Subsurface characterization of the California Wash fault, southern Nevada, using seismic,
potential field, and resistivity survey methods
Elijah T. Mlawsky
Alison Dorsey
Eve Marie Hirt
Nicholas Hobart
Paul Schwering
Janina Stampahar
John N. Louie
Abstract:
The California Wash fault zone contains westward-dipping, normal faults located roughly
50 miles northeast of Las Vegas, NV. Five geophysical exploration techniques comprise a twoday survey across the north-central portion of this fault zone, with the objective of assessing the
potential seismic hazard posed to the nearby Las Vegas basin. Applied survey methods include
reflection and refraction, refraction microtremor, resistivity, gravity, and magnetics. Four of the
five surveys support a proposed graben structure running along the fault of interest to the west;
the only method that cannot lend its support is refraction, due to uncertainty in the west-of-fault
data. The graben measures 100-200 m across its upper surface and sits at 30-80 m depth.
Additionally, gravity, magnetics, and refraction microtremor data suggest a horst located on the
east side of the fault zone. As modeled, the horst fault dips at 65°, westward. Refraction data
taken from east of the fault reveals 1-2 m of vertical slip occurring in the most recent earthquake
event. The accumulation depth of low-velocity sediments atop higher-velocity, recent alluvium
sufficiently justifies this claim. This mapping and hazard assessment of the California Wash fault
follows previous case reports by Saldaña and others, and by the USGS.
Introduction:
This report details the implementation and findings of five geophysical surveys with the
objective of mapping and characterizing a fault of interest within the California Wash fault zone.
The fault is located in the Basin and Range province, roughly 50 miles northeast of Las Vegas
(Fig. 1). As documented in the USGS Qfaults database, the fault measures 32 km in length and is
dipping to the west with a normal sense of movement. It constitutes a concave-westward contact
between the California Wash Quaternary alluvial basin to the west, and Tertiary to Mesozoic
sediments of the Muddy Mountains to the east (Anderson, 1999). Scarps in this fault zone are
likely Holocene, and are capable of generating M 6-7 earthquakes (Saldaña et al., 2004). The
California Wash basin includes Tertiary sediments (fine-grained basin fill) underneath
Quaternary pediment gravel. The scarp varies in height along the fault zone, ranging from 2.19.8 m, indicating a history of multiple displacement events (Anderson, 1999). Previously
compiled work characterizes the fault zone as being young and multi stranded – comprised of
“up to 15 distinct strands and splays, with as many as nine strands across strike” (Taylor, 2001).
An ongoing investigation of seismic hazard potential in the Las Vegas basin and the surrounding
region promotes interest in the fault, and warrants the intensive survey. We intend for our results
to provide a better understanding of subsurface geometry, segmentation, and earthquake history.
Methods:
Instrumentation:
The fault investigation employs five geophysical survey techniques, each receiving equal
care in their implementation. The instrumentation included: a Bison Galileo-21 48-channel
engineering seismograph; a research-grade Lacoste and Romberg gravimeter, G-509; a laser
theodolite; a pair of Scintrex roving and stationary proton-precession magnetometers; a 48channel seismic data cable with geophone takeouts at a maximum of 3-m intervals; a 12-channel
seismic data cable with takeouts at 10 m intervals; 48 100-Hz high-frequency geophone arrays
(in groups of six); 12, 4.5 Hz low-frequency geophones; a handheld GPS unit; an L & R
Instruments MiniRes resistivity meter; four steel electrode probes; four reels of high-gauge
electrical wire with alligator clips; a 7-kg sledgehammer; a steel striking plate; and a bull’s-eye
bubble level. At least two field party members worked on each of the surveys; however,
successful data acquisition is possible with only one operator. Consultations with geologists such
as C. dePolo established the locations and GPS coordinates of fault scarps within the survey area,
guiding survey placement. The procedure details of each survey are as follows:
Reflection and Refraction:
The field party conducted surveys along two 48-channel, 2 m takeout-offset lines of highfrequency geophone groups. The two line locations overlapped, running perpendicular to a
previously documented normal fault east of US Interstate 15, near the Ute Exit 80, 75 km north
of Las Vegas. Both lines crossed the fault – one extending to the east, and one to the west. Each
takeout connected to a six-geophone group array, evenly spaced and cable-parallel. The party set
each geophone visually level in the topsoil, and attached a 48-channel Bison Galileo-21
seismograph to one end of the line. A 7 kg sledgehammer, equipped with a trigger accelerometer,
initialized data collection when struck against a steel plate on the ground. By this method, the
party recorded 14 off-end hammer source points at 4 m intervals, leading up to the westernmost
geophone array along the line’s trend. Upon reaching the first takeout, the party recorded the
hammer at each of the 48 channel intervals, at 2 m spacing. Past the eastern end of the geophone
line were 12 additional off-end source points at 4 m spacing. In total, both surveys contained 74
individual hammer records; each of these consisted of ten, stacked hammer hits placed
approximately 1 m beside the line. The resulting reflection data received processing in the form
of an 80-400 Hz band-pass filter and a trace-equalization gain function. Refraction data required
no special processing prior to first-arrival picking and velocity analysis.
