An array of four seismographs operating in northwestern Kansas and southwestern Nebraska detected 31 earthquakes in the magnitude range 0.6 to 2.9 from March 1979 to March 1980. Of these, 22 were recorded at enough stations to allow accurate location, and at least three of them were felt by local residents. Sixteen of the located events occurred in the immediate vicinity of the most productive oil field in Nebraska: the Sleepy Hollow Field. A mail canvass revealed a well-constrained “felt” area about 10 km across which coincides remarkably well with the oil field. Water injection has been used as a method of enhanced recovery in this field since 1966. Some of the earthquakes occurred on possible downward extensions of faults in the deeper sedimentary rocks and granite basement, implying that some of the seismicity may be induced. Deeper events and other supporting data, however, suggest current tectonic activity on the Chadron-Cambridge Arch. The recently installed eight-station Sleepy Hollow Seismic Network should provide the data necessary to test these hypotheses.

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