Role of slow slip events in vertical deformation of the Cascadia forearc

Slow slip events (SSEs), characterized by aseismic slip spanning several weeks, have been detected on the Cascadia subduction zone. Understanding SSE processes is important to understanding stress accumulation along the subduction interface and the seismic threat it poses for the Pacific Northwest. Occurrence of SSEs at depth implies the presence of accumulated stress below the seismogenic zone. However, it is not fully understood if SSEs fully release the stress imposed on the subduction interface downdip of the seismogenic zone, and what effect they have on landscape evolution of the overlying forearc.

We calculate inter-SSE velocities from Pacific Northwest Geodetic Array GPS position time series after removing the offsets from 31 SSEs between 1996 and 2017, by using a MIDAS robust trend estimator that adjusts for seasonal variation. Using inter-SSE velocities separates the effect of the slow slip events from the interseismic strain accumulation process and permits models to extend slip deficit on the deep portion of the plate interface.

Our results show two distinct zones of coupling: one that is likely coincident with the seismogenic zone and deeper region hosting periodic events of slow slip. These zones of high coupling are separated by a band of low coupling around 30 km depth that could potentially define the lower limit of a future rupture event. Studies simulating coseismic slip show a maximum downdip rupture limit that coincides with our modeled band of low coupling and a range of coseismic slip consistent with our cumulative modeled slip deficit.

Our models predict a vertical deformation signature from SSEs that, spatially, resembles the long-term geologic uplift observed on the Olympic peninsula. While inter-SSE interseismic slip deficit predicts subsidence across the Olympic Peninsula and uplift further inland, adding the SSEs' uplift signature effectively shifts westward the boundary between uplift and subsidence. The resultant uplift pattern is more consistent with observed interseismic coastal uplift but contains a secondary uplift maximum from SSEs that is not seen in tide and leveling data. This inconsistency could be explained by time-variable coupling throughout the interseismic period.