Summary of a paper

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

By Seeber, Armbruster, Kim, Barstow, and Scharnberger

Introduction:

The 1994 earthquakes near Reading are better known as the Cacoosing Valley earthquakes. Cacoosing Valley is about 10km west of Reading, Pennsylvania. The mainshock mb (Lg) of 4.6 occurred on January 16, 1994. Most of the permanent damage from the earthquake triggered nontectonic deformation such as sinkholes, which are common in the karst environment. The foreshock was recorded as mb (Lg) 4.0 and occured an hour prior to mainshock. The environment of the Cacoosing Valley consists of many preexisting structures; the Paleozoic thrust and fold belts, Mesozoic rifting which created the Atlantic Ocean, and Jurassic igneous dikes. All of these structures create different strain rates and possible planes of failure. Though the presence of preexisting structure enables weak planes of strain, the biggest factor in the Cacoosing earthquake is the mechanical unloading of rock. The unloading of the quarry, which then was abandoned and flooded with water, created enough strain to cause the rupture.

Lancaster Seismic Zone:

The Lancaster seismic zone (LSZ) refers to the area approximately 50x50km in southeastern Pennsylvania that has a two-century history of earthquake epicenters. The LSZ has had several 4.0 events, including the 4.6 event of Cacoosing Valley. The LSZ also marks the southwest terminus of the larger Newark Basin Seismic Zone (NBSZ) that extends along the Appalachians from southeastern Pennsylvania to southern New York. The largest earthquake recorded in the NBSZ was a Mfa (felt area) 5.0 in 1884. There seems to be a reactivation of many of the preexisting faults, which cover a broad area of the NBSZ, including the massifs (Reading and Manhattan Prong), the exotic terranes to the southeast, and the Paleozoic suture line.

The main geologic structures in the Newark Basin Seismic Zone, which includes the Lancaster Seismic Zone, are 1) thrust faults and folds associated with shortening and thickening during a sequence of Paleozoic collisional orogenies and 2) extensional faults associated with Mesozoic rifting. Many of the extensional faults follow the same trend as the preexisting compressional faults and suggest reactivation of Paleozoic thrust faults into Mesozoic normal faults. One more important structure are the many Jurassic dikes that cut across the internal Appalachians and the Mesozoic extensional basins. These dikes are associated with many brittle faults and large planes of weakness suggesting that they too have an effect on the amount of seismicity in the LSZ. It seems that most of the seismicity in the LSZ is occurring on secondary faults at high angles to the main structures of the Appalachians

Earthquake:

The epicenter of the Cacoosing earthquake was probably in the low-grade metamorphic carbonate rocks of early Paleozoic age, and located in the foot-wall of the border fault of the Newark-Gettysburg extensional basins. The first stage of events that led to the 1994 Cacoosing earthquake is the tectonic stress. Though the region of the East Coast is known as a passive margin and does not lie on a plate boundary, there is still a small tectonic stress from the mid-ocean ridge about two thousand miles to the east in the Atlantic ocean. The second stage is the process of mechanical unloading, or quarrying. Quarrying began in the 1940's, which began decreasing the stress on the fault down below the quarry. The first rupture occurred in 1954. The third and next stage of events was the dewatering of the quarry. The engineers pumped the groundwater out so they could quarry deeper and get more rock. Taking out the water lowers the pore pressure, which actually counteracts the effect of unloading making the fault more resistant to failure. The fourth and final stage of events was when the quarry shut down in 1992. The pumps were turned off letting the quarry flood. This raises the pore pressure and brings the fault closer to failure, at a much faster rate than unloading. The presence of fluids and the great amount of pore pressure lower the strength of the rock and enable it to fail at a much lower stress.

Conclusion:

The earthquake ruptured very shallow, at about two kilometers, dipping 54 degrees, striking southeast, and slipping reverse and left lateral. The rupture developed in one or more stacked Appalachian thrust sheets containing low-grade metamorphic carbonate rocks. The earthquake is oblique to all preexisting brittle faults mapped in the area but parallel to the main joint set. The earthquake was probably triggered by the unloading of the rock quarry, which is centered above the hanging wall of rupture. Five months after the pumps were turned off seismicity began, which is thought to appear from the pore pressure rise. The largest amount of stress the quarry feels is the change from lowering the water table until dewatering ceases, to the flooding and rise of the water table to the original conditions. A large stress change can be responsible for triggering preexisting planes of weakness and causing earthquakes.