The location of the electronic version of this document is:
http://www.seismo.unr.edu/ftp/pub/louie/talks/lvsh/96agu.html
Objectives
We employ the seismic reflection, magnetic, and electromagnetic geophysical techniques to locate segments of the Pahrump Valley fault zone (PVFZ) and examine their subsurface geometry. Geophysical techniques can provide clues to segmentation and rates of activity in advance of detailed trench studies, and can uncover deeper and older displacements.
1) Project location map of the Pahrump Valley fault zone (PVFZ). The
PVFZ is the longest seismogenic structure within 100 km of the Las Vegas
metropolitan area, only 50 km distant at its closest reach. Possible
rupture lengths range from 60 to 150 km, with potential for events
having magnitudes between 6.9 and 7.2.
2) Shallow ground conductivity and magnetic measurements across the
PVFZ in southern Pahrump Valley. 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 their respective fault traces.
3) View of southern Pahrump Valley, looking SW over scarp 1. The red
line follows a tephra bed, not yet identified, exposed by headward
erosion from a branch of the fault.
4) Attempts to model the magnetic anomaly at scarp 1. Model A shows
a vertical fault displacement of a magnetic layer; Model B shows a
magnetic body as an inclusion within the steeply-dipping fault plane.
The inclusion model fits the symmetry of the anomaly better.
5) Analysis of three time-domain electromagnetic (TEM) soundings
shows distinct
high-conductivity layers at about 10 m depth away from the fault zone at
scarp 1, with only evidence of a very shallow conductivity high at the scarp.
The exposed, conductive tephra at the scarp agrees well with the coincident
shallow ground conductivity high.
6) Two-dimensional seismic reflection profiles of scarps 1 and 2.
The sections confirm the lack of absolute vertical offset of the
lake beds and tephra layers by the PVFZ. The slumping evident in the
profiles appears due to negative flower structures within the PVFZ.
The scarp 2 section may suggest either substantial slumping or absolute
vertical offset of more than 10 m.
Note that the vertical scale bar applies only to the scarp profile.
Click on the image for a large JPEG-format enhanced air photo, or
here for an annotated large JPEG photo.
7) Low-sun-angle aerial photography of the PVFZ at the southern end of
Stewart Valley. The PVFZ is shown by a series of continuous scarps
within 200 m of the state line. The highway running E-W across the
middle of the photo is California 178/Nevada 372.
8) This section of the air photo shows the locations of our
geophysical surveys in Stewart Valley. The 3-d seismic reflection survey
area is outlined in yellow; shallow ground conductivity and magnetic
measurements in red; and TEM measurements in blue.
9) Gravity and conductivity measurements across the PVFZ in Stewart
Valley.
The possible fault locations are noted on the topographic profile.
Several anomalies also exist that we have not been able to associate with
any fault break.
10) Stewart Valley TEM survey. No discrete conductive layers were
identified, possibly due to the relatively shallow water table. At fault
2 a low-conductivity anomaly near the surface (blue) suggests abundant
silica cementation within a Pluvial spring mound.
11) Ultra high-resolution three-dimensional seismic reflection surveying
across fault 1 of the PVFZ in Stewart Valley.
Depth imaging yielded this reflectivity volume, rendered to emphasize
the positive reflectivities of greatest amplitude as opaque 3-d objects
in warm 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.
12) The upward curving of deeper reflectors near the sides of the volume is an
artifact of low fold coverage there.
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.
13) 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 Pluvial 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.
Conclusions
Geophysical surveys across two sections of a major right-lateral strike-slip fault zone on the California/southern Nevada border have established that the Pahrump Valley Fault Zone maintains an almost completely strike-slip character from southern Pahrump Valley to southern Stewart Valley. Despite apparent changes in tectonic setting that suggested segmentation, the PVFZ is straight, continuous, purely strike-slip, and shows Holocene activity over a distance of almost 100 km.While this length of the fault may be a segment of a longer system possibly extending south into Mesquite Valley and north into Ash Meadows, segmentation hypotheses would propose that the main 100 km length in Pahrump Valley could rupture completely, producing an earthquake having a moment magnitude Mw as large as 7.2.
Contrary to current assessments of regional seismic hazards to the Las Vegas metropolitan area, the 18 m minimum Holocene dextral displacement found by high-resolution 3-d seismic surveying in Stewart Valley establishes a displacement rate much greater than the average for faults in southern Nevada, and likely above the 0.1 mm/yr average for faults in the Great Basin overall.
As little as 50 km from the metropolitan area, the Pahrump Valley Fault Zone could pose the most significant seismic hazard to Las Vegas after the very active 4 mm/yr Death Valley fault system.