S wave velocity structure of the Northern Cascadia Subduction Zone

Abstract

Teleseismic receiver functions from an array of portable broadband seismograph stations located in southwestern British Columbia are interpreted to estimate the S wave velocity structure to upper mantle depths across the northern Cascadia subduction zone. At our westernmost station on central Vancouver Island, a prominent low‐velocity zone (ΔVs = −1 km s−1) with a high Poisson's ratio is estimated at 36–41 km depth. This feature correlates with the reflective “E” zone observed in LITHOPROBE reflection data, a region that also exhibits high electrical conductivity and has recently been interpreted as a fluid‐saturated shear zone above the subducting Juan de Fuca (JdF) plate. Analysis of data at our stations to the NE permits this zone to be mapped into the upper mantle, 54 km beneath the British Columbia mainland, and approximately 250 km from the locus of subduction. The subducting oceanic crust is imaged at 47–53 km depth dipping 15°± 5° in the direction N30°E ± 20° beneath central Vancouver Island. The dip angle increases to 22° ± 5° at a depth of 60–65 km beneath the Strait of Georgia. The results of this analysis provide the first definitive evidence for the location of the subducting plate in this region and indicate that the seismicity at depth occurs within the oceanic crust. The dip direction of N30°E at each station provides new evidence that the JdF plate is arched upward as it subducts in this region. A low‐velocity zone that coincides with the “C” reflectors beneath Vancouver Island is interpreted to extend east to the British Columbia mainland near the base of the North American crust. The top of this zone, near 20–26 km depth, lies near the lower limit of shallow seismicity. The continental Moho is estimated to be 36 km below the Strait of Georgia.