We conducted a microearthquake study of southwest Utah-northwest Arizona during June and July of 1982 in order to better determine the variation of the stress field in what appears to be a transition between stress domains. Eleven single event and composite fault plane solutions were determined, including two from a microearthquake swarm which occurred near Enterprise, Utah. The region can be divided into two domains defined by fault plane solutions. The western and southern parts contain no Intermountain Seismic Belt-type fault plane solutions, while the remaining area contains both the Intermountain Seismic Belt and Northern Basin and Range-type fault plane solutions. The mixed fault plane solutions may be caused by a low horizontal deviatoric stress which corresponds to the stress transition between the Northern Basin and Range-Colorado Plateau, and structural complexities which may locally affect the stress field. The average T axis direction determined from the fault plane solutions for the entire study area is approximately N65°W, as compared to the minimum horizontal principal stress direction of N65°W inferred from Holocene geologic indicators and N85°E calculated from seismic moment tensors found by other workers. In addition, we observed, as have other workers in the Intermountain Seismic Belt, no relationship between seismicity and Quaternary faulting. Finally, the microearthquake swarm, which is characteristic of southwest Utah-northwest Arizona, displayed two distinctly different fault plane solutions from events that migrated in time and space. The two fault plane solutions are inferred to represent stress release by the earlier microearthquakes which thereby altered the local stress field.

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