QUATERNARY CRUSTAL FAULTS IN THE CASCADIA FOREARC—TOWARD QUANTIFYING STRAIN BUDGETS

Strain accumulation across the Cascadia subduction boundary is typically assigned to the megathrust and used to estimate both the recurrence and magnitude of major interplate earthquakes. A significant portion of this contraction occurs on crustal faults above the megathrust which lie closer to populated areas such as Vancouver, Seattle, and Portland. Yet the strain energy and seismic hazard associated with these faults remains poorly understood. Cataloging active crustal faults in the Pacific Northwest is crucial for assessing their seismic hazard and identifying areas with unknown strain rates. This catalog in turn becomes a valuable tool for documenting the transfer of kinematic plate motions to specific crustal faults and fault networks in the forearc.

Comparing geologic strain rates with geodetic rates and plate kinematic rates is essential for calculating rates of aseismic displacement. Such comparisons also provide a framework for separating crustal geodetic strain signals from the much larger overprint of interplate strain. For example, plate convergence at the southern end of the Cascadia forearc is ~45 mm/y. About 15 mm/y of this relative convergence is observed geodetically onshore. Crustal fault slip accounts for ~10 mm/y of the measured contraction, suggesting that only 5 mm/y is accumulating on the megathrust and leaving as much as 30 mm/y to be accounted for as aseismic slip. Numerous active faults are driven by differential motion between forearc blocks, further complicating the evaluation of seismic hazards. For example, the Oregon Coast Range (OCR) block translates northward at ~8 mm/y in coastal Washington and a fault zone at its NW boundary converts 2-3 mm/y of this translation into permanent crustal shortening. The remaining 5-6 mm/y of differential motion may be accommodated in the Olympic Mountains which are uplifting as much as ~3 mm/y or as broad-scale buckling of the OCR basement beneath southern Washington. These examples demonstrate the complexity of crustal faulting in the forearc, and in particular, emphasize that crustal strain is concentrated at block boundaries. The catalog of Quaternary faults helps to discern those areas with high crustal strain, and in turn helps to evaluate the associated seismic hazard.