Polarization Analysis of the September 2005 Northern Cascadia Episodic Tremor and Slip Event

The region of Northern Cascadia, extending from the Olympic Mountains and Puget Sound to southern Vancouver Island, down-dip of the subduction "locked" zone has repeatedly experienced episodes of slow slip. This episodic slip, observed to take place over a period of two to several weeks, is accompanied by a seismic tremor signal. Based on the average recurrence interval of 14 months, the last episodic tremor and slip (ETS) event was expected to occur in September, 2005. Indeed, it began on September 3. In order to record this event, we deployed an array of 11 three-component seismometers on the northern side of the Olympic Peninsula augmenting Pacific Northwest Seismographic Network stations as well as the first few EarthScope BigFoot stations and Plate Boundary Observatory borehole seismometers. This seismic array was comprised of six short-period and five broadband instruments with average spacings of 500 m and 2200 m respectively. In conjunction with this Earthscope seismic deployment, we also installed a dense network of 29 temporary, continuous GPS stations across the entire Olympic Peninsula to integrate seismic and geodetic observations. Based on past geodetic observations, a dominant assumption for the source of tremor is fault-slip in the direction of subduction, which can be tested using polarization of the seismic tremor. Using waveform cross- correlation to invert for the direction of slowness, we observed the tremor signal to migrate directly under our array. As the source passed beneath the array, tremor polarization stabilized to coincide with the direction of subduction. During a four day period starting September 8, the normalized eigenvalue associated with the dominant linear polarization jumped from ~0.7 to a stable 0.9 value. Also during this time, the polarization azimuth stabilized to a value of 57 +/- 8 degrees, close to the angle of subduction (56 degrees) suggesting that the tremor is caused by slip in the direction of relative plate motion on one or more faults.