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

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 acqui red aeromagnetic data show several large northwest-trending anomalies, incl uding one associated with the Mount Angel fault. The magnetic signature ind icates that the fault may actually extend 70 km across the Willamette Valle y to join the Newberg and Gales Creek faults in the Oregon Coast Range. We collected 24-fold high-resolution seismic reflection data along two transec ts near Woodburn, Oregon, to image the offset of the Miocene-age Columbia R iver Basalts (CRB) and overlying sediments at and northwest of the known ma pped extent of the Mount Angel fault. The seismic data show a 100-200-m off set in the CRB reflector at depths from 300 to 700 m. Folded or offset sedi ments appear above the CRB with decreasing amplitude to depths as shallow a s were imaged (approximately 40 m). Modeling experiments based on the magne tic data indicate, however, that the anomaly associated with the Mount Ange l fault is not caused solely by an offset of the CRB and overlying sediment s. Underlying magnetic sources, which we presume to be volcanic rocks of th e Siletz terrane, must have vertical offsets of at least 500 m to fit the o bserved data. We conclude that the Mount Angel fault appears to have been a ctive since Eocene age and that the Gales Creek, Newberg, and Mount Angel f aults should be considered a single potentially active fault system. This f ault, as well as other parallel northwest-trending faults in the Willamette Valley, should be considered as risks for future potentially damaging eart hquakes.