[HCG37-P06] Relationship between subsurface structure and large-scale fissures in the northwestern region in Aso valley caused by the 2016 Kumamoto earthquake
Two station spatial auto-correlation (2ST-SPAC) method (Hayashi and Craig, 2016) was applied to estimate subsurface structure using ambient noises. We succeeded to estimate the S-wave velocity structure to the depth of 130 m in and around the regions with fissures. In the regions where large scale fissures were developed, a layer with S-wave velocity less than 150 m/s lay from the surface to the depth of 60 m, followed by two layers with 250 m/s and 300 m/s at depths of 60-90 m and 90-130 m, respectively. This low-velocity layer was considered to represent soft sediments in the caldera lake due to Aso-4 eruption and consistent with the nearby boring profile. Two relatively higher layers might correspond to lava layers after Aso-4 eruption. Moreover, the S-wave velocity at the top surface to the depth of 5 m was so slow as 80 m/s. We continue to estimate the distribution of the soft sediments and lava structure beneath them, to elucidate how fissures were generated in this area.