Estimating near-surface shear wave velocities in Japan by applying seismic interferometry to KiK-net data

We estimate shear wave velocities in the shallow subsurface throughout Japan by applying seismic interferometry to the data recorded with KiK-net, a strong motion network in Japan. Each KiK-net station has two receivers; one receiver on the surface and the other in a borehole. By using seismic interferometry, we extract the shear wave that propagates between these two receivers. Applying this method to earthquake-recorded data at all KiK-net stations from 2000 to 2010 and measuring the arrival time of these shear waves, we analyze monthly and annual averages of the near-surface shear wave velocity all over Japan. Shear wave velocities estimated by seismic interferometry agree well with the velocities obtained from logging data. The estimated shear wave velocities of each year are stable. For the Niigata region, we observe a velocity reduction caused by major earthquakes. For stations on soft rock, the measured shear wave velocity varies with the seasons, and we show negative correlation between the shear wave velocities and precipitation. We also analyze shear wave splitting by rotating the horizontal components of the surface sensors and borehole sensors and measuring the dependence on the shear wave polarization. This allows us to estimate the polarization with the fast shear wave velocity throughout Japan. For the data recorded at the stations built on hard rock sites, the fast shear wave polarization directions correlate with the direction of the plate motion.