Dispersion Measurements of P Waves and their Implications for Mantle Q_p

摘要

We analyze the anelasticity of the earth using group delays of P-body waves of deep (>200 km) events in the period range 4-32 s for epicentral distances of 5-85 degrees. We show that Time Frequency Analysis (TFA), which is usually applied to very dispersive surface waves, can be applied to the much less dispersive P-body waves to measures frequency-dependent group delays with respect to arrival times predicted from the CMT centroid location and PREM reference model. We find that the measured dispersion is due to: (1) anelasticity (described by the P-wave quality factor Q_p), (2) ambient noise, which results in randomly distributed noise in the dispersion measurements, (3) interference with other phases (triplications, crustal reverberations, conversions at deep mantle boundaries), for which the total dispersion depends on the amplitude and time separation between the different phases, and (4) the source time function, which is dispersive when the wavelet is asymmetrical or contains subevents. These mechanisms yield dispersion ranging in the order of one to 10 seconds with anelasticity responsible for the more modest dispersion. We select 150 seismograms which all have small coda amplitudes extending to ten percent of the main arrival, minimizing the effect of interference. The main P waves have short durations, minimizing effects of the source. We construct a two-layer model of Q_p with an interface at 660 km depth and take Q_p constant with period. Our data set is too small to solve for a possible frequency dependence of Q_p. The upper mantle Q_1 is 476 [299-1176] and the lower mantle Q_2 is 794 [633-1064] (the bracketed number indicate the 68 percept confidence range of Q_p~(-1)). These values are in-between the AK135 model (Kennett et al., 1995) and the PREM model (Deiewonski and Anderson, 1981) for the lower mantle and confirm results of Warren and Shearer (2000) that the upper mantle is less attenuating than PREM and AK135.