault induced rupture of the Emerson
Fault near their intersection(Figure lid). We suggestthat
since dextral slip on the Northern CrossFault shouldhave
HVF
NCF
WS
EF a
3 m
1.2 m
20 cm
1.5 m
300
200
40
260
increased the normal stress on the Emerson
EF b
3m
Fault north of the
m
m
m
m
...
Bedrock
Offset
300 m
...
...
...
4 km
Number of
Events
100
160
200
170
1300
intersection and decreased it to the south, the southern tail of
the Emerson Fault ruptured next, propagatingtoward the
southeast.This rupture would best fit into Wald and Heaton's
[1994] sequencewithin the period 14-16 s. Next, the Eastern
Splaysaccommodatethe strainsbuilt up at the southeastern
tail of the EmersonFault andbeginto rupturesouthward,back
toward the Homestead Valley Fault (Figure lie). Stresses
producedby failure of that portionof the EmersonFault south
of the Northern CrossFault inducedslipon the EmersonFault
north of the stepoverabout 16 s into the earthquake,at which
time the rupturefront acceleratedand resumedits rapid northwestwardpropagationat about 4 km/s (Figure 11f). This sequenceof faultingmightaccountfor WaldandHeaton's[1994]
observationof somererupturingtowardthe southeaston some
fault(s) in the stepoverarea as the main rupturefront continues northwestpast the stepover.
This scenariois, of course,speculative,but it is consistent
with the seismological
inversionof WaldandHeaton[1994]and
the staticslip data. Ostensibly,this model conflictswith the
best fitting inversionsof Coheeand Beroza[1994].Coheeand
Beroza[1994]alsohavethe rupturepropagatingsouthto north
through the stepover11-15 s into the earthquake,and the
rupture front on the HomesteadValley Fault slowingdown
slightlyas it approachesthe stepoverregion and accelerating
again acrossthe region of highest slip. However, they have
rupture fronts movingsimultaneously
alongboth the Homestead Valley and the Emerson Faults in the stepover.They
interpret this to mean rupture occurredon a throughgoing
fault. However,it may alsomean simplythat rupture occurred
simultaneously
on two or more independentfaults.At anyrate,
the resolutionof seismologicaldata is suchthat they cannot
rule out the possibilityof back rupture.
If slip asymmetrydoescorrelatewith propagationdirection
and indicates
the source of the shear stresses that induced
failure, then perhapsthe surficialslip signaturecan help resolvesomeof the details of rupture dynamicsthat seismological methods
cannot.
Implications of the 1992 Rupture for Long-Term Fault
Behavior and Earthquake Recurrence
As noted above,severaloffsetgeomorphicand bedrockfeatures on the faults constitutingthe transfer structureindicate
that the faults have experienceda total of 100-200 times the
amount of slip associatedwith the 1992 event. Table 1 summarizes these long-term offsetsand comparesthem with the
1992 values.
The similarity in the number of 1992-1ikeslip events required to create the total offsetson the Homestead Valley
Fault, Northern CrossFault, and WesternSplaysuggests
they
and the stepoverstructureas a wholehavebeenbehavingas a
systemsince their inception. It seemsreasonableto assume
that when the Northern CrossFault ruptureswith the Home-
The approximatenumberof 1992-1ikeeventsthat haveoccurredon
eachfault segmentwas calculatedby assumingthe 1992 offsetsto be
"average"and dividingthe 1992slip into the cumulativeoffset.
aWithin the stepoverregion.
b15km northof thestepover.
steadValley Fault, the northernEmersonFault alsoruptures,
and slip transfer,in general,is as completeas it was in 1992.
Whether the northern Emerson Fault also always ruptures
with the HomesteadValley Fault is another matter, sincewe
believe its much larger total offsetsuggeststhat the Emerson
Fault is older and had a distincthistoryprior to inceptionof
the HomesteadValley Fault and the stepover.
