The 1968 Adel, Oregon, earthquake swarm

Abstract

Historical seismic activity in Oregon is investigated; it is found that earthquake swarms are not atypical of seismic activity in southern Oregon. Swarms tend to occur in regions of heterogeneous geologic structure, or at low stress levels. A swarm of earthquakes occurred during May, June, and July of 1968 near Adel, Oregon; the largest of which had magnitude 5-1 on May 30. The focal mechanism of this shock indicates left-lateral oblique- slip motion on a plane that dips 80° E and strikes N E. The vertical component of motion oh this plane is reverse fault motion, which implies a change from extensional to compression stress. 169 events were selected for detailed analysis: epicenters fell within a rectangle about 15 km long (N-S) and 6 km wide; depths averaged about 9 km. A duration-magnitude relationship was established for the tripartite array; the value of b was found to be I.13.