SUBSURFACE STRUCTURES OF COASTAL SOUTHWESTERN WASHINGTON USING POTENTIAL-FIELDS GEOPHYSICAL MODELING

Crustal faults in the forearc of Cascadia present a hazard for communities and infrastructure in the Pacific Northwest. Recent work indicates known faults in southwestern Washington may be longer than previously mapped (and thus capable of producing larger magnitude earthquakes) and reveals additional, previously unmapped faults and folds. Some of these newly-identified structures, as well as previously mapped offshore faults, may project into coastal southwestern Washington, although dense vegetation and limited access prevent detailed structural mapping. We model new regional gravity and aeromagnetic data to gain insight about the extent of subsurface faults and folds. We present three geophysical models crossing structures in the region around Aberdeen, Raymond, and Oakville. In this study, major faults we characterize through potential-field modeling include the Blue Mountain fault and North River fault. Potential-field modeling of the geophysical gradient across the North River fault suggests an east-dipping reverse fault with 1.97 km of vertical throw. The Blue Mountain fault is a curvilinear east-dipping thrust fault and modeling in this study indicates 1.94 km of vertical throw. We use geophysical models to estimate slip rates since 47.6 Ma, with rates of 0.08 mm/yr for the North River fault and 0.1 mm/yr for the Blue Mountain fault. Our work provides key insight to slip rates of regional crustal faults, contributes to a broader understanding of the Cascadia forearc, and adds detail to a structural model for southwestern Washington.