Mechanical and Geodetic Constraints on the Northern Cascadia Megathrust

Abstract:

Kinematic inversions of GPS and tide gauge/leveling data display an unresolved gap between the down-dip limit of the locked region and the top of the ETS zone in northern Cascadia. Physics-based models of slow slip events (SSE) result in nearly constant shear stress when averaged over the SSE cycle, implying additional slip-deficit within the ETS zone. Assuming the fault is locked to the top of the ETS zone, the predicted deformation-rates from such models do not adequately fit long-term GPS velocities, and especially the uplift rates.

We analyze long-term deformation rates, concentrating the attention on the uplift rates, to determine how much interseismic slip deficit accumulates on the megathrust. We first show that the use of heterogeneous Green's functions, which include a stiff oceanic mantle, compared to homogeneous half-space, does not sufficiently bias the predicted slip rates to explain the gap identified in kinematic inversions. We then explore physics-based models with velocity-strengthening regions up-dip of the ETS zone, to account for steady creep within the gap. These models improve the fit to the geodetic data overall, but still misfit the uplift rates.

As an intermediate step between kinematic and fully physics-based inversion, we invert for the distribution of shear stress rate on the megathrust that best fits the data. We find that a small decrease in shear stress within the ETS zone, reaching 5 kPa/year at a depth of ~30 km, is required to fit the data. Possible explanations for this include a slow decrease in normal stress with time, possibly due to an increase in pore pressure, or a reduction in fault friction. We explore these hypotheses, and study their impact on both the SSE characteristics (e.g. recurrence time, slip), and on the seismic cycle, using 2D quasi-dynamic simulations with rate-and-state friction and isothermal v-cutoff models for generating slow slip events.