S-wave velocity structure in the central Japan derived from adjoint tomography.

In Japan, seismic velocity structures have been estimated by using first arrival tomography method. Many significant crustal structures such as the coordinate of the subducted Philippine Sea plate has been revealed by seismic tomographic images. In this study, we applied the adjoint tomography including full numerical simulation and finite frequency sensitivity kernels for the area of central Japan. The study area is characterized by the very heterogeneous geologic structures. We used 72 natural earthquakes in this study. Because the dominant phase used in our analysis is the surface wave, only S-wave velocity was inverted. We tried to minimize the time-frequency phase misfit between observed and calculated waveforms with the frequency of 0.033~0.1Hz. Based on the checker bord test, our inversion scenario resolved the upper and lower crust. From the results, we identified more heterogeneous structures compared to those from the first arrival tomography. The estimated S-wave velocity model clearly resolved the low velocity anomalies around the active volcanoes. Furthermore, the velocity boundaries agree with the main tectonic lines in the central Japan.