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Geophysics Department Seminar - "Tectonic Geomorphology of Subaqueous Strike-Slip Faults Offshore Western North America" - Dr. Danny Brothers, USGS Pacific Coastal and Marine Science Center
Tectonic Geomorphology of Subaqueous Strike-Slip Faults Offshore Western North America
Seismic and geodetic monitoring of active fault systems does not typically extend beyond one seismic cycle, hence it is challenging to link the characteristics of individual earthquakes with long-term fault behavior. This is particularly true for fault systems that are located offshore, where even geomorphic evidence for past fault movement and associated dating of faulted landforms can be difficult to obtain. In this talk I will discuss results from two marine geophysical/geological studies aimed at examining the tectonic geomorphology, regional strain partitioning, and slip-rates along submerged strike-slip faults of the Pacific-North American plate boundary. The first study, a targeted AUV- and ROV-based investigation of the right-lateral Palos Verdes Fault (PVF) offshore southern California, will set the stage for a description of preliminary results from a broad-scale study of the Queen Charlotte-Fairweather Fault (QCFF) of southeastern Alaska. Both faults display similar characteristics in terms of tectonic geomorphology despite having an order-of-magnitude difference in slip-rate (<2 mm/yr for the PVF versus >50 mm/yr for the Queen Charlotte). New bathymetric imagery of the QCFF provides stunning views of a single, continuous and relatively narrow fault zone for more than 400-km. Linear fault valleys, step-overs, and laterally offset submarine canyons, gullies, and ridges characterize the surface morphology. We are constructing a catalog of lateral piercing points along the fault to better understand long-term behavior, particularly along sections that have generated large (M7+) historical earthquakes. Although subtle changes in fault strike suggest a southward increase in convergence, the horizontal slip-rate appears to remain between 50–55 mm/yr, which is essentially the entire plate motion budget. One question is whether observed spatial alignments amongst large historical ruptures, fault step-overs and subtle patterns in the along-strike character of the QCFF can be used to delineate sections of the fault with distinctive, long-term (>10 kyr) earthquake characteristics.