CONTEMPORARY SEISMICITY IN THE EASTERN SNAKE RIVER PLAIN, IDAHO BASEDON MICROEARTHQUAKE MONITORING

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