GEOPHYSICAL IMAGING OF THE PORTLAND HILLS FAULT: A NEWLY CLASSIFIED ACTIVE FAULT

The Pacific Northwest is a seismically active region, in part due to subduction-related tectonics. Recent focus has been on assessing earthquake hazards from active crustal faults within the urban corridors. The Portland metropolitan area (population 1.5 million) contains many potentially active faults, including the Portland Hills fault (PHF), East Bank fault, and Oatfield fault, but no direct evidence suggested these faults were active. This is in part due to the paucity of subsurface information and the 12-15 ka Missoula floods that reconstructed the area terrain and stratigraphy. We are presently characterizing the PHF at multiple sites using high-resolution geophysical imaging techniques. High-quality geophysical images are providing the long-term fault history and deformational style and has suggested that the PHF is active. A new exposure along a seismic profile shows folded and offset of Missoula flood silts and confirms that the PHF is active.

We employed multiple geophysical methods, including high-resolution seismic reflection, ground penetrating radar (GPR), and magnetic profiling. The seismic profiles provide detailed images of the upper 100 m of the stratigraphic section, and identify significant offset in the Miocene-age Columbia River basalts (CRB) and overlying sediments. The seismic data show a strong amplitude, steeply dipping horizon that correlates with the top of the CRB sequence. Reflections from younger sediments (Tertiary to Latest Pleistocene deposits) also dip steeply and appear faulted. Away from the fault zone, reflections associated with young sediments appear flat lying and undisturbed. Ground-based magnetic profiles along the seismic transects correlate with offset and orientation in the volcanic basement and provide key reconnaissance information to locate potentially active structures. The magnetics, well logs, and ground truthing show that CRBs crop out near the southwest portion of the PHF and that offsets in the Missoula flood deposits appear immediately below the land surface. Continued analysis of the geophysical data should provide details of deformational style and history of faulting, and improve the earthquake hazards assessment for the Portland metropolitan area.