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Seismic structure across the active subduction zone of western Canada Spence, George Daniel


The Vancouver Island Seismic Project (VISP) was conducted in 1980 to study the structure of the subducting oceanic Juan de Fuca plate and the overriding continental America plate. The principal seismic refraction line (line I) was a 350 km onshore-offshore profile perpendicular to the continental margin. An array of 32 receivers was located on the America plate on the mainland and across Vancouver Island, and extended offshore with 3 ocean bottom seismometers (OBS's). Two shots were fired at the eastern end of the line, and 17 shots were located along the westernmost 100 km of the profile. Control for the interpretation of the onshore-offshore profile was provided by a reversed refraction profile along the length of Vancouver Island (McMechan and Spence 1983) and by the marine refraction profile recorded on the OBS's (Waldron 1982). To aid in the modeling of the seismic structure of this complex region, two practical techniques have been developed and applied in the interpretation of line I . The first procedure was an iterative inversion technique for traveltimes from explosions in which shots at several locations are recorded on the same set of receivers. Traveltimes for the initial model and for subsequent iterations are computed using two-dimensional ray tracing. The model is represented by one or more blocks in which the velocity, the velocity gradient, and specified boundary positions are allowed to vary. Perturbations to the parameters are then determined simultaneously using a damped least squares method. Second, a fast, efficient algorithm based on asymptotic ray theory has been developed for the calculation of synthetic seismograms through two-dimensional media. The same ray tracing scheme is used as in the traveltime inversion method, in which the velocity model is represented by large polygonal blocks, each with a uniform velocity gradient. Simple analytic expressions are thus used for both the ray tracing and for the amplitude computations. Amplitudes may be calculated for head waves, refractions, pre-critical and wide-angle reflections, surface reflections and multiples. The traveltime inversion procedure and synthetic seismogram algorithm were both applied in the interpretation of the onshore-offshore profile. The major features of the refraction structural model are as follows: (1) The oceanic lithosphere dips at 3° or less beneath the continental slope, so the bend in the subducting slab occurs landward of the foot of the slope. (2) The subducting crust dips at 14-16° beneath the continental shelf until it passes beneath the continental Moho at 37 km depth below western Vancouver Island. (3) An upper mantle reflector may correspond to the base of the subducting lithosphere. (4) A segment of high-velocity material above the downgoing crust, with velocity 7.7 km/s and depth range ~20-25 km, may represent a remnant of subducted lithosphere, perhaps detached when the subduction zone jumped westward to its present position.

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