On Q and seismic discrimination

The vagaries of seismic attenuation, or Q, in the Earth render uncertain the estimates of many seismic source parameters made by the unwary. In particular, the discrimination of waves produced by earthquakes from those of explosions, if the waves have travelled dissimilar paths, and the characterization of a seismic source from the seismic-wave amplitude spectrum require concern for the spatial and frequency dependence of Q. Surface-wave amplitudes are most affected by attenuation at periods long enough (30 s and greater) so that much of the energy propagates in the low-Q asthenosphere, and at periods short enough (less than 15 s) so that near-surface structure is important. Thus the standard surface-wave magnitude is largely insensitive to anelastic losses, though for the surface-wave spectral shape to be diagnostic of source depth, such losses must be considered. The amplitudes of body waves that have penetrated the low-Q asthenosphere reflect the extreme variations of Q within that zone. Seismic discrimination criteria based on the body-wave magnitude should be restricted to comparison of events from the same tectonic province. Further, estimates of such source properties as source dimension and fractional stress drop depend critically on properly accounting for the frequency dependence of Q.