Moment Tensor Solutions of the 1994 to 1996 Double Spring Flat,
Nevada, Earthquake Sequence and Implications for Local
Tectonic Models
GENE A. ICHINOSE1,
KENNETH D. SMITH,
and
JOHN G. ANDERSON1
University Nevada Reno Seismological Laboratory
Mackay School of Mines
1Also at the University Nevada Reno, Dept. Geological Sciences
Mail Stop-174
Reno, NV, 89557-0141
phone (702) 784-4265
fax (702) 784-1833
email ichinose@seismo.unr.edu
url http://enigma.seismo.unr.edu
Bulletin Seismological Society of America, Vol 88. No. 6, pp. 1363-1378, December 1998
Abstract
The September 12, 1994 Mw 5.8 Double Spring Flat, Nevada earthquake initiated
at the intersection of a northeast and northwest striking set of conjugate
faults within an overlapping zone between the Genoa and Antelope Valley faults zones,
of the eastern Sierra Nevadan range frontal fault system.
The mainshock ruptured on the northeast striking fault plane. Eight
days after the mainshock, the aftershock activity migrated from the mainshock
fault plane to the northwest striking conjugate fault. Over the next two years,
aftershocks migrated southward onto another set of conjugate faults and then
onto the Antelope Valley fault zone. The focal mechanisms of seventeen M > 4
aftershocks were estimated from a time domain moment tensor inversion using regional
broadband data. The T-axis (minimum stress direction) is oriented east-west
(N80°E to N100°E) for the (M > 4) events as is, commonly observed along the
eastern Sierra Nevadan range front in northwestern Nevada. From these results,
we make some general points that can be considered in seismic hazard assessment.
The maximum magnitude in overlapping normal fault zone is limited
to the size of the overlapping zone.
This makes small to moderate size (M<6) strike-slip earthquakes more likely than
large range-front (M>7) earthquakes. The seismicity within this overlapping zone may indicate
interseismic strain accumulation from east-west extension mainly through strike-slip
deformation. The apparent scarcity of modern normal faulting earthquakes along the Sierran
range front faults suggests a characteristic model, while a Gutenberg & Richter model
for the recurrence behavior of earthquakes applies to the overlap zone of the normal faults.
The pattern of seismicity and principle stress directions from
the aftershock fault plane solutions suggest a tectonic
model of changing fault geometry for the overlapping zone between
the Genoa and Antelope Valley fault zones. Two plausible long-term tectonic outcomes
may develop with this model: a normal fault growth model
where the overlapping segments of the Genoa and Antelope Valley faults
eventually become "hard-linked" (form a throughgoing fault), or a normal fault growth model
where the overlapping segment of the Genoa fault system
grows southward while the
Antelope Valley fault is isolated in the formation
of new basins and ranges.
Introduction
The Double Spring Flat earthquake (Mw=5.8) occurred at 12:23 GMT (5:23 AM PST)
on September 12, 1994. Its location was 30 km south of Carson City, Nevada, and
it was felt strongly throughout the Reno-Carson City region (pop. approximately 400,000).
The damage was fortunately light because the
epicentral area was not inhabited. This paper reports
on the locations and mechanisms of the earthquake and its aftershock sequence, and seeks
to understand the role this sequence plays in the
regional tectonics. The study is also
important because of its impact on understanding the local seismic hazard.
The 1994 Double Spring Flat (DSF) earthquake
(dePolo et al., 1994; Ramelli et al, 1994) occurred within the
overlap of two major range bounding faults,
the Genoa and Antelope Valley fault zones, and it has increased the concern for a major
earthquake near the population centers of northwestern Nevada.
The Antelope
Valley and Genoa fault zones are part of the Sierra frontal fault system, which are capable
of an M 7.5 to 7.8 earthquake (Ryall and VanWormer, 1980).
Although not associated with active volcanism or
geothermal centers, the 1994 initiated cross-fault triggering (Ichinose et al., 1997b)
similar to the 1987 Superstition Hills earthquake (Hudnut et al., 1989). The earthquake
sequence has included seventeen M 4 or larger aftershocks in the two
years following the mainshock
and the seismicity has migrated nearly 20 km to the south of the mainshock area
along several pairs of conjugate faults.
Migration and clustering of seismicity have also been observed for large earthquakes in
the Central Nevada Seismic belt, like the 1954 Fairview Peak, Dixie
Valley, Stillwater, and Rainbow Mountain earthquakes (Doser, 1986) and may perhaps suggest that
clustering and migration is the result of triggering by static or dynamic stress
changes (Ichinose et al., 1997b; Jaume, 1998, in preparation) or by viscoelastic
response from the lower or upper crust (Wesson, 1987; Scholz, 1977).
Regional Tectonic Setting
The earthquake occurred in the Walker Lane belt (WLB), described by Stewart (1988)
as a broad northwest trending zone of diverse topography and a combination
of strike-slip and normal faulting. The WLB is located
east of the Sierra Nevadas and west of the central Basin and Range province.
It is about 700 km long and about 100 to 300 km wide, and is characterized
by Quaternary faults in a complex distribution of deformation
extending from the Garlock fault northward into northeastern California (Fig. 1a).
The Inyo-Mono Section, in the southern WLB, contains major strike-slip faults
including the Owens Valley, Death Valley-Furnace Creek, and Fish Lake Valley fault zones
of the northern portion of the Eastern California Shear Zone, where 9 to 23% of the
dextral shear along the Pacific-North
American plate boundary is accommodated (Dokka and Travis, 1990).
A slip rate of 6-12 mm/yr (Dokka and Travis, 1990; Dixon et al., 1995) is accounted for
through the Eastern California Shear Zone.
The Excelsior-Coaldale section
is characterized by east-west striking faults with possibly recent left-lateral
and dip slip motion (Doser, 1988) differing in structural grain from the Inyo-Mono
Section to the south and the Walker Lake Section to the north.
The Walker Lake Section of the central WLB
includes predominantly north-northwest striking range front faults east of the Sierran range front
to approximately the Wassuk range and Walker Lake whereas right-lateral strike-slip faults
lie east of the Wassuk range. These northwest striking
strike-slip faults extend in a narrow band from Mina, Nevada to the Carson Section
along the eastern edge of the WLB.
The northeast oriented faults in the Excelsior-Coaldale section
may represent the transfer of slip from the Inyo-Mono
section eastward, to the northwest oriented strike-slip faults in the Mina area (Figure 1).