Only the northernhalf of the EmersonFault rupturedduring the 1992 earthquake. The half that lies south of the
stepoverhasbeen activein the late Holocene(D. P. Schwartz,
personalcommunication,1994) but did not rupture in 1992.
We suggestthat early on, before the initiation of the HomesteadValley Fault and the stepover,the southernand northern
portionsof the EmersonFault must have formed a solitary,
throughgoingfault. More recently, perhaps in responseto
changingstressfieldsor overall rotation of the Mojave block
[Dokka and Travis,1990a;Nur et al., 1986], the Homestead
Valley Fault developed.This fault then siphonedall or a portion of the sliprate that had been carriedby the southernhalf
of the Emerson Fault. Thus the long-term slip rate of the
EmersonFault north of its juncturewith the HomesteadValley Fault would now equal the sum of the slip rates of the
southernEmerson and the Homestead Valley Faults.
This structuralevolution,the partitioning of slip rates between the two faults,and the 1992 slip valuessuggestseveral
scenariosfor the long-terminteractionof the HomesteadValley and EmersonFaults.One possibilityis that the Homestead
Valley Fault and northern half of the Emerson Fault usually
fail in a singleearthquakesequence,as they did in 1992. Paleoseismicdata do indicatethat the last prehistoriceventson
both faultswere within a thousandyearsof each other in the
early Holocene [Rubin and Sieh, 1993; Hecker et al., 1993;
Rockwellet al., 1993;D. P. Schwartz,personalcommunication,
1994;T. K. Rockwell,personalcommunication,1994].If thisis
the case,then the HomesteadValley Fault shouldprovide3 m
of slip to the EmersonFault. However, in 1992, north of the
stepover,the EmersonFault had 6 m of slipor 3 m that did not
come from the HomesteadValley Fault. Obviously,this slip
distributioncannotcontinueindefinitelywithout producinga
slip deficit. Therefore we need another sourceof slip. One
possibilityfor that sourceis the southernsegmentof the Emerson Fault. Thus interspersedbetween earthquakesof the
Landers-1992typewouldbe earthquakeson the SouthernEmersonFault only.A typicalslip functionfor a southernEmerson event might then exhibit a northward decreasein slip
ZACHARIASEN
AND SIEH: SLIP TRANSFER
15
:
ß
,
12
Recurrence
penultimate.
EF-HVFevent
••
knowledgeof the averagerecurrenceintervalwould enable
one to determinehow long the HomesteadValley Fault has
been active.Sauberet al. [1994],usinggeodeticdata to comparethe coseismic
strainreleasein the Landersearthquake
to
.
:
.
i
.
ß Transfer
Emerson Fault '
ß
ß
Zone
Homestead
ß Valley Fault
the interseismicstrain accumulationin this region,estimatea
recurrence interval of 3500-5000 years for Landers-type
events.This is markedlyshorterthan recurrenceintervalsin-
15
12
1992 L.andersevent
'
ß
-9
ß
ß
3
N. •nd S. Emersonevent
ß
,
'•'""'",,,,.,,,,•__.[
S.Emeison
event
ß
.
EF-H.VFevent
.
ß
.
i
.
ß
Emerson
Faulti
Homestead
Transfer
Zone
:
Valley Fault
18
ß
future
SiEmerson
event
ß
.
1992 Landers event
15
:
ß
•
Intervals
ß
ß
•
15,299
If indeedthe 1992 Landersearthquakeis a "typical"event,
S.Emerson
event
ß
EF-H.VFevent
c
FAULTS
steadValley eventstwiceas often asEmersoneventswithout
involvingthe SouthernEmersonat all. However,giventhe
paleoseismological
evidence
thatthe southernEmersonis still
active(D. P. Schwartz,personalcommunication,
1994),we
preferto includeit in our favoredscenarios.
