John Louie, Gordon Shields, Gene Ichinose, Michael Hasting, Gabriel Plank, and Steve Bowman
Seismological Lab (174), University of Nevada, Reno, NV 89557-0141
(775) 784-4219; fax (775) 784-1833; louie@seismo.unr.edu
Clicking on the thumbnail images below will get a large JPEG image or
high-resolution Adobe Acrobat PDF
file.
The location of the electronic version of this document is:
http://www.seismo.unr.edu/ftp/pub/louie/talks/lvsh/lvsh.html
Although hidden on many maps by its location usually within 200 m of the
California/Nevada state line, the Pahrump Valley fault zone (PVFZ) is the longest
seismogenic structure within 100 km of the Las Vegas metropolitan area.
Only 50 km distant at its closest reach, it extends at least 60 km from Stewart
Valley to southern Pahrump Valley (Joanne Hoffard, 1991),
equal in length to any one of the four segments of the Death Valley
fault system proposed by Thomas Sawyer (LVSH meeting, 1996).
The PVFZ may well extend north of Stewart Valley into Ash Meadows and
Amargosa Valley as proposed by Diane Donovan (1991).
To the south, it extends through Mesquite Valley (MIT Field Geophys. Course,
1985)
and possibly into Sandy and even Ivanpah Valleys (Burt Slemmons, pers. comm.
1996).
Thus possible rupture lengths range from 60 to 150 km, impying the potential
for events with Mw magnitudes between 6.9 and 7.2.
Such an event on the PVFZ could produce rock-site accelerations in the
Las Vegas metropolitan area of up to 20% g, and possibly larger spectral
accelerations at periods of one Hertz and greater (Feng Su, LVSH meeting, 1996).
Given the 12 mm/yr of the 56 mm/yr of Pacific/North America plate motion that diverts into the Eastern Mojave shear zone (Dokka and Travis, 1990) and the Walker Lane, Burt Slemmons (LVSH meeting, 1996) accounts for 2 mm/yr on the Owens Valley fault system, 2 mm/yr in Panamint Valley, and now 4 mm/yr on the Death Valley fault system (Thomas Sawyer, LVSH meeting, 1996). This leaves 4 mm/yr for all Basin and Range motions east of Death Valley. Structures such as the Eglington scarp and the Frenchman Mountain fault in Las Vegas Valley show inconclusive evidence of rates as high as 1 mm/yr. However, the PVFZ is the only candidate for a structure east of Death Valley large enough to show a rate as large as 1 mm/yr. Craig dePolo (LVSH meeting, 1996) finds rates in southern Nevada to average near 0.01 mm/yr, an order of magnitude below the 0.1 mm/yr average deformation rates for the Great Basin. Seismicity rates presented by Ken Smith (LVSH meeting, 1996) support such rates in southern Nevada except in association with Lake Mead reservoir induced seismicity, activity associated with the Little Skull Mountain and Rock Valley sequences, and a cluster of seismicity in southern Pahrump Valley.
We will examine the PVFZ at two locations, in Southern Pahrump Valley at
the Old Spanish Trail Highway, and in Stewart Valley near California Highway
178. Each of these localities crosses a section of the PVFZ that appears to
differ from the other in its apparent style of faulting, and type of scarp
exposure.
The authors all contributed to a UNR course in Geophysical Applications that
performs field exercises in the area every two years.
Our objective is to investigate how inexpensive shallow geophysical exploration
methods may allow some characterization of fault displacement amounts and
styles on these two parts of the PVFZ, and describe localities most appropriate
for more detailed paleoseismic investigations.
In southern Pahrump Valley, the PVFZ divides into three fault-line scarps,
each dissected by headward erosion of the uplifted playa and alluvial surfaces
(Hoffard, 1991).
The elevation profile at bottom left, along the Old Spanish Trail Highway,
shows the three scarps, with the state border near the 1 km distance mark.
Scarp 1 appears geomorphically yougest and sharpest, with about 10 m of
relief, while scarps 2 and three, while about twice as high, have gentler
slopes and appear more eroded.
At left above are the results of shallow (< 3 m) ground conductivity
measurements made with a Geonics EM-31 instrument
along the road (solid line), together with total-field magnetometer measurements
(dashed line). Shields
et al. (1996) discuss the data collection, and establish the repeatability
of both the conductivity and magnetic measurements, including the fact that
the strike of the anomaly at scarp 1 follows the strike of the scarp.
Note that both types of anomaly suggest that all three scarps are fault-line
scarps, with the topographic scarps having eroded back between 50 and 300 m
from the fault break locations suggested by the anomalies.
The photo at left is a view toward the southwest of southern Pahrump Valley,
looking over scarp 1 from the edge of the Old Spanish Trail Highway.
