Mechanical and Geodetic Constraints on the Gap between the Locked Zone and the ETS Region in Northern Cascadia

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. Moreover, 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 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, 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. This yields ~1cm/year of creep within the gap, improving the fit to the geodetic data, however these models 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 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.