This mb = 4.9 earthquake was located about 20 km north of Prescott in the Transition Zone between the Basin and Range and Colorado Plateau provinces. Several lines of approach were used to determine the focal mechanism: P-wave first motions; surface wave amplitude and phase spectra; and synthetic seismograms. The preferred focal mechanism displays northwest trending nodal planes with predominantly normal movement. The inferred fault plane strikes 120°, dips 40°SW, with a slip of − 100° and a depth of 10 to 15 km. The seismic moment is 1.0 × 1023 dyne-cm. The azimuth of least-horizontal stress is inferred to be 37°. Lithostatic stress generated by variations in elevation and density between the crusts of the southern Basin and Range and Colorado Plateau may contribute to the inferred stress orientations. Uplift and resultant extension in this zone of thin lithosphere and relatively high heat flow may also be important. Microearthquake studies in the Chino Valley during the summers of 1978 and 1979 indicate a microseismicity rate of 0.3 events per day. This is an order of magnitude less than that observed in the Intermountain seismic belt.