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[J02-P-05] Relationship between subsurface structure and large-scale fissures in the northwestern region in Aso valley caused by the 2016 Kumamoto earthquake
In accompanied with the 2016 Kumamoto earthquake, fissures with the length and the height a few hundred meters and two meters, respectively, emerged in Aso valley (inside the Aso caldera). In this area, the top layer consists of sediments in the caldera lake created by Aso-4 eruption with the thickness a few tens of meters. InSAR and seismic data show that the region with the size of 1-4 square meter moved 1-2 m northward horizontally during strong motion of the mainshock (Fujiwara et al., 2016; Doi et al., 2016). We investigated the mechanism of the movement of these regions by estimating subsurface structure beneath this area.
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.
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.