First results from a dense semi-continuous GPS deployment during the September 2005 northern Cascadia slow slip event

In August of 2005, we deployed 29 GPS stations in the Olympic Peninsula with the goal of recording crustal motion associated with a slow slip event on the Cascadia subduction interface. This is the first semi-continuous GPS network established with the aim of providing a high resolution image of crustal motion associated with a slow slip event. We recorded data during the period from August 2 to October 20, roughly centered on the slow slip event that began between September 3-6 and lasted for approximately 14 days. We observe transient surface motions associated with this event at 27 stations bounded by the surface projection of the 20-70 km slab contours. The total observed surface displacements range from about 2 to 9 mm. The precise time of onset of transient motion is difficult to discern from the observed coordinate time series, but our results are consistent with the onset of crustal motion coinciding with the time of first tremor detection. Maximum surface displacements are centered on the Straight of Juan de Fuca, including southern Vancouver Island and the northern Olympic Peninsula, the region directly overlying the arch in the Juan de Fuca plate. Coastal sites located above the 20 km and shallower slab contour record little or no deformation during the 2005 event, providing a new constraint on the down-dip limit of the locked part of the subduction interface. We present simple elastic dislocation models to illustrate the dependence of the observed motions on the time of rupture initiation, propagation rate and direction.