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TitleMagnitude conversion relations for very short distances: The Charlevoix, Quebec, Seismic Zone
AuthorBent, A L; Vadnais, F
SourceGeological Survey of Canada, Open File 7945, 2016, 17 pages,
PublisherNatural Resources Canada
Documentopen file
Mediaon-line; digital
File formatpdf
NTS21M/01; 21M/02; 21M/03; 21M/07; 21M/08; 21M/09; 21M/10; 21M/16; 21N/04; 21N/05; 21N/12; 21N/13; 21N/14
AreaCharlevoix; Kamouraska; La Malbaie; Baie-St-Paul
Lat/Long WENS-71.2500 -69.2500 48.0000 47.0000
Subjectsgeophysics; seismic zones; seismicity; earthquakes; earthquake magnitudes; epicentres; amplitude spectra; Logan Line; Charlevoix Seismic Zone
Illustrationslocation maps; plots; histograms
ProgramCanadian Hazard Information Service, Canadian Hazard Information Service
Released2016 04 29
AbstractThe Nuttli (MN) scale is the most commonly used magnitude scale in eastern Canada. It is based on the amplitude of the Lg phase and therefore is not appropriate for distances of less than 50 km where the Lg phase is not developed. The original Richter, ML, scale developed for use in California and known to be inappropriate for eastern North America, is used only when the Lg phase is non-existent or highly attenuated, generally for earthquakes recorded at distances of less than 50 km or earthquakes occurring in oceanic crust. In the Charlevoix, Quebec, Seismic Zone the station density is such that it is possible to routinely locate earthquakes of magnitude 1.0 or smaller. Magnitudes for these smallest earthquakes are usually ML. For many slightly larger earthquakes, the MN values are based on readings at a single station, most often DAQ, as the earthquakes are usually not well-recorded at greater distances. Establishing a magnitude relation between MN calculated at appropriate distances and MN or ML calculated at close distances would enable more magnitude readings to be used for the small earthquakes and a magnitude recurrence relation to be established over a wider magnitude range. Using data from earthquakes occurring within the Charlevoix Seismic Zone for which MN was reported as the preferred magnitude we calculated ML and MN from stations at less than 50 km from the epicenter and compared them to the published or event magnitudes. ML underestimates the magnitude by about 1 magnitude unit whereas the MN (close) values were only about 0.2 units smaller than the presumed magnitude. These results suggest that MN would be a better measure of the earthquake size even when calculated out of range and not from an Lg wave. We also evaluated the effect of using hypocentral distance instead of epicentral distance for earthquakes at less than 50 km but found that the difference was not significant in most cases. An evaluation of DAQ single station versus multi-station magnitudes suggests that DAQ station MN magnitudes may overestimate the size of the earthquake by about 0.2 magnitude units.
Summary(Plain Language Summary, not published)
The Nuttli MN scale is the most commonly used scale to measure the magnitude of eastern Canadian earthquakes. It cannot be used for distances of less than 50 km because the Lg wave on which it based is not developed. Thus the original Richter (ML) magnitude must be used even though it is know to be inappropriate for eastern Canada. To compare earthquakes and use them in hazard assessments, they must be measured using a common magnitude scale. Using earthquake data from the Charlevoix Seismic Zone we develop conversion relations for ML to MN. We find that ML underestimates the size of the earthquake by approximately 1.2 magnitude units. If we calculated MN at closer distances than those for which the scale is defined, we underestimate the earthquake size by 0.2 magnitude units. The MN magnitudes for many Charlevoix earthquakes are based on a single station. By employing our conversion relations we can use data from more stations, which should result in a more reliable magnitude estimate.