The Mineral, Virginia (USA), earthquake of 23 August 2011 was an unusually strong seismic event in the eastern United States. It caused widespread structural damage to residential property near the epicenter. An analysis of residential property damage reports, in conjunction with visits to some damaged residences, reveals a 40 km 2 area of concentrated damage centered 11 km south of the town of Louisa. This area is west of the earthquake’s epicenter and may be in the immediate hanging wall of a northeast-striking, moderately southeast dipping causative fault suggested by seismic data. The degree of damage in this area is consistent with a maximum Modified Mercalli Intensity (MMI) of VIII. A surrounding area of ~550 km 2 reported damage that is consistent with an MMI intensity of VII. A statistical analysis of dwelling characteristics confirms that home age and condition were factors that influenced the frequency and severity of reported property damage. The median damage to homes constructed between 1900 and 1973, relative to assessed value, was approximately twice that of homes constructed after 1973 in Louisa County, and three times greater within areas of MMI intensity VI, VII, and VIII.

Abstract The karst of the central Shenandoah Valley has characteristics of both shallow and deep phreatic formation. This field guide focuses on the region around Harrisonburg, Virginia, where a number of these karst features and their associated geologic context can be examined. Ancient, widespread alluvial deposits cover much of the carbonate bedrock on the western side of the valley, where shallow karstification has resulted in classical fluviokarst development. However, in upland exposures of carbonate rock, isolated caves exist atop hills not affected by surface processes other than exposure during denudation. The upland caves contain phreatic deposits of calcite and fine-grained sediments. They lack any evidence of having been invaded by surface streams. Recent geologic mapping and LIDAR (light detection and ranging) elevation data have enabled interpretive association between bedrock structure, igneous intrusions, silicification and brecciation of host carbonate bedrock, and the location of several caves and karst springs. Geochemistry, water quality, and water temperature data support the broad categorization of springs into those affected primarily by shallow near-surface recharge, and those sourced deeper in the karst aquifer. The deep-seated karst formation occurred in the distant past where subvertical fracture and fault zones intersect thrust faults and/or cross-strike faults, enabling upwelling of deep-circulating meteoric groundwater. Most caves formed in such settings have been overprinted by later circulation of shallow groundwater, thus removing evidence of the history of earliest inception; however, several caves do preserve evidence of an earlier formation.