GEOLOGIC, SEISMOLOGIC, AND ENGINEERING LESSONS LEARNED FROM THE 2011 MINERAL, VIRGINIA EARTHQUAKE

The M_w5.7-5.8, 23 August 2011 Mineral, VA earthquake was the largest and most damaging in the central and eastern U.S. since the 1886 Charleston, SC earthquake. Aftershock monitoring and reconnaissance fieldwork were followed by robust seismologic studies and detailed bedrock and surficial geologic investigations. New airborne geophysical surveys (magnetic, gravity, radiometric, LiDAR) were done, and seismic-reflection data were reprocessed to image subsurface geology at hypocentral depths. Damage assessments and geotechnical studies from the epicentral area to Washington, DC were also completed.

Seismic data indicate that the earthquake rupture occurred on a SE-dipping reverse fault and consisted of three subevents that progressed NE and up dip. USGS “Did You Feel It?” intensity reports throughout the eastern U.S. and southeastern Canada, rockfalls triggered at distances up to 245 km, and regional groundwater-level changes are all consistent with efficient propagation of high-frequency seismic waves in eastern North America due to weak anelastic attenuation as previously suggested.

Damage included cracked or shifted foundations and broken walls or chimneys, notably in unreinforced masonry and brick facades. The earthquake triggered the first automatic shutdown of a nuclear power plant in the U.S. Although shaking exceeded the plant’s design basis earthquake, damage to safety-related structures, systems, and components was superficial. Damage to structures 130 km to the NE in Washington, DC is consistent with source directivity, soft-soil ground-motion amplification, and anisotropic wave propagation with least attenuation parallel to the northeasterly 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 previously unknown Quail fault zone, and shallow aftershocks defined outlying faults. The earthquake induced minor liquefaction sand boils, but there was no evidence of a surface fault rupture. Possible evidence of previous Quaternary(?) and older deformation near the South Anna River is under investigation. This event is a reminder that earthquakes of similar or larger magnitude pose a real hazard in eastern North America.