18
1992 Landers event
BETWEEN
12
HVF.:event
ß
_
ferred from paleoseismic
studiesof individualfaults in and
near the stepover.Paleoseismic
studieson the Homestead
Valley Fault southof this studyarea by Heckeret al. [1993]
revealtwo faultingevents,one 5700-8500 yearsago and one
12,500-14,000yearsago.If 7000 yearsis a typicalrecurrence
interval,then it impliesthat the HomesteadValley Fault has
been activefor about 1 m.y. This is consistentwith an age of
1.5-0.7 m.y.for the seriesof NW strikingfaultsthroughoutthe
Mojaveblockdetermined
bycrosscutting
relationships
[Dokka
and Travis,1990b].Paleoseismic
studiesby Rubin and Sieh
[1993]in the playaon the EmersonFault in thisarea(Figure
3c) likewiseindicatetwopreviousevents,oneabout9000years
agoand the otherbetween14,800and 24,000yearsago.The
EmersonFault agesare slightlyolderthan HomesteadValley
Fault agesbut stillimplyan ageof about1 m.y.for the HomesteadValley Fault.
ß
N. and S. Emerson event
ß
ß
,
•'-',,,.,•...' S.Emeison
event
ß
similarto that of the EmersonFault, 0.2-0.6 mm/yr[Rubinand
.
EF-HyF event
ß
ß
,
!
:
Emerson Fault '
ß
Accordingto thesepaleoseismic
studies,the Homestead
ValleyFault sliprate of 0.4-0.6 mm/yr[Heckeret al., 1993]is
.
Transfer
Zone
Homestead
ß
Valley Fault
Figure 12. Plausiblescenarios
of earthquake
recurrence
and
slipin the Homestead
Valley-Emerson
Faultstepover
region.
(a) Earthquakes
alternatebetweenLanders-type
eventsthat
rupturethroughthe stepover,
andeventson the southern
EmersonFault,whichdid not rupturein 1992.(b) Earthquakesas
in Figure 12a but alternatingwith eventsthat rupture the
Sieh,1993],yet the cumulativeoffseton the EmersonFault is
an order of magnitudehigher than that on the Homestead
Valley Fault. This similarityof modern rates supportsour
previoussuggestion
that the EmersonFaultactedaloneuntil
the HomesteadValley Fault wasborn.
The aboveexerciseis usefulfor placinggeneralconstraints
on the ageandactivityof the faultsin thisarea.On the other
hand, if this earthquaketeachesus anything,it is that we
shouldbe waryof assuming
characteristic
andrepetitivefaulting
behavior.
Patterns
in
space
are
clearly
complicated
and
southern and northern sections of the Emerson Fault and
irregular,
so
there
is
no
reason
to
expect
patterns
in
time
to
be
bypassthe HomesteadValley Fault. (c) Variety of rupture
combinationsoccurring: Landers-styleand full Emerson anydifferent.We cansay,however,on the basisof large-scale
events and Southern Emerson or Homestead Valley events offsets,that while the detailsof timing may elude us, rupture
patternsdo repeatoverlongperiodsand are responsible
for
only.
creatinglarge-scale
geomorphicand tectonicfeatures.
wherethe fault boundsthe easternflank of the stepover(Fig-
ure 12a).The frequencyof eventson the southernEmerson
Fault andthe averageoffsetsassociated
with themwoulddependon the averagerate of slipon the southernhalf of the
fault andon the strengthof the fault.None of thesequantities
is known,althoughwe use3 m per eventfor illustrativepurposesin Figure12. Otherpossibilities
includethe ruptureof
the entireEmersonFaultnorthof the stepoverfailsalternating
withLanders-type
eventsandSouthernEmersonevents(Figure 12b), and eventsin whichthe HomesteadValley Fault
ruptures
aloneaftera fullEmerson
Faultrupture(Figure12c).
