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Seismicity and contemporary tectonics of the Helena, Montana area

DEPARTMENT OF GEOLOGY AND GEOPHYSICS UNIVERSITY OF UTAH, SALT LAKE CITY, UTAH 84112
DEPARTMENT OF GEOLOGICAL SCIENCES SOUTHERN METHODIST UNIVERSITY, DALLAS, TEXAS 75 275

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.

Footnotes

^* Present address: Union Oil Company, Midland, Texas.

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