The California border is about 100 m ahead.
The red line highlights a tephra bed exposed by headward erosion from the
scarp. The tephra appears to slump about a meter into the fault zone.
We have not yet identified this tephra, or its age. Based on work by Roger
Morrison (1991) and J. W. Hillhouse (1987) in the Tecopa and Chicago Valleys
immeditely to the west of Pahrump Valley, active lacustrine deposition ended
no earlier than 0.16 Ma, with prominent tephras deposited at 0.5 (Bishop),
0.9, and 2.01 (Huckleberry Ridge) Ma.
The pluvial lake in Pahrump Valley drained north to or was contiguous with
a lake in Stewart Valley, with drained north in turn to Ash Meadows and the
Amargosa River, and so was at least occasionally tributary to pluvial Tecopa
Lake.
Despite the evidence above for vertical offset of tephra layers and other
lacustrine beds at scarp 1, our attempts at left to model the magnetic anomaly at
the scarp with a vertical fault displacement of a magnetic layer were not
successful.
Given the orientation of our survey with respect to Earth's magnetic field,
the displacement anomaly cannot match the symmetry of the magnetic high
in the data.
A model placing a magnetic body as an inclusion within the steeply-dipping
fault plane fits the symmetry better.
Shields et al. (1996)
propose that pluvial spring activity (as discussed by Quade et al., 1995)
produced mineralization of the fault plane allowing the conductivity and
magnetic observations.
Under this mineralization hypothesis, scarp 1 appears to have the most
recent motion and best preserved mineralization, with the largest anomalies,
while scarps 2 and 3 appear to be associated with older and more degraded
fault mineralization.
In addition to the shallow conductivity and magnetic measurements, we
conducted more deeply-penetrating transient electromagnetic (TEM) soundings
at three locations: at scarp 1; and 200 m southwest and 200 m northeast of
scarp 1.
The photo at left shows students laying out the 40 m square TEM transmitter loop
at scarp 1, looking south
with the eroded edge of the tephra layer highlighted in red, as well as a piece
of tephra lying as float near the instrument.
Analysis at left of the three TEM soundings shows distinct high-conductivity
layers at about 10 m depth away from the fault zone, with only evidence of a very
shallow conductivity high at scarp 1. The TEM soundings average over the 40 m
squared area of the transmitter loops. The sounding 200 m northeast of scarp
1 shows the apparently conductive tephra layer at the same absolute elevation
as the layer exposed at scarp 1, since the ground surface at the sounding to
the notheast is about 10 m higher in elevation than the surface at the scarp 1
sounding. The surface elevation at the sounding 200 m southwest of scarp 1 is
at about the same elevation as the scarp 1 sounding.
The exposed, conductive tephra at the scarp agrees well with the coincident
shallow ground conductivity high shown above.
The TEM technique and the time-domain depth inversion we use is not, however,
expected to be sensitive to any layers below the uppermost conductive layer.
Two-dimensional seismic reflection profiles confirmed the lack of absolute
vertical offset of the lake beds and tephra layers by the PVFZ at scarp 1.
The reflection at 40 ms two-way travel time in the unmigrated section at left
shows an approximately constant subsurface elevation after correction for
surface elevation statics, with some slumping and disruption within a 70 m
wide zone on the southwest edge of the surface fault-line scarp.
The slumping appears to be part of a negative flower structure along an
almost purely strike-slip PVFZ at scarp 1.
Note that the vertical scale bar applies only to the scarp profile; the 40 ms two-way travel time implies a depth to the reflective bed of about 30 m. This reflective bed thus cannot be the tephra layer exposed at scarp 1 and buried 10 m away from the scarp; it is likely to be an older tephra buried about 20 m deeper. The seismic section at scarp 2 also suggests a slumped tephra layer at about 60 m depth, more centered on the topographic scarp, with at least twice the vertical displacement. The image cannot rule out an absolute offset of the tephra layer of 40 m at scarp 2.
The geophysical investigations in southern Pahrump Valley suggest the PVFZ
has its most recent, and almost purely strike-slip, motion at scarp 1, closest
to the California-Nevada border.
The other two scarps show older motions, possibly with larger proportions of
dip slip.
Click on the image for a large JPEG-format enhanced air photo, or
here for an annotated large JPEG photo.
Low-sun-angle aerial photography shows the PVFZ as a series of continuous scarps that follow within 200 m of the state line from southern Pahrump Valley to the southern end of Stewart Valley, directly west of the town of Pahrump. The highway running across the middle of the photo at left is California 178/Nevada 372, and Ash Meadows Road extends to the north along the east side of the PVFZ. The intersecting road is part of a residential development, and has been paved since the photo was taken about 1987. Several homes are now occupied within the development.
