Magnitude, recurrence interval, and near-source ground-motion modeling of the Mineral, Virginia, earthquake of 23 August 2011
Martin C. Chapman, 2015. "Magnitude, recurrence interval, and near-source ground-motion modeling of the Mineral, Virginia, earthquake of 23 August 2011", The 2011 Mineral, Virginia, Earthquake, and Its Significance for Seismic Hazards in Eastern North America, J. Wright Horton, Jr., Martin C. Chapman, Russell A. Green
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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°N, 77.975°W and depth was 8 km. The widely reported moment magnitude (Mw) was 5.7 ± 0.1. The mb (teleseismic short-period) magnitude estimated here using 26 global network stations is 5.77 ± 0.23, in agreement with mb values reported by national and international data centers, and not significantly different from Mw. However, the mbLg magnitude of the earthquake determined here using 386 stations in eastern North America is 6.28 ± 0.26. The mbLg magnitude is a short-period magnitude correlated with mb 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 The mbLg magnitude was the primary magnitude appearing in catalogs of eastern United States earthquakes until superseded recently by Mw. The catalog of previous earthquakes in central Virginia is keyed to mbLg, rather than Mw. 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 mbLg 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.