Refraction microtremor and optimization methods for site strength and 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., Reno, NV 89502-7140

Evaluation of shallow shear velocity is important to both
earthquake-hazard assessment and efficient foundation design.
Borehole-based methods (downhole, crosshole) require both drilling and
measurement activities.  This makes them expensive point measurements,
unsuitable for many preliminary investigations.  We tested two
alternative surface-based methods for estimating shallow shear
velocities with seismic refraction equipment.  We demonstrate
comparisons at the sites of several boreholes in California and Nevada,
at sites of precariously balanced rocks near the 1857 rupture of the
San Andreas fault, and on weathered rock at the crest of Yucca
Mountain, Nevada.  The sites ranged from hard to soft (NEHRP hazard
classes A to D). The first method, refraction microtremor (ReMi), 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.  The second method, SeisOpt(R)@2d by
Optim LLC, takes standard (ASTM D5777) seismic refraction arrival picks
and finds an optimized 2-d P-velocity model.  Laterally heterogeneous
velocity models are available within an hour of recording.  We compare
the effects of downhole, refraction-microtremor, and SeisOpt(R)
velocity results on surface spectra.  For the sites tested, shear
velocities estimated from both surface methods are just as effective as
borehole velocities for two purposes: estimating 3-meter depth-averaged
shear velocity for foundation design; and estimating the spectral
seismic site-transfer function for earthquake-hazard evaluations of sites.