Effect of slow slip on uplift across the Olympic Peninsula and implications for the downdip extent of locking on the Cascadia Subduction Zone

The Cascadia Subduction Zone, extending from Northern California to Vancouver Island, has not experienced a major earthquake since 1700 and presents a seismic risk for the Pacific Northwest. Slow slip events (SSE), which have been detected on the subduction interface, are characterized by aseismic slip and occur over several weeks to months, rather than the seconds to minutes duration of an ordinary earthquake. Slow slip events are thought to occur downdip of the interseismically locked zone on the subduction interface, calling into the question the source of the stresses causing this slip. We want to understand what causes the stress that is released by SSEs, the long-term effect of deep, aseismic slip on fore-arc deformation patterns, and the spatial disparity between the zone of locking and the zone of slow slip.

We use GPS observations from the Plate Boundary Observatory to estimate the spatial distribution of coupling and slow slip. We calculate interseismic velocities as a 20-year average after removing displacements from each slow slip event, so they only reflect motion between SSEs. Including SSE displacements in the velocity field calculation would underestimate the truly interseismic velocities that reflect the strain accumulation process.

Models confining subduction zone coupling to the shallowest 30 km of the plate interface predict vertical displacements consistent with long-term net uplift of the Olympic Peninsula, but do not predict downdip slip deficit that could explain the recurrence of SSEs. Models permitting coupling deeper than 30 km estimate slip deficit on the deep extent of the interface, providing a source of stress that may be released by SSEs. These models also predict interseismic subsidence across the Olympic Peninsula, which is inconsistent with observed coastal uplift and past records of coseismic subsidence. Adding the vertical displacement signatures of slow slip events to these models, however, yields deformation more consistent with long-term coastal uplift, effectively shifting the boundary between uplift and subsidence westward. These results could indicate that the zone of coupling extends farther downdip than originally thought and is separated into a seismogenic zone and a zone characterized by the periodic occurrence of slow slip events.