Refraction Microtremor:
The field party established five survey lines 100-150 m apart, each consisting of 12, 4.5
Hz vertical geophones. The refraction microtremor data cable measures 120 m in length, with
takeouts in 10 m intervals. Lines ran north-south, parallel to one another, across the proposed
graben structure and north of the Ute quarry road (Fig. 2). Party members used a bull’s-eye
bubble level to set each geophone. The Bison unit recorded low-frequency seismic noise at 2-ms
intervals, brought about by driving an equipment truck on the road near the lines. Each line
produced ten data sets, which were converted to SEG-Y file format prior to processing.
Resistivity:
Electrical resistivity testing utilized the same five-line configuration as the refraction
microtremor survey (Fig. 2). At each location along the graben, survey operators constructed a
line consisting of two mirrored Wenner arrays (one outer electrode that carries electric current,
and one inner electrode that samples electric potential). Planted into the ground at least 10 cm,
each electrode connects to a central L & R MiniRes resistivity meter. The meter operator took
measurements of potential and induced polarization (IP) across an expanding set of electrode Aspacings, ranging from 1 to 32 m.
Gravity:
Using a Lacoste and Romberg Model G gravimeter, the field party first established a base
station to correct for tidal and drift variations in gravity data. Surveyors took gravity readings at
this station several times throughout the day, amid taking readings along a 20-station line
crossing the fault and graben along the Ute quarry road (Fig. 3). Every station along the survey
line was subject to a minimum of three individual readings, preformed by at least two party
members, and in agreement to within 0.1 mGal. This process assured consistent measurements,
notwithstanding the high sensitivity of the gravimeter to movement and temperature. A
stationary laser theodolite marked the relative elevation and position of each gravity station.
Surveyors accounted for terrain correction with estimates of local relief within a 2-to-50 m
radius.
Magnetics:
The field party established a reference base station to control against diurnal variations in
magnetic field data. A stationary Scintrex magnetometer was intended to serve this purpose,
while also providing a base-level value; however, a diminished battery resulted in a loss of base
station data. As an alternative, analysts downloaded data from a USGS magnetic base station in
Fresno, Calif. Ultimately, there was no application of a diurnal correction, as the diurnal
variation in Fresno was found to be particularly low. Surveyors used a roving Scintrex
magnetometer to sample across four, mile-long, east-west transects and their connecting northsouth jogs, with the jogs roughly 400 m in length. This resulted in a serpentine survey path that
crossed over the fault scarp four times (Fig. 3). Sampling took place at 50-meter intervals,
marked with a handheld GPS unit. In anomalous areas, sampling increased in frequency –
performed as often as every 5 m. The survey control implements a NAD83 UTM Zone 11N
metric coordinate system.
