Refraction Microtremor as an Alternative to Boreholes for Earthquake Hazard Assessments

John N. Louie
Rasool Anooshehpoor
Glenn Biasi
Seismological Laboratory, Mackay School of Mines, University of
Nevada 174, Reno, NV; 775-784-4219; fax 775-784-1833;
louie@seismo.unr.edu

Robert E. Abbott
Instrumentation Development Dept., MS 1168, Sandia National Labs,
Albuquerque, NM 87185-1023

Harold E. Beeston
Black Eagle Consulting, 1345 Capitol Blvd. Ste. A, Reno, NV 89502-7140

Evaluation of shallow shear velocity is important to earthquake-hazard
assessment.  Borehole-based methods (downhole, crosshole) require both
drilling and measurement activities.  This makes them expensive point
measurements, unsuitable for many preliminary investigations.  We
tested an alternative surface-based method for estimating shallow shear
velocities with seismic refraction equipment at the sites of several
boreholes in California and Nevada:  the Newhall Fire Station; the
Pinon Flat Observatory; Keenwild, California; and the I-580 extension
in Reno, Nevada. The method was also demonstrated on weathered rock at
the crest of Yucca Mountain, Nevada.  The sites ranged from hard to
soft (NEHRP hazard classes A to D). The refraction microtremor method
uses ambient ground noise (recorded as in ASTM D5777), and wavefield
analysis to identify Rayleigh-wave phase velocities.  It works well in
dense urban areas and transportation corridors.  At quiet rural sites
with hard rock, estimating phase velocities below a few meters depth
requires a simple energy source such as a weight drop or a rolling
truck.  Depth-averaged shear velocities are available in the field
within a few minutes of recording.  We compare the effects of downhole
and refraction-microtremor velocities on surface spectra.  For the
sites tested, shear velocities estimated from refraction microtremor
are just as effective as borehole velocities for estimating the spectral
seismic site-transfer function for earthquake-hazard evaluations of sites.