OBSERVATIONS OF SEISMIC-WAVE ATTENUATION AND SITE RESPONSE FROM THE AUGUST 23, 2011 MW 5.8 EARTHQUAKE SEQUENCE

An Mw 5.8 earthquake occurred on August 23, 2011 (17:51:04 UTC) near Mineral, Virginia. The reverse faulting earthquake occurred on a northeast-striking plane within a region of diffuse seismicity known as the Central Virginia Seismic Zone (CVSZ) and was widely felt along the eastern US. The USGS received over 148,000 felt reports through the “Did You Feel It?” (DYFI?) Community Internet Intensity website, suggesting that this earthquake was felt by more people than any other in U.S. history. An interesting feature of the DYFI map pattern is the clear azimuthal dependence of reported intensities, with higher levels of shaking trending to the northeast toward the large urban population centers surrounding Washington, DC. The distribution of felt reports suggests that more efficient seismic energy propagation occurs along the strike of the tectonic fabric associated with the Appalachian Mountains.

The mainshock was recorded by permanent seismic stations in the Advanced National Seismic System (ANSS) backbone, by regional seismic networks, and by the Earthscope Transportable Array. In the days following the mainshock, 46 portable seismic stations were deployed by several organizations, including 8 stations that were installed in time to record the M4.5 aftershock on August 24th. In the 10 months following the mainshock, nearly 450 aftershocks with magnitude 1 and larger were recorded, making the aftershock sequence the best-resorded in the eastern US to date. These records provide a rare opportunity to study seismicity, attenuation and local site response within an intraplate environment.

We will describe the CVSZ portable aftershock deployment and present site amplification and regional frequency-dependent Lg wave attenuation results. We observe an azimuthal distribution of seismic wave attenuation that is consistent with the structural trend of the Appalachian Mountains, and that explains observations of northeast trending felt distributions as observed in the DYFI? results.. We compare preliminary Vs30 and depth to bedrock measures to resonance frequency and amplification and find that sapprolite soils, common in the CVSZ produce significant site amplification in the 1-5 Hz frequency band. This is expected to contribute increased hazard to structures with heights in the 2 to 10 story range.