PBO STRAINMETER OBSERVATIONS OF SLOW SLIP EVENTS IN THE CASCADIA SUBDUCTION ZONE:  ASSESSING THEIR IMPLICATIONS FOR TIME-­VARYING EARTHQUAKE PROBABILITY

Cascadia SSEs recorded to date can be modeled as a few cm of convergence-parallel slip on the subduction interface, over days or weeks. SSEs recur along the entire forearc, but slip (and accompanying tremor) appears limited to depths down-dip from the seismogenic locked zone. An SSE transfers shear stress up-dip at an above-background rate. If the locked zone can rupture immediately at a critical threshold stress, daily earthquake probability is higher during an SSE. Alternatively, if rate-weakening locked-zone rheology delays rupture, then probabilities could rise after the SSE ends. In either case, an SSE of greater net slip and/or shallower up-dip extent transfers more stress to the locked zone. To date, Cascadia SSEs have differed in length and migration speed, but not greatly in up-dip extent or net slip.

Tide gauge and leveling data from 1925 to 2006 indicate an incompletely locked Cascadia "transition zone" extending 10-15 km up-dip from the shallow limit of tremor to the locked zone. SSEs recorded with PBO BSMs have been limited to depths below this transition zone. In contrast, SSEs preceding the 1944 and 1946 Nankai M8 earthquakes, the 1960 M9 Chile earthquake, and the 2011 M9 Tohoku earthquake occurred adjacent to their hypocentral regions. PBO BSMs near the coast in northern Oregon and northernmost Cascadia are well-positioned to detect potentially significant transition-zone slip.

In numerical simulations, SSEs can reach progressively shallower depths, encountering higher effective stress where shear heating can destabilize slip, causing dynamic rupture. Other models also predict evolving SSE behavior as a great earthquake approaches, with shorter waits between SSEs, further along-strike propagation, and/or higher slip speed. An SSE of greater net slip, shallower up-dip extent, or unusual propagation pattern can be recognized using PBO BSM data.