HIGH-RESOLUTION GEOPHYSICAL SURVEYS OF THE MOUNT ANGEL FAULT, NORTHERN WILLAMETTE VALLEY, OREGON

The Mount Angel fault extends ~55 km northwestward across the northern Willamette Valley and coincides with the projected rupture surface of the 1993 Scotts Mills earthquake (M 5.7), the most costly instrumentally-recorded earthquake in Oregon. The Mount Angel fault, defined by aeromagnetic anomalies, seismic-reflection data, and earthquake focal mechanisms, has both right-lateral and northeast-side-up reverse components of offset. To precisely define the near-surface characteristics of the fault, we have conducted detailed ground-magnetic, ground-penetrating-radar (GPR), and global-positioning system topographic surveys at several locations near the town of Woodburn, Oregon. At the Miller Road site, earlier seismic-reflection and refraction profiles show nearly 200 m of vertical offset of Columbia River basalt (CRB) at 300 to 450 m depth, and 20- to 30-m amplitude folding of overlying Plio-Pleistocene deposits. Our detailed topographic survey of the Miller Road site reveals a subtle (0.1 to 0.2 m) topographic scarp coincident with the mapped trace of the Mount Angel fault. Our ground-magnetic survey collected at the same site shows a sharp, linear, 60-nT magnetic anomaly coincident with the scarp. A cross-sectional model based on our new ground-magnetic survey and high-resolution topography, constrained by earlier seismic-reflection results, is consistent with 17 m of vertical offset of late Pleistocene (22-34 ka) Linn Gravels known to lie at about 30 m depth, an average vertical slip rate of 0.5 to 0.8 mm/y. The correspondence between aeromagnetic and ground-magnetic anomalies, high-resolution topography, and seismic-reflection data indicates that the Mount Angel fault was active after emplacement of CRB in the Miocene and as recently as late Pleistocene time. Despite the compelling agreement of these disparate datasets, GPR profiles across the topographic scarp failed to show conclusive evidence for fault-related deformation within <10 m of the surface. Future LIDAR and paleoseismic investigations are planned to help resolve the Holocene slip history of the Mount Angel fault.