Results:
Reflection and Refraction:
Reflection analysis utilized a cvstack process examining images at constant stacking
velocities in the 500-3,000 m/s range. Velocity picks made in the 1,100-1,600 m/s range allowed
for the generation of a common-midpoint stack or cmpstack (Fig. 4). The cmpstack shows a
shallow, discontinuous reflector that truncates to the east. The associated reflections have twoway travel times of roughly 0.05 s. There are two deeper reflectors at 0.1 s two-way times (about
75 m depth), that appear on the eastern and western ends of the survey line. These are coincident
with the top edges of high-velocity (3,500+ m/s) regions revealed in the cross section of
optimized refraction velocities (Fig. 5). NMO velocity calculation gives a depth for these
reflectors of about 80 m.
Line 1 refraction models (centered east of fault, Fig. 5, 6) depict shallow p-wave velocity
in the 600-900 m/s range; calculations return an average value of 880 m/s. The velocity beyond
the first refractor interface is roughly 1,400 m/s. Analysis suggests an estimated refractor depth
of up to 30 m, with a slight eastward dip. Line 2 models (centered west of fault, Fig. 5, 7) show a
shallow p-wave velocity of 200-400 m/s, again consistent with calculations, at 270 m/s. Velocity
beyond the first interface is roughly 1,200 m/s, giving an estimated refractor depth of 1-2 m with
minimal dip.
Refraction Microtremor:
Refraction microtremor inferences rely on hand picking p-f plots, which map wave
slowness against frequency. Many of the resulting p-f plots were corrupt and had to be ignored.
Of the usable data: shallow velocity analysis of all five survey lines was inconclusive; however,
analysis to the depth of a hypothetical third velocity layer correlates to expected results for the
proposed graben. The depths of the three-layer analysis increase from either end of the survey
area toward the middle survey line. The velocity analysis for the two above layers returned
results similar to one another. Effectively, a three-layer analysis was conducted on what is now
assumed to be a two-layer section of ground.
Resistivity:
Figure 8 shows the apparent resistivity at ascending A-spacings for the five survey lines.
Survey lines increase by number from west to east, and run parallel to the test fault. The plot
depicts an overall downward trend that continues to the 8 m A-spacing (where apparent
resistivity ranges from 200-300 Ωm), followed by a slight increase. This indicates a lack of
groundwater in the near-surface region. Line 5 is anomalous in this respect. This finding is
further support for the existence of a graben; line five lies on the back half of the wash, as
suggested by refraction microtremor analysis, and is likely to demonstrate higher resistivity. See
the discussion section below for an interpretation of the anomalous trend.
Gravity:
Figure 9 illustrates the complete Bouguer anomaly gravity profile with respect to easting.
There is an increasing trend from west to east, with a local maximum of -108 mGal near the
center. The rightmost spike is modeled on a reading taken several hundred meters south of the
others, and is ignored in calculations due to a lack of adjoining data. Gravity data, combined with
magnetic data below, lead to the construction of a geologic cross section model (Fig 10). The
model features horst faults that dip at 65°, inward.
Magnetics:
The California Wash fault zone is subject to low-relief magnetic signals of approximately
40 nT total. Magnetic readings decrease to the east, displaying a steep drop off toward the middle
of the survey area (Fig 3). Drop offs trend along the foothills between the western basin and
eastern range front. This is indicative of a fault that trends NNE-SSW. A horst structure
following this trend fits the surface physiography well. As noted above, the horst faults dip at
65°. Figure 10 depicts a geologic cross section, derived from gravity and magnetic data. The
density and magnetic susceptibility contrasts are subtle in the modeled lithologies.
Discussion and Conclusions:
Data suggest that the fault of interest trends NNE-SSW and exhibits 1-2 m of recent
offset. This is most evident in the reflection and refraction survey results. The accumulation
depth of low-velocity sediments atop higher-velocity, recent alluvium supports a vertical-offset
interpretation, west side down. In addition to the surveyed fault, reflection data revealed a fault
on the other side of the graben. If true, this might imply a flower structure in the subsurface.
