QUATERNARY GEOLOGIC AND GEOMORPHIC MAPPING ALONG THE MOUNT ANGEL FAULT USING HIGH-RESOLUTION LIDAR DATA, NORTHERN WILLAMETTE VALLEY, OREGON

The 1993 M_L 5.7 Scotts Mills earthquake revealed one of several shallow crustal faults in the northern Willamette Valley of Oregon that threaten the Portland Metropolitan region. The Mount Angel fault (MAF), the likely source of the Scotts Mills earthquake, is a 17-km-long NW-striking NE-dipping fault zone that accommodates dextral oblique reverse slip. Probabilistic seismic hazard models consider the MAF to be part of the larger Gales Creek-Mount Angel fault zone, capable of producing a M_w 6.8 earthquake.

The study involved multidisciplinary geologic and geophysical investigations of the MAF, including Quaternary geologic and geomorphic mapping using detailed LiDAR data. Structure contour maps and geologic cross sections indicate a structural high where bedrock and Pleistocene gravels are preserved in the hanging wall of the MAF confirming previous observations. Geologic cross sections suggest the MAF vertically displaces these gravels 20 to 61 m.

Quaternary geomorphic mapping using LiDAR data revealed pre-and post-Missoula flood (12-22 ka) surfaces preserved across the MAF with little sign of recent surface rupture. Evidence for potential Holocene deformation includes topographic and terrace profiles with minor elevation changes of the post-flood surface (12-22 ka) and changes in sinuosity and gradient of the Pudding River across the MAF. The overall lack of post-flood fault-related geomorphic features suggests the fault has not produced surface fault rupture since the Missoula floods despite recent seismicity associated with the MAF and inferred displacements of Pleistocene gravels and bedrock. The absence of geomorphic evidence for surface fault rupture could indicate the fault has not accumulated enough strain to produce a large post-Missoula flood surface-rupturing earthquake. Alternatively, these observations may indicate that the MAF is blind or has a complex geometry near the surface leaving little surficial evidence of past surface ruptures.