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

Abstract

Summary The Strait of Georgia is a topographic depression straddling the boundary between the Insular and Coast belts in southwestern British Columbia. Two shallow earthquakes located within the strait (M = 4.6 in 1997 and M = 5.0 in 1975) 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 the Strait of Georgia to record shots from a marine source within the strait. We apply a tomographic inversion procedure to first‐arrival traveltime data to derive a minimum‐structure 3‐D P‐wave velocity model for the upper crust 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 variations within the major Cretaceous to Cenozoic Georgia Basin, which underlies the Strait of Georgia. The 3‐D velocity model clearly delineates the structure of the Georgia Basin. Taking the 6 km s−1 isovelocity contour to represent the top of the underlying basement, the basin thickens from between 2 and 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 Nanaimo Group. Velocities to the south are lower (3–6 km s−1) because of the additional presence of the overlying Tertiary Huntingdon Formation and more recent sediments, including glacial and modern Fraser River deposits. In contrast to the relatively smoothly varying velocity structure of the basin, velocities of the basement rocks, which comprise primarily Palaeozoic to Jurassic rocks of the Wrangellia Terrane and possibly Jurassic to mid‐Cretaceous granitic rocks of the Coast Belt, show significantly more structure, probably an indication of the varying basement rock lithologies. The 2‐D velocity model more clearly reveals the velocity layering associated with the recent sediments, Huntingdon Formation and Nanaimo Group of the southern Georgia Basin, as well as the underlying basement. We interpret lateral variation in sub‐basin velocities of the 2‐D model as a transition from Wrangellian to Coast Belt basement rocks. The effect of the narrow, onshore–offshore recording geometry of the seismic experiment on model resolution was tested 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.

Seismic Traveltime Tomography of the Crust and Lithosphere

“…recorded by an in-line array of sources and receivers), although recent studies (Zelt & Barton, 1998;Zelt et al, 1999;Day et al, 2001;Rawlinson et al, 2001b;Zelt et al, 2001) indicate that 3-D wide-angle surveys are becoming more frequent. In 2-D experiments, data coverage is often quite dense, and tomographic-style interpretation techniques are usually designed to allow both interface structure and layer velocities to be constrained by the data.…”

“…Ritsema et al (1998) used several different synthetic models rather than checkerboards and analyzed the accuracy of their recovery. This kind of analysis is not commonly used in wide-angle studies, although recently, Zelt (1998), Zelt et al (1999Zelt et al ( , 2001 and Day et al (2001) implemented checkerboard tests for analyzing the resolution of velocity structure , also use it for interface structure) derived from wide-angle traveltime inversion. In local earthquake tomography, checkerboard tests have been implemented by Chiarabba et al (1997) and Graeber & Asch (1999); Walck & Clayton (1987) and Walck (1988) used synthetic reconstructions with anomalies positioned in key localities (i.e.…”

“…For example, numerous studies have been carried out in Canada (e,g, Hole et al, 1992;Kanasewich et al, 1994;Clowes et al, 1995;Zelt & White, 1995;Morozov et al, 1998;Zelt et al, 2001), the U.S. (e.g. Jarchow et al, 1994;Zhu & Ebel, 1994;Parsons et al, 1996;Wang & Braile, 1996;Lizarralde & Holbrook, 1997), Europe (e.g.…”

“…Also using finite-difference techniques, Riahi et al (1997) invert wide-angle reflections for Moho structure beneath the Gulf of Bothnia and Zelt et al (1996) invert both reflection and refraction traveltimes for velocity and interface structure beneath the southwestern Canadian Cordillera. Other studies to use finite difference solutions of the eikonal equation in 3-D wide-angle traveltime inversions include Zelt & Barton (1998), Zelt et al (1999Zelt et al ( , 2001 and Day et al (2001). Parsons et al (1996) use a finite difference approach in the inversion of wide-angle traveltimes for the 2-D crustal structure of the Colorado Plateau.…”

Modified Finslerian spaces for more effective ray-tracing. Application to seismic travel-time tomography

“…[1] were originally designed for deep crustal subsurface imaging and have been applied in several interesting case studies [2][3][4][5][6][7][8][9]. In crustal studies, the velocity contrasts are low with smooth gradients so that the algorithms function without major difficulties and bugs.…”