SEGJ Technical Conference

Elucidation of shallow structure in the western extension of the Futagawa fault by AMT survey

Abstract At occurrence of the 2016 Kumamoto earthquake, the large ruptures appeared on the ground surface along the eastern part of Futagawa fault zone, called as the Futagawa segment. The aftershock activity was also high along the segment. On the other hand, the western part of Futagawa fault zone, called as the Uto Peninsula north-coast segments, has no fault displacement at the earthquake and no aftershock activities. This feature implies that the western segment still has the risk of future earthquakes with strong shakings and large surface displacement. Therefore, we analyzed audio-frequency magnetotelluric data obtained in the Uto Peninsula, and estimated the shallow resistivity structure across the Kamiouda fault, a part of Uto Peninsula north-coast segments. We found a low resistivity zone distributed along the active fault, interpreted as sub-surface damaged zone with a large amount of pore water. It is also found that the spatial scale of the low resistivity zone is smaller than ones along the Futagawa segments, having a obvious surface ruptures at the Kumamoto earthquake. As conclusion, we infer that the activity of the Uto Peninsula north-coast segments is not high, and the possibility of the surface rupture at future earthquake will low.