Using Refraction Microtremor (ReMi) to detect Quaternary
Extensional Faults under the University of Nevada, Reno Campus
Introduction
Previous Refraction Microtremor (ReMi) and high resolution Reflection/Refraction surveys have traced a
north-south trending fault through downtown Reno, Nevada (Clark, 2005; Scott et al., 2004). During the
spring of 2015, a ReMi survey was conducted to find the hypothesized continuation of this fault zone
under the University of Nevada, Reno campus. ReMi technology uses multichannel refraction arrays to
measure Rayleigh waves propagating through subsurface strata (Louie, 2001). These surface waves are
generated by urban “noise” and are used to calculate volume-averaged shear-wave velocities at 30-m
depths (VS30s). The physical characterization of the fault is beyond the scope of the ReMi technique.
However, significant contrasts in the VS30s measured on opposite sides of the hypothesized fault zone
may allow for its detection. The VS30s have been ranked using the National Earthquake Hazards
Reduction Program (NEHRP) site classification system for the International Building Code (IBC).
Methods
Twenty-six survey lines were selected across the University of Nevada, Reno campus. The locations were
dependent on the ability to accommodate an 88-m ReMi array. At each survey line, the seismic data
were collected using twelve 4-Hz geophones at 8-m spacings (see Figure 1 for reference). The
geophones collected several thirty second records measuring the relative amplitude of surface waves.
The seismic records were processed using ReMiVspect and ReMiDipsert. Through Rayleigh phasevelocity dispersion picking (Louie, 2001), a shear-velocity model, shown in Figure 2, was made to
approximate a VS30 for each location. Three subsurface layers were modeled and assumed to have a
density of 2.0 g/cm3.
Results
The processed seismic records from the twenty-six ReMi surveys, shown in Figure 3, returned shear
wave velocities rangining from a minimum of 289.9 m/s along the freeway to a maximum of 455.9 m/s
in front of the postoffice on N. Virginia St. In general, the calculated VS30s fell under the IBC Site Class C
or D, or can be classified as “very dense” to “stiff” soil profiles (International Code Council, 2000). A
number of transects were plotted to discover trends in the VS30s across the University of Nevada, Reno
campus, as a north-south fault would have a change in VS30s in the east-west direction. Figure 4 shows
an east-west transect with a relatively small change in VS30s, the only profile to show this behavior. Just
north of this transect, the east-west profile in Figure 5 shows the shear wave velocities behaving
differently. A north-south transect (shown in Figure 6) revealed noteworthy change in that direction.
Conclusions
The calculated VS30s, the relative location of each IBC Site Class, and the transect profiles do not
provide significant evidence for the existence of a north-south trending fault under the University of
Nevada, Reno campus. The different VS30s correleated roughly with the Reno basin geology (shown in
Figure 7) described by Remelli et al. (2011). The narrow range in calculated VS30s suggests that the
there is not a signicant transition in shear wave velocities in near surface layers. The rests of this study
do not disprove the existence of an underlying fault. Future work, including high-resolution
reflection/refraction suveys, resistivity arrays, and more ReMi surveys, might reveal a Quaternary
extensional fault. Integrating the results of future work and increasing the extent of ReMi information
may limit the attributable error in this level of geophysical research.
Clark, M., 2008, Hydrologic and geophysical investigation of a fault as a hydrologic barrier in Reno,
Nevada [M.S. Thesis]: University of Nevada, Reno.
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Microtremor Arrays: http://quake.seismo.unr.edu/ftp/pub/louie/papers/disper/refr-pp.html
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