\nThis small seismic zone in western Ohio has had moderately frequent earthquakes at least \nsince the first one was reported in 1875. The largest earthquake (magnitude 5.1) caused \ndamage in 1937. Moderately damaging earthquakes strike the Anna seismic zone every \ntwo or three decades, and smaller earthquakes are felt two or three times per decade.\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 of the \nAnna seismic zone's bedrock was formed as several generations of mountains rose and \nwere eroded down again a billion or more years ago.\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 Anna \nseismic zone is far from the nearest plate boundaries, which are in the center of the \nAtlantic Ocean and in the Caribbean Sea. Several faults are known in the seismic zone. \nSome of the earthquakes in the zone appear to coincide with the Anna-Champaign fault, \nalthough other earthquakes occur far from any known fault. Numerous smaller or deeply \nburied faults may remain undetected. Even the known faults are poorly located at \nearthquake depths. Accordingly, few earthquakes in the seismic zone can be linked to \nnamed faults. It is difficult to determine if a known fault is still active and could slip and \ncause an earthquake. As in most other areas east of the Rockies, the best guide to \nearthquake hazards in the Anna seismic zone is the earthquakes themselves.\n
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