\nSince colonial times, people in the Lancaster seismic zone of southeastern Pennsylvania \nhave felt small earthquakes and suffered damage from larger ones. Earthquakes are felt \nonce or twice per decade, with some decades having none and the 1990s having as many \nas six.\n
\n\nEarthquakes in the central and eastern U.S., although less frequent than in the western \nU.S., are typically felt over a much broader region. East of the Rockies, an earthquake \ncan be felt over an area as much as ten times larger than a similar magnitude earthquake \non the west coast. A magnitude 4.0 eastern U.S. earthquake typically can be felt at many \nplaces as far as 100 km (60 mi) from where it occurred, and it infrequently causes \ndamage near its source. A magnitude 5.5 eastern U.S. earthquake usually can be felt as \nfar as 500 km (300 mi) from where it occurred, and sometimes causes damage as far \naway as 40 km (25 mi).\n
\n\n\nEarthquakes everywhere occur on faults within bedrock, usually miles deep. Most \nbedrock beneath the seismic zone was assembled as continents collided to form a \nsupercontinent about 500-300 million years ago, raising the Appalachian Mountains. \nMost of the rest of the bedrock formed when the supercontinent rifted apart about 200 \nmillion years ago to form what are now the northeastern U.S., the Atlantic Ocean, and \nEurope.\n
\n\nAt well-studied plate boundaries like the San Andreas fault system in California, often \nscientists can determine the name of the specific fault that is responsible for an \nearthquake. In contrast, east of the Rocky Mountains this is rarely the case. The Lancaster \nseismic zone is far from the nearest plate boundaries, which are in the center of the \nAtlantic Ocean and in the Caribbean Sea. The seismic zone is laced with known faults but \nnumerous smaller or deeply buried faults remain undetected. Even the known faults are \npoorly located at earthquake depths. Accordingly, few, if any, earthquakes in the seismic \nzone can be linked to named faults. It is difficult to determine if a known fault is still \nactive and could slip and cause an earthquake. As in most other areas east of the Rockies, \nthe best guide to earthquake hazards in the Lancaster seismic zone is the earthquakes \nthemselves.\n
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