[S03-5] Imaging of heterogeneities in the Earth with seismic scattered waves and ambient noise V

[S03-5-04] Anisotropic S-wave velocity change in the shallow subsurface associated with the 2016 Kumamoto earthquakes

^○Kaoru Sawazaki, Tatsuhiko Saito, Tomotake Ueno, Katsuhiko Shiomi (National Researh Institute for Earth Science and Disaster Resilience, Tsukuba, Japan)

In this study we focus on anisotropic S-wave velocity change at KiK-net station KMMH16, where PGV exceeding 100 cm/s was recorded when the 2016 Kumamoto earthquake, Japan, occurred. We use 217 earthquake waveforms registered by station KMMH16, where S-wave coda is used to avoid apparent velocity change due to difference in incident angles. After correcting azimuthal bias, we rotate two horizontal components every 10 degree for both surface and borehole bottom (255 m depth) records to compute polarized waveform pairs. Then we deconvolve the polarized waveform on the ground surface by that at the borehole bottom to extract Green's function between the two sensors. After applying 4-20 Hz bandpass filter to the deconvolved waveform, we apply the stretching technique using lag-times of 0.2-0.9 s to extract relative velocity change before and after the Kumamoto earthquake for each polarization direction.
In the period within 7 minutes to 26 hours after the MJ6.5 foreshock on April 14, we find the maximum velocity change of -7.0% for N50E polarization direction, while the minimum velocity change is -3.6% for N10W direction. The N50E direction is close to the along-strike direction of the Futagawa-Hinagu fault system. Within 53 minutes to 14 hours after the MJ7.3 mainshock on April 16, the velocity further reduce to -7.4% for N50E direction. The velocity recovers to -4.8% by two months after the mainshock, while no further recovery is found within 2-9 months after the mainshock.