STRUCTURE OF THE NORTHERN CASCADIA SUBDUCTION ZONE

As part of the POLARIS (Portable Observatories for Lithospheric Analysis and Research Investigating Seismicity - www.polarisnet.ca) project, we have initiated a broadband seismic experiment across southwestern British Columbia and northwestern Washington. The objective of this work is to better understand the structure of the subducting plate and mantle wedge in northern Cascadia, and its relation to intra-slab (Wadati-Benioff) seismicity. There are currently 30 broadband stations extending across southern Vancouver Island, the Gulf and San Juan Islands, Watcom county and the British Columbia lower mainland in an approximately linear array. We have employed the P-wave coda from 28 teleseismic events recorded on this array in a formal inversion to image fine-scale shear-velocity structure. Preliminary results indicate a structure very similar to that identified across a comparable profile in Oregon (Bostock et al. session 152). The continental Moho is evident at the eastern end of the profile near 30 km depth but disappears towards the Georgia Strait/Puget sound. Farther west, a horizontal boundary at a similar depth with an "inverted" shear-velocity contrast appears and persists to its projected intersection with the subducting plate. The oceanic crust is clearly evident below southern Vancouver Island as a dipping low-velocity zone but its signature diminishes at depths below 45 km. As for Oregon, we interpret these observations to signal the presence of a serpentinized forearc mantle hydrated, in part, through eclogitization of oceanic crust.