Shallow crustal structure beneath the Juan de Fuca ridge from 2 seismic refraction tomography

In May, 1982 a seismic refraction survey, using a 32 I airgun and a radio telemetering sonobuoy system with direct digital recording, was carried out in Georgia Strait. The objectives of this experiment were determination of the upper crustal structure beneath the Strait and investigation of the proposed existence of a major fault separating Vancouver Island from the continental mainland. Three reversed profiles across the Strait and reversed lines along it on either side were shot. In addition, the southernmost line from northern Galiano Island to Point Grey was recorded digitally from a telemetered land-based station on Galiano Island. The refraction data are supplemented by several high resolution reflection profiles from previous experiments. Two-dimensional models of the crustal structure across the Strait have been constructed using a forward modelling ray trace and synthetic seismogram algorithm to match the travel times and amplitude characteristics of the data. Generally, these models consist of 3 layers. The first consists of unconsolidated sediments and Pleistocene glacial deposits varying in thickness up to 1 km. The velocity of this layer is assumed to be 1.6 km/s due to lack of velocity information. The second layer represents Upper Cretaceous sediments of the Nanaimo Group and possibly Chuckanut Formation, having velocities ranging from 3.6 km/s to 4.2 km/s. It varies in thickness up to 2 km, thinning toward the mainland side of the Strait. The third layer consists of a thin transitional zone, with velocity 6.0-6.1 km/s at the surface and gradient 0.5 km/s/km, beneath which the velocity gradient is decreased to 0.1-0.15 km/s/km. This layer dips at angles of 2°-16° toward Vancouver Island. This layer is likely the extension of the Coast. Range intrusives. The Malaspina fault proposed for the Strait of Georgia is not observed, although some uncertainty remains due to termination of seismic coverage short of the mainland. A local fault with calculated throw of 0.55 km has been located approximately 15 km northeast of Galiano Island. The dip and strike of the fault are poorly constrained. Maximum depth of penetration obtained in this study is about 3 km.Science, Faculty ofEarth, Ocean and Atmospheric Sciences, Department ofGraduat