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Pennsylvania lies athwart these two zones. The northwestern zone crosses the northwest corner of Pennsylvania; the southeastern one passes through the Philadelphia area. A subsidiary zone of enhanced seismicity, the Lancaster seismic zone, lies to the northwest of the latter.
The Pennsylvania Geological Survey has prepared a catalog of reported seismic events in and near Pennsylvania. Three hundred seventy events from 1724 to the present are recorded in this database. For each event, the database contains the time and location, the magnitude, intensity, and depth of focus (where known), and a list of references. Foreshocks and aftershocks are listed separately from main shocks. Magnitudes range from 0.4 to 5.1, the largest being the 1998 Pymatuning earthquake in northwestern Pennsylvania.
An epicenter map of Pennsylvania, keyed directly to the catalog, portrays the areal distribution of seismic events. Map symbols reflect the magnitude of the largest event at each location. The natural events are subdivided into four categories based on whether the epicenter is within or outside of Pennsylvania and the type of record (instrumental or pre-instrumental). A fifth category contains quarry blasts and mine collapses that were incorrectly recorded as natural seismic events in other catalogs.
This local, detailed catalog provides Pennsylvania's regulatory agencies a means for evaluating seismic activity as an environmental hazard to be considered in the design and construction of structures.