Bulletin of the Seismological Society of America
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Bulletin of the Seismological Society of America; October 2005; v. 95; no. 5; p. 1994-2000; DOI: 10.1785/0120050028
© 2005 Seismological Society of America
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Short Note

Moment Magnitude–Local Magnitude Calibration for Earthquakes in Western Canada

John Ristau1, Garry C. Rogers1 and John F. Cassidy1

1 Geological Survey of Canada
Pacific Geoscience Centre
P.O. Box 6000
Sidney, British Columbia V8L 4B2
Canada

School of Earth and Ocean Sciences
University of Victoria
Victoria, British Columbia V8L 3P6
Canada
jristau{at}nrcan.gc.ca
grogers{at}nrcan.gc.ca
jcassidy{at}nrcan.gc.ca

Local magnitude (ML) is the primary magnitude scale calculated for western Canada by the Geological Survey of Canada (GSC). Moment magnitude (Mw), 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 Mw–ML throughout much of western Canada. Continental crust events in the Canadian Cordillera and Vancouver Island/Puget Sound region are found to have Mw = ML for earthquakes with ML ≥ 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 Mw systematically larger than ML by nearly 0.6 magnitude units. The calibrations of Mw with ML are an important result that will allow the western Canadian earthquake database to be used more effectively for tectonic studies and seismic hazard analysis.






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