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TitleSite corrections and residual analysis for seismograph stations used for magnitude calculations in eastern Canada
LicencePlease note the adoption of the Open Government Licence - Canada supersedes any previous licences.
AuthorBent, A L
SourceGeological Survey of Canada, Open File 8436, 2019, 159 pages, Open Access logo Open Access
PublisherNatural Resources Canada
Documentopen file
Mediaon-line; digital
File formatreadme
File formatpdf (Adobe® Reader®); rtf; xlsx (Microsoft® Excel®)
ProvinceNewfoundland and Labrador; Prince Edward Island; Nova Scotia; New Brunswick; Quebec; Ontario; Nunavut; Northwest Territories; Manitoba; Saskatchewan; British Columbia; Alberta; Saskatchewan; Manitoba; Ontario; Quebec; New Brunswick; Nova Scotia; Prince Edward Island; Newfoundland and Labrador; Northwest Territories; Yukon; Nunavut; Canada
NTS1; 2; 3; 10; 11; 12; 13; 14; 15; 16; 20; 21; 22; 23; 24; 25; 26; 27; 28; 29; 30; 31; 32; 33; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46; 47; 48; 49; 52; 53; 54; 55; 56; 57; 58; 59; 63L; 65; 66; 67; 68; 69; 75; 76; 77; 78; 79; 85; 86; 87; 88; 89
AreaCanada; United States of America; Greenland; Denmark
Lat/Long WENS-115.0000 -50.0000 80.0000 40.0000
Subjectsgeophysics; seismology; seismographs; seismological network; earthquakes; earthquake magnitudes; attenuation; Canadian National Seismograph Network; Canadian National Earthquake Database
Illustrationstables; location maps; plots
ProgramCanadian Hazard Information Service
Released2019 06 13
AbstractEarthquake magnitudes are generally determined by taking an average (most often, the arithmetic mean) of magnitudes calculated at many individual seismograph stations. While some variation in station magnitudes stems directly from the seismic source (for example, radiation pattern or directivity) conditions beneath the recording station also affect the calculated value. For example, soft soils tend to amplify the seismic signal resulting in an apparent magnitude that is higher than the true value. By analyzing the differences between the magnitude determined at a specific station and the average magnitude for a large number of earthquakes, a site correction for the station can be determined and then applied as part of the magnitude calculation. Station corrections have been determined for more than three hundred seismographs used in the calculations of magnitudes in eastern Canada. In most cases, the site corrections are small but several stations with significant corrections were identified. Magnitudes were recalculated applying the corrections. When the earthquake catalog is considered as a whole, the effect is negligible but there are many individual earthquakes for which it is significant. The remaining residuals after the application of the site corrections were further evaluated to determine whether they are dependent on parameters such as distance, azimuth or frequency. A consistent pattern of residuals with respect to distance is seen at a large number of stations spanning the region, suggesting that the attenuation relation used in magnitude calculations may need to be modified. In most regions azimuthal dependence is minimal but a few regions have been targeted for further study. Residuals are near zero for periods of ~0.02 - ~0.5 sec and then slowly increase with increasing period, raising questions about the validity of using the magnitude equation over a wider range of frequencies than that for which it was originally intended.
Summary(Plain Language Summary, not published)
Earthquake magnitudes are calculated by taking the average from magnitudes calculated at many individual stations. The magnitude equations assume average site conditions at recording stations but some sites may routinely record higher or lower amplitudes. The site effects must be taken into account to produce reliable magnitudes. This report determines the site corrections for all seismograph stations used in magnitude calculations in eastern Canada. Further analysis of magnitude residuals suggests that further research is needed to correct for the dependence of residuals on distance and frequency and to determine the overall effect of azimuth.

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