Geological Society of America

68 D7. Recent Advances in Geophysics/Geodynamics

Tuesday, 27 October 2020
1:30 PM - 5:30 PM
Meeting Rooms

A DOUBLE DIFFERENCE TOMOGRAPHY STUDY OF THE WASHINGTON FOREARC: DOES SILETZIA CONTROL CRUSTAL SEISMICITY?

Paper number: 68-3
Program: Technical Sessions
Time: 2:00 PM – 2:15 PM

Authors

Reid Merrill Presenting Author
Vancouver, BC
Michael G. Bostock
The University of British Columbia, Vancouver, BC
Simon M. Peacock
University of British Columbia, Vancouver, BC
A.J. Calvert
Simon Fraser University, Burnaby, BC
Nikolas I. Christensen
University of Wisconsin-Madison, Madison, WI

Abstract

We present new seismic tomography of the Washington forearc using a suite of manually picked regional earthquake phase arrivals. The recovery of similarly sampled P- and S-velocity permits the robust calculation of Poisson’s ratio throughout the region. The seismological signature of Siletzia, an accreted oceanic plateau that crops out in Washington as the Crescent Formation, is evident in our models as a continuous high Poisson’s ratio body that coincides with subsurface structures estimated from potential field maps. Relocated earthquakes preferentially locate in low Poisson’s ratio regions in the forearc crust and, in particular, in a diffuse layer located at 15-25 km depth beneath relatively aseismic Siletzia. Our imaging of the Puget Sound is consistent with previous interpretations of the architecture of major faults and blocks, as evident by distinct Poisson’s ratio signatures that distinguish sedimentary basins from mafic rocks of the Crescent Formation. We speculate that seismicity below the Puget Sound is promoted by slab-derived fluids that are localized beneath Siletzia as a result of intrinsic low vertical permeability.

Geological Society of America Abstracts with Programs. Vol. 52, No. 6

doi: 10.1130/abs/2020AM-353785

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