Abstract

A new model for average earthquake S-wave source spectra in eastern North America (ENA) is derived. The model is consistent with fundamental elasto-dynamic principles, as well as the familiar spectral scaling laws appropriate to sources having geometrical and dynamical similarity. Observed characteristics of S-wave spectra in ENA are fully explained as simple consequences of directivity effects entailed by the classical crack rupture model (Kostrov, 1964; Madariaga, 1976), with “normal” effective stresses of the order of 100 bars and fractional stress drop (Brune, 1970): abnormally high stress drops are not required. Recognition of the fundamental role of directivity explains most features of observed data that are currently interpreted as “anomalous”. Among other things, neglect of such effects is shown to be responsible for the conclusion by Boatwright and Choy (1992) and Boore and Atkinson (1992) that high-frequency spectral levels for the magnitude 6 Saguenay earthquake of 1988 are anomalously large. Other important implications of the new model are that some previously inferred systematic variations in Brune stress drop, with magnitude, are simply artifacts of the invalid use of the standard Brune (1970, 1971) model assumption and that current estimates of Q in ENA, which depend on this assumption, are likely to be significantly too large for high frequencies. Associated uncertainties allow the possibility that effective stress does not vary significantly with magnitude, for earthquakes having magnitudes in the range of − 4 to + 7.

The new model suggests that some currently favored source spectra may under-estimate spectral amplitudes of future larger earthquakes in ENA, by as much as a factor of 6, for some frequencies. The proposed new spectra are practically identical with those of the Brune standard model for an effective stress of 100 bars, for frequencies up to about twice the Brune corner frequency, but then increase to values 3 times larger over a range of about 4 in frequency. As such, the high-frequency spectral levels are equivalent to standard Brune model values for earthquakes of one full magnitude unit larger, with consequent important implications for seismic hazard estimates in the region.

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