2:00 PM - 2:15 PM
[SSS15-16] Paleo-seismological survey on the Idenokuchi fault, Nishihara village, Kumamoto prefecture
Keywords:2016 Kumamoto earthquake, Idenokuchi fault, Paleo-seismological survey, K-Ah tephra, Normal fault
We excavated a 14-m-long, 6-m-wide, 3-m-deep trench on the antithetic fault of the Idenokuchi fault, where we found 30-60 cm vertical offset associated with the 2016 event. In the laboratory, we conducted tephra analysis and radiocarbon dating. Now, we are measuring radiocarbon dates, and thus will estimate depositional and event ages in our presentation.
On the trench walls, we recognized clear, but complex normal and reverse faults. A main normal fault with a high dip angle bounded basement rocks and soil just below the 2016 surface rupture. On the hanging wall side, we found many antithetic normal faults and reverse faults. In the soil sediments, we identified K-Ah tephra (thickness of 40 cm) based on volcanic glass morphology and refractive index. Soil sequence on the hanging wall side is shown from the surface: surface soil, brown soil, black soil (paleosol1), brown soil, K-Ah, black soil (paleosol2) and yellowish soil.
At present, we identified at least four events including the 2016 event since paleosol2. Especially, after K-Ah deposition, we confidently three events including the 2016 event, corresponding with the frequency of the Futagawa fault activity (Kumahara et al., 2017; Toda et al., 2018; Tsutsumi et al., 2018; Ueta et al., 2018). This preliminary result suggests that the Idenokuchi fault ruptured with the Futagawa fault. Additionally, we also identified a few probable events and will discuss this with radiocarbon dates in the presentation.