The structural grain of the Carson Section is similar to the
Excelsior-Coaldale section but differs from sections to the north and south in that
it does not have any mapped right-lateral strike-slip faults,
which occur in the adjacent Walker Lake and Pyramid Lake Sections.
Further northward,
right-lateral strike-slip faults reappear in the Pyramid Lake section and
extend north-northwestward from the Carson Section
through the Pyramid Lake fault zone (Slemmons et al., 1965)
and then northwestward several hundred km into northern California.
A combination of sub-parallel strike-slip and extensional faulting
in the Walker Lake Section (from the latitudes of Reno to
Walker Lake) may operate
under one net extension direction and thus be explained by a slip partitioning model
(Wesnousky and Jones, 1994; Oldow, 1992) rather than by a spatially and/or temporally
varying stress field.
Slip partitioning has been described as a mode of deformation in
the southern WLB by Wesnousky and Jones (1994) for the Owens Valley and
Independence fault and has been proposed in kinematic models for the
central WLB deformation (Oldow et al., 1994). The
model is motivated by the need to explain the proximity
of both strike-slip and normal faults with similar strike orientations.
Faults of the Walker Lake Section north of the Mina,
Nevada, area strike sub-parallel to the major normal fault systems to the west.
Right lateral strike-slip motion in the WLB is
consistent with northwest directed oblique extension, whereas the orientation of the
normal fault systems in the Walker Lake Section is consistent with east-west directed
extension.
Based on the stress regime, the Walker Lake Section can be
regarded as a separate domain, isolated between the Sierran block and Basin and Range block.
In this context, the displacement field of the Walker Lake Section
can be considered to be partitioned between strike-slip motion along the eastern edge
of the Walker Lake Section and predominantly normal displacement in the western Walker Lake Sec
Whether the sum of these two displacement fields accounts for the northwest
migration of the Sierran block (Wright, 1976) may only be resolved with continued
deformation monitoring with GPS. In this interpretation, the normal fault systems
in the Walker Lake Section would be accommodating displacement, in an east-west sense, not
accounted for in the transfer of sinistral strike-slip motion through the
Excelsior-Coaldale Section (Oldow et al., 1994). In other words,
the major normal fault systems of the
Walker Lake Section may represent the collapse of the central eastern Sierran block in
response to a large conjugate shear system developed to accommodate slip transfer
through the Excelsior-Coaldale Section and Walker Lake Section.
At latitudes near the Double Spring Flat earthquake,
and longitudes west of Walker Lake, the Quaternary faults are
generally north-south striking major range bounding fault systems with
predominantly normal displacement.
Some of these Sierra range front
normal faults show as much as 11 m of Holocene offset (Ramelli et al., 1996; 1997).
However, focal mechanisms of small to moderate sized earthquakes in this region
have shown right- or left-lateral strike-slip motion on high angle
faults with a generally east-west oriented extension direction
(VanWormer et al., 1980; Rogers et al., 1991; Horton et al., 1997; Figure 2).
This differs from mechanisms along the eastern WLB, that show a N55°W extension
direction (Zoback and Zoback, 1980).
VanWormer and Ryall (1980) and Rogers et al. (1991)
describe the western region of the WLB as the "Sierra Nevada Great Basin
Boundary Zone" because of the differences in the local extension direction.
Figure 1. (a) A shaded relief map of eastern California and western Nevada
showing the boundaries of the Walker Lane Belt (Stewart, 1988) in bold
dashed lines. Normal faults are shown in thin solid lines and strike-slip
faults in dash-dotted lines. We only label five sections of Waker Lane Belt
(Stewart, 1988). Physiographic features: PL-Pyramid Lake, PLFZ-Pyramid Lake fault zone,
DVFZ-Dog Valley fault zone, LT-Lake Tahoe, GFZ, Genoa fault zone,
AVFZ-Antelope Valley fault zone, FLVFZ-Fish Lake Valley fault zone,
NDV-FCFZ-Northern Death Valley-Furnace Creek fault zone, OVFZ-Owens
Valley fault zone, ML-Mono Lake, WL-Walker Lake, LVC-Long Valley Caldera.
One tectonic problem involves explaining the small to moderate size
earthquakes that contribute to the seismicity database of the western and
central WLB (Fig. 1b) in the presence of large range bounding faults. These earthquakes
are generally concentrated between the end segments
of predominant range bounding faults, and show mostly
strike-slip motion (e.g., Horton et al., 1997), except for aftershocks
of 1954 and 1932 earthquakes. VanWormer and Ryall, (1980)
discussed two examples of concentrations of activity at the ends of the Genoa and
Dog Valley fault zones near the Reno area. These may be the expressions
of transitions, or accommodation
zones, which are hypothesized to transfer extension (Wright, 1976) between the range
bounding faults along the central Sierra (Bursik and Sieh, 1989). In
this paper this idea will be examined in the context of the Double Spring Flat earthquake.
Another tectonic problem involves accounting for all of the right-lateral strike-slip
motion through the central WLB. According to VLBI results (Dixon et al., 1995), about
1 cm/yr of the Pacific-North American plate motion takes place east of the Sierra
Nevadas. North of our study region, the Pyramid Lake
fault zone, within the Pyramid Lake Section has a possible
32 km of right lateral slip (Bonham, 1969; Fontaine, 1997).
South of our study region,
the eastern part of the Walker Lake Section shows a cumulative
right-lateral slip of about 48 km (Slemmons et al., 1979). Even farther
south, there may be as
much as 40 to 50 km of right-lateral motion accounted for along the Fish Lake Valley,
Furnace Creek, and Owens Valley fault systems
(Stewart, 1988; Reheis and McKee, 1991), in the Inyo-Mono Section. In contrast,
the Carson Section, adjacent to the Sierra Nevadas
at the latitudes studied in this paper, lack mapped throughgoing
strike-slip features with recent Quaternary offsets that are large enough
to account for the approximately 30 to 50 km of strike-slip deformation
(Oldow et al., 1994; Oldow, 1992; Hardyman and Oldow, 1991).
Figure 2.
Earthquakes of (M > 3.5) from 1860 to 1997
plotted at the same map scale as Figure (1a). Focal mechanisms are from preliminary
moment tensor inversion for (M > 4) from 1997 in Nevada and
eastern California.
