Regional spectral analysis of moderate earthquakes in northeastern North America—Final Report to the Nuclear Regulatory Commission, Project V6240, Task 3

Abstract

We analyze the Fourier spectra of S+Lg+surface wave groups from the horizontal and vertical components of broadband and accelerogram recordings of 120 small and moderate (2< Mw <6) earthquakes recorded by Canadian and American stations sited on rock at distances from 3 to 600 kilometers. There are seven Mw 4.0-4.5, six Mw 4.5-5.0, and three Mw ≥5 earthquakes in this event set. We test the regional spectral analysis by comparing the moment magnitudes with the moment magnitudes from the earthquake moment tensors determined by Bob Herrmann (St. Louis University) for 27 events, obtaining dMw=0.004±0.074. We determine the Lg attenuation in seven regions within northeastern North America: Charlevoix, lower St. Lawrence, Maine, Northern New York, lower Great Lakes, Ontario, and Nunavut. These attenuation estimates yield an average attenuation Q=(368±13)f (0.54±0.02) for the Appalachian region, a stronger attenuation Q=(317±16)f (0.54±0.03) for the Appalachian lowlands, and a weaker attenuation Q=(455±20)f (0.51±0.02) for Ontario and western Quebec. For events in Nunavut and northernmost Quebec, we estimate a similar attenuation for r <450 km, but a weaker attenuation Q=(773±70)f (0.27±0.06) for Lg propagation for 450< r <1700 kilometers. This farregional attenuation allows us to analyze recordings of the 1989 Ungava and Payne Bay earthquakes obtained in Ontario and southern Quebec. We use these regional attenuations to determine the corner frequencies, stress drops, and radiated energies of the 120 earthquakes.

The search for hard-rock kappa (κ) in NGA-East: A semi-automated method for large, challenging datasets in stable continental regions

“…In Eastern Quebec, there are indications of depth (but not magnitude) dependence of stress drop (Boatwright, 2014). Figure 1 (right) shows the depths of all events in the NGA-East dataset, based on the flatfile.…”

“…In Boatwright (2014), the average stress drop over their entire dataset is 114 bar, and the regional averages are 215, 88, and 39 bar for Eastern Quebec, Western Quebec, and Northeastern United States, respectively. The latter two regions also include some events with stress drops as low as 20 bar.…”

“…Horizontal dashed blue lines mark the chosen maximum and minimum credible values, Δ σ min and Δ σ max , taken as constant in all the NGA-East CENA regions except for Eastern Quebec. Inclined dashed orange lines indicate the depth-dependence in Eastern Quebec as per Boatwright (2014). Right: Magnitude-depth distribution for all events in the NGA-East dataset, with events from Eastern Quebec shown in red circles.…”