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 strongmotion 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 shearwave polarization directions correlate with the direction of the plate motion.