MAGNITUDE, RETURN PERIOD AND STRONG-MOTION MODELING OF THE AUGUST 23, 2011, MINERAL, VIRGINIA EARTHQUAKE

The Mineral, Virginia earthquake occurred at 17:51:3.9 UTC on August 23, 2011, with hypocenter at 37.905°N, 77.975°W and depth 8 km. The widely reported moment magnitude (M_w) was 5.8. The short-period teleseismic body wave m_b magnitude estimated here using 26 global network stations is 5.77 +/- 0.23, nearly equivalent to M_w and in agreement with m_b values reported by national and international datacenters. However, the m_bLg magnitude of the earthquake determined here using 386 stations in eastern North America is 6.28 +/- 0.26. This difference between m_b and m_bLg is significant, and is in part due to large variations in Lg phase attenuation. The catalog of previous earthquakes in central Virginia is keyed to m_bLg, rather than M_w. The expected value for the return period of m_bLg 6.3 and larger earthquakes in the central Virginia seismic zone is 752 years, with 95% confidence interval 385 - 1,471 years.

The Mineral earthquake caused MMI VIII damage in the epicentral area, with several instances of partial and total collapse of masonry chimneys and walls. A finite-fault, full-wavefield simulation of the motions within 30 km of the epicenter accurately reproduces the recordings at the only strong-motion station in that distance range. The strongest motions are predicted to have occurred in two lobes offset to the northwest and southeast of the epicenter, within which peak ground accelerations may have approached 2g, and peak velocities were probably well in excess of 20 cm/s. The only factor mitigating damage in this earthquake was the brief (less than 3 seconds) duration of strong shaking.