Microearthquakes in Western Ohio, 1980-1981
(from Winter 1982 Ohio Geology)

Nine small earthquakes were recorded during 1980 and 1981 in western Ohio by the seismic array maintained in that part of the state by the University of Michigan Seismological Observatory (this array operated from 1977 to 1992). These events and data pertaining to them are listed in the accompanying table.

 Western Ohio Microearthquakes

*UTC, Universal Coordinated Time, is equal to Greenwich Mean Time (GMT) and is used to standardize all earthquakes. To convert to local time (Eastern Standard Time) subtract 5 hours (4 hours for Daylight Savings Time)from UTC. Note that events that occur in early morning hours on UTC are listed a day later than local time.

Six of the earthquakes occurred within a 4-mi2 area defined by the Shelby County communities of Anna, Botkins, and Jackson Center. This area lies close to a proposed north-south fault (Auglaize fault) and is the general area commonly mentioned in historic seismic literature. The reason for the rather sudden beginning and end to this cluster of events is uncertain. None of the earthquakes were reported to be felt by local residents.

Shelby County and vicinity, commonly referred to as the Anna area because of the historic seismic activity experienced by this community, has reported at least 35 felt earthquakes in the last century.

Included in this number are the two most severe earthquakes to strike Ohio, the events of March 2 and 9, 1937. The March 2 earthquake was assigned a Modified Mercalli intensity of VII, whereas the March 9 event was estimated to have an intensity of VIII and a Richter magnitude of 5.5. Earthquake-caused damage in the community of Anna included toppled chimneys and cracked plaster. The relatively new school building at Anna was so severely cracked that it was declared unsafe and torn down.

In addition to the Shelby County activity, two small earthquakes were recorded in 1980-81 in Putnam County and one event was recorded on the border of Clark and Greene Counties. These three events, like the Shelby County earthquakes, were of very small magnitude and not reported to he noticed by local residents. Unlike the Shelby County events, however, these earthquakes occurred in areas where little or no historic seismicity has been reported and no fault has been identified or postulated. The only record of any possible activity occurred in 1845 on a farm in Putnam County where some type of displacement caused a ridge of ground to be shifted 4 feet. No shock was reported to be felt (Bradley, E. A., and Bennett, T. J., 1965, Earthquake history of Ohio, Bulletin of the Seismological Society of America, v. 55, p. 745-752).

It should be noted that the 1980-81 earthquakes represent the smallest magnitudes that have been reported from Ohio. Such small events undoubtedly occur from time to time in western Ohio and probably elsewhere in the state, but because they do not have sufficient intensity to be felt by local residents they are never reported. It has been only since the installation of the University of Michigan seismic array (nine remote stations) in western Ohio in 1977 that sufficient instrumentation has been available to record such events. Although such small earthquakes are in themselves of no danger, they do provide important clues to geologic structures (and potential economic mineral resources associated with these structures) and delineate areas where future seismicity may occur.

--Michael C. Hansen