Poster of the Potomac-Shenandoah Region Earthquake of 16 July 2010 - Magnitude 3.6

Tectonic Summary

Earthquakes in Maryland and Northern Virginia are uncommon but not unprecedented. The earthquake on July 16th, 2010 occurred in a part of the Eastern Seaboard that is less seismically active than central Virginia, New England, and the area surrounding New York City. Since 1980, 14 earthquakes have been felt within 80 km (about 50 miles) of the July 16th earthquake. All were smaller than this event. Other earthquakes have been reported in that area as far back as at least 1758.

The most recent earthquake to have been widely felt in the Washington area occurred west of Richmond, Virginia on December 9, 2003, in the Central Virginia Seismic Zone. It had a magnitude of 4.3 and was felt throughout the Washington-Baltimore area.

The largest earthquake on record in Virginia is a magnitude 5.9 in Giles County on May 31, 1897. More recently in the broader area, a magnitude 4.1 earthquake struck Lancaster County, Pennsylvania on April 22, 1984. This Pennsylvania earthquake caused slight damage at Conestoga, Lampeter, Mt. Nebo and New Providence and was felt from West Virginia to Connecticut. A series of small, felt earthquakes spanning March to July 1993 occurred near Columbia, Maryland, within 33 km (20 miles) of today’s earthquake. The largest in this series was a magnitude 2.7 in March, 1993. On May 5, 2003 a magnitude 3.9 event struck near Cartersville, Virginia about midway between Charlottesville and Richmond. That earthquake was felt widely in central Virginia and in parts of Maryland. The most recent earthquake in this region was a magnitude 2.0 event on May 6, 2008 near Annandale, Virginia.

Earthquakes occur on faults. However, east of the Rocky Mountains it is very difficult to determine which fault generated a particular earthquake with much confidence. As a result, it is unlikely that the fault responsible for the July 16th, 2010 earthquake will be identified.

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