2009 ETS in Cascadia, Crustal Deformation and its effect on Mt. St. Helens

In 2009 there was excellent geodetic and seismic data coverage throughout the Cascadia Subduction Zone, recording multiple Episodic Tremor and Slip (ETS) events that span the length of the Cascadia Subduction Zone. Here we inspect surface displacements using GPS displacement vectors provided by the Pacific Northwest Geodetic Array (PANGA) combined with a spline in tension technique to help image areas of along strike and along dip displacement. We focus on the August 2009 event that produced tremor and slip between the middle of Washington to Southern Oregon, where the subducting slab is geometrically simple and the largest displacements were recorded. We model the geodetic data with slip on the plate interface in an elastic half space at the location defined by tremor provided by the Pacific Northwest Seismic Network (PNSN). Assuming a Gaussian slip distribution with a half width of 20 km, the peak slip is ~65 mm. The azimuth of the slip vector is 250°, indicating that the slip is oblique to the N-S trending slab contour by about 20°. At 46°N, -125.5°W, the Juan de Fuca - North America Euler pole of DeMets et al. [1990] produces a plate convergence rate of 41.1 mm/yr at an azimuth of 62.3°, thus the direction of plate conversion is approximately opposite of the direction of the model result. The estimated moment magnitude of this event is 6.8. In addition, modeled residuals indicate that Mt. St. Helens may be ‘stretched’ during the event, possibly decreasing conduit pressure.