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TitleToward an improved understanding of the MN-Mw time dependence in eastern Canada
AuthorBent, A; Greene, H
SourceBulletin of the Seismological Society of America vol. 104, no. 4, 2014 p. 2125-2132,
Alt SeriesEarth Sciences Sector, Contribution Series 20130413
PublisherSeismological Society of America (SSA)
Mediapaper; on-line; digital
File formatpdf
ProvinceOntario; Quebec; New Brunswick; Newfoundland and Labrador; Nova Scotia; Prince Edward Island
NTS21; 22; 31; 32; 40; 41; 42; 52; 53
Lat/Long WENS-96.0000 -50.0000 55.0000 42.0000
Subjectsgeophysics; seismological network; seismology; earthquake magnitudes
Illustrationslocation maps; plots; histograms
ProgramCanadian Hazard Information Service
Released2014 07 08
AbstractAn earlier study by Bent (2011) established that the mN-MW relation for eastern Canada changed by 0.12 magnitude units in approximately 1995. Although several possible factors were identified that might have contributed to the change the exact cause was not determined. This study picks up where the earlier one left off and tests the effects of several changes made in the 1990s that might have effected magnitude calculations. Changes made to the instrumentation and sampling rate can explain approximately 60% of the observed change and are likely related to magnitudes being calculated at higher frequencies than they were in the past. Changes to the network distribution may be contributing an additional, approximately, 10% of the difference but the difference is not statistically significant. Increased precision of amplitude and period measurements can be ruled out as a factor. We have not been able to determine whether some of the additional difference is due to a change in the software package used for routine magnitude and epicentral determinations. The seismograph network in Canada has undergone several significant changes or upgrades during the past century and the results of this study raise the question whether there may have been other magnitude relations in the past. Preliminary results using MLg(f) as a proxy for MW show a significant change in the mN-MLg(f) relation over time with several steplike increases that correspond well with network upgrades raising the question whether magnitudes, in general, have remained constant with time.
Summary(Plain Language Summary, not published)
A change in the relation between the mN (used for routine earthquake monitoring in eastern Canada) and the Mw magnitude scales (preferred magnitude for seismic hazard assessments) of 0.12 magnitude units occurred in 1995. The difference has an observable effect on seismic hazard calculations. This study evaluates changes made during the early 1990s that might have affected magnitude calculations to understand the reasons for the change in the magnitude relation. 60% of the difference is related to an upgrade in the seismic monitoring network that led to (mN) magnitudes being calculated at higher frequencies. 10% may be due to changes in the station distribution. A change in software cannot be verified or ruled out as a factor. As the change is related to changes in the network, it raised the question whether there were other such changes in the past. Initial investigations provide evidence that this is the case. These results have potential implications beyond the study area as many monitoring networks worldwide have undergone similar changes with time.

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