Cascadia low frequency earthquakes at the base of an overpressured subduction shear zone

Andrew J Calvert,Martyn J Unsworth,Geneviève Savard,Michael G Bostock

doi:10.1038/s41467-020-17609-3

Abstract

In subduction zones, landward dipping regions of low shear wave velocity and elevated Poisson’s ratio, which can extend to at least 120 km depth, are interpreted to be all or part of the subducting igneous oceanic crust. This crust is considered to be overpressured, because fluids within it are trapped beneath an impermeable seal along the overlying inter-plate boundary. Here we show that during slow slip on the plate boundary beneath southern Vancouver Island, low frequency earthquakes occur immediately below both the landward dipping region of high Poisson’s ratio and a 6–10 km thick shear zone revealed by seismic reflections. The plate boundary here either corresponds to the low frequency earthquakes or to the anomalous elastic properties in the lower 3–5 km of the shear zone immediately above them. This zone of high Poisson’s ratio, which approximately coincides with an electrically conductive layer, can be explained by slab-derived fluids trapped at near-lithostatic pore pressures.

Highlights

In subduction zones, landward dipping regions of low shear wave velocity and elevated Poisson’s ratio, which can extend to at least 120 km depth, are interpreted to be all or part of the subducting igneous oceanic crust
Southern Vancouver Island lies in the areas of investigation of the LITHOPROBE project[28] and the 1998 SHIPS (Seismic Hazards Investigation in Puget Sound) survey, which acquired marine seismic reflection data in the Strait of Juan de Fuca, Strait of Georgia and Puget Sound that were recorded at wide-angle by onshore stations and ocean-bottom seismometers[29,30,31]
Travel times from local earthquakes and low-frequency earthquakes6 (LFEs) recorded by all post-2002 seismograph stations, together with first arrivals from the SHIPS survey have been used in a double-difference tomographic inversion (TomoDD) to Strait of Georgia

Summary

Introduction

Landward dipping regions of low shear wave velocity and elevated Poisson’s ratio, which can extend to at least 120 km depth, are interpreted to be all or part of the subducting igneous oceanic crust. We show that during slow slip on the plate boundary beneath southern Vancouver Island, low frequency earthquakes occur immediately below both the landward dipping region of high Poisson’s ratio and a 6–10 km thick shear zone revealed by seismic reflections. The plate boundary here either corresponds to the low frequency earthquakes or to the anomalous elastic properties in the lower 3–5 km of the shear zone immediately above them This zone of high Poisson’s ratio, which approximately coincides with an electrically conductive layer, can be explained by slab-derived fluids trapped at near-lithostatic pore pressures. In Cascadia, detection of an approximately coincident 3–5 km thick, landward dipping zone with anomalously high Poisson’s ratio of 0.3–0.4 has led to the proposal that the upper oceanic crust is maintained at near-lithostatic pore pressure by a low-permeability inter-plate boundary immediately above the LVZ21,22, and a similar interpretation has been made in Japan[23,24]. In Cascadia, interpretations of the inter-plate boundary using receiver functions derived from teleseismic data are systematically shallower than interpretations based on active source, e.g., normal-incidence reflection and wide-angle, seismic surveys[9,10]

Methods

Results

Conclusion