USING HIGH-RESOLUTION SURFACE SEISMIC IMAGING TO STUDY EARTHQUAKE SITE RESPONSE

As part of the USGS earthquake ground motion studies in Memphis, TN, Seattle, WA, San Jose, CA, and Los Angeles, CA, we acquired P- and S-wave reflection/refraction data at various sites in these areas. High-resolution seismic-refraction/reflection data are a fast, non-invasive, and inexpensive alternative to borehole data for 1) determining the average S-wave velocity to 30-m depth (Vs30) and thus the NEHRP site classification, 2) revealing significant impedance boundaries in the upper 70 m that could produce damaging site resonances, and 3) measuring the depth of the water table. Recently, we have also found a high correlation between Vs30 and travel times of first-arrival phases at 100 m offset from the seismic source. This correlation suggests that a simple interpretation of travel time can map NEHRP classifications. After assigning NEHRP classifications in Seattle, we found that the variations in site classification appear to control part of the strong ground motion during the 2001 Nisqually earthquake. Except for one class D site, Nisqually earthquake results suggest that only class E sites in Seattle were associated with the highest amplification factors of 4 to 5.5 in the 0.5- to 1.0-Hz frequency range. In this frequency range, site class C site amplification values range from 1.3 to 2.4. In Sherman Oaks, Calif., the areas of extensive building damage and ground motion from the January 1994 Northridge Earthquake correlate to a thick sequence of low Vs (~ 10 m thick at <200 m/s). Potential earthquake resonance frequencies, usually in the 1 to 10 Hz range, are directly determined from the reflection data based on a quarter-wavelength conversion of the reflection two-way travel time. Resonances observed in earthquake seismograms recorded in Seattle, Wash., Memphis, and Sherman Oaks, CA, are correlated with the site’s near-surface seismic reflections. In all of these cases, the resonance accounts for the highest amplitude shaking above 1 Hz. Some resonances observed during the Nisqually earthquake may be generated by impedance boundaries in the upper 60 m.