Three-dimensional crustal S wave velocity structure in Japan using microseismic data recorded by Hi-net tiltmeters

We developed a three-step method for three-dimensional (3-D) S wave velocity tomography by fitting synthetic cross spectra to the observed ones of ambient seismic noise. We applied this method to the recording of Hi-net tiltmeters in Japan at 679 stations from June 2004 to December 2004. First, we calculated normalized cross spectra between radial components and those between transverse components for every pair of stations. The first step is local 1-D S wave velocity inversion for each station assuming small lateral heterogeneity under a 100-km circle of a station. We measured the dispersion curves of fundamental Rayleigh waves, fundamental Love waves, and first overtone of Love waves by fitting the synthetic cross spectra to the observed ones between pairs of stations within the circle. We inverted the measured dispersion curves for obtaining a 1-D S wave velocity model. The second step is the inversion of the observed cross spectra for obtaining path-averaged 1-D S wave velocity structure. The third step is the inversion of the resultant path-averaged structures for obtaining 3-D S wave velocity structure (0.1 degrees x 0.1 degrees x 1 km grid from the surface to a depth of 50 km) using ray approximation. The resultant S wave velocity structures show clear low-velocity anomalies along tectonic lines from the surface to a depth of 20 km. In particular, along the Hidaka mountain range, we observed S wave perturbation more extreme than -20%. They also show low-velocity anomalies under volcanoes in Kyusyu and Tohoku. In the southwestern part of Shikoku, our results show a clear low-velocity anomaly corresponding to an accretional belt (Shimanto belt). Below 20 km, we observe a low-velocity anomaly in the center of Japan, which suggests a thick crust.