Abstract

Examination of seismicity and late Quaternary faults in Montana and Idaho north of the Snake River Plain shows a geographic correspondence between high seismicity and 24 faults that have experienced surface rupture during the late Quaternary. The Lewis and Clark Zone delineates the northern boundary of this tectonically active extensional region. Earthquakes greater than magnitude 5.5 and all identified late Quaternary faults are confined to the Montana-Idaho portion of the Basin and Range Province south of the Lewis and Clark Zone. Furthermore, all 12 Holocene faults are confined to a seismologic zone (Centennial Tectonic Belt) parallel to the northern flank of the Snake River Plain in extreme southwestern Montana and adjacent Idaho. Fault trends, strain data, and focal mechanisms suggest that both late Quaternary faulting and seismicity in the region are primarily products of two distinct stress regimes: (1) overall Basin and Range extension along a S45°W direction relative to the mid-continent; and (2) localized effects of the Yellowstone hot spot which appears to have the least principle stress axis oriented horizontally at about S4°W. The three largest historic earthquakes north of the Snake River Plain are indicative of three different stress provinces—the 1983 Borah Peak earthquake occurred in the Montana-Idaho basin and range; the 1959 Hebgen Lake earthquake is associated with the Yellowstone hot spot area; and the 1925 Clarkston Valley event appears to be associated with the stress field of the United States mid-continent.

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