Elastic Wave Constraints on the Slow-Slip Inter-Plate Boundary in the Northern Cascadia Subduction Zone

Session: Amphibious Seismic Studies of Plate Boundary Structure and Processes
Type: Oral
Date: 4/29/2020
Time: 05:00 PM
Room: 120 + 130
Description: In subduction zones, landward dipping regions of low shear wave velocity and elevated Poisson’s ratio can extend to at least 120 km depth; they are interpreted to be all or part of the subducting igneous oceanic crust, which is considered to be overpressured owing to fluid trapped beneath an impermeable seal along the overlying inter-plate boundary. P and S wave velocity models for southern Vancouver Island have been developed using 3-D double-difference tomographic inversion of travel times from local earthquakes, low frequency earthquakes (LFEs), the 1998 SHIPS wide-angle survey and all post-2002 stations. The velocity models extend from the landward end of the locked zone at 25-30 km depth to the forearc mantle. To view spatial variations more clearly, we have also calculated sections displaying density ratios of the Lamé elastic moduli (lambda and mu). Relocated LFEs, which exhibit particle motions consistent with low-angle thrusting, occur immediately below a 6-12 km thick package of landward dipping high amplitude, normal incidence P wave reflections imaged in both onshore and offshore surveys. These so-called E reflections are interpreted to be a shear zone because they truncate overlying structures such as the Leech River fault. The LFEs also occur at the base of a landward dipping layer where a Poisson’s ratio of 0.3-0.4 is inferred from scattered teleseismic waves; in the tomographic models, this layer corresponds to a Poisson’s ratio is 0.26-0.28 and is attributable to elevated lambda. Magnetotelluric data reveal a landward dipping 30-80 ohm-m electrical conductor that is approximately coincident with the shear zone and terminates landward close to the downdip termination of the teleseismic low velocity zone and the predicted onset of eclogitization in the subducting slab. Near-lithostatic pore-fluid pressures inferred from anomalous elastic properties close to the base of the shear zone are likely a consequence of slab-derived fluids that are trapped above a thin megathrust or, more likely, within a vertically distributed inter-plate boundary zone.

Presenting Author: Andrew J. Calvert

Authors