Crustal Heterogeneity in the Eastern Chugoku Region, Japan, Estimated by Coda Envelope Inversion: Toward Mapping the Nationwide Heterogeneity Using the Hi-net Data

Recently coda envelopes from local earthquakes are inverted to estimate a 3D heterogeneous structure in the crust (Nishigami, 1991, 2000; Asano and Hasegawa, 2003). These studies suggest that the data of dense seismographic network is effective to image crustal heterogeneities such as the fractured zone along active faults, magma or melted bodies below active volcanoes, and reflective zones in the lower crust. Recently High Sensitivity Seismograph Network (Hi-net) has been constructed by NIED covering all over Japan. We are aiming at estimating systematic variation in the crustal heterogeneity in Japan and trying to detect a possible structure related to the earthquake generation, using the Hi-net data. First of all we analyzed the eastern Chugoku region, Japan. Coda envelopes from 138 local earthquakes were measured at 23 Hi-net stations, and their deviation from the average decay curve assuming a single isotropic scattering model was estimated as the observational data. The analysis area, about 200 km in horizontal and 60 km in depth, was divided into 5800 small blocks, and 3D distribution of relative scattering coefficient was obtained by solving the observational equation using a recursive stochastic inversion. The preliminary result shows strong scattering along the Yamasaki fault system and also the high seismicity zone along the Japan Sea coast. The resolution will be improved by adding more data including the University and JMA stations, and also the analysis area will be enlarged toward the nationwide mapping of the heterogeneity.