Estimation of the frequency dependence of Q from ScP and ScS phases

A measurement of the relative frequency content of ScP and ScS phases from deep earthquakes provides a simple indication of the average value for Q in the mantle. When the ScP converts to the compressional mode at the core-mantle boundary it begins to attenuate at a slower rate than the ScS. Assuming that the core reflections are frequency-independent, the spectral ratio of the two phases provides a measure of the spectral ratio of the P and S attenuation operators. The average value of mantle Q can be inferred from this information if it is assumed that negligible losses occur in pure compression. Both rays travel downward through the anomalous source region as S. Q is measured on the upward leg of the raypath well away from the subduction zone. The two raypaths diverge somewhat in the lower mantle, so they sample different portions of the core-mantle boundary. They converge closely again in the upper mantle, however, where most attenuation takes place. Clear ScP-ScS observations have been collected from the short period records at 25 WWSSN stations in a variety of tectonic regimes. The relative waveshapes have been analysed in terms of a standard linear solid Q operator. The low-frequency level of Q was set to be consistent with Q measurements from multiple ScS phases. The parameter controlling the increase of Q at high frequency, τm has been determined by minimizing a quantitative measure of fit between ScP waveforms filtered with standard linear solid Q operators and observed ScS waveforms. The frequency band of the observations is from about 0.5 to 1.5 Hz. The observations constrain τm to be between 0.0 and 0.2s. This implies that the rapid increase of Q with frequency does not occur until frequency exceeds 1 Hz. Good quality ScP-ScS pairs appear to be much more common in the WWSSN record library than good quality ScS–ScSm, pairs. As more ScP-ScS data are analysed, we should, therefore, be able to constrain the average mantle Q better in the ScP-ScS frequency band than in the multiple ScS band. No clear correlation exists between the ScP–ScSQ measurements and the tectonic regime of the recording sites. ScS phases are not more strongly attenuated with respect to ScP at oceanic island sites than at stable continent sites. This is interpreted to mean that other types of lateral variation often overwhelm the attenuation bias patterns induced by surface tectonics. These other lateral variations could be anelastic (deep seated low Q zones) or elastic (lateral velocity variations which focus or defocus seismic waves).