Data
The DSF sequence was recorded by the
Western Great Basin Seismic Network operated by the
University Nevada Reno Seismological Laboratory (Fig. 2). Seven portable
recorders were deployed for two months following the mainshock
to obtain additional phase and
waveform data. Phase data from Calnet stations in California were used in the
fault plane solutions and regional broadband data from the 14 stations of the
Berkeley Digital Network were used in the
moment tensor inversion
(Romanowicz et al., 1992, 1994). These stations were located in northern and central
California and at Mammoth Lakes, California (Fig. 2).
These regional broadband stations are equipped with 3-component
Streckeisen STS1 or STS2 type broadband seismometers sampling at 20 sps.
The DSF earthquake sequence occurred within the
overlap of two major range bounding faults. The earthquake
sequence has included seventeen M 4 or larger aftershocks in the two
years following the mainshock
and the seismicity has migrated nearly 20 km to the south of the mainshock area.
This southward migration has been predominantly into the footwall
of the Antelope Valley fault zone (Ichinose et al., 1997a);
with the exception being one normal faulting
M 4 event (other than those near or on the mainshock plane) that
occurred on the northern end of the Antelope Valley fault. Table 1 lists the
20 largest events of the DSF sequence through December 1996. The hypocentral
coordinates and fault plane solutions of these earthquakes have been derived
from travel time data and first motions from the local short-period
network supplemented by six temporary digital recorders and by
moment tensor inversion (Dreger and Helmberger, 1993) using regional broadband data.
Figure 3.
University Nevada Reno,
Berkeley Digital Seismic Network, and Terrascope broadband
station locations. Short period station locations were used in
relocations and first motion fault plane solutions.
Earthquake Relocations and First Motion Focal Mechanisms
We used an iterative scheme to constrain a 1-D crustal velocity model using
phase data from portable and network stations shown in
Figure 3. In the first step, the shallowest
layer of the model was adjusted according to arrival times
at three near source portable stations from a small number of well recorded
aftershocks.
More distant stations were subsequently added to constrain deeper layer velocities
while the layer thicknesses were kept constant.
The layer velocity which produced the lowest residual after relocation
was applied to that layer. Only events with good station coverage were used
in this process. The final velocity model (Table 2), with associated station delays,
was then used to relocate events from September 1994 to 1997.
We selected 581 of the highest quality events, with horizontal errors less than 1.0
km and
vertical errors less than 2.5 km, to compare orientations of first motion fault plane solutions
with structures defined by the hypocentral distribution.
The fault plane solutions were determined with FPFIT (Reasenberg and Oppenheimer, 1985)
and only include
events with 15 or more first motions (Fig. 5). The relocated events and focal
mechanisms of M > 4 events are shown in Figure 5a. The apparent fall-off in activity toward
the southeastern end of the aftershock zone, near
Topaz Lake from, 1995 to 1997
(Fig. 4a) is the result of uneven station coverage following removal of the portable
instruments, which in turn limited the detection threshold and location quality of
later smaller events.
Figure 4.
Snap-shots of UNR catalog seismicity from pre-1994 and the evolution of the Double Spring Flat
sequence to 1997. The panels from 9/12/1994 to 10/01/1997 show more
clearly the southward migration of activity along sets of strike-slip
conjugate faults between the overlapping normal fault segments. See Figure 5
for fault zone abbreviations.
Figure 5.
(a) Relocated seismicity of the 1994 Double Spring Flat earthquake
sequence. Focal mechanism are estimated from first motions and
plotted by lower hemisphere projection.
The mechanisms are numbered according to table 1.
Major faults: Genoa fault zone (gfz), Eastern Carson Valley fault zone (ecvfz),
Slinkard fault (sf), Antelope Valley (avf), East Tahoe fault (etf), Waterhouse
Peak fault (wpf), Wabuska lineament (wl), Double Spring Flat fault zone (dsffz),
and Smith Valley fault zone (svfz).
(b) Cross section A-A'.
Figure 6.
First motion focal mechanism solutions plotted on lower hemisphere
equal angle stereonets. The mechanisms are numbered according to table 1.
Event 13 has a multiple solution.
Table 1
Source Parameters of 20 largest events in the sequence through 1997
---------------------------------------------------------------------------------------------
# Date Latitude Longitude Z1 Z2 Md Mw Strike- Dip Rake %dc
y-m-d o o km km o o o
---------------------------------------------------------------------------------------------
1 1994-09-12 38N48.88 119W38.65 6.4 10 5.9 5.8 48 82 -10 97
139 82 -170
2 1994-09-12 38N49.39 119W38.84 5.2 4.2
3 1994-09-12 38N47.02 119W40.00 7.3 4.7
4 1994-09-13 38N45.90 119W41.35 6.3 12 3.9 4.0 44 88 14 93
313 76 178
5 1994-09-13 38N46.94 119W36.74 7.9 10 3.7 3.8 40 87 -7 96
130 87 -173
6 1994-09-13 38N50.51 119W36.97 6.2 16 4.0 4.0 43 68 25 85
303 67 156
7 1994-09-20 38N45.36 119W35.76 9.5 8 3.9 4.4 235 87 -3 95
325 87 -173
8 1994-09-20 38N45.51 119W35.13 7.4 4 4.1 4.4 230 84 -33 96
324 57 -173
9 1994-10-10 38N46.15 119W35.99 7.1 4.4
10 1995-01-06 38N47.01 119W39.52 5.4 4 4.1 4.1 47 86 -30 97
139 60 -175
11 1995-02-18 38N47.26 119W39.69 9.1 6 4.0 4.0 21 61 -59 73
150 41 -133
12 1995-04-22 38N47.40 119W40.85 3.3 8 4.4 4.2 9 54 -78 93
169 38 -107
13 1995-12-22 38N44.06 119W35.43 9.7 18 4.7 4.8 35 80 -24 73
130 66 -169
14 1995-12-23 38N44.78 119W35.37 9.0 14 4.6 4.7 224 87 -19 90
315 71 -177
15 1995-12-28 38N43.63 119W36.68 11.7 16 4.9 4.7 225 67 5 83
133 86 157
16 1996-01-02 38N44.80 119W35.50 9.6 10 4.0 3.7 4 61 -69 85
146 35 -123
17 1996-03-09 38N43.28 119W36.24 7.9 10 4.2 3.8 29 76 -22 94
125 69 -165
18 1996-06-27 38N38.07 119W28.24 14.3 14 4.3 4.3 341 51 -118 91
202 47 -59
19 1996-12-12 38N40.42 119W31.42 7.6 6 4.4 4.4 230 85 -19 90
321 71 -175
20 1996-12-13 38N40.20 119W30.36 7.7 6 4.2 4.1 227 80 -14 92
320 76 -170
---------------------------------------------------------------------------------------------
Z1 is the relocated hypocenter depth and Z2 is best depth from
moment tensor inversion.
