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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