Mantle Structure Beneath Three Subduction Zones Derived From ScS Reverberations in a Ray Summation Method

We study mantle structure beneath three subduction zones (Bolivia, Tonga, and Japan) by modeling multiple ScS and sScS reverberations and their reflected waves at the first-order discontinuities in the mantle. Our methodology is to model waveforms in the time domain by means of ray summation. The ray summation method was used by Kato et al. (2001) who determined the local variation of the mantle structure beneath Japan. By applying the same method to the other subduction zones, we investigate what determine characteristics of the mantle structure globally. We obtained Q values and perturbations of average S-wave speed separately for the upper and lower mantle. Depths and reflection coefficients of the Moho, 410-km and 660-km discontinuities were also determined. The present method is most effective for waveforms at small epicentral distances. Using broadband data archived by the IRIS DMC, we examined records of large deep-focus earthquakes, which are greater than Mw7.0 and deeper than 500 km, all over the world during the period between 1990 and 2004. We thus found that the records of deep-focus earthquakes in Tonga and Bolivia as well as Japan were suitable for our analysis. In this study, we also investigate effects of earthquake source process on the seismograms, which were not considered in Kato et al. (2001). We preferred displacement waveforms to velocity in order to reduce weight for higher-frequency oscillations which can be more affected by source effects. We searched source duration as well as mantle structure. The addition of the source process parameter improves match between observed and synthetic waveforms. This is especially significant for the great Bolivian earthquake (Mw8.2), whose duration we estimated was about 50 seconds. The most intriguing in our results is the attenuation structure. The Q value in the upper mantle of Bolivia is estimated to be around 200, while Q in the lower mantle is close to 500. In Japan, however, the Q values in the upper and lower mantle are around 100 and 150-350, respectively (Kato et al. 2001). The Q values estimated for Tonga are similar to those in Japan. The Q values for Bolivia are thus larger than those of Tonga and Japan. These variations in the mantle structure may reflect different characteristics of the subduction zones (e.g., temperature, content of liquid, morphology of plates, tectonic setting).