The Intermountain seismic belt, a north-trending zone of seismicity in the western United States, is interpreted as a boundary between subplates of the North American plate. The seismic activity closely follows the boundary between the Great Basin and the Colorado Plateau–Middle Rocky Mountains and extends northwestward into the Northern Rocky Mountains. Seismicity is characterized by shallow focal depths, most less than 15 km, and by earthquake swarms that are coincident in some cases with geothermal features and areas of high heat flow. A secondary zone of earthquakes extends from southwestern Utah along the southern margin of the Great Basin. The Northern Rocky Mountains are separated from the Great Basin and the Snake River Plain by an east-trending seismic zone in Idaho. Together, these patterns of seismicity outline the Great Basin and Northern Rocky Mountain subplates. The contemporary motions of the subplates with respect to one another and with respect to the North American plate are inferred from fault-plane solutions of earthquakes around their borders. The Northern Rocky Mountain and Great Basin subplates are moving relatively west with respect to the stable part of the North American plate. The Idaho seismic zone exhibits north-south extension, the same as that postulated for the Snake River Plain. The overall motions of the subplates of the Intermountain West may be explained as a response to a mantle plume that tracked beneath the Snake River Plain and that is now beneath Yellowstone Park. On the basis of a comparison of the calculated and observed rates of the motion for the Yellowstone plume with respect to that for the North American plate, we conclude that the Yellowstone plume has remained stationary relative to the Hawaiian plume. Deformation along the southern and western margins of the Great Basin is dominated by northwest-southeast extension and appears to be primarily related to the relative motion between the Pacific and North American plates.

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