Stress rotation across the Cascadia megathrust requires a weak subduction plate boundary at seismogenic depths

Earth and Planetary Science Letters
By: , and 

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Abstract

The Mendocino Triple Junction region is the most seismically active part of the Cascadia Subduction Zone. The northward moving Pacific plate collides with the subducting Gorda plate causing intense internal deformation within it. Here we show that the stress field rotates rapidly with depth across the thrust interface from a strike-slip regime within the subducting plate, reflecting the Pacific plate collision, to a thrust regime in the overriding plate. We utilize a dense focal mechanism dataset, including observations from the Cascadia Initiative ocean bottom seismograph experiment, to constrain the stress orientations. To quantify the implications of this rotation for the strength of the plate boundary, we designed an inversion that solves for the absolute stress tensors in a three-layer model subject to assumptions about the strength of the subducting mantle. Our results indicate that the shear stress on the plate boundary fault is likely no more than about ∼50 MPa at ∼20 km depth. Regardless of the assumed mantle strength, we infer a relatively weak megathrust fault with an effective friction coefficient of ∼0 to 0.2 at seismogenic depths. Such a low value for the effective friction coefficient requires a combination of high fluid pressures and/or fault-zone minerals with low inherent friction in the region where a great earthquake is expected in Cascadia.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Stress rotation across the Cascadia megathrust requires a weak subduction plate boundary at seismogenic depths
Series title Earth and Planetary Science Letters
DOI 10.1016/j.epsl.2018.01.002
Volume 485
Year Published 2018
Language English
Publisher Elsevier
Contributing office(s) Earthquake Science Center
Description 10 p.
First page 55
Last page 64
Country United States
State California