The Locking State of the Cascadia Megathrust offshore Vancouver Island

John A. Collins*1, Jeff McGuire1, Earl Davis2, Keir Becker3, Martin heesemann4

1Department of Geology and Geophysics, Woods Hole Oceanographic Institution, USA (, 2Pacific Geoscience Centre, Geological Survey of Canada, Canada, 3University of Miami, USA, 4Ocean Networks Canada, Canada

The Cascadia subduction zone has a history of great earthquakes but the extent of inter-seismic locking at the up-dip end of the megathrust is poorly constrained by onshore geodetic measurements. In July 2016, WHOI, U. Miami, and ONC installed a suite of seismic, geodetic and geothermal sensors in IODP Hole U1364A on the Accretionary Prism offshore Vancouver Island with the objective of providing long-term, real-time monitoring of this part of the megathrust. U1364A is located ~20 km from the deformation front and sits ~4.5 km above the thrust interface. The borehole observatory was connected to the ONC cabled network in June 2017. The borehole sensors consist of a broadband seismometer and two geodetic-quality tilt sensors clamped to the borehole casing ~277 m below the seafloor. Borehole temperature is monitored via a 24-thermistor cable. U1364A also hosts an ACORK (Advanced Circulation Obviation Retrofit Kit) that allows monitoring of rock formation pressure. We use the seismic-geodetic data to show that the Cascadia megathrust does not appear to slip in triggered tremor or slow-slip events when subjected to dynamic stress transients. Tilt and seismic data from four recent teleseismic M7.6-8.1 earthquakes demonstrate a lack of triggered slow-slip even at the Mw 4.0 level and an absence of triggered tremor despite shear-stress transients of 1–10 kPa that were sufficient to trigger tremor on the downdip end of the interface.  Our observations are most consistent with a model in which the Cascadia fault offshore Vancouver Island is locked all the way to the trench.