Magnitude, recurrence interval, and near-source ground-motion modeling of the Mineral, Virginia, earthquake of 23 August 2011

The Mineral, Virginia (USA), earthquake occurred at 17:51:3.9 UTC (Coordinated Universal Time) on 23 August 2011; the hypocenter was at 37.905 degrees N, 77.975 degrees W and depth was 8 km. The widely reported moment magnitude (M (sub w) ) was 5.7 + or - 0.1. The m (sub b) (teleseismic short-period) magnitude estimated here using 26 global network stations is 5.77 + or - 0.23, in agreement with m (sub b) values reported by national and international data centers, and not significantly different from M (sub w) . However, the m (sub bLg) magnitude of the earthquake determined here using 386 stations in eastern North America is 6.28 + or - 0.26. The m (sub bLg) magnitude is a short-period magnitude correlated with m (sub b) that is based on the amplitude of the Lg phase at regional distances. Lg is a crust-guided phase that represents the largest amplitudes observed on short-period seismograms at regional distances in eastern North America. The m (sub bLg) magnitude was the primary magnitude appearing in catalogs of eastern United States earthquakes until superseded recently by M (sub w) . The catalog of previous earthquakes in central Virginia is keyed to m (sub bLg) , rather than M (sub w) . The Mineral shock reveals large regional variations in the Lg phase attenuation in the eastern United States. The expected value for the return period of m (sub bLg) 6.3 and larger earthquakes in the Central Virginia seismic zone is 752 yr, with a 95% confidence interval of 385-1471 yr. The Mineral earthquake caused Modified Mercalli Intensity (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 fits the velocity recordings and Fourier spectral amplitudes in the 1-10 Hz frequency band, at the only strong-motion station in that distance range. The strongest motions are predicted to have occurred in two areas offset to the northwest and southeast of the epicenter, within which peak ground accelerations may have approached 2 g, and peak velocities were probably well in excess of 20 cm/s. The only factor mitigating damage in this earthquake was the brief (<3 s) duration of strong shaking.