Separation of scattering and intrinsic attenuation for the Kanto‐Tokai region, Japan, using measurements of S‐wave energy versus hypocentral distance

Abstract

Determining the relative amount of attenuation caused by scattering and intrinsic absorption is important to the understanding of wave propagation and attenuation in the heterogeneous lithosphere. A recently developed model based on radiative transfer theory provides a method for quantifying the ratio of scattering loss to total attenuation, which is called seismic albedo. The basic assumption of this model is that the medium is composed of a uniform distribution of isotropic scatterers. The method is based on a theoretical result showing that the variation with sourcereceiver distance in the seismic energy integrated over time is directly related to albedo and total attenuation, Qt−1. We introduce an improvement in the previously used method which results in more reliable estimates of seismic albedo and Qt−1 which can be used to calculate the losses due to scattering and intrinsic absorption. We call our new method multiple lapse‐time window analysis. The improvement is based on the observation that the relationship between integrated energy and distance is strongly dependent on the time duration over which energy is integrated. We show that parameters describing media attenuation can be estimated from measurements of two ratios from the integrated energy versus distance relations compiled using two time windows for integration. One ratio is the energy integrated from 0 to 15 s after the S‐wave arrival observed at 50 km source‐receiver distance divided by the energy in the same time interval observed at 150 km distance. The second ratio is the energy integrated from 0 to 15 s observed at 150 km divided by that from 30 to 100 s observed at the same source‐receiver distance. Integrated energy calculated for many source‐receiver pairs may be corrected for relative site amplification and relative source amplitude determined using the coda‐wave method. These corrections allow us to use data from many sourcereceiver pairs to find a well‐constrained energy versus distance relation. Site amplifications relative to a reference station are calculated for three frequency bands by determining the ratio of the spectral amplitude in each band at one station for a 10 s time window to that at the reference station in the same 10 s time window. Ratios are calculated for many 10 s time windows for each of 10 events. For each station, we found little scatter in the ratios among the windows and events used. The average of all the ratios obtained for each station was taken to be the site amplification relative to the reference station. We applied the multiple lapse‐time window analysis method and source‐site correction procedures to data from the Kanto‐Tokai region of Japan and found that intrinsic attenuation Qi−1 is larger than scattering attenuation Qs−1 over three frequency bands; 1–2, 2–4, and 4–8 Hz. We found that estimates of coda‐wave attenuation Qc−1 made using the coda‐wave method, are similar to the intrinsic attenuation Q; for the frequency range of 2–8Hz. We were unable to fit data for a third integration time window for the lowest frequency band studied. Our inability to fit all the data might be due to the assumptions that scattering is isotropic, that scatterers are uniformly distributed, and that only body waves are observed. We found that the site amplifications and seismic albedos determined for vertical and north‐south components of motion are similar. Copyright © 1992, Wiley Blackwell. All rights reserved