Historic Earthquakes

Lancaster County, Pennsylvania
1984 April 23 01:36:00 UTC (local April 22)
Magnitude 4.4

This earthquake was centered near Marticville. It caused minor damage at Conestoga, where a garage shifted 1.3 cm off its foundation; plaster fell from a ceiling; and cracks formed in windows, concrete basement walls, and a cistern. Similar kinds of damage occurred at Lampeter, Mount Nebo, and New Providence. Also felt in Connecticut, Delaware, District of Columbia, Maryland, New Jersey, New York, Virginia, and West Virginia.

One foreshock occurred 5 days earlier and many slight aftershocks occurred. Aftershock data suggested a north-northeast fault dipping steeply east, with reverse, right-lateral slip consistent with a horizontal east-northeast axis of maximum compression. The geometry of the 1984 rupture conforms to the strike of Jurassic dikes and associated faults in the epicentral area.

Abridged from Seismicity of the United States, 1568-1989 (Revised), by Carl W. Stover and Jerry L. Coffman, U.S. Geological Survey Professional Paper 1527, United States Government Printing Office, Washington: 1993.