Changes in the density of the shallow crust has been previously related to co-seismic strain release during earthquakes, however, the influence of inter-seismic deformation on crustal density variations is poorly understood. Here we present gravity observations from the iGrav superconducting gravimeter in southern Vancouver Island, British Columbia, Canada which reveal a substantial gravity increase between July 2012 and April 2015. We identify a negative correlation between this gravity increase and crustal dilatation strain derived from horizontal GPS velocities. The overall increasing gravity trend is caused by the gravity increase during and immediately before and after episodic tremor and slip events, which is partially compensated by gravity decrease occurring between the events. We conclude that the observed gravity increase results from a density increase due to crustal compression and that this is mostly a result of inter-seismic strain accumulation during the subduction of the Juan de Fuca plate beneath the North American plate.

There is presently insufficient information to accurately assess the hazard posed by a tsunamigenic megathrust earthquake rupturing the Cascadia Subduction Zone in the Pacific Northwest. We report our plans to deploy a Northern Cascadia Subduction Zone Observatory (NCSZO) offshore Vancouver Island, Canada, which will comprise a seafloor network of seven new Global Positioning System sites to measure deformation caused by the subduction of the oceanic Juan de Fuca Plate beneath the continental North America Plate, and a Deformation Front Laboratory at the oceanic trench to monitor aseismic creep events on the shallow part of the plate interface. The NCSZO will make effective and sustainable use of the Ocean Networks Canada NEPTUNE observatory, and be complemented by the highest density of land-based seismic and high-rate geodetic stations in Canada. We expect the NCSZO will enable significant improvements of tsunami and earthquake risks assessments after only a few years of measurements.