\nSince at least 1828, people in the Giles County seismic zone of southwestern Virginia and \nadjacent West Virginia have felt small earthquakes and suffered damage from infrequent \nlarger ones. The \nlargest damaging earthquake\n (magnitude 5.9) in the seismic zone \noccurred in 1897. Smaller, slightly damaging earthquakes occur at variable intervals, but \nin the zone they tend to occur a few decades apart. Still smaller earthquakes that cause no \ndamage are felt once or twice a decade in the seismic zone.\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 southwestern Virginia and adjacent West Virginia was assembled as \ncontinents collided to form a supercontinent about 500-300 million years ago, raising the \nAppalachian Mountains.\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 Giles \nCounty seismic zone is far from the nearest plate boundaries, which are in the center of \nthe Atlantic Ocean and in the Caribbean Sea. The seismic zone is laced with known faults \nbut numerous smaller or deeply buried faults remain undetected. Even the known faults \nare poorly located at earthquake depths. Accordingly, few, if any, earthquakes in the \nseismic zone can be linked to named faults. It is difficult to determine if a known fault is \nstill active and could slip and cause an earthquake. As in most other areas east of the \nRockies, the best guide to earthquake hazards in the seismic zone is the earthquakes \nthemselves.\n
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