- The first nodal plane is the preferred fault plane.
Moment Tensor Inversions
Fault plane solutions and seismic moments were estimated using three component records
at regional distances (approximately 100 to 400 km) from multiple broadband stations
in a full waveform time domain moment tensor inversion (Dreger and Helmberger, 1993).
The moment tensor inversion procedure is constrained to only the deviatoric tensor
and neglects any volumetric changes of the source.
The data are inverted in the 10 to 100 seconds band but the signal
to noise ratio varies within this band with the size of the earthquake.
At regional distances, the source dimension is much smaller
than the wavelengths modeled. The source
time function is simplified to a delta function so that lower order faulting
processes can be observed without any higher order dynamic effects.
The objective function in the inversion from Pasyanos et al. (1996) is;
fit = RMS(data-synthetic) / % double couple,
where the RMS residual is modulated over the percent double couple.
The percent double couple (%dc) component
is determined from the isotropic, major double-couple
and compensated linear vector dipole terms (Table 1).
A frequency-wave number integration code written by Saikia (1994)
is used to compute Greens functions from simple 1D velocity models.
The sensitivity of the moment tensor solutions was tested by
using a local velocity model defined in the relocation procedure
and a regional velocity model routinely used
for southern California earthquake locations (Table 2). The use of long period
energy avoids the need for modeling complex crustal structure. The
large epicentral distances usually allows the use of simple 1D velocity
models because the travel paths for long-period phases like
$P sub n$ and $S sub n$ are in the lower crust and upper mantle,
most likely, along laterally homogeneous material.
Table 2
Local velocity model used in hypocenter relocations and moment
tensor inversions
------------------------
a Depth p
(km/s) (km) (g/cc)
------------------------
3.20 0.0 2.2
5.50 1.0 2.4
5.55 3.0 2.75
5.80 5.0 2.75
6.00 10.0 2.75
6.80 35.0 2.8
7.80 40.0 3.32
------------------------
Southern California Velocity Model (Dreger and Helmberger, 1990)
------------------------
a Depth p
(km/s) (km) (g/cc)
------------------------
5.5 0.0 2.4
6.3 5.5 2.67
6.7 16.0 2.8
6.8 37.0 3.0
------------------------
This type of moment tensor inversion does not solve for the centroid location.
Horizontal mislocations up to 15 km can be accounted for by shifting
the synthetics and absorbing travel-time errors by aligning the
synthetics with the data. The regional body
waves are sensitive to the vertical
mislocations because of changes in relative amplitudes of
the up and down going energy at the source,
although changes in the relative amplitudes of these
waves can help in determining source depth by an iterative process
(Dreger and Helmberger, 1993). A search for the optimal depth is done by
using Greens functions computed at 2 km depth increments.
The moment tensor solution depth with the lowest
variance is selected as the source depth. In cases of poor station coverage
or noisy data, a minimum variance point might not exist, in which case a
minimum or maximum depth can only be resolved.
Moment Tensor Inversion Results
Figure 7 shows the moment tensor solutions for the 12 September 1994 mainshock
and the 27 June 1996 event that we interpret to be on the Antelope
Valley fault (events 1 and 18 in Table 1 and Figure 7).
Typical waveform fits
are also shown along with the depth-variance curves. The source depths resulting
from the waveform modeling procedure are within the 2.5 km vertical error
for the depths estimated from the travel time data for these two events.
The fault plane solutions in Table 1 report both planes, with the first nodal plane listed
representing the likely fault plane
based on the geometry of the aftershock
distribution or on geologic constraints.
Figure 7.
Two examples of time domain moment tensor inversion with
waveform fits from regional stations and their depth-variance
curves for finding best fit hypocenter depths.
Double Spring Flat mainshock (event 94255122341) and
an event on the Antelope Valley fault (event 96179054902).
The moment tensor solutions were all found to be insensitive to the use of
the local or southern California velocity models. Variations in strike, dip, and rake
differed by less than 10° and
seismic moment estimates were within a factor of 2 for both model solutions.
We selected moment tensor solutions which gave the lowest variance from the two models.
For example, the well constrained M 5.8 mainshock shows
a small variation in source parameters
but a 4 km difference in source depth between the local model and
the southern California model. We feel that the moment
tensor solution is insensitive to path propagation
effects because of the high signal to noise ratio for the mainshock record.
The solution did not significantly differ in strike, dip, rake and seismic
moment when applying either velocity model.
The 1994 mainshock is the best example of source depth optimization
because a well defined minimum point exists in the depth-variance curve
at 10 km depth for the local velocity model. The southern California model gives a
higher residual and a greater depth (14 km) because this model is generally
faster. Some smaller size events do not have a minimum variance, which is most likely
due to the poorer signal quality at long periods for these events.
Three aftershocks consisted of double events separated by about 1
second and could not be modeled using a full waveform inversion. The first motion
fault plane solutions of the first event of these double events (events 2, 3, and 9)
are shown in Figure 5a.
Figure 8 shows the complete set of moment tensor solutions; these are
generally similar to the first motion solutions in Figure 5a except for
the solutions for events 5 and 14.
Moment tensor solutions for all but four of the M > 4 events show
strike-slip motion with a small normal-oblique component.
The orientation of the T-axis ranges between
N80°E to N100°E.
The east-west T-axis orientation is common along the Sierra Nevadas, but not
typical within the eastern WLB which exhibits
a T-axis of approximately N60°W (Zoback and Zoback, 1980; Rogers et al., 1991).
The moment tensor solution of the more recent
June 28, 1996 Mw 4.3 earthquake (event 18 in Table 1) indicates north-northwest
striking normal-oblique motion which is slightly different from its nearly
pure normal-slip first motion fault plane solution. Events 3, 11, and 12, within
the aftershock zone near the mainshock at the north end of the sequence,
also have normal-oblique mechanisms, indicating either shallow
north-south striking normal faults at depth above the more steeply
dipping mainshock fault plane or
normal-oblique slip events on the mainshock fault plane itself. More
confidence can be placed on fault plane
solutions determined from moment tensor inversions
over first-motion solutions because the entire waveform is being incorporated
to resolve the overall average faulting process
rather than using just the phase information which only resolves the
faulting process at the initiation of rupture. This might also be the
origin of source depth differences between the moment tensor inversions
and travel-time locations. It is possible for the rupture process to
initiate at a depth different than the overall centroid depth on the fault
surface. A large difference in source depth is not expected for small and
moderate sized earthquakes
so that these differences may be due to signal noise and lateral velocity variations
near the source region.
