\nSince at least 1877 people in the urban corridor have felt small earthquakes. They occur \nabout once per decade, although some decades have none and the 1990s had three. None \nare known to have caused damage since the arrival of European colonists. The corridor is \nbetween more seismically active regions to the southwest and northeast, and residents of \nWashington or Baltimore have felt several earthquakes that caused damage in those \nother, more active regions.\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 Washington-Baltimore urban corridor was assembled as continents \ncollided to form a supercontinent about 500-300 million years ago, raising the \nAppalachian Mountains. Most of the rest of the bedrock formed when the supercontinent \nrifted apart about 200 million years ago to form what are now the northeastern U.S., the \nAtlantic Ocean, and Europe.\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 \nWashington - Baltimore urban corridor is far from the nearest plate boundaries, which \nare in the center of the Atlantic Ocean and in the Caribbean Sea. The urban corridor is \nlaced with known faults but numerous smaller or deeply buried faults remain undetected. \nEven the known faults are poorly located at earthquake depths. Accordingly, few, if any, \nearthquakes in the urban corridor can be linked to named faults. It is difficult to \ndetermine if a known fault is still active and could slip and cause an earthquake. As in \nmost other areas east of the Rockies, the best guide to earthquake hazards in the \nWashington - Baltimore urban corridor is the earthquakes themselves.\n
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