The 2011 mineral, virginia, earthquake, and its significance for seismic hazards in eastern north america-overview and synthesis

Abstract

The 23 August 2011 Mw (moment magnitude) 5.7 ± 0.1, Mineral, Virginia, earthquake was the largest and most damaging in the central and eastern United States since the 1886 Mw 6.8-7.0, Charleston, South Carolina, earthquake. Seismic data indicate that the earthquake rupture occurred on a southeast-dipping reverse fault and consisted of three subevents that progressed northeastward and updip. U.S. Geological Survey (USGS) "Did You Feel It?" intensity reports from across the eastern United States and southeastern Canada, rockfalls triggered at distances to 245 km, and regional groundwater-level changes are all consistent with effi cient propagation of high-frequency seismic waves (~1 Hz and higher) in eastern North America due to low attenuation.Reported damage included cracked or shifted foundations and broken walls or chimneys, notably in unreinforced masonry, and indicated intensities up to VIII in the epicentral area based on USGS "Did You Feel It?" reports. The earthquake triggered the fi rst automatic shutdown of a U.S. nuclear power plant, located ~23 km northeast of the main shock epicenter. Although shaking exceeded the plant's design basis earthquake, the actual damage to safety-related structures, systems, and components was superfi cial. Damage to relatively tall masonry structures 130 km to the northeast in Washington, D.C., was consistent with source directivity, soft-soil ground-motion amplifi cation, and anisotropic wave propagation with lower attenuation parallel to the northeast-trending Appalachian tectonic fabric. The earthquake and aftershocks occurred in crystalline rocks within Paleozoic thrust sheets of the Chopawamsic terrane. The main shock and majority of aftershocks delineated the newly named Quail fault zone in the subsurface, and shallow aftershocks defi ned outlying faults. The earthquake induced minor liquefaction sand boils, but notably there was no evidence of a surface fault rupture. Recurrence intervals, and evidence for larger earthquakes in the Quaternary in this area, remain important unknowns. This event, along with similar events during historical time, is a reminder that earthquakes of similar or larger magnitude pose a real hazard in eastern North America.