Tectonic tremor: the chatter of mafic underplating beneath southern Vancouver Island?

Abstract

Tremor is a low-amplitude seismic signal that usually temporally coincides with episodic slow fault slip at plate boundaries worldwide. Since the discovery of tremor in Cascadia, significant effort has been devoted to understanding its relationship to slow slip. However, its source mechanism has been widely debated, owing in large part to the challenge of locating sources accurately in depth. We assemble a tremor catalog of 4,851 events for a ~ 10 X 20 km2 area on southern Vancouver Island from slow slip episodes in 2003–2005 using a cross-station detection method adapted from previous studies, which we extend to use both P- and S- waves, thereby recovering accurate depths. Tremor occurs in distinct, quasi-planar clusters in the plate boundary region at a depth near 39 km, just beneath a layer of high reflectivity and within a zone of elevated Poisson’s ratio. We interpret this tremor to represent mafic underplating, wherein shearing generates tremor and continuously erodes basaltic material of the upper few hundred meters of the oceanic crust. Comminuted basalt with an increasingly anisotropic fabric is gradually plated onto the overriding lithosphere to form the highly reflective layer. Localized areas of material transfer within the subduction zone may manifest the distinct tremor clusters.