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TitleEarthquake magnitude and Lg Q variations between the Grenville and northern Appalachian geologic provinces of eastern Canada
AuthorPerry, H K C; Bent, A L; McNamara, D E; Crane, S; Kolaj, M
SourceBulletin of the Seismological Society of America vol. 110, no. 2, 2020 p. 698-714,
Alt SeriesNatural Resources Canada, Contribution Series 20190088
PublisherSeismological Society of America
Mediapaper; on-line; digital
File formatpdf; html
ProvinceNewfoundland and Labrador; New Brunswick; Nova Scotia; Prince Edward Island; Quebec; Ontario
Lat/Long WENS -83.0000 -52.0000 60.0000 40.0000
Subjectsgeophysics; Nature and Environment; Science and Technology; seismology; earthquakes; earthquake magnitudes; attenuation; seismic waves; seismic velocities; seismological network; models; seismicity; seismic risk; earthquake risk; Grenville Province; Appalachian Province
Illustrationsgeoscientific sketch maps; histograms; spectra; plots; tables
ProgramCanadian Hazard Information Service
Released2020 02 04
AbstractThis article assesses the ability of regionally specific, frequency-dependent crustal attenuation (1/Q) to reduce mean magnitude discrepancies between seismic stations in the northern Appalachian and Grenville provinces (NAP and GP) of Canada. Lg Q(f) is an important parameter in ground-motion models used in probabilistic seismic hazard analysis. Discrepancies in regional magnitude estimates have long been noted to exist between stations in the two provinces for common event origins. Such discrepancies could arise from systematic site condition variations between the geologic provinces or from varying crustal attenuative properties. To evaluate the effect of frequency-dependent anelastic attenuation, Lg Q(f) on estimated magnitudes, we analyze Lg amplitudes from > 6000 waveforms recorded by Grenville and northern Appalachian receivers from 420 natural earthquakes of MN magnitude 3-5.6. Waveform analysis is strictly limited to analyst-reviewed, vertical-component waveforms in which Lg is clearly identified, ensuring that the datasets exhibit dominant, high-frequency energy in the Lg velocity window. Lg Q(f) is found to be higher in the GP than in the northern Appalachians. In the Grenville, Q(f)=761±145f(0:25±0:014), and in the northern Appalachians, attenuation is higher: Q(f)=506±172f(0:33±0:310). Earthquake magnitude determined using the peak amplitude of the Lg phase (mbLg) for eastern Canada is corrected to incorporate the frequency-dependent, regionally specific Lg Q(f) determined in this study. Using the new Lg Q(f) values diminishes and nearly resolves magnitude discrepancies between the provinces. Correcting regional magnitude discrepancies between provinces is critical for reliable regional seismic hazard estimates because magnitude error in a particular region could lead to increased uncertainty in seismic hazard models.
Summary(Plain Language Summary, not published)
The intensity of felt and reported ground shaking from earthquakes in eastern Canada depends on the geographic region. In this paper, we show that differences in perceived ground shaking between the Proterozoic Grenville Province and the Paleozoic northern Appalachian Province is likely related to the way in which seismic energy dissipates with distance from the earthquake source in the two regions. We use a large dataset of over 6000 seismic records from 420 earthquakes to measure the distribution of the ground displacement amplitudes with distance from the earthquake. Measured amplitudes are shown to diminish more rapidly with distance in the northern Appalachian Province compared to the Grenville Province for earthquakes of comparable magnitude. We carry out an inversion procedure to determine the relative contributions of 1) the earthquake source rupture, 2) the recording seismometer site conditions and 3) wave scattering and energy loss to amplitude loss of seismic waves in the two regions. The results are used to improve regional magnitude estimates in the two regions.

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