The Mw 5.8 Mineral, Virginia, Earthquake of August 2011 and Aftershock Sequence: Constraints on Earthquake Source Parameters and Fault Geometry

Abstract

The Mw 5.8 earthquake of 23 August 2011 (17:51:04 UTC) (moment, M0 5:7 × 10 17 N·m) occurred near Mineral, Virginia, within the central Virginia seis- mic zone and was felt by more people than any other earthquake in United States history. The U.S. Geological Survey (USGS) received 148,638 felt reports from 31 states and 4 Canadian provinces. The USGS PAGER system estimates as many as 120,000 people were exposed to shaking intensity levels of IV and greater, with approximately 10,000 exposed to shaking as high as intensity VIII. Both regional and teleseismic moment tensor solutions characterize the earthquake as a northeast- striking reverse fault that nucleated at a depth of approximately 7 2 km. The distri- bution of reported macroseismic intensities is roughly ten times the area of a similarly sized earthquake in the western United States (Horton and Williams, 2012). Near- source and far-field damage reports, which extend as far away as Washington, D.C., (135 km away) and Baltimore, Maryland, (200 km away) are consistent with an earthquake of this size and depth in the eastern United States (EUS). Within the first few days following the earthquake, several government and aca- demic institutions installed 36 portable seismograph stations in the epicentral region, making this among the best-recorded aftershock sequences in the EUS. Based on modeling of these data, we provide a detailed description of the source parameters of the mainshock and analysis of the subsequent aftershock sequence for defining the fault geometry, area of rupture, and observations of the aftershock sequence mag- nitude-frequency and temporal distribution. The observed slope of the magnitude- frequency curve or b-value for the aftershock sequence is consistent with previous EUS studies (b 0:75), suggesting that most of the accumulated strain was released by the mainshock. The aftershocks define a rupture that extends between approxi- mately 2-8 km in depth and 8-10 km along the strike of the fault plane. Best-fit modeling of the geometry of the aftershock sequence defines a rupture plane that strikes N36°E and dips to the east-southeast at 49.5°. Moment tensor solutions of the mainshock and larger aftershocks are consistent with the distribution of aftershock locations, both indicating reverse slip along a northeast-southwest striking southeast- dipping fault plane.