Coupling on the northern Cascadia subduction zone from geodetic measurements and physics-based models

Abstract

Kinematic inversions of GPS and tide gauge/leveling data display an unresolved “gap” between the downdip limit of the locked megathrust and the top of the episodic tremor and slip (ETS) zone in northern Cascadia. This work combines physics-based models of slow-slip events with both mean ETS displacements and decadal-averaged deformation rates to explain the gap and determine how interseismic stress accumulates on the megathrust. While physics-based predictions match the average ETS displacements, they significantly misfit long-term rates, implying faster slip rates within both the gap and the ETS region. Heterogeneous Green's functions or velocity-strengthening friction within the gap cannot explain the decadal rates. The observed uplift rates require steeper gradients in slip rate at the base of the locked zone. We invert for the smallest possible shear stress rate on the creeping megathrust below a locked zone that satisfactorily fits the data. A nonzero shear stress rate within the ETS zone, reaching −2.5 kPa/yr at a depth of 25–30 km, is required. Finally, of all the models that adequately fit both horizontal and vertical data, only those with deep locking depths, around 21 km, significantly improve the fit to the uplift rates.