On January 31, 1986, an earthquake of Richter magnitude 4.9 occurred in northeastern Ohio some 17.0 km south of the Perry Nuclear Power Plant (PNPP) and 12.0 km south of the Calhio injection wells. Accelerometers on site at the PNPP recorded accelerations as high as 0.19 to 0.23 g. Many instruments tripped due to high-amplitude vibrations. Microearthquake networks have recorded 16 microearthquakes within 5.0 km of the injection wells with focal depths ranging from 1.0 to 3.0 km. A hydrological model of an anisotropic reservoir 7.2 km wide and 18.4 km long indicates a pressure buildup of 5.3 MPa at the epicenter and 11.8 MPa at the injection well. The assumption of an anisotropic reservoir is consistent with available geophysical and geologic data. A pressure increase of 11.8 MPa, based on stress ratio estimates in crustal rocks in the region, is more than sufficient to induce failure to a depth of 5.0 km. Furthermore, brittle faults and extensive fracture permeability within the basement rocks would allow for the migration of pressure transients to hypocentral distances. The indicated pressure buildup of 5.3 MPa at the epicenter may have been sufficient to trigger the January 31, 1986, earthquake.

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