The 2011 Virginia earthquake: What are scientists learning?

Nearly 1 year ago, on 23 August, tens of millions of people in the eastern United States and southeastern Canada were startled in the middle of their workday (1:51 P.M. local time) by the sudden onset of moderate to strong ground shaking from a rare magnitude ( M ) 5.8 earthquake in central Virginia. Treating the shaking as if it were a fire drill, millions of workers in Washington, D. C., New York City, and other eastern cities hurriedly exited their buildings, exposing themselves to potentially greater danger from falling bricks and glass; “drop, cover, and hold” would have been a better response. Fortunately, the strong shaking stopped after about 5 seconds and did not cause widespread severe damage or serious injuries. The central Virginia earthquake, among the largest on the eastern seaboard during the approximately 400‐year historic record, occurred as the result of reverse slip on a previously unrecognized north‐to‐northeast striking fault within the Central Virginia seismic zone (CVSZ) (Figure 1a). Many old faults are mapped in the CVSZ, yet no individual strands were previously confirmed to be active. However, persistent low‐level seismicity has been observed during historical times, and instrumental recordings since about 1970 detect ongoing distributed seismicity within the CVSZ [ Bollinger and Hopper , 1971], which has been identified by the U.S. Geological Survey (USGS) as an area of elevated earthquake hazard since 1976 [ Algermissen and Perkins , 1976].