Abstract

Hypocenters of regionally recorded earthquakes that occurred in central Idaho from 1944 through October 1985 have been recomputed with travel times calibrated by locally recorded aftershocks of the Borah Peak, Idaho, earthquake of 1983. The effect of the relocation is to define more sharply the sizes and relative positions of seismic source zones in central Idaho and to move epicenters systematically south, often by more than 10 km, from their previously cataloged positions. The distribution of epicenters of the Borah Peak main shock and early aftershocks suggests that the fault segment that ruptured in the main shock was approximately a parallelogram with one pair of sides parallel to the zone of surface fault scarps associated with the earthquake and with the other pair of sides parallel to the slip vector of the earthquake. Such a rupture shape would be expected on geometrical grounds if the 1983 rupture zone were terminated on the north and south by intersection with adjacent segments of the Lost River fault that have the same slip vector as the 1983 rupture. Epicenters of earthquakes occurring before 1983 define a seismic zone, the White Cloud Peaks zone, that is approximately parallel to the Borah Peak aftershock zone but situated from 30 to 40 km to the west. The relocated epicenters may also be interpreted as defining a north-northeast trending seismic zone near Seafoam and a north-northeast trending seismic zone that is situated north and west of Challis, here called the Twin Peaks-Myers Cove zone. The Seafoam zone includes the epicenter of the largest instrumentally recorded central Idaho earthquake prior to the Borah Peak earthquake, the magnitude 6.1 shock of 12 July 1944. The region within 25 km of the epicenter of the Borah Peak main shock was quiescent for at least two decades before the main shock for magnitudes [mb(Lg] of 3.5 and greater.

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