Abstract

An earthquake with body wave magnitude 4.5 occurred in the Anna Ohio Seismogenic Zone near the town of St. Marys, Ohio on 12 July 1986. The St. Marys earthquake had a seismic moment of 4.5 × 1022 dyne-cm and a maximum intensity of VI in the epicentral area. It is the largest event to occur in the Anna, Ohio region since the events of 2 March 1937 (total felt area magnitude=4.7) and 9 March 1937 (total felt area magnitude=4.9). The focal mechanism for the St. Marys earthquake indicates a nearly pure strike-slip motion with one nodal plane approximately parallel to the proposed Anna-Champaign Fault and a nearly horizontal p-axis oriented east-northeast. Although the locations of the 1937 events are not well-determined, reports of maximum intensities for the 1937 events near Anna, Ohio indicate that they occurred in a distinctly different location from the St. Marys earthquake. The seismicity in the Anna Seismogenic Zone indicates that much of the seismic activity is concentrated in a northwest to southeast elongate region, extending from Anna, Ohio northwest past Celina, Ohio. This trend parallels the proposed Anna-Champaign Fault as well as lineaments which can be detected on Landsat images, which suggests that this fault may be controlling the seismic behavior in this region.

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