A seismic zone on the Oregon-Idaho border: The powder river earthquakes of 1984

Two magnitude 3.8 earthquakes occurred south of the Wallowa Mountains in northeastern Oregon in August and September 1984. Locations of 15 aftershocks recorded on portable seismographs cluster around the Oregon-Idaho border near the mouth of the Powder River. A composite fault-plane solution indicates normal faulting on planes striking north or north-northwest, consistent with stresses observed in the Basin and Range Province to the east. The record of historical earthquakes in the region suggests periodicity with intervals of 11 to 21 yr between cycles of events consisting of several main shocks within a 3-yr time span. A magnitude 3.6 earthquake on 29 September 1981 may have begun the present cycle.