Measurement of body-wave dispersion and estimation of related attenuation from broadband stations

Abstract

Abstract Attenuative parameters of the Earth's mantle are obtained for the frequency range 0.5–2 Hz by means of an iterative nonlinear fit to observed body-wave dispersion. Dispersion curves are obtained by directly measuring the arrival time of some selected frequency components of P- and S-wave pulses. The frequency components are obtained by narrow band-pass filtering the signal, and the arrival time is determined as the expectation value of the first pulse, which can be interpreted as the arrival time of the center of gravity of its energy contents. Before any interpretation, arrival time must be corrected by the group delay of filter and instrumental response. The apparent Q, as a function of frequency, is retrieved from broadband records of the Network of Autonomously Recording Stations array corresponding to earthquakes which occurred in the Japan-Kurile Islands region, and interpreted in terms of the continuous relaxation model (CRM) and the power law model (PLM). For the CRM the following parameters are obtained, corresponding to average values of the mantle: for P waves, τ 2 = 0.064s andQ m = 587 ; for S waves, τ 2 = 0.281andQ m = 399 . In both cases, τ 1 has been given an arbitrary value of 1.0E + 05. Using these parameters, the values of Q P = 608andQ S = 464 are obtained for a period of 1 s. For the PLM, the values of Q 0 = 611andγ = 0.07 are obtained for P waves, and Q 0 = 714andγ = 0.6 for S waves, with the same numerical 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 dispersion proves to be an independent method and a powerful complement to classical well-established methods for the study of attenuation based on amplitude analysis.