Here is an update on the continuing seismic activity near the borough of Dillsburg, in northern York County, PA. The swarm began on Friday, October 3, and has continued until at least Saturday, November 8. Residents of Carroll Township, just southeast of Dillsburg, report hearing and feeling many "bumps" that resemble explosions. Sometimes these occur in clusters and sometimes singly. At least 100 of these small earthquakes have been reported since the swarm began. Some have been large enough to be detected by stations in the LCSN. The largest recorded so far occurred early Sunday morning, October 19. It had a coda magnitude of 2.1. (Coda magnitude is calculated by measuring the duration of the decaying signal following the initial impulse, and applying an empirically determined formula that is different for different regions.) A local network of four temporary stations has been deployed around the area where the earthquakes are being felt. Records from these stations should provide highly accurate times and locations for these events. The fact that these very small earthquakes are being felt so distinctly by local residents implies that they are originating at a very shallow depth.
Similar swarms of small earthquakes have occurred in other locations in the Northeast, notably in Columbia, Maryland, in 1993, and Moodus, Connecticut, in the 1980s (and going back to colonial times.) The Moodus swarm was investigated by John Ebel of Boston College (Seismological Research Letters, vol. 60, No. 4, Oct.-Dec., 1989, pp. 177-184). The hypocenters were found to range in depth from 0.4 to 2.4 km., but could not be associated with any obvious fault or other geologic structure. The focal mechanism was found to be reverse faulting with the axis of maximum compressive stress oriented WNW-ESE. Moodus, like Dillsburg, is located in a Mesozoic basin where Triassic age sedminetary rocks have been intruded by diabase.
About 200 local residents attended a public meeting at the Carroll Township building on October 28. Some were fearful that this swarm of small earthquakes may be a precursor to a large, damaging quake. In the light of experience with similar swarms elsewhere, this seems highly unlikely. In all probability, the swarm will continue for a few more weeks or months, and then stop as mysteriously as it began.