M 2.3 - Ohio

Earthquakes in the Anna Seismic Zone

This small seismic zone in western Ohio has had moderately frequent earthquakes at least since the first one was reported in 1875. The largest earthquake (magnitude 5.1) caused damage in 1937. Moderately damaging earthquakes strike the Anna seismic zone every two or three decades, and smaller earthquakes are felt two or three times per decade.

Earthquakes in the central and eastern U.S., although less frequent than in the western U.S., are typically felt over a much broader region. East of the Rockies, an earthquake can be felt over an area as much as ten times larger than a similar magnitude earthquake on the west coast. A magnitude 4.0 eastern U.S. earthquake typically can be felt at many places as far as 100 km (60 mi) from where it occurred, and it infrequently causes damage near its source. A magnitude 5.5 eastern U.S. earthquake usually can be felt as far as 500 km (300 mi) from where it occurred, and sometimes causes damage as far away as 40 km (25 mi).

Faults

Earthquakes everywhere occur on faults within bedrock, usually miles deep. Most of the Anna seismic zone's bedrock was formed as several generations of mountains rose and were eroded down again a billion or more years ago.

At well-studied plate boundaries like the San Andreas fault system in California, often scientists can determine the name of the specific fault that is responsible for an earthquake. In contrast, east of the Rocky Mountains this is rarely the case. The Anna seismic zone is far from the nearest plate boundaries, which are in the center of the Atlantic Ocean and in the Caribbean Sea. Several faults are known in the seismic zone. Some of the earthquakes in the zone appear to coincide with the Anna-Champaign fault, although other earthquakes occur far from any known fault. Numerous smaller or deeply buried faults may remain undetected. Even the known faults are poorly located at earthquake depths. Accordingly, few earthquakes in the seismic zone can be linked to named faults. It is difficult to determine if a known fault is still active and could slip and cause an earthquake. As in most other areas east of the Rockies, the best guide to earthquake hazards in the Anna seismic zone is the earthquakes themselves.