Stalking the September 2005 Cascadia Episodic Tremor and Slip event: Results from a Dense GPS Deployment

Episodic tremor and slip (ETS) events on the Cascadia subduction zone occur at remarkably regular intervals. Based on the average recurrence interval, an ETS event in northwestern Washington State was expected in September 2005 plus or minus a month. The fact that Cascadia ETS events may be predicted in advance presents an opportunity to install temporary GPS and seismic networks in a configuration optimized for ETS observations. A temporary network of 29 GPS instruments supplied from the Earthscope/PBO campaign pool was deployed on the Olympic Peninsula and northern Puget Sound region during the first half of August 2005 in anticipation of the next ETS event. The spatial distribution of this dense GPS array was designed to better resolve the updip and downdip extent of the slip regions, as well as the southern terminus. By utilizing a simple antenna mount -- a threaded rod driven into the ground to refusal -- the deployment (including site selection and permitting activities) was accomplished in about 25 person-days. The GPS instruments are programmed to log data to internal memory and operate using batteries recharged by solar panels. The instruments will be retrieved at the end of October 2005 and the stored data offloaded and processed in November. The predicted ETS tremor activity did start on September 3, 2005, and is continuing as this abstract is being written. Assuming that the ETS event (and the GPS experiment) unfolds as anticipated, analysis of the evolution and distribution of deformation accompanying this event will be completed in November and results presented at this meeting.