Upper Crustal Velocity Structure of the Southwestern Canadian Cordillera from Explosion Recordings on the WCTN Earthquake Seismic Net

Abstract

—The digital seismic network used to monitor earthquakes in southwestern British Columbia recorded explosions from Lithoprobe’s Southern Cordillera Refraction Experiment (SCoRE) in 1989 and 1990 and from the USGS Pacific Northwest Refraction Survey in 1991. Both P and S waves were recorded. The geographical distribution of the shots and receiver stations enabled a large three-dimensional area to be sampled. For ray paths considered to be in the top 10 km of the crust, we used a tomographic inversion procedure to estimate the 2-D distribution of upper crustal P- and S-wave velocities within blocks of a refracting velocity grid, located beneath a near surface velocity layer of uniform thickness. The P-wave velocity distribution mapped the subsurface location of several terranes in the southwest Cordillera. Volcanic rocks of Wrangellia terrane, which is the principal component of the Insular Belt, were characterized by upper crustal velocities > 6.3 km s ⁻¹, whereas plutonic rocks of the Coast Belt had velocities < 6.3 km s⁻¹. East of southern Vancouver Island, the velocity distribution indicated that Wrangellia extends in the subsurface beneath the Coast Belt for about 130 km eastward of the surface location of the Insular/Coast Belt boundary. A portion of the southern boundary of Wrangellia was also delineated by the P-velocity map, which suggested that metamorphic rocks of the Northwest Cascades Thrust System may extend beneath sedimentary cover as far west as south-central Georgia Strait. Inverted S-wave velocities were typically 3.6–3.7 km s⁻¹. Corresponding values of Poisson’s ratio were 0.27–0.28 over Vancouver Island and 0.20–0.24 in the Coast Belt. The higher values for Vancouver Island likely reflect the high mafic content of Wrangellia volcanics, compared to the high quartz content of the Coast Belt plutonics.