THE DILLSBURG, PENNSYLVANIA 2008-2009 EARTHQUAKE SWARM

In early October, 2008, residents of Carroll Township, just southeast of the Borough of Dillsburg, in northern York County, Pennsylvania, began to feel and report small earthquakes. As of the end of November, 2009, more than 800 earthquakes had been reported. Residents reported booms and sulfur smell with the tremors. Twenty seven of these earthquakes were large enough to be recorded at regional seismic stations, the closest of which is approximately 40 km (25 mi) distant. The largest event, on October 25, 2009, had a magnitude of 2.9. Two temporary seismograph networks were operated, one in the fall of 2008, the other in the summer of 2009, to gather data on this swarm. The earthquakes are occurring in the Mesozoic Gettysburg Basin. Hypocenters appear to be located within and just below a 350 m (1150 ft) thick sheet of York Haven diabase. The Jurassic diabase intrudes sandstone, shale, and limestone fanglomerate of the Triassic Gettysburg Formation, which has been metamorphosed and mineralized by the intrusion. Formerly, magnetite was mined where the current earthquake swarm is occurring. What has triggered this swarm is unknown. Factors under consideration are the area’s mining history, hydrologic conditions, and stress concentrations at the contact between diabase and metasediments.