Ground Motion Site Effects from Multi-Method Shear-Wave Velocity Characterization at Seismograph Stations Deployed for Aftershocks of the August 2011 Mineral, Virginia, Earthquake

Abstract:

We characterize shear-wave velocity-versus-depth (Vs profile) at 16 portable seismograph sites through the epicentral region of the 2011 M_w5.8 Mineral earthquake to investigate ground motion site effects in the area. We used a multi-method acquisition and analysis approach, where active-source horizontal shear (SH)-wave reflection/refraction as well as active-source multichannel analysis of surface waves (MASW) and passive-source refraction microtremor (ReMi) Rayleigh wave dispersion were interpreted separately. The time-averaged shear-wave velocity to a depth of 30 m (Vs30), interpreted bedrock depth and site resonant frequency were estimated from the best-fit Vs profile of each method at each location for analysis. Using the median Vs30 value (which range from 270 m/s to 715 m/s) as representative of a given site, we estimate that all 16 sites are NEHRP site class C or D. Based on a comparison of simplified mapped surface geology to median Vs30 at our sites we do not see clear evidence for using surface geologic units as a proxy for Vs30 in the epicentral region, although this primarily may be because the units are similar in age (Paleozoic) and may have similar bulk seismic properties. We compare resonant frequencies calculated from ambient noise horizontal-to-vertical spectral ratios (HVSR) at available sites to predicted site frequencies (generally between 1.9 Hz and 7.6 Hz) derived from the median bedrock depth and average Vs to bedrock. Robust linear regression of HVSR to both site frequency and Vs30 demonstrate moderate correlation to each, and thus both appear to be generally representative of site response in this region. Based on Kendall tau rank correlation testing, we find that Vs30 and the site frequency calculated from average Vs to median interpreted bedrock depth can both be considered reliable predictors of weak-motion site effects in the epicentral region.