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The corner frequency shift, earthquake source models, and Q

U.S. GEOLOGICAL SURVEY, MS 77, 345 MIDDLEFIELD ROAD, MENLO PARK, CALIFORNIA 94025

Abstract

The very common seismological observation that the P waveform is enriched in high-frequency motion relative to the S waveforms of the same earthquake manifests itself, in spectral studies of the earthquake mechanism, as the "corner frequency shift," the general although not ubiquitous tendency for the P-wave corner frequency f_o(P) to be greater than the S-wave corner frequency f_o(S). In point source, time-domain modeling studies of the earthquake mechanism which follow the recipe of D. V. Helmberger and C. A. Langston that explicitly suppresses the corner frequency shift, it is an equally common result that the synthetic S waves are systematically enriched in high-frequency motion relative to the observations. Almost three dozen spectral analysis and time-domain modeling studies are recapitulated in this one to conclude: (1) the corner frequency shift is a very common condition of the far-field body waves of earthquakes, with no discernible dependence on earthquake source strength, hypocentral distance or depth, or recording device; and (2) the corner frequency shift is the manifestation of an intrinsic property of earthquakes, source finiteness. Anelastic attenuation estimates for shear waves determined on the basis of a source model derived from P waves are likely to be strongly biased to high values of Q^-1, if the shear excitation is estimated directly from the equivalent far-field compressional excitation, i.e., with no allowance for source finiteness. The corner frequency shift, moreover, places strong constraints on admissible earthquake source models; point-source and Haskell-type dislocation models will not be among them.

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