MEASUREMENT OF BODY-WAVE DISPERSION AND ESTIMATION OF RELATED ATTENUATION FROM BROAD-BAND STATIONS

Attenuative parameters of the Earth's mantle are obtained for the freq uency range 0.5-2 Hz by means of an iterative nonlinear fit to observe d body-wave dispersion. Dispersion curves are obtained by directly mea suring the arrival time of some selected frequency components of P- an d S-wave pulses. The frequency components are obtained by narrow band- pass filtering the signal, and the arrival time is determined as the e xpectation value of the first pulse, which can be interpreted as the a rrival time of the center of gravity of its energy contents. Before an y interpretation, arrival time must be corrected by the group delay of filter and instrumental response. The apparent Q, as a function of fr equency, is retrieved from broadband records of the Network of Autonom ously Recording Stations array corresponding to earthquakes which occu rred in the Japan-Kurile Islands region, and interpreted in terms of t he continuous relaxation model (CRM) and the power law model (PLM). Fo r the CRM the following parameters are obtained, corresponding to aver age values of the mantle: for P waves, tau2 = 0.064 s and Q(m) = 587; for S waves, tau2 = 0.281 and Q(m) = 399. In both cases, tau1 has been given an arbitrary value of 1.0E + 05. Using these parameters, the va lues of Q(P) = 608 and Q(S) = 464 are obtained for a period of 1 s. Fo r the PLM, the values of Q0 = 611 and gamma = 0.07 are obtained for P waves, and Q0 = 714 and gamma = 0.6 for S waves, with the same numeric al values for Q at 1 s. The predicted values at 1 s agree with average published data for the mantle, except the value Q(S) = 714 predicted by the PLM, which is 50% higher. Thus, the measurement of body-wave di spersion proves to be an independent method and a powerful complement to classical well-established methods for the study of attenuation bas ed on amplitude analysis.