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Using deformation rates in Northern Cascadia to constrain time-dependent stress- and slip-rate on the megathrust

Lucile Bruhat, & Paul Segall

Published July 26, 2016, SCEC Contribution #6371, 2016 SCEC Annual Meeting Poster #161

In northern Cascadia, estimates of interseismic slip deficit on the megathrust are complicated by the poorly understood “gap” between the down-dip limit of the locked region and the top of the Episodic Tremor and Slip (ETS) zone, as revealed by kinematic inversions of deformation rates. Physics-based models of slow slip events (SSE) match the average ETS displacements, and predict nearly constant shear stress when averaged over many SSEs. Yet, when the fault is locked to the top of the ETS zone, such models predict too much locking when compared to observed decadal-scale velocities, especially uplift rates.

Minimum norm inversions of GPS and tide gauge/leveling rates show that larger slip rates are necessary within both the gap and the ETS zone relative to the model with constant shear stress, requiring non-stationary shear stress rates. We invert for the distribution of shear stress rate on the fault below a locked zone that best fits the data. We find that negative shear stress rates within both the gap and the ETS region, reaching -3kPa/yr at a depth of 25km, and locking depths around 21km are required by the data.

To explain the negative shear stress rates, we investigate models where both the size of the creeping region and the slip profile evolve with time. We explore whether the gap and ETS region act a quasi-static crack, potentially driven up-dip by plate motion at the down dip limit of the ETS region. Stress reaches a peak at the crack tip, and then decreases over a cohesive region. Using MCMC we solve for the locking depth, the crack friction parameters, and the up dip propagation velocity. Best fitting models reveal that a very slow up dip propagation is needed to fit the data and that the gap seems to act as a region of fault weakening. Possible explanations for this include a slow decrease in normal stress within the gap, possibly due to an increase in pore pressure, or a long-term reduction in fault friction.

Bruhat, L., & Segall, P. (2016, 07). Using deformation rates in Northern Cascadia to constrain time-dependent stress- and slip-rate on the megathrust. Poster Presentation at 2016 SCEC Annual Meeting.

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