Microearthquake monitoring by a permanent network operated since 1971 and two recent temporary surveys performed by the Idaho National Engineering Laboratory suggest that infrequent, small-magnitude earthquakes are characteristic of eastern Snake River Plain (ESRP) seismicity. Although a total of only 19 earthquakes have been observed to date, and not all have well-constrained focal depths, their relatively shallow occurrence at depths of 8 km is consistent with the hypothesis that elevated crustal temperatures in the ESRP confine the brittle portion of the crust, and hence seismogenesis, to the upper 6 to 10 km. A composite focal mechanism of two microearthquakes located near the axis of the ESRP indicates normal faulting with a minor component of strike-slip motion. The northeast-southwest-trending T-axis indicated by this composite mechanism is consistent with the direction of extension indicated by Holocene volcanic rift zones within the ESRP and normal faults in the adjacent Basin and Range province to the northwest. Based on this low level of small-magnitude seismicity, deviatoric stresses appear to be small in the ESRP.

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