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< M w <6) earthquakes recorded by Canadian and American stations sited on rock at distances from 3 to 600 kilometers. There are seven M w 4.0–4.5, six M w 4.5–5.0, and three M w ≥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 d M w =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 far-regional 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.