As the PVFZ enters southern Stewart Valley it turns to a more northerly strike,
and may become the basin-bounding fault between Stewart Valley and High Peak
to the east.
Landowners along the PVFZ in central Stewart Valley report the water table
at 9 or 10 m depth, and the fault is marked by groves of tamarisk and other
phreatophytes.
The main trace of the PVFZ is visible near the left edge of the photo as a continuous
vegetation lineament.
Additional traces to the right follow a series of spring mounds (Quade et al.,
1995), or possibly terraces in the lake beds cut by wave action in the pluvial
lake. We targeted our work in Stewart Valley to a relatively simple stretch
of the fault at the northward bend, between more complex sets of traces to
the north and south.
This location appears about midway along the fault trace between the highway
and the development road.
View north-northeast along the principal vegetation lineament, highlighted
in red. Note the 10 cm deep linear depression in the foreground, transverse
to the right-to-left drainage direction.
Our surveys at this site were centered at the bush just beyond the depression
(station EM344).
In the background on the left is a house, and the roof of another towards
the right about 600 m away that stands at the intersection of the development
road with Ash Meadows Road.
The section of air photo at left shows the locations of our geophysical
surveys in Stewart Valley.
The preceding view was from station EM344, on the south side of our 3-d
seismic reflection survey area (yellow), in the middle of the gravity
line, and on one of three lines where we took Geonics EM-31 shallow ground
conductivity and magnetic measurements (red).
The northern of the three lines was the basis for our layout of eight adjacent
40 m square TEM transmitter loops (blue).
The main trace of the PVFZ at the most continuous vegetation lineament
is the hatched line at left (fault 1); while the hatched line to the right denotes a
topographic scarp and second trace cutting the spring mounds,
possibly a pluvial lake terrace (fault 2).
This view southwest toward the Nopah Range shows students surveying
shallow ground conductivities with a Geonics EM-31 instrument atop the
possible scarp through the spring mounds locating fault 2.
The students in the middle distance are making
total-field magnetic measurements just beyond the main trace (fault 1)
at the vegetation lineament.
In the distance at the extreme left is a vehicle on Calif. Highway 178, and
another is visible near the right edge of the photo.
Although the surface at this locality exposes pluvial lacustrine sediments
and/or spring deposits, it is covered with a desert pavement of volcanic
float washed from the hills to the east, about 300 m behind this viewpoint.
The volcanic cobbles and rubble rendered no useful magnetic signal from the
fault scarps at this locality; the rapidly varying field from surface float
blocks swamped any anomalies from subsurface structures.
The gravity results at left (top, triangles) can match a synthetic (dashed)
model putting a small, approximately 50 m deep basin or shelf between
fault 1 and fault 2. The possible fault locations are noted on the topographic
profiles at left (bottom).
As in southern Pahrump Valley, the EM-31 shallow conductivity measurements
(center) produced clear anomalies centered on the surface fault breaks and
continuous along fault strike.
However, anomalies also appear that we have not been able to associate
with any possible fault break on the ground or in low-sun-angle aerial
photography.
Unlike in southern Pahrump Valley, TEM surveys in Stewart Valley did not
identify discrete conductive layers, possibly because of the relatively
shallow water table.
Combining the eight TEM soundings into the pseudosection at left, fault
1 appears to mark the edge of conductive lacustrine sediment filling the
basin to the west (orange, versus lower conductivities in green to the
northeast).
At fault 2 a low-conductivity anomaly near the surface (blue) suggests
abundant silica cementation within the spring mound.
The apparent very high conductivity below (red) may be an artifact of the
low-conductivity anomaly.
The EM-31 shallow conductivity measurements only cover the very top layers
of the pseudosection, and there does appear to be some correspondence between
higher conductivities in the TEM results and in the EM-31 profiles at fault 1.
Ultra high-resolution three-dimensional seismic reflection surveying we
carried out across fault 1 in Stewart Valley reveals the details of fault
geometry and displacement.
We laid out 11 lines across the fault spaced at 3.05 m (10 ft), and recorded
each line individually.
Each line consisted of 48 fixed 100 Hz single-phone receivers, buried about
20 cm and tamped with soil.
A source consisting of a 5 kg (12 lb) sledgehammer was hit against a 30 cm
square 2 cm thick steel plate, set on the surface, 10 times at each receiver point on each line.
The 10 hits at each point were stacked by a Bison Galileo-21 seismic recorder,
generously donated to the UNR Mackay School of Mines by the W. M. Keck Foundation.
Data reduction consisted of minimal bandpass filtering followed by true
three-dimensional imaging using an interval velocity profile derived
from analysis of a suite of constant-velocity stacked sections.
