Three-dimensional crustal velocity structure beneath the Strait of Georgia, British Columbia

The Strait of Georgia is a topographic depression straddling the boundary b etween the Insular and Coast belts in southwestern British Columbia. Two sh allow earthquakes located within the strait ((M=4.6 in 1997 and M=5.0 in 19 75) and felt throughout the Vancouver area illustrate the seismic potential of this region. As part of the 1998 Seismic Hazards Investigation of Puget Sound (SHIPS) experiment, seismic instruments were placed in and around th e Strait of Georgia to record shots from a marine source within the strait. We apply a tomographic inversion procedure to first-arrival traveltime dat a to derive a minimum-structure 3-D P-wave velocity model for the upper cru st to about 13 km depth. We also present a 2-D velocity model for a profile orientated across the Strait of Georgia derived using a minimum-parameter traveltime inversion approach. This paper represents the first detailed look at crustal velocity variation s within the major Cretaceous to Cenozoic Georgia Basin, which underlies th e Strait of Georgia. The 3-D velocity model clearly delineates the structur e of the Georgia Basin. Taking the 6 km s(-1) isovelocity contour to repres ent the top of the underlying basement, the basin thickens from between 2 a nd 4 km in the northwestern half of the strait to between 8 and 9 km at the southeastern end of the study region. Basin velocities in the northeastern half are 4.5-6 km s(-1) and primarily represent the Upper Cretaceous Nanai mo Group. Velocities to the south are lower (3-6 km s(-1)) because of the a dditional presence of the overlying Tertiary Huntingdon Formation and more recent sediments, including glacial and modern Fraser River deposits. In co ntrast to the relatively smoothly varying velocity structure of the basin, velocities of the basement rocks, which comprise primarily Palaeozoic to Ju rassic rocks of the Wrangellia Terrane and possibly Jurassic to mid-Cretace ous granitic rocks of the Coast Belt, show significantly more structure, pr obably an indication of the varying basement rock lithologies. The 2-D velo city model more clearly reveals the velocity layering associated with the r ecent sediments, Huntingdon Formation and Nanaimo Group of the southern Geo rgia Basin, as well as the underlying basement. We interpret lateral variat ion in sub-basin velocities of the 2-D model as a transition from Wrangelli an to Coast Belt basement rocks. The effect of the narrow, onshore-offshore recording geometry of the seismic experiment on model resolution was teste d to allow a critical assessment of the validity of the 3-D velocity model. Lateral resolution throughout the model to a depth of 3-5 km below the top of the basement is generally 10-20 km.