A microearthquake survey of the Snake River plain and the Stanley-Sunbeam area of central Idaho was conducted during the summer of 1972 employing highgain, high-frequency portable seismographs. In 3 weeks of recording at various localities in the Snake River plain, a major east-west trending zone of Pliocene to Holocene basalt and rhyolite, no earthquakes were observed. However, in 8 days of recording near Stanley, at the east edge of the Idaho batholith, more than 40 microearthquakes were recorded, of which 18 were accurately located. All of the events in the Stanley area occurred in the uppermost part of the crust, with focal depths of less than 6 km. A single focal mechanism cannot be determined by a composite plot of first motions. The events cluster in space and time, suggesting earthquake swarm development perhaps associated with the geothermal activity of the Sunbeam hot-springs district.

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