Onshore/offshore structure of the Northern Cascadia subduction zone from Bayesian receiver function inversion

Date

2012-05-01

Authors

Brillon, Camille

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Abstract

This study applies Bayesian inversion to receiver functions (RF) to estimate local shear wave velocity (Vs) structure of the crust and upper mantle beneath two ocean bottom seismometers (OBS) offshore, and two land-based seismometers onshore Vancouver Island, British Columbia, Canada. We use passive seismic data recorded on NC89, a permanent NEPTUNE (North-east Pacific Time-series Undersea Networked Experiments) OBS located on the continental slope, and on a temporary autonomous KECK foundation OBS, KEBB, located at the Endeavour segment of the Juan de Fuca Ridge (JdFR). The two land based seismometers (OZB and PGC) are located on Vancouver Island and are part of the Canadian National Seismograph Network (CNSN). The introduction of NEPTUNE has helped to fill a gap in offshore seismic monitoring, however; due to high noise levels and a relatively short deployment time, few useful events have been recorded (to date) for RF analysis. In this study, we utilize three-component, broadband recordings of large (M6+), distant (30 -100 degrees) earthquakes to compute RFs due to locally generated P (compressional) to S (shear) converted waves. RFs are then inverted using a non-linear Bayesian approach which yields optimal profiles of Vs, Vp (compressional wave velocity), and strike and dip angles, as well as rigorous uncertainty estimates for these parameters. Near the JdFR a thin sediment layer (<1 km) is resolved overlying a 2 km thick oceanic crust. The crust contains a large velocity contrast at the depth of an expected axial magma chamber. The oceanic crust thickens to 10 km at the continental slope where it is overlain by 5 km of sediments. At the coastal station (OZB) a low velocity zone is imaged at 16 km depth dipping approximately 12 degrees NE. Evidence for this low velocity zone is also seen beneath southern Vancouver Island (PGC) at a depth consistent with previous studies. Determining such models at a number of locations (from the spreading ridge to the coast) provides new information regarding local structure and can aid in seismic hazard analysis.

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Keywords

Receiver Functions, Ocean Bottom Seismometers, Juan de Fuca, Oceanic Plate, Bayesian, NEPTUNE, Cascadian Subduction Zone

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