STRAIN SIGNALS ASSOCIATED WITH THE JANUARY 2007 CASCADIA EPISODIC TREMOR AND SLIP EVENT AS RECORDED ON PBO BOREHOLE STRAINMETERS

The Plate Boundary Observatory (PBO) was designed to study the strain fields resulting from the tectonic deformation at active plate boundaries, including deep tremor and geodetic slip, labeled Episodic Tremor and Slip (ETS), in the Cascadia subduction zone. Though temporally correlated, it is not known whether the same source causes both the seismically-measured tremor and the geodetically-measured slow slip during ETS events. In September 2005, the first ETS-related strain event was recorded on two of the borehole strainmeters located on the Olympic Peninsula. Thirteen PBO borehole strainmeters were operational during the next ETS event in January 2007. Preliminary analyses of the 1-Hz data on strainmeter B018 (near Delphi, WA) show a strain event beginning around January 16, 2007. The event first manifests as a shear event, and then as E-W extension. The strain changes are roughly concurrent with the onset of strong tremor in the southern to central Puget Sound, and with geodetic changes recorded on the regional GPS network and the long-baseline tiltmeter at Shelton, WA (observed by scientists at Central Washington University). Preliminary analyses of 1-Hz data on strainmeter B004 (on the northern Olympic Peninsula) show a strain event that begins around January 26, 2007, later than at B018 and consistent with the migration of the tremor epicenters observed by scientists at the University of Washington. Here the reverse pattern is observed: the event first manifests as an E-W compression event, and then as shear. The availability of 20-Hz strainmeter data will allow for better evaluation of the timing of the slow slip event than is possible from the GPS data and may reveal strain associated with the tremor.