Reflection survey lines 1 and 2 in conjunction suggest that both faults dip inward at roughly 65°
– a finding that is supported by the gravity and magnetic model. The dip is close to being in
agreement with a previous study, which places dip at 75° WNW (Saldaña et al., 2004). Saldaña’s
investigation took place 40 km further north along the fault, and claims 3-5 m of vertical
displacement. A 2 m offset could pose significant seismic hazard to the nearby Las Vegas basin.
The two or more faults in the area each have the potential to generate M 7 earthquakes.
Several past studies demonstrate the aptitude of reflection and refraction surveys in
characterizing subsurface grabens. In particular, Mueller (et al., 2005) uses two-dimensional
seismic reflection analysis to successfully model the Rough Creek Graben in western Kentucky;
however, this method is highly subjective in its interpretation. Disagreements in the reflection
and refraction data limit the placement of the California Wash graben to a range of 30-80 m in
depth. Additional research is necessary to rule out the possibility of an edge effect in the
refraction model (Fig. 5), and thereby confirm the existence of a high-velocity region. There is
little evidence for dip over the surface of the graben regardless of depth.
Resistivity data presents an anomalous trend about survey line 5. This line lies on the
back half of the wash, or graben, and is higher in elevation by several meters. Damming of
westward groundwater flow by the west side of the graben structure may have led to greater
moisture retention in the shallow subsurface – matching that of the other lines at depth. The
[10?] m higher elevation of the resistivity array explains the deeper drop off in resistivity on line
5, while fitting the horst model on the east side of the graben [?] from magnetic interpretation.
The horst block is also suggested by the local physiography.
The considerably low magnetic and gravity relief merits a greater span and density of
data collection. Additional east-west magnetic profiles would better grid the area; decreasing the
transect interval along the fault from 400 m to 200 m would be sufficient. Additional profiles to
the north and south would also be of interest in following the eastern [?] horst block. The
existence of a graben is essential for the structural integrity of a horst and range-front model.
Gravity readings taken from further east of the fault would confirm this structure, and would also
provide information on basement structure in relation to the uplifted range front.
Acknowledgements:
We thank Dr. Craig dePolo of the Nevada Bureau of Mines and Geology for illuminating
discussion of the fault and where it can be best viewed. The Dept. of Geological Sciences and
Engineering at the Univ. of Nevada, Reno provided generous and crucial support for class travel
to southern Nevada.
References:
Anderson, R. Ernest (1999). Fault number 1118: California Wash Fault, Quaternary fault and
fold database of the USGS.
Mueller, S. E., J. L. Sexton, J. P. McClain, and M. Silverman (2005). Two-dimensional seismic
reflection modeling of shallow faulting associated with the Rough Creek graben in
western Kentucky. Rocky Mountain Section - 57th Annual Meeting. Mesa State College:
Saccomanno Lecture Hall, Grand Junction, CO. May 2005.
Saldaña, S. C., C. M. Snelson, W. J. Taylor, and A. C. Hirsch (2004). Geophysical investigation
of the California Wash fault, Nevada. American Geophysical Union, Fall Meeting 2004.
Abstract #S43C-1025.
Taylor, W. J., Arriola, T., Artz, Z., Beyene, M., Evans, T., Kayvon, L., Nie, W., Seibenaler, S.,
Zellner, G (2007). Paleoseismicity of the California Wash fault, southern Nevada:
implications for neotectonics and seismic hazard. American Geophysical Union, Fall
Meeting 2007. Abstract #T33A-1142.
Figure Captions:
Figures:
{1 – site map}
{2 – ReMi/Resistivity map}
{3 – Grav/Mag map}
{4 – cmpstack}
{5 – seisopt 2d refraction model overlay L1/2}
{6 – refraction time-dist plot L1}
{7 – “ L2}
{8 – resistivity plot}
1000
Resistivity of California Wash
Aparent Resistivity (Ωm)
Line 2
Line 1
Line 3
Line 4
Line 5
100
1
{9 – CBA}
10
A-Spacing (m)
100
{10 – Grav/Mag model}