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 (jcollins@whoi.edu), 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.

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