abstract

Most historical earthquakes near the Three Forks Basin, Montana have concentrated in the southwestern and northern portions of the basin and not along the spectacular escarpment of the Bridger Range fault which bounds the basin on the east.

Relocation of earthquakes from 1974 to 1977 using earthquake data from regional seismograph networks and three microearthquake surveys (1974, 1976, and 1977) show that the most seismically active area is the Clarkston Valley, 10 km north of the Three Forks Basin. Reanalysis of travel-time data confirms Pardee's (1926) hypothesis that the large Montana earthquake of 1925 (Magnitude 634) also occurred in Clarkston Valley. The seismologically determined focal mechanism for this earthquake differs from fault mechanisms of recent events in the valley.

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