Time-lapse change in near-surface shear-wave velocities caused by rainfall and large earthquakes detected by applying seismic interferometry to earthquake data

Shear-wave velocity is an indication of the shear strength of the ground since the velocity is related to the shear modulus. Therefore, monitoring the velocities is useful for site characterization and disaster prevention. We estimate the time-lapse change in shear-wave velocity as well as shear-wave splitting 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 is on the surface and the other is in a borehole. Using seismic interferometry, we extract the shear wave that propagates between these two receivers. Because KiK-net has continuously recorded strong motion seismograms since the end of 1990s, the data are available for time lapse measurements. After the Tohoku-Oki earthquake, the shear-wave velocity decreases about 5% in a region 1200 km wide and anisotropy increases more than 10%. From seasonal averages, we find the velocity and precipitation have a negative correlation.