J.-P. Mercier

Morphology of the Explorer–Juan de Fuca slab edge in northern Cascadia: Imaging plate capture at a ridge-trench-transform triple junction

The Explorer plate is a young oceanic microplate that accommodates relative motion between the Pacific, Juan de Fuca, and North America plates near northern Vancouver Island, Canada. The northern limit of Explorer plate–Juan de Fuca subduction and the fate of the slab in northern Cascadia are poorly understood. We use passive teleseismic recordings from an array of POLARIS broadband seismic stations to image crustal and upper mantle structure beneath northern Vancouver Island into the interior of British Columbia. A clear signature of subducted material extends northeast from the Brooks Peninsula at crustal levels, beneath Georgia Strait and the mainland deep into the mantle to 300 km depth. Complexity in slab morphology results from Juan de Fuca ridge subduction and toroidal flow around the slab edge, in agreement with geophysical and geological data. We propose a tectonic model for the Explorer plate in which its separation from the Juan de Fuca plate is caused by the thermomechanical erosion of the slab edge and slab thinning at shallow levels, both of which slow convergence with North America and lead eventually to plate capture.

Improved Green’s functions for passive-source structural studies

Over the past two decades, teleseismic receiver functions have proved to be a useful tool to investigate crustal structure. Because they represent a first-order approximation to the S -wave component of the teleseismic- P Green’s function, receiver functions provide valuable information on physical properties related to shear modulus. However, the implicit use of the P -component seismogram as a proxy for the source precludes the recovery of information on discontinuous structure involving contrasts in compressional modulus. By deconvolving improved estimates of complex source time functions generated by earthquakes, one may move beyond the conventional receiver function paradigm to a more accurate approximation of the earth’s Green’s function. Using a new deconvolution method, we present estimates of the P -component of the teleseismic- P Green’s functions at several stations of the Canadian National Seismic Network (CNSN) that clearly show the receiver-side pure P -wave crustal multiple. The identificat...