Estimating the velocity structure in the crust in southwest Japan by using body waves and surface waves

In this research, we extract the characteristic heterogeneity beneath the hypocentral region. Two approaches are taken to estimate the heterogeneous structure. We used reflection method to determine the velocity contrast and local heterogeneities in the crust. In order to estimate the average velocity in the crust, we used surface waves group velocity. We chose the Chugoku Region in southwest Japan as the analysis field, where there occurred seven M7 class earthquakes within recent 150 years with their spacing intervals of 10-50 kilometers. It is suitable to compare the heterogeneities beneath between their source regions and the other regions. In the reflection method, we made NMO (normal moveout) correction for 2,341 waveforms from 121 local, crustal earthquakes recorded at Hi-net and university network stations in Japan. At several stations in the eastern part of the Chugoku Region, we detected three reflection layers at depths of 20-25 km, 35-40 km, 50- 55 km. Doi et al. (2004) suggested that the Philippine Sea Plate (PHP) is subducting just beneath to the west of the region where these reflection phases are detected. This shows the wide PHP subduction and it has large meaning to think the stress accumulating system for crustal large earthquakes in the Chugoku Region. For the second approach, we calculate the Rayleigh waves group velocity at 0.2 Hz from two M6 events occurred about 100 km away from the Chugoku Region. We manually picked the arrival time of the energy of the 0.2 Hz waves at the KiK-net stations spacing the other stations about 20 km apart. Assuming the surface waves as plane waves, we calculated the group velocity by least square method at the grids set at 0.1 degree interval in the Chugoku Region. We obtained the Rayleigh waves group velocity as 2.5-3.4 km/s. This group velocity is thought to stand for the (weighted) average velocity from the surface to 10-15 km depth. However, this obtained value include the effect of sedimental layer and the errors in the estimation. We estimated the wide-area heterogeneous structure in the Chugoku Region. Furthermore, we will investigate the spreads of the reflection layers and their shapes to discuss the earthquake occurrence environment.