Upper crustal velocity structure of the Southwestern Canadian Cordillera from explosion recordings on the WCTN earthquake seismic net

The digital seismic network used to monitor earthquakes in southwestern Bri tish Columbia recorded explosions from Lithoprobe's Southern Cordillera Ref raction Experiment (SCoRE) in 1989 and 1990 and from the USGS Pacific North west Refraction Survey in 1991. Both P and S waves were recorded. The geogr aphical distribution of the shots and receiver stations enabled a large thr ee-dimensional area to be sampled. For ray paths considered to be in the to p 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 bloc ks of a refracting velocity grid, located beneath a near surface velocity l ayer of uniform thickness. The P-wave velocity distribution mapped the subs urface location of several terranes in the southwest Cordillera. Volcanic r ocks of Wrangellia terrane, which is the principal component of the Insular Belt, were characterized by upper crustal velocities > 6.3 km s(-1), where as plutonic rocks of the Coast Belt had velocities < 6.3 km s(-1). East of southern Vancouver Island, the velocity distribution indicated that Wrangel lia extends in the subsurface beneath the Coast Belt for about 130 km eastw ard of the surface location of the Insular/Coast Bell boundary. A portion o f the southern boundary of Wrangellia was also delineated by the P-velocity map, which suggested that metamorphic rocks of the Northwest Cascades Thru st 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 Isl and 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.