PRELIMINARY PASSIVE- AND ACTIVE-SOURCE VS30 RESULTS AT 18 PORTABLE SEISMOGRAPH STATIONS DEPLOYED TO STUDY THE MW 5.8 MINERAL, VA, 23 AUGUST 2011 EARTHQUAKE

The 2011 Mw 5.8 Mineral, Virginia earthquake, widely felt from Maine to Georgia and inland as far as Chicago, caused significant damage from the epicentral area to as far away as Washington D. C. (135 km). Within days following the earthquake, 46 portable seismograph stations were deployed by several organizations, making this the best recorded aftershock sequence in the eastern U.S. In May 2012, the USGS used both active- and passive-source surface- and body-wave methods to acquire shallow shear-wave velocity (Vs) data at 18 of the portable seismograph stations. Methods used include refraction microtremor (ReMi), both P- and SH-wave refraction/reflection, multi-channel analysis of surface-waves (MASW) and horizontal/vertical spectral ratio (H/VSR). These data provide information needed to construct Vs30 (average Vs to 30 m depth), NEHRP site classification, as well as depth to and seismic velocity of bedrock. Sites where data was acquired are located within the Piedmont Province and overlay bedrock of the Chapawasmic Terrain group which consist of mid- to late-Ordovician metamorphosed volcanic and sedimentary rocks and granitic plutons. Preliminary results of the ReMi data indicate that NEHRP site classifications are predominantly C with five D sites. Depth to bedrock (9 to 28 m) and the wide range of bedrock velocities, from 700 to 1200 m/s, are significant site conditions that will affect site response. Vs versus depth plots for the various methods are presented, and preliminary Vs30 results are compared with spectral parameters obtained from HVSR analysis using aftershocks and noise. Preliminary results indicate a good correlation of increasing Vs30 with both HSVR peak frequency and sediment thickness. Based on these correlations it is speculated that site specific data near the portable seismograph stations can be used to predict site conditions at stations without Vs measurements that are located on similar geologic units. Using a multi-method acquisition approach with collocated arrays to acquire V_S data helps corroborate results, thus adding confidence that reliable site characterization information has been obtained. Importantly, inconsistencies in results may be a warning sign of poor data quality and may indicate the need for further data analysis.