The 3-d prestack depth imaging technique is almost identical to the Kirchhoff-sum
migration of Louie et. al (1988), with operator aliasing controls as described
by Lumley et al. (1994), but using boxcar instead of triangle antialias filters.
Depth imaging yielded the 73 m deep, 27 m thick, and 73 m deep volume at
left, which we rendered to emphasize the positive reflectivities of greatest
amplitude as opaque 3-d objects in warm colors.
Near-zero reflectivies are rendered transparent, and strong negative reflectivities
take cool colors.
The front face of the image volume shows interruptions in flat reflectors
between 24 and 48 m depth that locate the subsurface fault break with a
near-vertical dip, surfacing at the center of the face.
The upward curving of deeper reflectors near the sides of the volume is an
artifact of low fold coverage near the ends of the survey lines.
No measurable vertical offset of any of the layers is apparent, limiting the
dip slip of SVFZ fault 1 in Stewart Valley to less than one meter.
The depth slice at 48 m shows the interruption of a layer by the fault trace
at that depth, without vertical displacement.
The depth slice at 24 m shows a lateral discontinuity on the northeast side
of fault 1 that could arise at a fluvial channel wall, a facies change, or
the side of a spring mound structure.
The layer on the southwest side of the PVFZ fault 1 shows no similar lateral
discontinuity within the image volume, proving that the discontinuity was
dextrally displaced a minimum of 18 m into the image volume by PVFZ fault
motion.
This image establishes a minimum fault displacement on a sedimentary
structure of unknown age.
Quade et al. (1995) establish pre-Wisconsin Rancholabrean ages of
10 ka to less than 450 ka for spring mounds in Stewart and Pahrump Valleys.
Since the pluvial lake in Stewart Valley served as an outlet for all discharge
from the western Spring Mountains certainly as late as the Wisconsin pluvial
period, near-surface lacustrine deposits in Stewart Valley could be as young
as 5-10 ka.
The lateral offset at 24 m depth may well represent fault displacement of
the top of a pre-Wisconsin age spring mound, within Wisconsin-age lake
deposits. Thus the 18 m minimum offset could represent a cumulative
displacement rate as high as 1.8 mm/yr, if the spring mound has the 10 ka
minimum age.
Spring activity may have peaked earlier in southern Nevada, at 100-150 ka,
suggesting that the displacement rate is likely above 0.1 mm/yr, about average
for faults in the Great Basin (C. dePolo, LVSH meeting, 1996).
Putting the spring mound at the earliest possible Rancholabrean age, we see
that the Quaternary displacement rate on the PVFZ cannot be less than
0.03 mm/yr, well above the rate of smaller faults in southern Nevada.
Donovan, Diane E., 1991, Neotectonics of the southern Amargosa Desert, Nye County, Nevada and Inyo County, California: University of Nevada, Reno M.S. thesis 2755, 151 pp.
Hillhouse, J. W., 1987, Late Tertiary and Quaternary geology of the Tecopa basin, southeastern California: U.S. Geological Survey Miscellaneous Investigations Map I-1728, scale 1:48,000, 1 sheet, 16 p. text.
Hoffard, Joanne L., 1991, Quaternary tectonics and basin history of Pahrump and Stewart Valleys, Nevada and California: University of Nevada, Reno M.S. thesis 2747, 138 pp.
Lumley, D. E., J. F. Claerbout, and D. Bevc, 1994, Anti-aliased Kirchhoff 3-d migration: Soc. Explor. Geophys. Ann. Internat. Mtg., Los Angeles, October, Expanded Abstracts, p. 1282-1285.
MIT Field Geophysics Course, 1985, A Geophysical Study of Mesquite Valley: Nevada-California Border, J. Geophys. Res., v. 90, p. 8685-8690.
Morrison, R. B., 1991, Quaternary stratigraphic, hydrologic, and climatic history of the Great Basin, with emphasis on Lakes Lahontan, Bonneville, and Tecopa, in Morrison, R. B., ed., Quaternary non-glacial geology, conterminous U.S.: Boulder, Colorado, Geological Society of America, Geology of North America, v. K-2, p. 283-320.
Quade, Jay, Mifflin, Martin D., Pratt, William L., McCoy, William, and Burckle, Lloyd, 1995, Fossil spring deposits in the southern Great Basin and their implications for changes in water-table levels near Yucca Mountain, Nevada, during Quaternary time: Geological Society of America Bulletin, v.107, no. 2, p. 213-230.
Schweickert, R. A., and Lahren, M. M., 1994, Amargosa fault system near Yucca Mountain, Nevada: Abstracts with Programs, Geological Society of America, v. 26, no. 7, p. 250.