Figure 8.
Focal mechanisms from moment tensor inversion results
are numbered according to table 1. Dashed lines represent
estimated locations of block boundaries inferred from earthquake
locations and sense of slip given the geology and the
lineation of aftershock seismicity.
Interpretation of Faulting Processes
The mainshock occurred at the intersection of a set of N50°E and
N40°W striking conjugate
fault planes. The location of eight foreshocks along the northeast
striking plane, 3 days before
the mainshock, and the alignment of concentrated aftershock activity immediately following
the mainshock, provide evidence that the Mw 5.8 event occurred on the
N50°E striking structure. The mainshock first-motion focal mechanism (event 1 on Figure 5a
indicates mainly left-lateral strike-slip with a small component of normal-oblique slip
for the northeast striking nodal plane.
Over the next two year period, the earthquake sequence migrated to the
southeast generally along the northwest striking conjugate
right-lateral fault plane, and then further southward
onto a northeast striking plane parallel
to the mainshock plane. Figure 4 illustrates the progression of seismic
activity in time panels selected to best show the evolution of the
sequence.
The earthquake sequence has produced seventeen (Mw > 4.0)
events with eleven of these events occurring along fault systems other than
the northeast striking mainshock fault plane.
Three Mw 4.7 to 4.8 (events 13, 15, and 17 in Figure 5a) strike-slip
earthquakes, and associated aftershock activity, occurred in December 1995
on a fault 10 km south of the mainshock
and striking nearly parallel to the mainshock fault plane
near Holbrook Junction. In
December of 1996, Mw 4.1 and 4.4 earthquakes occurred
10 km south of Holbrook Junction beneath Topaz Lake (events 19 and 20).
The first motion fault plane solutions
and locations of related aftershock activity suggest the activation of an
additional northeast striking fault.
An Mw 4.3 normal faulting event on 27 June 1996 (event 18) appears to have
occurred at the northern end of the Antelope Valley fault at a depth of 14 km.
The Antelope Valley fault is of some importance to seismic hazard because it's
paleoseismic expression suggests recent Quaternary activity and its
approximate rupture length of 30 km
indicates a possible Mw 7.0 event (Wells and Coppersmith, 1994).
The initiation of a sequence at the intersection of conjugate faults has been observed
for other WLB earthquakes. For example, the M 5.8 1984 Round Valley (Priestley et al.,
1988), the M 6.2 1986 Chalfant (Cockerham and Corbett, 1987; Smith and Priestley, 1988), and
possibly the M 5.0 1978 Diamond Valley earthquake (Somerville et al., 1979; 1980)
occurred at the intersection of conjugate fault systems that were activated during
these sequences.
The DSF earthquake appears to have played a similar role in the
local tectonic framework, as did the 1978 Diamond Valley earthquake that occurred
about 10 km to the west. Somerville et al. (1980) also noted a southward migration
of aftershocks following the Diamond Valley earthquake
and Somerville et al. (1979) favored a N50°E striking fault
plane showing left-lateral strike-slip motion based on geologic observations but,
at that time, an actual fault plane was ambiguous. A
conjugate fault system seems to have been activated for Diamond Valley sequence, which can
be interpreted from the University of Nevada Reno
seismicity catalog (1978 to 1997). The M 6.0 1966 Truckee
(Tsai and Aki, 1970) and the M 5.0 1990 Lee Vining
earthquake (dePolo and Horton, 1991; Horton et al., 1997)
occurred on northeast trending structures within the northern and central
WLB, respectively, but these sequences may not have exhibited activity along
conjugate fault systems. The Lee Vining event may not have been large enough to
activate a fault system that could be imaged; and considering the state of monitoring
in the 1960's, the aftershock sequence of the 1966 earthquake may not have been well sampled.
Tectonic models
Deformation models can be evaluated with regard to the stress regime,
style of faulting, and expression of Quaternary faulting in the
DSF area and the central WLB. Three models are considered;
(1) a simple steady state block model, (2) a normal fault growth model
where the overlapping segments eventually become "hard-linked" (Trudgill and Cartwright, 1994),
(3) a normal fault growth model
where one overlapping segment increases in length while the other segment
is isolated and does not continue to extend its length and (4) vertical-axis block
rotation.
A simple steady state block tectonic model consists of normal
fault segments offset by transform faults, such as in
mid-ocean ridge environments. Figure 9 shows the hypothesis under which a transform
fault with a similar geometry connects the Genoa and Antelope Valley normal faults.
This block model implies that the
Sierra Nevada block is rigid and the Basin and Range block extends
eastward by simple extension. The focal mechanism for the DSF mainshock is
inconsistent with this
model, and thus the block model does not apply.
The faulting patterns from the DSF aftershock sequence and the
1978 Diamond Valley sequence show deformation
within a set of northwest oriented right-lateral and northeast oriented left-lateral
high angle faults rather than on a single through going strike-slip fault.
This block model is the only model in which the geometry of the
faults can be in steady state and thus since this model is rejected by the DSF
earthquake, the faults must have an evolving geometry.
Another model must be considered in which
strike-slip motion on high-angle faults, operating in east-west extension in
conjunction with range bounding faults, acts to modify the geometric relationships
between the normal faults.
The relationship between the Genoa and Antelope Valley fault zones could be
consistent with tectonic processes that would eventually "hard-link" these structures.
The faulting in the overlap zone, which may currently represent a transfer
of slip through extensional deformation by a "soft-link",
might eventually link by breaching the volume between the normal faults.
In continental extension, the overlap zone has been described by
some as relay structures (Larsen, 1988),
accommodation zones (Bosworth, 1985), fault bridges (Ramsey and Huber, 1987),
or strain transfer zones (Morley et al., 1990).
This model provides a possible explanation for the segmented features
observed in the relocated seismicity from 1994 to 1997
which appears to show this faulting pattern in the aftershock activity.
The mechanism for
transfer of normal displacement may occur by lateral extension across jointed
surfaces described by Trudgill and Cartwright (1994).
The topographic features
in the overlap zone between the Genoa and Antelope Valley faults
are also consistent with the presence
of intrabasin highs through the overlap zone and
down-dropped basins adjacent to the overlapping normal faults (Anders and Schlische, 1994).
