Moment Magnitude-Local Magnitude Calibration for Earthquakes in Western Canada

Abstract

Local magnitude ( M L ) is the primary magnitude scale calculated for western Canada by the Geological Survey of Canada (gsc). Moment magnitude ( M w ), derived from moment tensor analysis, provides a more robust estimate of the magnitude of earthquakes but is more demanding to calculate. Moment tensor analysis of regional seismic data for earthquakes with magnitudes larger than 3.5 in western Canada is now possible owing to the installation of more than 40 three-component broadband stations in western Canada, the Pacific Northwest of the United States, and southeast Alaska. More than 100 regional moment tensor solutions have been calculated in the Canadian Cordillera and Vancouver Island/Puget Sound region for 1996–2004 at the gsc. These solutions, along with 45 prior solutions, allow the calibration of M w – M L throughout much of western Canada. Continental crust events in the Canadian Cordillera and Vancouver Island/Puget Sound region are found to have M w = M L for earthquakes with M L ≥ 3.6. In contrast, earthquakes located within the subducting slab in the Vancouver Island/Puget Sound region, where there are complex source–receiver travel paths, have M w systematically larger than M L by nearly 0.6 magnitude units. The calibrations of M w with M L are an important result that will allow the western Canadian earthquake database to be used more effectively for tectonic studies and seismic hazard analysis.