A 3-D seismic velocity model across the south-central Cascadia subduction margin

Interplate coupling along the Cascadia subduction zone is segmented along-strike. In central Oregon compared to southern Oregon/northern California, plate motions in the locked zone are less strongly coupled and there is reduced episodic tremor and slip in the downdip direction. The relationship of variations in plate coupling to the structure of the overlying crust and underlying subducting plate is still poorly understood.

This study presents a high resolution 3-D seismic velocity model of the downdip portion of the locked zone in the region between the central and southern Oregon segments of Cascadia. This area includes the transition in upper plate structure from the Siletz terrane in central Oregon to the Klamath and Franciscan terranes in southern Oregon.

We use controlled source, wide angle seismic data acquired during the Cascadia2021 onshore-offshore experiment to develop a P-wave tomographic model. Offshore airgun sound sources were recorded on Fairfield nodal seismometers deployed across the coast ranges in two densely spaced lines (~1 km spacing) imbedded within a larger grid of sites (~10 km spacing). We used travel times for crustal refractions, Pg, reflections from the Moho of the oceanic crust, PmP, and head waves in the upper mantle of the oceanic plate, Pn. An initial P-wave velocity model was built that smoothly merged velocity models from several previous studies.

Our tomographic P-wave velocity model suggests a complex 3-D structure. This model shows the detailed geometry and internal velocity variations of the Siletz terrane including the backstop boundary. Preliminary results also suggest there is a layer of sediments underthrusting the Siletz terrane. Oceanic P-wave crustal velocities appear to be reduced in the propagator wakes, 5.5 to 6 km/s from 6.1 to 6.8 km/s. There may be lower mantle velocities (7.8 to 8 km/s compared to 8.1 km/s) west of the Siletz terrane indicating the presence of fluids, serpentinization, and/or low density mantle.