Tectonics of the Intermountain Seismic Belt , Western United States : Microearthquake Seismicity and Composite Fault Plane Solutions

During the summer of 1969, six portable seismographs were operated at 82 sites along the Intermountain Seismic Belt from southwestern Utah to northwestern Montana. This survey followed a well-known seismic belt along the eastern physiographic boundary of the Basin and Range province, and within the middle and northern Rocky Mountains. In general, the 120 microearthquakes located in this study follow the same spatial trend as the macroseismic earthquakes reported by the NOS (formerly USCGS). Most of the microearthquakes clustered in time and space along well-known fault zones on which late Tertiary or younger movements have occurred. All of the accurately located hypocenters occurred between the surface and a 20 km depth. Composite fault plane solutions along the Hurricane and Sevier fault zones (southwestern Utah), Tushar and Sevier fault zones (Marysvale area, Utah), and Wasatch and East Cache fault zones in central and northern Utah indicate vertical-motion on steeply dipping fault planes. These motions may be indicative of differential movements between the Basin and Range province and the Colorado Plateau-Rocky Mountains. Composite fault plane solutions (CFPS) in the Caribou Mountains, southeastern Idaho, and Flathead Lake area, northwestern Montana, show normal faulting on less steeply dipping planes and have west-northwest trending extensional axes. Swarm activity was also observed in the above two regions. Between the above two areas of uplift and extension lies a region of complicated geology and seismicity.