Abstract

Earthquake studies of the Basin and Range-Colorado Plateau transition in central Utah are being carried out by the University of Utah, motivated by an increasing interest in the geodynamics of the transition zone. A 10-week microearthquake field study (12 instruments, 36 sites, 700 km²) was completed during the summer of 1979 between 39° and 40° N latitude (110°30′-112°30′ W longitude). Results from this study, together with fixed-network data and the results of a 1980 aftershock study (following a shock of magnitude 4.4 on May 24, 1980, in Goshen Valley), provide new information for: (1) resolution of crustal seismicity in central Utah, (2) correlation of seismicity with geologic structure, and (3) mapping of changes in stress orientation from fault-plane solutions.

Diffuse shallow seismicity (depth <15 km) and normal faulting focal mechanisms characterize the Basin and Range structural region west of the Wasatch Plateau. Scattered microearthquakes occur throughout the western and central parts of the Wasatch Plateau itself, which is broken by a series of en echelon N-S-trending Cenozoic graben. Along the eastern boundary of the Wasatch Plateau, intense, very shallow microseismicity (depth < 4 km) is associated with extensive underground coal mining. Epicentral scatter observed across the Basin and Range-Colorado Plateau transition is not simply due to poor resolution. Although poorly understood, the likely causes of diffuse seismicity include complex geologic structure and the mechanism of crustal deformation.

Fault-plane solutions along the eastern Basin and Range province indicate the predominance of seismic slip on fracture planes with moderate to relatively high-angle dip. Two fault-plane solutions suggest that horizontal compression along a northerly trend apparently characterizes the eastern Wastach Plateau—spatially identifying a transition between basin-range extension to the west and horizontal compression along a more easterly trend thought to typify the inner core of the Colorado Plateau to the east.