Microgravity effect of inter-seismic crustal dilatation

Abstract

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.