abstract

An earthquake survey conducted during 1973 in the Helena, Montana area indicates that there may be a possible relationship between the local seismicity and the nearby Marysville high heat-flow area. Earthquakes were located in two clusters northwest of Helena along a N50°W trend that is parallel to the Intermountain Seismic Belt. Focal depths were generally shallow with maximums of 17 km. The Marysville high heat-flow anomaly 30 km northwest of Helena lacked detectable earthquakes, but corresponded to an area of low P-wave velocity. Composite fault-plane solutions for the Helena region showed both strike-slip and normal fault mechanisms, but with consistent northeast-trending T axes. The occurrence of both types of faulting could be interpreted as a result of stress reorientation with depth or the result of a simple shear acting along a northwest-trending zone of pre-existing weakness. The trends of the T axes at Helena are intermediate to north-south trends at the Hebgen Lake-Yellowstone Park area, 200 km to the southeast, and east-west trending T axes at Flathead Lake, 150 km to the northwest. The rotation of T axes across western Montana probably reflects a reorientation of the regional stress field along the Intermountain Seismic Belt produced by the relative motion of the Northern Rocky Mountain subplate with respect to the North American plate.

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