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125 DUNBAR ROAD SOUTHWATERLOO, ONTARIO CanadaN2L 2E8
U. S. GEOLOGICAL SURVEY, MAIL STOP 977, 345 MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94025
Abstract
The sequence of earthquakes that began in October 1985 near the Nahanni River, Northwest Territories, Canada, provides an exceptional opportunity to study the variation with magnitude of the source spectra from intraplate earthquakes. We study the spectral scaling for the seven largest events, using P waves recorded at stations of the Western Canada Telemetred Network (WCTN), the Eastern Canada Telemetred Network (ECTN), and the Global Digital Seismograph Network (GDSN). These earthquakes range in size from M = 4.8 to M = 6.8. We compare predictions from various source-scaling models with observed spectral ratios and source acceleration spectra for the suite of events. The spectral ratios show a strong azimuthal dependence, which is probably due to differences in the focal mechanisms or rupture characteristics of the events. Taking the average of the ratios from individual GDSN stations as the best representation of the spectral ratios, an 
-squared model with a constant stress parameter or one that increases slightly with moment magnitude gives a simultaneous fit to both the spectral ratios and to the absolute spectra. The high-frequency spectral levels, by themselves, imply a somewhat stronger dependence of the stress parameter on moment than found from fitting the overall spectra. The stress parameters for the scaling laws are less than 35 bars for earthquakes as large as M = 7. Recently proposed scaling laws in which the corner frequency is proportional to the inverse quarter root of seismic moment for an -squared model are not consistent with the data and neither are -cubed models.
The spectral scaling for the sequence of Nahanni earthquakes may not be representative of intraplate magnitude scaling in general, since there is some evidence that aftershocks do not exhibit the same scaling as main shocks. The high-frequency spectral levels of the aftershocks of both the Nahanni and the 9 January 1989 Miramichi, New Brunswick, earthquakes imply stress parameters that increase with magnitude. By contrast, the spectral levels for a series of main shocks in eastern North America show less magnitude dependence of the stress, and generally higher stress levels when compared to aftershocks of the same magnitude.
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