Spherically symmetric attenuation within the Earth from normal mode data

Abstract

SUMMARY Observations of the attenuation of free oscillations after large earthquakes are used to determine the spherically symmetric dissipative structure within the Earth. We model intrinsic attenuation of seismic energy between 0.3 and 10mHz with a frequency-independent Q model. The resolving power of our data indicates that simple models of Q can explain our observations and that we do not have enough information to constrain detailed absorption band models. The average shear Q is relatively well constrained for the mantle and is 250 f 2 per cent. The shear Q in the inner core is mostly constrained by new observations of PKIKP equivalent modes and is 110 f 25 per cent. This is lower than in some previous models, which have used possibly erroneous observations of core modes, though the new models still predict too little attenuation to match the observations of strong differential attenuation of the BC and DF branches of PKP at frequencies of about 1 Hz. Agreement between the mode and body-wave data sets should probably not be expected since it now seems likely that the PKIKP data sense an absorption band which is distinct from that sensed by the mode data. The distribution of bulk attenuation in the Earth remains ill-constrained though bulk attenuation is required somewhere to explain the attenuation of the radial modes. Successful models tend to put significant bulk Q in the upper mantle.