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TitleAccurate determination of local magnitude for earthquakes in the Western Canada Sedimentary Basin
AuthorBabaie Mahani, A; Kao, HORCID logo
SourceSeismological Research Letters vol. 90, no. 1, 2018 p. 203-211,
Alt SeriesNatural Resources Canada, Contribution Series 20180221
PublisherSeismological Society of America (SSA)
Mediapaper; on-line; digital
File formatpdf (Adobe® Reader®); html; csv
ProvinceAlberta; British Columbia; Yukon; Northwest Territories
NTS82I; 82J; 82K; 82L; 82M; 82N; 82O; 82P; 83; 84; 85B; 85C; 85D; 85E; 85F; 85L; 92I; 92J; 92K; 92L; 92M; 92N; 92O; 92P; 93; 94; 95; 103A; 103H; 103I; 103J; 103O; 103P; 104A; 104B; 104G; 104H; 104I; 104J; 104O; 104P; 105A; 105B; 105G; 105H; 105I; 105J; 105O; 105P
Lat/Long WENS-132.0000 -112.0000 64.0000 50.0000
Subjectsgeophysics; fossil fuels; seismology; earthquakes; earthquake magnitudes; earthquake risk; seismicity; seismic waves; attenuation; amplitude spectra; petroleum industry; hydrocarbon recovery; seismological network; seismographs; earthquake catalogues; models; Western Canada Sedimentary Basin; monitoring
Illustrationslocation maps; plots; tables
ProgramEnvironmental Geoscience Shale Gas - induced seismicity
Released2018 11 07
AbstractIn 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 forWCSB 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(Rhypo)/100) + 0.0016 × (Rhypo - 100) + 3.0 Rhypo less than or equal to 85 km
-0.1385 × log(Rhypo/100) + 0.0016 × (Rhypo - 100) + 3.0 Rhypo >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 Rhypo less than or equal to 50 km (0.27 unit) and varies slightly for larger Rhypo: 0.08 unit for 50 km < Rhypo less than or equal to 100 km, 0.12 for 100 km < Rhypo less than or equal to 200 km, and 0.10 for Rhypo > 200 km.
Summary(Plain Language Summary, not published)
Precise determination of earthquake magnitude is especially important for events in the Western Canada Sedimentary Basin. In case of an event with local magnitude (ML) 4.0 or larger, regulators in both British Columbia and Alberta issue suspension orders to temporarily stop injection operations. Besides the economic impact of ML overestimation for the oil and gas industry, underestimation of ML can also have significant impact with regard to public safety. A recent study reported that the distance correction term used by NRCan in the calculation of ML is not suitable for WCSB. In this study, we investigate this issue using more than 4000 relocated events that occurred in WCSB. We propose a new formula for the distance correction term that can better account for the attenuation effect of seismic wave propagation and deliver more consistent ML values.

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