Upper crustal structure of southwestern British Columbia from the 1998 Seismic Hazards Investigation in Puget Sound

Abstract

[1] This paper applies nonlinear three-dimensional travel time tomography to refraction data recorded during the 1998 Seismic Hazards Investigation in Puget Sound (SHIPS) to derive the first large-scale, high-resolution upper crustal velocity model for southwestern British Columbia. A minimum structure P wave velocity model is constructed using 175,000 first arrival travel times picked from data recorded by 58 temporary onshore stations. The model details forearc crustal structures related to terrane accretion and subsequent basin formation to a depth of about 10 km. The Metchosin igneous complex

Gas hydrate on the northern Cascadia margin: regional geophysics and structural framework

“…Adjacent onshore studies of the LITHOPROBE program (e.g., Clowes et al, 1987aClowes et al, , 1987b, as well as large-scale seismic reflection profiling along the margin allowed the construction of continuous onshore-offshore structural cross sections (e.g., Hyndman, 1995). These earlier undertakings were continued in the forearc by the Seismic Hazard Investigations in Puget Sound (SHIPS) project in 1998 (Fisher et al, 1999), which resulted in new multichannel seismic (MCS) reflection lines and higher resolution structural maps of the forearc part of the subduction zone, including largescale seismic tomography studies (e.g., Ramachandran et al, 2004Ramachandran et al, , 2005.…”

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

“…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.…”

“…The thickness of the oceanic crust was set to 5 km. The 3D sedimentary basin structure in the Georgia basin region is primarily constrained by the tomographic V P model (1 km resolution) of Ramachandran et al (2004Ramachandran et al ( , 2006. The V P =V S ratio for Quaternary basin sediments varies from 2.5 at the surface to 2.2 at 1 km depth.…”

“…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).…”