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
65 km northeast of Las Vegas, NV. Five geophysical exploration techniques comprise a two-day
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 about 300 m across its upper surface and contains 30-80 m of
alluvium. 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. Offset estimates suggest that the fault is capable
of generating a M 7 earthquake every 3,000-6,000 years. 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 65 km 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 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 (about 40 m depth). There are two deeper reflectors at 0.1 s
two-way times (70-80 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, 6). NMO velocity calculation gives a
depth for these reflectors of about 80 m.
The line 1 refraction model (centered east of fault, Fig. 5) depicts 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. 6)
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 12 m with minimal dip. Figure 5 also reveals lateral velocity discontinuities in the 20-40 m depth
range.
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.
Velocity analysis of the usable data resulted in uncertainty with respect to shallow interface
depth. Analysts assumed a velocity interface at 12.75 m, based on p-f dispersion picks; though,
the two resulting layers appear very similar. The average velocity of the two layers to 30 m depth
is 850 m/s. Further analysis, to the depth of a hypothetical third velocity layer, correlates to
expected results for the proposed graben. Effectively, a three-layer analysis was conducted on
what is now assumed to be a two-layer section of ground. The depths of the three-layer analysis
increase from either end of the survey area toward the middle survey line. The model for this
depicts a pair of inward dipping faults (Fig 7, upper).
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 upper 10 m subsurface. 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 proposes horst faults that dip at 65°, outward.
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 confirmed a fault
on the other side of the graben. Both reflection survey lines, in conjunction with refraction
microtremor interpretation, suggest that the faults dip inward at roughly 65° – a finding that is in
disagreement with the gravity and magnetic model. A horst-graben pair is more likely, and
accommodates both data sets (Fig. 7, lower). The primary dip angle of 65° coincides with a
previous study, which places dip at 75° WNW (Saldaña et al., 2004); however, Saldaña models
both faults as dipping in the same direction. The prior investigation took place 5 km further north
along the fault, and claims 3-5 m of vertical displacement. An earthquake event consistent with
the recent 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 (Saldaña et al.,
2004); though, a disagreement in fault geometry suggests segmentation somewhere between the
two investigation sites. This would effectively shorten the length of the fault from 30 km
(Anderson, 1999) to 25 km, and reduce the potential earthquake severity.
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 placement of alluvium depth in the graben to a range of 30-80 m.
Assuming the graben contains 40 m more alluvium than its surroundings, and an age of the
deepest alluvium in the graben of 120 ka (Sangaman interglacial), the vertical fault offset rate is
0.3 m per thousand years. On average, the fault would generate a M 7 earthquake every 3,0006,000 years. Additional research is necessary to rule out the possibility of an edge effect in the
refraction models (Fig. 5, 6), and thereby confirm the existence of a high-velocity region.
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 a depth of
around 15 m. The 12 m higher elevation of the east-most resistivity array explains the deeper
drop off in resistivity on line 5, while fitting the horst model on the east side of the graben. 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, making the
lower model of figure 7 preferable to that of figure 10. 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. Dr. Satish Pullammanappallil of Optim provided important technical
guidance, and Optim SDS donated their SeisOpt® ReMi™ analysis package for class use. The
Univ. of Nevada, Reno Applied Geophysics (GPH 492/692) class of Spring 2011 contributed to
all data acquisition, analysis, and interpretation under the instruction of Dr. John Louie. The
students are: Reka Aczel, Josh Beckwith, Peter Bernhard, Alison Dorsey, Stephen Hein, Eve
Marie Hirt, Nick Hobart, Mason Kreidler, Jonathan Payne, Greg Saunders, Paul Schwering,
Janina Stampahar, and Braden Walsh.
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:
Figure 1: All surveys took place within the bounded area. The site is roughly 50 miles northeast
of Las Vegas.
Figure 2: GPS-based map of the five fault-parallel survey lines used in refraction microtremor
and resistivity.
Figure 3: Color-contour map of gridded magnetic data overlain on satellite photo. Red dots
indicate magnetic rover station locations and green dots indicate gravity station locations.
Figure 4: Common-midpoint stack representation of reflection data. West-east cross-section,
time scale is 0-0.25 s.
Figure 5: SeisOpt® velocity cross-section.
Figure 6: SeisOpt® velocity cross-section.
Figure 7: Model of the proposed graben based on refraction microtremor and resistivity data
(upper) and amended to accommodate gravity and magnetic data (lower). Numbers correspond
to survey lines from Fig. 2. Not to scale.
Figure 8: Plot of apparent resistivity with respect to A‐ spacing for the five survey lines.
Figure 9: Profile of the complete Bouguer anomaly, displayed from west to east.
Figure 10: Magnetic and gravity-calculated models, and resulting cross-section.
Figures:
Figure 1: Site Map
Figure 2: Refraction Microtremor and Resistivity Survey Lines
Figure 3: Magnetic and Gravity Survey Locations, Magnetic Gradient
Figure 4: Reflection Cmpstack
Figure 5: Two-Dimensional Refraction Model for Survey Line 1, East
Figure 6: Two-Dimensional Refraction Model for Survey Line 2, West
Figure 7: Graben and Horst-Graben Models
Figure 8: Apparent Resistivity
Aparent Resistivity (Ωm)
1000
Line 2
Line 1
Line 3
Line 4
Line 5
100
1
Figure 9: Complete Bouguer Anomaly
10
A-Spacing (m)
100
Figure 10: Gravity and Magnetic Data Models