SEISMOTECTONICS OF WESTERN CANADA FROM REGIONAL MOMENT TENSOR ANALYSIS
RISTAU, J., ROGERS, G.C., AND CASSIDY, J.F., Geological Survey of Canada, Pacific Geoscience Centre, Sidney, BC, Canada, V8L 4B2, and School of Earth and Ocean Sciences, University of Victoria, Victoria, BC, Canada, V8W 2Y2, firstname.lastname@example.org, email@example.com, firstname.lastname@example.org
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 broadband stations in western Canada, the U.S. Pacific Northwest, and southeast Alaska. Regional moment tensor (RMT) analysis using robust waveform fitting techniques are employed to routinely calculate source mechanisms, moments, and depths of earthquakes with M >~ 3.5–4.0 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 370 RMT solutions have been calculated for western Canada and adjacent regions for the years 1995–2003. These solutions have provided new insights into a number of tectonic problems in western Canada. Local magnitudes (ML) have been calibrated with moment magnitudes (Mw) 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. Focal mechanisms from RMT solutions have also been used to constrain the motions of the Explorer plate, a small oceanic plate off the coast of British Columbia. Rotation poles calculated from earthquake slip directions show that the Explorer plate is undergoing intense internal deformation, and strain tensors calculated from RMT solutions give a deformation rate of 5.5 mm/yr over 100 km. Stress tensors calculated for western Canada from RMT solutions show stress pattern changes throughout western Canada. Most of western Canada is being compressed in a NE-SW direction except in the region opposite the Cascadia subduction zone where the stress regime changes to N-S compression. Principal strain and stress directions for the northern Canadian Cordillera have similar orientations to one another suggesting that the earthquakes occur on faults which are favourably oriented for failure and are not particularly weak.