SEISMICITY AND SEISMIC HAZARD IN THE MID-ATLANTIC REGION

The mid-Atlantic region is considered here to be the area bounded by latitudes 37 and 41 north, and longitudes 71 and 79 west. This region extends from Long Island southward to almost the southern border of Virginia, and westward to approximately the longitude of Johnstown, PA, and Lynchburg, VA. Three named seismic zones (SZ) are located in the mid-Atlantic region: the Pennsylvania-New Jersey SZ, the Lancaster SZ, and the Central Virginia SZ, which was the site of the largest known earthquake in the region—the magnitude 5.8 event in August of 2011. The NEIC catalog lists 91 earthquakes of magnitude at least 2.5 in this region over the past 40 years. A plot of number of events versus magnitude suggests that this list is reasonably complete down to that magnitude. The second largest mid-Atlantic earthquake, a magnitude 4.6 event in Berks County, PA, in January of 1994, very likely was induced by flooding of a recently abandoned quarry. One of the more interesting seismic occurrences was a swarm of more than 1000 small earthquakes in northern York County, PA, beginning in the fall of 2008 and continuing for more than a year. A similar, though shorter-lived, swarm occurred in a suburb of Harrisburg, PA, in the fall of 2011.

As is usually the case with earthquakes in eastern North America, mid-Atlantic earthquakes cannot be tied unambiguously to known faults or other geologic structures, although fractures associated with Mesozoic basins, diabase intrusions, and the southwestern termination of the New England crystalline province have been proposed by various authors to be of seismogenic significance.

The mid-Atlantic region includes 4 of the 20 largest Metropolitan Statistical Areas in the United States, with a combined population of over 33 million. The national earthquake hazard maps identify the two areas of greatest hazard as being in the New York metropolitan area, and in the Central Virginia SZ, near Richmond, VA, and Washington, DC. Despite the large population and extensive built environment at risk, earthquake preparedness in this region remains relatively low.