On 30 September 1977, a magnitude 4.5 ML earthquake occurred in a relatively aseismic area about 135 km east of Salt Lake City, Utah, near the boundary between the southern flank of the Uinta Mountains and the Uinta Basin. Aftershocks were recorded with a temporary local network for 15 days after the main earthquake, and 173 hypocenters were located using these data. Computed standard errors for the hypocenters are less than 0.5 km horizontally and 0.75 km vertically. Projections in plan and cross-section show that the aftershocks occurred in three clusters, which together define a roughly rectangular zone that strikes N10°E and dips 45°ESE. The zone extends about 2.5 km NE-SW, 3.7 km NW-SE, and is 1 km thick. The time-space history of the aftershocks indicates that, following the largest aftershock (11 October 1977, ML 4.0), the rupture zone was extended in the up-dip direction into a zone that was previously seismically inactive. Composite focal-mechanism solutions for the aftershocks indicate normal faulting; one nodal plane of each solution corresponds to the planar zone defined by the aftershock hypocenters.

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