THE LANCASTER SEISMIC ZONE OF SE PENNSYLVANIA IN RELATION TO THE GETTYSBURG-NEWARK BASIN

The Lancaster Seismic Zone (LSZ) of southeast Pennsylvania is one of the most active zones in the eastern United States. Most well-located epicenters in the LSZ lie just outside the east-west trending Mesozoic Gettysburg-Newark Basin. The epicenters of 11 LSZ events since 1964 in the western part of the LSZ define a north-south trend that intersects the juncture between the Gettysburg and Newark sub-basins. This juncture is a hinge around which the two sub-basins subsided, resulting in east-west oriented tensile stress. Numerous north-south trending fractures and diabase dikes are consistent with this hypothesis. It is likely that seismicity in at least the western part of the LSZ is due to present-day NE-SW compressional stress which is activating these Mesozoic fractures, with dikes perhaps serving as stress concentrators. It is probable that some recent earthquakes in the LSZ have been triggered by surface mining.