Correlation of porosity variations and rheological transitions on the southern Cascadia megathrust

The unknown onshore extent of megathrust earthquake rupture in the Cascadia subduction zone represents a key uncertainty in earthquake hazard for the Pacific Northwest that is governed by the physical state and mechanical properties of the plate interface. The Cascadia plate interface is segmented into an interseismically locked zone located primarily offshore that is expected to rupture in large earthquakes, a region of aseismic slow slip at greater depth, and an intervening transition zone of uncertain rupture potential. Here we image the evolution of the ratio of seismic compressional to shear wave velocities from the locked zone to the transition zone, which is related to changes in fluid content of the plate boundary zone, using a dense onshore–offshore seismic dataset from southernmost Cascadia. The locked zone shows evidence of high fluid content implying a high porosity, yet the downdip transition zone shows an order of magnitude lower porosity. This strong variation is consistent with models that contain a ductile region between the earthquake rupture and slow slip zones that would inhibit onshore propagation of future large earthquake ruptures and hence reduce seismic hazard.