Between April 1982 and June 1984, 311 earthquakes were detected, and 176 earthquakes were located in the vicinity of the Sleepy Hollow oil field, southwestern Nebraska. The oil field is being injected with water to enhance oil production, and the earthquakes may be related to production. There is geologic evidence, however, to suggest that the earthquakes are tectonic in origin and are related to uplift along the axis of the Cambridge Arch, on which the field is located. First motion data from these events do not yield a well-constrained fault plane solution. Furthermore, first motion detections are inconsistent for earthquakes with nearly coincident hypocenters, indicating that more than one faulting mechanism is active in the oil field. Injection pressures do not correlate well with earthquake occurrence.

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