Intensities and Structural Setting of the Earthquakes of 19 April and 23 April, 1984 Lancaster County, Pennsylvania

Abstract

Two small earthquakes and a series of aftershocks occurred in April–May 1984 in a north-south belt in faulted Cambrian and Ordovician rocks on the projection of a known north-south fault whose mapped portion ends eleven kilometers north of the recent epicenters. The April 23 earthquake was one of the largest which has occurred in Pennsylvania, being felt from Westchester County, New York to Cambria County in western Pennsylvania and south to Washington, D.C. and Sussex County, Delaware with a maximum intensity of Mercalli VI (magnitude = 4.2). The earthquakes were notable for their strong audible component, which may have extended the reported isoseismals beyond where they would normally have been mapped.