TREMOR CONSTRAINTS ON MOMENT RELEASE DURING THE 2007 ETS EVENT FROM SURFACE AND BOREHOLE SEISMOMETERS

The 2007 ETS event, which began around January 15 beneath the southwestern Puget Basin and ended around Feb 5 beneath southern Vancouver Island 200 km north, was well-recorded on local surface seismic arrays, EarthScope borehole-seismometers, strainmeters and long-baseline tiltmeters, and continuous GPS of the PANGA and PBO networks. Seismic tremor, however, offers the highest resolution for studying moment release through time, since tremor bursts lasting less than 10-seconds are often visible across stations. To test the hypothesis that tremor and transient deformation are two different manifestations of the same faulting process, and to quantify the relative contribution of moment release during times of strain-transients versus other times, we systematically analyze the tremor bursts during the 2007 event and compare those to the strain and long-baseline tiltmeter data.

Our methodology is as follows. We first consolidate daily seismic files from the northern Puget Basin of Washington State and SW British Columbia, where GPS density is highest. Seismic traces are included from the Pacific Northwest Seismic Network, the Plate Boundary Observatory borehole seismic network, and the Earthscope-funded CAFÉ experiment. We then remove instrument gain, decimate the data to 10 sps, rectify it, compute its envelope using a Hilbert transform, and average the envelopes from regionally adjacent stations to provide a single metric indicative of tremor activity. This process is effective in quantifying small tremor bursts lacking GPS-inferred deformation and accurately identifies timing and duration of known events. We then convert tremor duration to equivalent moment slip inversions of corresponding GPS-derived deformation.

As the 2007 event has only recently concluded, data analysis is still ongoing and results will be presented. In addition to correlating the tremor bursts with transient strain and tilt, we are also attempting to correlate shorter tremor bursts with sustained ones to test whether impulsive source time functions might be derived that can be used to better locate tremor.