New constraints on subduction zone structure in northern Cascadia

A detailed passive seismic experiment was carried out across southwestern British Columbia and northwestern Washington to investigate the structure of the subducting Juan de Fuca plate and mantle wedge in Cascadia and its relation to intraslab seismicity. As part of the POLARIS project, 31 three-component broad-band stations were deployed in an approximately linear array spanning southern Vancouver Island, the Gulf and San Juan Islands, Watcom county and the British Columbia lower mainland. P-wave coda from 41 teleseismic events have been employed in formal inversions for fine-scale shear-velocity structure. Our results indicate a structure very similar to that identified across a comparable profile in central Oregon. The continental Moho is evident at the eastern end of the profile near 35-km depth but disappears towards Georgia Strait/Puget Sound. A prominent low-S-velocity zone is clearly evident below southern Vancouver Island dipping eastwards through Georgia Strait/Puget Sound, and coincides with the E-reflection zone originally identified in LITHOPROBE studies. Structure below the E-layer is much less prominent and varies intermittently along the array. Based on the observations and interpretations of similar structures beneath Oregon, Alaska and South America, and its projection to mantle depths, we suggest that the low-velocity E-layer represents the dehydrating oceanic crust of the subducting Juan de Fuca plate. This interpretation is consistent with recent seismicity studies that place shallow Wadati-Benioff events within the oceanic mantle, and implies that the oceanic crust is 6-8 km shallower beneath Vancouver Island than previously assumed. As in Oregon, we interpret the diminished signature of oceanic crust below a depth of 45 km to signal the presence of eclogitization, which in turn supplies water to serpentinize the overlying forearc mantle.