Aseismic Slip Phenomena in Southern Cascadia

Date: 4/24/2019

Time: 03:15 PM

Room: Cascade II

Southern Cascadia experiences an array of interesting aseismic phenonema. Here we report on both a new technique used to study Episodic Tremor and Slip (ETS) in Cascadia, along with it’s application to constraining local tectonics, and a novel observation of a new aseismic phenomenon: enduring coupling changes induced by dynamic stresses associated with regional earthquakes. Using daily GNSS timeseries provided by UNAVCO and the Pacific Northwest Seismic Network Cascadia tremor database, we estimate the ETS velocities at GNSS stations in southern Cascadia averaging over the full timeseries at each station (~10-14 years). We invert this velocity field to obtain the long-term average ETS slip rate on the plate interface, accounting for elastic heterogeneity. We find that ETS produces up to 40 mm/year of westward motion, far above the 25 mm/year of convergence predicted by the Juan de Fuca – North America Euler pole given by MORVEL. The direction of convergence is also not well described by the Euler pole, with a difference of >30° at the southern end. This implies significant internal deformation of the subducting Gorda sub-plate, which is likely to be an important effect in assessing earthquake hazards in the region.

Modeling of velocity changes detected in the GNSS time series not associated with ETS suggests the existence of a “spot” updip from the ETS zone on the southern Cascadia interface which experiences abrupt changes in coupling. The inferred increases and decreases in coupling are co-incident with the times of large (M_W >=6.5) offshore earthquakes. Known postseismic processes, such as afterslip and mantle relaxation following these events, cannot explain our observations. Mechanically, it appears that the plate interface undergoes enduring changes in strength triggered by dynamic stresses. This phenomenon, which has not previously been reported, implies that plate interface coupling models may need to be periodically re-assessed.

Presenting Author: Noel M. Bartlow

Authors