The 1994 Cacoosing Valley earthquakes near Reading, Pennsylvania: A shallow rupture triggered by quarry unloading

A m(b)(Lg) 4.6 mainshock on January 16, 1994, in the Cacoosing valley, 10 km west of Reading, southeastern Pennsylvania caused modified Mercalli VI-VII intensities and the highest earthquake damage (similar to$2 million) in the eastern United States since 1944. Aftershock hypocenters from a temporary local seismic network are confined to the upper 2.5 km of the crust. They are clustered around the periphery of a tabular 3x3 km zone that is interpreted to outline the mainshock rupture. This zone matches the nodal plane with reverse and left-lateral slip (strike 135 degrees, dip 54 degrees southwest, and rake 55 degrees) of a focal mechanism obtained from aftershock first motions and from mainshock waveforms. This rupture does not correlate with any of the faults mapped in the epicentral area, but it is parallel to the most prominent fracture set, including joints and small faults. Maximum possible strike-slip accumulated on the causative fault is no more than several tenths of meters. A large carbonate rock quarry is centered above the rupture. We calculate a small, but significant (0.13 MPa) Coulomb stress increase caused by the quarry on the shallow portion of the rupture. Most of this increase was caused by pore pressure rise after the quarry was abandoned in December 1992 and flooded. Seismicity started may 1993. We conclude that the 1993-1997 Cacoosing Valley sequence is probably triggered by the quarry. About 200 km northeast of Cacoosing, another quarry in early Paleozoic carbonate rocks triggered the 1974 m(b)(Lg) 3.0 Wappingers Falls earthquake.