High-resolution imaging of rapid tremor migrations beneath southern Vancouver Island using cross-station cross correlations

Abstract

We develop a cross‐station method to detect and locate tremor and low‐frequency earthquakes (LFEs), based on the original work of Armbruster et al. (2014) that compares waveforms from the same time window at stations separated by roughly 10 km. To improve the signal‐to‐noise ratio, we first rotate the horizontal components into the empirical shear wave particle motion direction. The large‐scale “rapid tremor reversals” beneath southern Vancouver Island are best recorded by stations that exhibit pronounced shear wave splitting, which obscures this optimal direction. We correct for splitting using the stacked templates of 11 low‐frequency earthquake families obtained in this region by Bostock et al. (2012). We find that the style of rapid tremor migrations (RTMs) evolves as the main front passes over a region. Very close to the main front, numerous small‐scale migrations occur with recurrence intervals far shorter than tidal periods. These usually propagate along the main front even when that is not parallel to dip. Several larger RTMs propagating along the main front have prominent elongation orthogonal to the propagation direction, inconsistent with the interpretation that their large propagation speed is an “apparent” velocity caused by the slow main front intersecting a preexisting linear structure on the plate interface. Farther behind the main front, RTMs gradually progress to being tidally modulated and have generally slower propagation speeds. Many reversal‐like RTMs are observed, some of which evolve from fronts initially propagating along the main front. These reversals are sometimes coherent across regions of low tremor density.