New aeromagnetic data reveal large strike-slip (?) faults in the northern Willamette Valley, Oregon

High-resolution aeromagnetic data from the northern Willamette Valley, Oreg on, reveal large, northwest-striking faults buried beneath Quaternary basin sediments. Several faults known from geologic mapping are wed defined by t he data and appear to extend far beyond their mapped surface traces. The Mo unt Angel fault, the likely source of the Richter magnitude (M-L) 5.6 earth quake in 1993, is at least 55 km long and may be connected in the subsurfac e with the Gales Creek fault 25 km farther northwest, Northeast of the Moun t Angel fault, a 60-km-long, northwest-striking anomaly may represent a pre ciously unrecognized dextral-slip fault beneath the towns of Canby and Mola lla. Vertical offsets along the Mount Angel fault increase with depth, indi cating a long history of movement for the fault. Dominantly northwest-trend ing, relatively straight faults, consistent stepover geometrics, offset mag netic anomalies, and earthquake focal mechanisms suggest that these faults collectively accommodate significant dextral slip. The 1993 earthquake may have occurred on a left-stepping restraining bend along the Mount Angel-Gal es Creek fault zone.