Improving Magnitude Consistency in Eastern Canada Through Regionally Appropriate Attenuation Relations

Date: 4/25/2019

Time: 02:15 PM

Room: Pike

The Nuttli magnitude scale, MN or mbLg, developed for use in the intraplate regions of North America is the magnitude scale most commonly used for earthquakes occurring in eastern Canada. As part of a larger study to ensure uniformity of magnitudes across Canada for hazard assessments, MN magnitudes have been scrutinized and several problems, predominantly systematic inconsistencies, have been discovered. One of these is that for any given earthquake, stations in the Appalachians tend to underestimate magnitudes with respect to those in the Canadian Shield, suggesting possible differences in seismic attenuation. Analysis of site corrections and felt reports are consistent with this interpretation. Regional analysis of Lg amplitudes from high quality waveforms provides constraints on crustal attenuation with direct applications to magnitude scaling relations and strong ground motion estimates. The predominant uncertainty in the magnitude relations is the attenuation value prescribed. To reduce this uncertainty, velocity-windowed Lg rms amplitudes over a range of frequency bands (0.5-16Hz) and distances are used to determine crustal attenuation in both regions. An average, frequency-dependent Q(f) is determined using a single frequency Q inversion method for the Appalachian and Grenville provinces. The Lg-Q results show that attenuation is significantly higher in the Appalachians compared to the neighbouring Grenville (Shield) province. Magnitudes are recalculated for a suite of earthquakes in eastern Canada applying appropriate attenuation values for earthquake-station paths. Replacing the generic attenuation term with the new Lg-Q values consistently reduces and, in some cases eliminates the differences between Appalachian and Shield magnitudes. Our tests to date employ frequency-independent attenuation, and the next steps will be to evaluate the use of frequency-dependent attenuation in the magnitude relations.

Presenting Author: Claire Perry

Authors