U.S. Geological Survey
By G.C. Thomas,1 R.S. Crosson,1 D.L Carver,2 and T.S. Yelin3
The 25 March 1993 Ml = 5.7 crustal earthquake near Scotts Mills, Oregon, was the largest earthquake to occur in the Pacific Northwest in over a decade. The mainshock was located at 45.033 degrees N, 122.586 degrees W and at a depth of about 15.1 km, based on arrival time data from the short period Pacific Northwest Seismograph Network. Beginning about 12 hours after the mainshock, investigators from the U. S. Geological Survey deployed twenty-two digital seismographs to record aftershocks. Using data from the temporary and permanent stations, we analyzed a subset of 50 aftershocks with quality locations. Hypocenters of these aftershocks lie on a northwest-trending steeply-dipping plane (strike 290 +/- 10 degrees, dipping 60 +/- 5 degrees to the north-northeast), in agreement with the preferred slip-plane of the mainshock focal mechanism solution (strike 294 degrees, dipping 58 degrees to the north-northeast). The planar structure defined by the aftershock locations may be a southeast continuation of the Mount Angel Fault, a reverse fault identified from both surface and subsurface evidence. The mapped southeast extent of the Mount Angel Fault is located less than 10 km west of the Scotts Mills epicentral region. In addition, the mainshock focal mechanism solution, with a combination of reverse motion and right-lateral strike-slip, has a geometry and sense of motion consistent with the Mount Angel Fault. While aftershock focal mechanisms are varied, P axes are consistently oriented in a sub-horizontal north-south direction. This earthquake sequence, together with the geological and geophysical evidence for the Mount Angel Fault, suggests a significant crustal earthquake hazard for this region of northwest Oregon.
1 Box 351650, Geophysics Program, University of Washington, Seattle, Washington, 98195-1650
2 U.S. Geological Survey, 1711 Illinois St., Golden, Colorado, 80401
3 U.S. Geological Survey, Box 351650, Geophysics Program, University of Washington, Seattle, Washington, 98195-1650