Oral

ARE MOST EARTHQUAKES IN SOUTHEAST PENNSYLVANIA INDUCED BY MINING?

SCHARNBERGER, C. K., Department of Earth Sciences, Millersville University, Millersville, PA 17551, charles.scharnberger@millersville.edu.

Seeber and others (1998, JGR, 103, B10, p. 24,505) have demonstrated the high probability that the 1994 Cacoosing Valley earthquakes in Berks County, Pennsylvania, were triggered by recent flooding of a local limestone quarry. The same authors speculate that an earthquake in the same vicinity in 1954, with maximum intensity of VI, also was triggered, at a time when the quarry was being enlarged to meet increased demand for aggregate. I hypothesize here that most of the recent seismicity in five counties of southeastern Pennsylvania, viz., Berks, Lancaster, Lebanon, Lehigh and Montgomery, is mining-related. This conclusion is suggested by the following observations. 1) With the exception of the magnitude 4.1 Martic earthquake in Lancaster County in 1984, all 20th century epicenters are located close to active or recently active surface mines. This includes, besides the Berks County events mentioned above, earthquakes felt strongly in Allentown (Lehigh County), in northern Lancaster County, near Abington in Montgomery County in 1980, and at Cornwall, Lebanon County, in 1964. This last event occurred in the immediate vicinity of the largest open-pit iron mine in the eastern United States. 2) Almost all seismicity outside Lancaster County is a 20th century phenomenon. The one exception is the 1884 Allentown earthquake, whose date coincides with the beginning of large-scale quarrying of cement stone in that area. 3) Most earthquakes in the region have maximum intensities of V or VI, yet extremely small felt areas, probably an indication of very shallow focal depths. Most surface mines in southeast Pennsylvania do not have associated seismicity. This suggests that it is not merely unloading that causes earthquakes, but unloading in a locale with the right underlying conditions of stress and geologic structure. In this context, it is worth noting that all the apparently mining-related earthquakes have occurred less than 20 km from the borders of the Newark rift basin of Mesozoic age. Lancaster County has an earthquake history that stretches back into the 18th century, and the 1984 event, at least, had a more normal focal depth of about 5 km. Thus, it appears that structures in that county are capable of producing earthquakes of moderate size without inducement. This is in addition to the shallow, possibly mining-related events of the late 20th century.