DETAILS OF TRANSIENT STRAIN DURING AND BETWEEN NORTHERN CASCADIA EPISODIC TREMOR AND SLIP EVENTS FROM PLATE BOUNDARY OBSERVATORY BOREHOLE STRAINMETERS

Plate Boundary Observatory (PBO) borehole strainmeters augment continuous GPS data to reveal features of Episodic Tremor and Slip (ETS) events in northern Cascadia. During propagating ETS events in January 2007, May 2008, and May 2009, shear strain transients were observed as tremor activity migrated past each strainmeter along the strike of the subducting oceanic plate. The propagation direction and sense of slip can be inferred from the two horizontal strain components provided by any individual strainmeter. For all of these events, the shear strain excursions and net offsets are consistent with models inferred from the GPS time series that specify thrust displacement at depths of 35-45 km along the plate boundary. Strain signals can be observed when there is tremor activity within a 50-km horizontal radius. At strainmeter B018, directly above the inferred slip zone, abrupt onsets of strain signals coincide, to within 30-60 minutes, with initiation of tremor less than 50 km away, supporting the hypothesis that the tremor and strain both originate from a single underlying process. At strainmeter B004 in northern Washington, 70 km up-dip of the modeled slip zone, the time histories of the 2007, 2008, and 2009 ETS events are all consistent with slip propagating steadily northward, although the 2007 event propagated faster and produced strains about half as large as those during the other two events. The PBO borehole strainmeters can detect smaller slip events than GPS at periods of a few weeks or less. During the intervals between the large, recurrent ETS events in northern Cascadia, at least four bursts of tremor on northern Vancouver Island or in the south Puget Sound area were accompanied by a signal on one strainmeter. Little or no surface displacement from these inter-ETS tremor bursts was evident in GPS time series. Strain time series associated with two of these tremor bursts have the same time history for both shear components, and therefore they can be explained by increasing slip over a spatially fixed source, consistent with their failure to propagate.