The intrabasin highs are
possibly the result of along-strike deficits in normal fault displacement
(Anders and Schlische, 1994), which is consistent with a lower rate of deformation
occurring at the southern end of the Genoa fault (Ramelli et al, 1997); deficits
in normal slip near end-segments could leave high topography
in the overlapping zones.
This model implies
that the Genoa Fault is trying to link with the Antelope Valley
fault to form a nearly through going normal fault system.
As another alternative, the Genoa fault zone
may grow southward, into the Sierra Nevada block. In this model,
the older overlap zone may become
separated from the Sierra Nevada block and
raft away while newer overlap structures form and migrate southward,
similar to process of Overlapping Spreading Centers (Macdonald and Fox, 1983).
This second possible long term outcome occurs by continued growth of the Genoa fault zone
to the south leading to the creation of
a new isolated mountain range with the Antelope Valley fault on the east and
a new basin on the west.
The pattern of faulting in the Double Spring Flat region may not represent a long
term displacement configuration and, in this case,
present structures would have a relatively short lifetime.
We finally consider the block rotation as a tectonic model for the Double
Spring Flat region.
Paleomagnetic and structural analysis indicate horizontal rotations
of crustal blocks between parallel faults in the eastern California
shear zone within the Mojave Block (Dokka, 1992) and in the Pyramid Lake
and Carson sections of the Walker Lane (Fontaine, 1997).
Crustal block rotation requires parallel faults. The Sierran range
frontal fault system may not carry any right-lateral strike-slip, but it may suffice
to have strike-slip on one side, for example, along the Double Spring Flat fault
zone (Fig. 8). If so, as in Figure 9d, then block rotations may be responsible
for conjugate strike-slip features in the Walker Lane. This is not necessary
to explain the style of deformation in the Double Spring Flat region, and the
throughgoing strike-slip faulting has not been mapped in the area,
although there are some hints it may have been present (Vadurro, 1993).
Further paleomagnetic analysis
could rule out or support block rotations.
Cartoons of four models showing the current geometry and expected
slip of the normal faults as well as the eventual tectonic evolution
within the overlap zone between the Genoa Fault (GF) and Antelope Valley Fault (AVF)
The normal faults are shown in solid bold
lines with the ball on the down-dropped side. The +/- refers to the change
in topography due to slip transfer or block rotation within the overlap zone.
(a) A steady-state block tectonic model is shown with a fault geometry and expected
slip between an assumed
rigid Sierra Nevada (SN) block and Basin and Range (BR) block.
We can disregard the block tectonic model immediately because it has
an opposite sense of shear than shown by the 1994 Double Spring Flat
earthquake.
(b) The end segments of the normal faults within the overlap zone
eventually "hard-link" and may form a through going fault
(Anders and Schlische, 1994; Trudgill and Cartwright, 1994).
(c) The end segment of the Genoa fault may instead grow southward
and raft away the Antelope Valley fault. In this case, the overlapping
zone may also migrate southward.
Models shown in
panels (b) and (c) are more plausible long term outcomes. Clockwise
block rotations in
(d) may result from any past or present regional right-lateral strike-slip
between the Sierra Nevada block and the Basin and Range.
Implication for Seismic Hazard
Strike-slip earthquakes like Double Spring Flat, in the
overlap zone between offset normal faults, make up one component of the seismic
hazard in the Walker Lane. The maximum magnitude of this category of earthquakes would
be controlled by the diagonal distance between the normal faults. It may be
further limited by the jointed block pattern in the overlap zone, which in
the 1994 DSF case appears to have lengths ranging from 10-20 km between conjugate fault
intersections. The overlap zones may also limit the lengths of normal faulting
earthquakes, but since the normal faults between the overlap zones are generally
larger in size, the normal faulting earthquakes can be longer.
The concentration of small to moderate sized earthquakes in the overlap zones
along the Sierra Nevadan range suggest interseismic strain accumulation. Also,
the scarcity of normal faulting seismicity in the historical
catalog for these regions suggest a different recurrence behavior between the
range bounding fault systems and the overlap zones.
When resolvable, the focal mechanisms of small to moderate sized earthquakes (M<6) in the
central western Walker Lane belt over the period of seismic monitoring have shown predominantly
strike-slip motion (Horton et al., 1997; Vetter, 1984; Rogers et al., 1991). This
appears to be inconsistent with the presence of large range bounding normal faults that
show Quaternary and Holocene offsets and are capable of generating (M>7) earthquakes.
This observation may result from the different behavior of normal faults and
strike-slip faults in the central WLB. If the normal faults behave with characteristic
recurrence and the strike-slip activity is better characterized
by Gutenberg and Richter (1954)
recurrence relations, then the available focal mechanism data will be dominated by
small magnitude strike-slip
activity, in terms of numbers of events.
Also, the overlap zones may be characterized by short faults in a distributed zone resulting
in smaller maximum magnitudes and shorter recurrence intervals than the large
range bounding normal faults. Therefore,
characterizing the stress field based on the seismicity data
may result in models suggesting the horizontal orientation of the principle stresses,
whereas the moment release modeled through the seismic cycle will indicate primarily
east-west and vertical deformation from large normal faulting earthquakes.
Conclusions
We have relocated the seismicity of the 1994 Double Spring Flat
earthquake sequence, along with continuing seismicity through 1997, and estimated
fault plane solutions from first motions as well as from moment
tensor solutions for M > 4 events. There is general agreement between the
first motion and moment tensor results.
The moment tensor solutions are believed to be more reliable because
of the uncertainty involved in computing take-off angles used
for first-motion focal mechanisms. The fault
plane solutions and seismicity show mainly strike-slip
faulting occurring in a zone of deformation that appears to accommodate east-west
extension and the transfer of slip
between the Genoa and Antelope Valley faults through conjugate sets of high angle
strike-slip
faults. Assuming these small to moderate sized earthquakes are representative of the
deformation in this overlap zone, then they provide evidence that the fault
geometry is not stable and must change with time. Following Anders and Schlische (1994),
the two faults might eventually link, but another long-range
possibility is that the Genoa fault will grow towards the south into the
Sierra, eventually splitting off a new mountain range.
From these results, we make some general points which
can be considered in seismic hazard assessment. The maximum magnitude in overlapping
normal fault zones is limited to the
the size of the overlapping zone.
