Abstract

A microearthquake investigation of the Kaibab Plateau in northern Arizona revealed swarms of small to very small magnitude earthquakes. A 12-station network of portable seismographs recorded 296 events with magnitudes from −0.5 < ML < 2.5 during 6 weeks in the summer of 1981. Fifty-six of these events were large enough and close enough to the network to be recorded on four or more instruments. The microearthquakes ranged in depth from 5 to 20 km, and epicenters were spatially clustered near late Cenozoic surface faults. Hypocenters for the 56 located events, however, did not reveal individual faults. The compilation of T axes from these fault-plane solutions yields an average least principal horizontal stress direction of ENE-WSW. The inferred orientation of principal stresses found in this study indicates the Kaibab Plateau lies within a region of predominantly normal faulting which extends from central Arizona northward into southwestern Utah. Although data are sparse in the region surrounding the Kaibab Plateau, evidence for the location of a Basin-Range-Colorado Plateau stress boundary through the Kaibab Plateau is not supported by the results of this study.

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