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Integration of high-resolution seismic and aeromagnetic data for earthquake hazards evaluations: An example from the Willamette Valley, Oregon

Center for Geophysical Investigation of the Shallow Subsurface Boise State University, Boise, Idaho 83725lml{at}cgiss.boisestate.edu
College of Oceanic and Atmospheric Sciences Oregon State University, Corvallis, Oregon 97331-5503
U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 45025
Science Applications International Corp., 1227 South Patrick Drive, Satellite Beach, Florida 32937

Abstract

Aeromagnetic and high-resolution seismic reflection data were integrated to place constraints on the history of seismic activity and to determine the continuity of the possibly active, yet largely concealed Mount Angel fault in the Willamette Valley, Oregon. Recent seismic activity possibly related to the 20-km-long fault includes a swarm of small earthquakes near Woodburn in 1990 and the magnitude 5.6 Scotts Mills earthquake in 1993. Newly acquired aeromagnetic data show several large northwest-trending anomalies, including one associated with the Mount Angel fault. The magnetic signature indicates that the fault may actually extend 70 km across the Willamette Valley to join the Newberg and Gales Creek faults in the Oregon Coast Range. We collected 24-fold high-resolution seismic reflection data along two transects near Woodburn, Oregon, to image the offset of the Miocene-age Columbia River Basalts (CRB) and overlying sediments at and northwest of the known mapped extent of the Mount Angel fault. The seismic data show a 100-200-m offset in the CRB reflector at depths from 300 to 700 m. Folded or offset sediments appear above the CRB with decreasing amplitude to depths as shallow as were imaged (approximately 40 m). Modeling experiments based on the magnetic data indicate, however, that the anomaly associated with the Mount Angel fault is not caused solely by an offset of the CRB and overlying sediments. Underlying magnetic sources, which we presume to be volcanic rocks of the Siletz terrane, must have vertical offsets of at least 500 m to fit the observed data. We conclude that the Mount Angel fault appears to have been active since Eocene age and that the Gales Creek, Newberg, and Mount Angel faults should be considered a single potentially active fault system. This fault, as well as other parallel northwest-trending faults in the Willamette Valley, should be considered as risks for future potentially damaging earthquakes.

This article has been cited by other articles:

				Late Pleistocene and Holocene Tectonics of the Portland Basin, Oregon and Washington, from High-Resolution Seismic Profiling Bulletin of the Seismological Society of America, August 1, 2001; 91(4): 637 - 650.

				R. J. Blakely, R. J. Blakely, R. E. Wells, T. L. Tolan, M. H. Beeson, A. M. Trehu, and L. M. Liberty New aeromagnetic data reveal large strike-slip (?) faults in the northern Willamette Valley, Oregon Geological Society of America Bulletin, August 1, 2000; 112(8): 1225 - 1233. [Abstract] [Full Text] [PDF]