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Moment Magnitude–Local Magnitude Calibration for Earthquakes off Canada's West Coast

Geological Survey of Canada
Pacific Geoscience Centre
P.O. Box 6000
Sidney, BC V8L 4B2, Canada
ristau{at}pgc.nrcan.gc.ca
rogers{at}pgc.nrcan.gc.ca
cassidy{at}pgc.nrcan.gc.ca

Local magnitude (M_L) values of earthquakes off Canada's west coast are known to be underestimated by at least 0.5 magnitude units compared with other magnitude scales. Moment magnitude (M_w), derived from moment tensor analysis, provides the most robust estimate of the magnitude of earthquakes. Moment tensor analysis of regional seismic data 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. Moment tensor solutions are now possible down to M 4.0. More than 230 regional moment tensor solutions have been calculated off Canada's west coast at the Geological Survey of Canada for 1995–2002. These solutions, along with 14 previous solutions by Oregon State University in 1994–1995 and 13 Harvard solutions for 1984–1993, allow a systematic M_w–M_L calibration for earthquakes in this region. The study area extends from the Queen Charlotte Islands region in the north to the area off the west coast of southern Vancouver Island. At the northern end of the study area, where there is little oceanic crust in the source-receiver travel path, M_w is systematically larger than M_L by 0.28 ± 0.08 magnitude units. At the southern end of the study area, where there is a significant amount of ocean crust in the source-receiver travel path, M_w is systematically larger than M_L by 0.62 ± 0.08 magnitude units. Calibration of M_L with M_w will allow the western Canadian earthquake database to be used more effectively for tectonic studies and seismic hazard analysis.

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