Upper-crustal structure beneath the Strait of Georgia, Southwest British Columbia

Abstract

S U M M A R YWe present a new three-dimensional (3-D) P-wave velocity model for the upper-crustal structure beneath the Strait of Georgia, southwestern British Columbia based on non-linear tomographic inversion of wide-angle seismic refraction data. Our study, part of the Georgia Basin Geohazards Initiative (GBGI) is primarily aimed at mapping the depth of the Cenozoic sedimentary basin and delineating the near-surface crustal faults associated with recent seismic activities (e.g. M = 4.6 in 1997 and M = 5.0 in 1975) in the region. Joint inversion of firstarrival traveltimes from the 1998 Seismic Hazards Investigation in Puget Sound (SHIPS) and the 2002 Georgia Basin experiment provides a high-resolution velocity model of the subsurface to a depth of ∼7 km. In the southcentral Georgia Basin, sedimentary rocks of the Cretaceous Nanaimo Group and early Tertiary rocks have seismic velocities between 3.0 and 5.5 km s −1 . The basin thickness increases from north to south with a maximum thickness of 7 (±1) km (depth to velocities of 5.5 km s −1 ) at the southeast end of the strait. The underlying basement rocks, probably representing the Wrangellia terrane, have velocities of 5.5-6.5 km s −1 with considerable lateral variation. Our tomographic model reveals that the Strait of Georgia is underlain by a fault-bounded block within the central Georgia Basin. It also shows a correlation between microearthquakes and areas of rapid change in basin thickness. The 1997/1975 earthquakes are located near a northeast-trending hinge line where the thicknesses of sedimentary rocks increase rapidly to the southeast. Given its association with instrumentally recorded, moderate sized earthquakes, we infer that the hinge region is cored by an active fault that we informally name the Gabriola Island fault. A northwest-trending, southwest dipping velocity discontinuity along the eastern side of Vancouver Island correlates spatially with the surface expression of the Outer Island fault. The Outer Island fault as mapped in our seismic tomography model is a thrust fault that projects directly into the Lummi Island fault, suggesting that they are related structures forming a fault system that is continuous for nearly 90 km. Together, these inferred thrust faults may account for at least a portion of the basement uplift at the San Juan Islands.

Earthquake Ground Motion and 3D Georgia Basin Amplification in Southwest British Columbia: Deep Juan de Fuca Plate Scenario Earthquakes

“…The updated 3D physical-structure model can be further improved by: (1) incorporation of the 600 m gridded V P model of Dash et al (2007; only the 800 m surface was used here), (2) validation by comparison with future empirical earthquake recordings, and (3) increasing the maximum depth to ∼80 km for simulation of JdF plate earthquakes (constrained to maximum depth of 55 km here). Conclusions are limited to the simulations conducted here and are specific to the chosen epicenter locations and earthquake rupture style.…”

“…Structural studies in this region have shown that basement units surrounding the Late Cretaceous Georgia basin sediments correspond to V P of ∼5:5-6:0 km=s in the upper 2 km and ∼6:4-6:75 km=s at deeper depths ( White and Clowes, 1984;Zelt et al, 2001;Ramachandran et al, 2004Ramachandran et al, , 2006Dash et al, 2007). Expected reduction in seismic velocity of basement rock units in southwest British Columbia likely occurs in the upper tens of meters, which is not resolved by the chosen 250 m gridded velocity model.…”

“…Therefore, in order to update V P in the upper 1 km of the base elastic 3D model in the Georgia basin region for the modeling work conducted here, all nonconfidential government geological and geophysical datasets, as well as the higherresolution (600 m gridded) tomographic regional model of Dash et al (2007), were collected, converted to V P estimates (if required), and merged (details in Molnar, 2011). The V P =V S ratio is set to 2 for V P ≤ 5:5 km=s in the updated 3D basin model; the base of the Georgia basin is composed of Late Cretaceous Nanaimo Group rocks, inferred as the 5:5-6:0 km=s V P surface in regional tomographic V P models (Zelt et al, 2001;Ramachandran et al, 2004Ramachandran et al, , 2006Dash et al, 2007). This higher V P limit for the V P =V S ratio of 2 effectively causes low V S values to extend to greater depths in the updated model.…”

“…The Georgia basin is one in a series of basins spanning from California to south Alaska along the Pacific margin of North America ( England and Bustin, 1998), and is relatively wide and shallow (Tertiary dimensions of 130 by 70 by 5 km) in comparison to basins southward in Seattle (75 by 30 by 8 km; Frankel et al, 2007) and Los Angeles (50 by 30 by 5 km; Magistrale et al, 1996). Properties of the Late Cretaceous and Tertiary sedimentary rocks within the Georgia basin and its basement are known from seismic surveys (e.g., White and Clowes, 1984), particularly seismic tomography results of the 1998 Seismic Hazards Investigations in Puget Sound experiment (Zelt et al, 2001;Ramachandran et al, 2004;Dash et al, 2007).…”

Spectral acceleration basin amplification factors for interface Cascadia Subduction Zone earthquakes in Canada’s 2020 national seismic hazard model

“…The Georgia sedimentary basin in southwest British Columbia is one of a series of basins extending from California to southern Alaska along the Pacific margin of North America (England and Bustin, 1998) and is relatively shallow and wide (130 by 70 km wide by 5 km in depth) compared to basins further south in Seattle, WA (75 by 30 km wide by 8 km in depth; Frankel et al, 2007) and Los Angeles, CA (50 by 30 km wide by 5 km in depth; Magistrale et al, 1996). Properties of the Late Cretaceous and Tertiary sedimentary rocks within the Georgia basin and its basement are known from seismic surveys (White and Clowes, 1984), particularly seismic tomography results of the 1998 Seismic Hazards Investigations in Puget Sound experiment (Brocher et al, 2001; Dash et al, 2007; Ramachandran et al, 2004a). The presence of the Georgia basin is expected to cause amplification of ground motions in Metro Vancouver, based on amplification observed in areas with sediment-filled basins (Campillo et al, 1989; Frankel et al, 2009; Olsen, 2000).…”