[S07-P] Poster

[S07-P-21] Effect of shallow S-wave velocity structure on ground motion characteristics at temporary aftershock observation stations of the 2016 Kumamoto earthquake

^○Kosuke Chimoto¹, Hiroaki Yamanaka¹, Seiji Tsuno², Hiroe Miyake³, Nobuyuki Yamada⁴ (1.Tokyo Institute of Technology, Tokyo, Japan, 2.Railway Technical Research Institute, Tokyo, Japan, 3.The University of Tokyo, Tokyo, Japan, 4.University of Teacher Education Fukuoka)

Immediately after the 2016 Kumamoto earthquake, we have performed temporary strong motion observation for aftershocks of the event. Yamanaka et al. (2016) reported about this temporary observation and showed variety of the ground motions in the damaged areas in Kumamoto prefecture, Japan. It is observed that the stations where severe damage was observed shows larger ground motions than the less damaged areas. To understand the cause of variety in the ground motion characteristics, we have conducted microtremor array exploration at each temporary strong motion observation stations to reveal the S-wave velocity structure at near-surface. Chimoto et al. (2016) revealed that the low velocity layer at near-surface widely distributes in the damaged area from town of Mashiki to village of Minamiaso. This low velocity layer theoretically contributes to large amplification on ground motions. This study compares the ground motions observed due to aftershocks with the site amplification from shallow S-wave velocity structure. The spectral ratio was computed for site amplification characteristics calculated from the shallow S-wave velocity structures at each stations to that at the reference station as Yamanaka et al. (2016) did. Both spectral ratio for the observation and the calculation show peak at the period of about 0.2-0.3 seconds at most of the sites. However, large spectral ratio at the period around 0.5 sec. in observation was not observed in calculation especially at the heavy damage sites. This indicates the difference of deep structure which is not considered in this study. It is also investigated about the relationship between the average value of spectral ratio and AVS30. It is observed good correlation that large spectral ratio at the site with small AVS30. However, the value for observation shows larger value than calculation. This also indicates the large amplification may have been caused by not only shallow but also deep structure.