Seismotectonics of western Canada from regional moment tensor analysis

Moment tensor analysis of regional earthquakes (distances ∼<1000 km) in western Canada is now possible due to the installation of more than 40 three-component broad-band seismometers in western Canada and adjacent regions. In this study, regional moment tensor (RMT) analysis using robust waveform fitting techniques are employed to routinely calculate source mechanisms, moments, and depths of earthquakes with M ≥∼4.0 in and near western Canada. This has resulted in about 10 times as many solutions per year for this region than have been calculated with teleseismic methods which are limited to earthquakes about M > 5.0.; To date, more than 380 RMT solutions have been calculated in this study for western Canada and adjacent regions for the years 1995--2004. These solutions provide new insights into a number of tectonic problems in western Canada. Local magnitudes NO have been calibrated with moment magnitudes (M w) providing a more consistent estimate of the magnitude of an earthquake. This is particularly important in the offshore region of British Columbia where RMT analysis shows that ML is underestimated by 0.3--0.7 magnitude units compared with Mw depending on the amount of oceanic crust present in the source-receiver travel path. This has important consequences for seismic hazard analysis and tectonic studies. Focal mechanisms from RMT solutions are also used to constrain the motions of the Explorer plate, a small oceanic plate off the coast of British Columbia. Rotation poles are calculated by leaving Pacific/Explorer motion unconstrained, and by constraining Pacific/Explorer motion using moment release rates along the Pacific/Explorer boundary. The Pacific/Explorer rotation rate decreases by a factor of 2 if Pacific/Explorer motion is constrained. This changes the convergence direction of the Explorer plate relative to the North America plate from NE-SW in the unconstrained case to N-S in the constrained case. This suggests that Explorer plate motion cannot be modeled with a single rotation pole and cannot be treated as a rigid plate. The Explorer plate is likely undergoing intense internal deformation. The strain tensor for the Explorer plate, calculated from RMT solutions, gives a strain rate of 7.8 x 10-8 yr -1 in a N-S direction. (Abstract shortened by UMI.)...