This makes small to moderate size (M<6) earthquakes earthquakes more likely than
large (M>7) earthquakes. The seismicity within
this overlapping zone may indicate interseismic
strain accumulation from east-west extension mainly through strike-slip deformation. The
apparent scarcity of modern
normal faulting earthquakes along the Sierran range front
suggests a different recurrence behavior between
earthquakes in the overlapping normal faulting zones
(Gutenberg & Richter model) and the Sierra Nevadan range frontal faults
(characteristic model).
Acknowledgments
We acknowledge Rasool Anooshehpoor, Steve Horton and others involved with instrument deployment
and spending time servicing the sites. We would like to thank Doug Dreger for the
TDMT inversion codes. We appreciate
discussions with Patricia Cashman and Michael Ellis on their kinematic model.
This paper has been improved from comments by Diane Doser, Suzette Payne, and
an anonymous reviewer.
This work was made possible through financial support
provided by U.S. Geological Survey NEHRP grant 1434-94-G-2479.
Draft 12-1-1997
Submitted 1-1-1998
Revised 4-23-1998
References
Anders, M. H., and R. W. Schlische (1994). Overlapping faults, intrabasin
highs, and the growth of normal faults, The Journal of Geology 102, 165-180.
Bonham, H. F. (1969). Geology and mineral deposits of Washoe and Storey
Counties, Nevada, Nevada Bureau of Mines and Geology Bulletin 70, 1-140.
Bosworth, W. (1985). Geometry of propagating continental rifts, Nature 315,
625-627.
Bursik, M. and K. Sieh (1989). Range front faulting and volcanism
in the Mono basin, eastern California, J. Geophys. Res. 94, 15587-15609.
Cockerham, R. S., and E. J. Corbett (1987). The 1986 Chalfant Valley,
California, earthquake sequence, Bull. Seism. Soc. Am. 77, 280-289.
dePolo, D. M., and S. P. Horton (1991). A magnitude 5.0 earthquake near
Mono Lake, California (abstract), Seism. Res. Lett. 62, 52.
dePolo, D. M., K. D. Smith, R. Anooshehpoor, C. J. dePolo, and K.
Priestley (1994). Double Spring Flat earthquake, western
Nevada, 12 September 1994 (abstract), EOS Transactions,
American Geophysical Union, special session.
Dixon, T. H., S. Robaudo, J. Lee, and M. C. Reheis (1995). Constraints on
present-day Basin and Range deformation from space geodesy, Tectonics 14,
755-772.
Dokka, R. K., C. J. Travis (1990). Role of the eastern California shear
zone in accommodating Pacific-North America plate motion, Geophys. Res. Lett.
17, 1323-1326.
Dokka, R. K. (1992). The Eastern California Shear Zone and its role in the
creation of young extensional zones in the Mojave Desert region, in
Structure, Tectonics and Mineralization of the Walker Lane, Ed. J. Stewart,
Walker Lane Symposium Proceedings Volume, Geological Soc. Nevada, Reno, NV, 161-186.
Doser, D. I. (1986). Earthquake processes in the Rainbow Mountain-Fairview Peak-Dixie
Valley, Nevada, region 1954-1959, J. Geophys. Res. 91, 12572-12586.
Dreger, D. S., and D. V. Helmberger (1990). Broadband modeling
of local earthquakes, Bull. Seism. Soc. Am., 80, 1162-1179.
Dreger, D. S., and D. V. Helmberger (1993). Determination of
source at regional distances with single stations or space
network data, J. Geophys. Res. 98, 8107-8125.
Fontaine, S. A. (1997). Structural and paleomagnetic analysis of the Pyramid
Lake domain, northern Walker Lane, west-central Nevada, M. S. Thesis,
Univ. Nevada Reno, Reno, NV.
Gutenberg, B. and C. F. Richter (1954). \fISeismicity of the Earth and Associated Phenomena\fR,
Princeton, University Press.
Hardyman, R. F., and J. S. Oldow (1991). Tertiary tectonic framework and
Cenozoic history of the central Walker Lane, Nevada, in
\fIGeology and Ore Deposits of the Great Basin\fR,
Ed. Raines, G. L., Lisle, R. E.,
Schafer, R. W., and Wilkinson, W. H., Reno, Nevada, Geological Society of Nevada
Symposium Proceedings, 279-301.
Horton, S. P., D. M. dePolo, and W. R. Walter (1997). Source Parameters and
tectonic setting of the 1990 Lee Vining, California, earthquake sequence,
Bull Seism. Soc. Am. 87, 1035-1045.
Hudnut, K. W., L. Seeber, and J. Pacheco (1989). Cross-fault triggering in
the November 1987 Superstition Hills earthquake sequence, southern California,
Geophys. Res. Lett. 16, 199-202.
Ichinose, G. A., K. D. Smith, J. G. Anderson, and J. N. Brune
(1997a). The seismotectonics and source parameters of recent
earthquakes near Reno, Nevada (abstract), in Basin and Range
Province Seismic Hazards Summit, Reno, NV, Western States
Seismic Policy Council, San Francisco, CA.
Ichinose, G. A., J. G. Anderson, and K. D. Smith,
Static stress changes caused by the 1978 Diamond Valley, California
and 1994 Double Spring Flat, Nevada earthquakes
(abstract), EOS Transactions, American Geophysical Union, Vol. 78, 1997.
Jaume S. (1998). Stress Transfer in the Central Nevada Seismic Belt, 1915-1954,
(in preparation).
Larsen, P. H. (1988). Relay structures in a lower Permian
basement-involved extensional systems, J. Structural Geology 10, 3-8.
MacDonald, K. C., and P. J. Fox (1983). Overlapping spreading centers:
new accretion geometry on the East Pacific Rise, Nature 302, 55-58.
Morley, C. K., R. A. Nelson, T. L. Patton, and S. G. Munn (1990). Transfer
zones in the East African rift system and their relevance to hydrocarbon
exploration in rifts, American Association of Petroleum Geologists Bulletin,
74, 1234-1253.
Oldow, J. S. (1992) Late Cenozoic displacement partitioning in the
northwestern Great Basin, in
\fIStructure, Tectonics and Mineralization of the Walker Lane\fR,
Ed. J. Stewart, Walker Lane Symposium
Proceedings Volume, Geological
Soc. Nevada, Reno, NV, 17-52.
Oldow, J. S., G. Kohler, and R. A. Donelick (1994). Late Cenozoic
extensional transfer in the Walker Lane strike-slip belt, Nevada, Geology,
22, 637-640.
Pasyanos, M. E., D. S. Dreger, and B. Romanowicz (1996).
