Acceleration source spectra anticipated for large earthquakes in northeastern North America

A set of 16 large intraplate earthquakes are considered on the basis of their tectonic settings, compressive focal mechanisms, and shallow focal depths as possible analogs for large earthquakes that may be anticipated to occur in northeastern North America. The earthquakes range in seismic moment from 5 × 1023 to 3 × 1026 dyne-cm and include the mainshocks and largest aftershocks of the 1982 Miramichi, Canada, the 1985 to 1988 Nahanni, Canada, and the 1988 Tennant Creek, Australia, earthquake sequences. The teleseismic recordings of these earthquakes are spectrally analyzed by correcting the logarithmically averaged acceleration spectra for the frequency-dependent effects of attenuation and interference of the depth and direct phases. The resulting acceleration source spectra are generally flat at high frequencies; the mainshock spectra exhibit intermediate spectral falloffs (that is, | u α(ω)| ∝ ω−γ where γ ≈ 1) over frequencies that characteristically extend from 0.1 to 0.5 Hz. In contrast, the largest aftershocks of the three sequences named above exhibit ω2 spectral shapes. The high-frequency source spectral levels increase with seismic moment approximately as Ru α| ∝ M 1/3. The 1988 Saguenay, Canada, earthquake, which occurred significantly deeper than the other earthquakes, exhibits an anomalous spectral shape. Combining the teleseismic data with strong-motion recordings indicates that the intermediate spectral falloff for this earthquake extends from 0.2 to 3 Hz. The spectral amplitude at 3 Hz is a factor of 3 higher than the amplitude anticipated for an earthquake of this size from the other 15 earthquakes.