Finally,the slip discrepancy
betweenthe HomesteadValley
and EmersonFaultscouldbe accommodatedby havingHome-
Summary and Conclusions
We measured surface offsets in the area of the Landers 1992
rupturewheretheHomestead
ValleyFaultstepped
overto the
EmersonFault. We found that thesetwo primaryfaultswere
linkedby a seriesof obliquelystrikingright-lateralstrike-slip
faults. This stepoverstructureappearsto be a developing
strike-slipduplexin whichthe northernmost
crossfault is the
most mature transfer structure, and the other crossfaults are
less mature. The secondaryfaults and rotationseffectively
transferredall the slipfrom the HomesteadValleyFault to the
Emerson Fault. The stepoverdiscontinuitymay have presenteda smallimpedimentto ruptureovera 5-speriodbut did
15,300
ZACHARIASEN
AND SIEH: SLIP TRANSFER BETWEEN FAULTS
not prove to be an impedimentfor the statictransferof slip.
The observationthat slip distributionon the crossfaults is
asymmetricleadsus to speculatethat the asymmetrymay indicate the source of the stresses which induced failure
on the
faults in the stepoverand may correlatewith propagationdirection,with slip decreasingin the directionof rupture propagation.Using seismological
modelsof rupture evolution,we
proposea scenariofor rupturewithin the stepover,and speculate that possiblygeologicalobservations
of surficialslip can
provideinsightsinto rupture dynamics.
Finally,we comparelong-termbedrockand geomorphicoffsetsalongthe faultsin the stepoverregionto 1992offsets.The
HomesteadValley Fault and severalsmallerfaults showevidenceof havingbeen activethrough100-200 nominalLanders
events.This, in turn, indicates that the crossfaults have effec-
tively transferredslip acrossthe stepoversinceits inception
and that the stephasnot presentedan impedimentto rupture.
The Emerson Fault, which has an order of magnitudegreater
total offset,appearsto be a much older fault than the other
faults making up the stepover.On the basisof preliminary
paleoseismological
studiesof recurrenceintervalson the faults
in the area, theselong-termoffsetsindicatethat the faults in
the area have been active of the order of one to a couple of
million years.
sin ((9) = v/D = 36/204
so(9 = 10ø;i.e., the trend andplungeof the slipvectormustbe
N35øW/16øNand the strikeand dip of the NCF mustbe NSøE/
10øN.
Similar calculationselsewherealong the NCF yield even
shallowerfault dipsand slipvectorplunges,e.g.,midwayalong,
where the offsetsare 120 cm right lateral, 21 cm east sideup,
the dip and plunge are 8ø and 12ø respectively.Given that at
thispoint the fault cutsacrosssteepterrainwith a topographic
slopeof greaterthan 20ø, suchfault dipsare clearlyimpossible.
We concludethen that the slipvector azimuthof the NCF is
not in fact parallel to the HVF.
Acknowledgments.This studywas funded by the Jet Propulsion
LaboratoryDirector's DiscretionaryFund and the SouthernCalifornia
Earthquake Center/National Science Foundation grant 1EAR8920136.Ken Hudnut, ScottLindvail,SallyMcGill, CharlieRubin, and
Jim Spotilahelpedcollectseveralof the offsetdata in the daysimmediatelyfollowingthe earthquake.We thank the 1992Caltechadvanced
fieldgeologyclassfor itshelpin mappingthe bedrockof the SplitHills,
Egill Haukssonfor discussions
of aftershockdata,Dave Wald andTom
Heaton for a stimulatingpaperand conversations
aboutLandersrupture dynamics,and SteveWard for his modelingof triggeredslip on
secondaryfaults.Greg Beroza'sand Doug Morton'sthoroughreviews
inspiredsignificantchangesin the original manuscript.Finally, we
thankAnne Lilje for her work in compilingour Landerssurficialoffset
measurements into a coherent database. Contribution 5513, Division
Appendix
of Geologicaland PlanetarySciences,California Institute of Technol-
With reference to Figure 13, use the offset measurements
ogy.
rl = 154 and v = 36. The strike of NCF is NSøE, and we
assumethe slipvectorazimuthis N35øW,parallel to the strike
of the HVF.
Then
a = 40 ø. Thus
h = (rl)tan (a) = (154) tan (40) = 129
tan (4•) = v/h = 36/129
so&=
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16ø
b = v/sin (4•)= 134
tan (/3) = b/rl = 134/154
soD=
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