Toward real-time estimation of regional moment tensors,
Bull. Seism. Soc. Am. 86, 1255-1269.
Priestley, K. F., K. D. Smith, and R. S. Cockerham (1988). The 1984
Round Valley, California earthquake sequence, Geophys. J., 95, 215-235.
Ramelli, A. R., C. M. dePolo, J. W. Bell, C. D. Henry, J. C.
Yount, S. J. Caskey, K. D. Adams, M. W. Stirling, T. W. Rennie
T. Adams, P. Rogers, J. R. Humphrey, T. L. Sawyer, and
W. D. Page (1994). Secondary surface fracturing associated
with the Double Spring Flat Earthquake of September 12,
1994, EOS Transactions, American Geophysical Union, Special
Section.
Ramelli, A. R., Bell, J. W., and C. M. dePolo (1996) Late Holocene
activity of the Genoa fault, western Nevada, Geol. Soc. Am. Abstracts
with Programs, 28, 103.
Ramelli, A. R. (1997). Guidebook and road log for earthquake hazards and geothermal
areas along the Carson Range, western Nevada, National Association of Geoscience
Teachers Fall Conf., S. Lake Tahoe, CA.
Ramsey, J. G., and M. I. Huber (1987). \fIThe Techniques of Modern Structural Geology\fR,
Volume 2, London, Academic Press.
Reasenberg, P. A. and D. Oppenheimer (1985). FPFIT, FPPLOT,
FPPAGE: Fortran computer programs for calculating and
displaying earthquake fault-plane solutions,
U.S. Geol. Survey, Open File Rep. 85-739.
Reheis, M. C. and McKee, E. H. (1991). Late Cenozoic history of
slip on the Fish Lake Valley fault zone, Nevada and California:
U.S. Geological Survey Open File Report 91-290, 26-45.
Rogers, A. M., S. C. Harmsen, E. J. Corbett, K. Priestley,
and D. dePolo (1991). The seismicity of Nevada and some
adjacent parts of the Great Basin, in \fINeotectonics of North
America\fR, Ed., D. B. Slemmons, Geol. Soc. Am.,
Decade Map Vol. I, 153-183.
Romanowicz, B., L. Gee, and R. Uhrhammer (1992). Berkeley digital
seismic network: a broadband network for northern and central
California, IRIS Newslett. XI, 1-5.
Romanowicz, B., D. Neuhauser, B. Bogaert, and D. Oppenheimer (1994).
Accessing northern California earthquake data via Internet, EOS 75, 258-261.
Ryall, A. S., and J. D. VanWormer (1980). Estimation of maximum magnitude and
recommended seismic zone changes in the western Great Basin, Bull. Seism. Soc. Am.
70, 1573-1581.
Saikia, C. K. (1994). Modified frequency-wavenumber algorithm for regional
seismograms using Filon's quadrature: modeling of $L sub g$ waves in
eastern North America, Geophys. J. Int. 118, 142-158.
Scholz, C. H. (1977). A physical interpretation of the Haicheng earthquake
prediction, Nature 267, 121-124.
Slemmons, D. B., A. E. Jones, J. I. Gimlett (1965). Catalog of Nevada
earthquakes, 1852-1960, Bull. Seism. Soc. Am. 55, 519-566.
Slemmons, D. B., D. VanWormer, E. J. Bell, and M. L. Silberman (1979).
Recent crustal movements in the Sierra Nevada-Walker Lane Region of
California-Nevada: Part 1, rate and style of deformation, Tectonophysics
52, 561-570.
Smith, K. D., and K. F. Priestley (1988). Foreshock sequence to the 1986
Chalfant, California earthquake sequence, Bull. Seism. Soc. Am. 78, 172-187.
Somerville, M. R., W. Peppin, and J. D. VanWormer (1979). Wideband
near-field observations of aftershocks of the Diamond Valley, California
earthquake of 4 September 1978 (abstract), EOS Transactions 60, p 874.
Somerville, M. R., W. A. Peppin, and J. D. VanWormer (1980). An
earthquake sequence in the Sierra Nevada-Great Basin Boundary
zone: Diamond Valley, Bull. Seism. Soc. Am. 70, 1547-1555.
Stewart, J. H. (1988). Tectonics of the Walker Lane belt, western Great Basin:
Mesozoic and Cenozoic deformation in a zone of shear, in \fIMetamorphism
and crustal evolution of the Western United States\fR, Editor, W. G. Ernst,
Ruby Vol. VII, Prentice-Hall, Engelwood Cliffs, New Jersey, 683-713.
Trudgill, B., and J. Cartwright (1994). Relay-ramps forms and normal-fault linkages,
Canyonlands National Park, Utah, Geol. Soc. Am. Bull. 106, 1143-1157.
Tsai, Y., and K. Aki (1970). Source mechanism of the Truckee, California
earthquake of September 12, 1966, Bull. Seism. Soc. Am. 60, 1199-1208.
Vadurro, G. (1993). Strain analysis of the Genoa fault, Sierra Nevadan
frontal fault system, northwestern Nevada and northeastern California,
Senior Thesis, Humboldt State Univ., Eureka, Calif., 1-35.
VanWormer, J. D. and A. S. Ryall (1980). Sierra Nevada-Great
Basin Boundary Zone: Earthquake hazard related to structures,
active tectonic processes, and anomalous patterns of
earthquake occurrence, Bull. Seism. Soc. Am. 70, 1557-1572.
Vetter, V. R. (1984). Focal mechanisms and crustal stress patterns in
western Nevada and eastern California, Ann. Geophys. 2, 699-710.
Wells, D. L., and K. J. Coppersmith (1994). New empirical relationships
among magnitude, rupture length, rupture width, rupture area, and surface
displacement, Bull. Seism. Soc. Am. 84, 974-1002.
Wesnousky, S. G., and C. H. Jones (1994). Oblique slip, slip partitioning,
spatial and temporal changes in the regional stress field, and the relative
strength of active faults in the Basin and Range, western United States,
Geology 22, 1031-1034.
Wesson, R. L. (1987). Modeling aftershock migration and afterslip of the
San Juan Bautistia, California, earthquake of October 3, 1972, Tectonophysics
144, 215-229.
Wright, L. (1976). Late Cenozoic fault patterns and stress fields in
the Great Basin and westward displacement of the Sierra Nevada block,
Geology, 4 489-494.
Zoback, M. L., M. D. Zoback (1980). Faulting patterns in north-central Nevada
and strength of the crust, J. Geophys. Res. 85, 275-284.