ABSTRACT

In this study, we take a close look at the constituents of the Richter (1935) relation for calculation of local magnitude (ML), which is the basis for magnitude determination by Natural Resources Canada (NRCan) in the western Canada sedimentary basin (WCSB). Using a comprehensive catalog of Wood–Anderson amplitudes from earthquakes in northeast British Columbia and western Alberta, we first compare the distance correction terms log(A0) for the Richter magnitude scale previously obtained for WCSB and several other regions. We also formulate a new correction term specifically for NRCan’s routine ML calculation that better accounts for the attenuation of direct and refracted waves from events within WCSB. Based on a bilinear model for ground‐motion attenuation, our log(A0) is  
{0.7974×log(Rhypo100)+0.0016×(Rhypo100)+3.0Rhypo85  km0.1385×log(Rhypo100)+0.0016×(Rhypo100)+3.0Rhypo>85  km,
in which Rhypo is the hypocentral distance. Our log(A0) results in lower ML by an average of 0.29, 0.27, 0.12, and 0.34 units, respectively, compared with those obtained by Richter (1958; California), Hutton and Boore (1987; California), Brazier et al. (2008; Ethiopian plateau), and Bona (2016; Italy) over all distances, but gives higher ML values than those obtained by Yenier (2017; WCSB), with an average of 0.12 unit over all distances. The difference between our ML calculation and Yenier (2017) is more significant for Rhypo50  km (0.27 unit) and varies slightly for larger Rhypo: 0.08 unit for 50  km<Rhypo100  km, 0.12 for 100  km<Rhypo200  km, and 0.10 for Rhypo>200  km.
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