Tectonic tremor: The chatter of mafic underplating

Abstract

Tremor is a weak seismic signal accompanying slow fault slip at plate boundaries. The relationship between tremor and slow slip and the tremor source mechanism have been widely debated, owing largely to the challenge of accurately locating tremor in depth. We assemble a tremor hypocenter catalog of 4,851 events in a 10 x 20 km² area beneath Vancouver Island during three slow slip episodes between 2003 and 2005 using a cross-station detection method adapted from previous studies to recover accurate depths. Improved tremor locations provide key constraints on i) thickness of the tremorgenic zone, ii) the relative location of tremor to key structural features in the subduction complex, and iii) the geologic context and mechanism of tremor. Tremor occurs in quasi-planar clusters < 500 m thick at a depth near 39 km, beneath a high reflectivity layer and within a zone of elevated Poisson's ratio with P-wave velocities of ~7 km/s. We interpret tremor as originating in the fragmentation of the upper few hundred meters of basaltic oceanic crust. Comminuted and overpressured basalt with increasingly anisotropic fabric is underplated onto overriding lithosphere to generate high reflectivity. Tremor thus manifests areas of material transfer across the plate boundary during slow slip.