PBO BOREHOLE STRAINMETERS OBSERVE OVER A DECADE OF EPISODIC TREMOR AND SLIP ALONG THE CASCADIA SUBDUCTION ZONE: SOUTHERN VANCOUVER ISLAND, BC, CANADA THROUGH OLYMPIA, WA, USA

EarthScope’s Plate Boundary Observatory (PBO) borehole strainmeter network has been in operation for 12 years, beginning June 2005 when the first PBO strainmeters were installed on the Olympic Peninsula. The Cascadia PBO borehole strainmeter sites were identified as being key locations to observe the development and migration of Cascadian Subduction Zone (CSZ) Episodic Tremor and Slip (ETS). ETS are non-volcanic tremor swarms that occur over periods of weeks, often migrating along segments of the subduction zone, which can release the energy equivalent to a M7 or greater earthquake. Observing this subduction process may provide a greater understanding of strain release patterns along a portion of the subducting slab and help resolve questions regarding the location of the locked zone of the slab as well as what role ETS events may play in the CSZ earthquake cycle. A decade of borehole strainmeter data and continued observation enables detailed investigation of the temporal evolution of CSZ ETS events along with seismic and GPS data.

CSZ ETS events repeat every ~14 months along the southern Vancouver Island to Olympia, WA portion of the plate boundary. Each ETS event is different; initial propagation location, ETS movement, duration, and direction all vary. The 2017 ETS started in early February and progressed through early April. Beginning in the northern Olympic Peninsula, near Port Angeles, ETS migrated south towards Olympia over the course of a week. The event paused and resumed two weeks later under the Straits of Juan de Fuca and propagated northwest under Vancouver Island. There are 15 PBO borehole strainmeters along this segment, and ETS strain observations correlate with the Pacific Northwest Seismic Network (PNSN) and PBO GPS measurements. PBO borehole strainmeters are sensitive over great distances from the ETS epicenters, and thus record compression or extension relative to the ETS migration direction. Using twelve years of openly available PBO borehole strainmeter data, significant contributions to understanding the ETS process, including the determination that ETS slip is tidally modulated, have been made by the community.