08:30 〜 09:00
[S08-1-01] Paleoseismological evaluation and surface faults of the 2016 Kumamoto earthquake along Futagawa fault zone, central Kyushu, Japan
invited
Futagawa fault zone was a main part of the source fault of the 2016 Kumamoto earthquake of Mj 7.3, which was the first case that a large earthquake accompanied with surface fault occurred on an active fault zone evaluated by the Earthquake Research Committee of Headquarters of the Earthquake Research Promotions (HERP). Therefore it is good opportunity to compare an evaluation before earthquake with surface faulting accompanied with the 2016 Kumamoto earthquake.
Futagawa fault zone is located in Kumamoto prefecture in Central Kyushu. HERP published Longterm evaluation of this fault zone in 2002 and 2013. Paleoseismological surveys were conducted by Kumamoto prefecture in 1995 and by AIST in 2007 and 2015. Paleoseismological trenching survey at Tanaka site by Kumamoto prefecture showed that the accumulated offset of sediments including AT volcanic ash (28 ka) was 5.2 m. They estimated average interval as 0.25 m/ka. And they also interpreted this offset was formed by 2 or 3 faulting events based on the observations of deformation of geological layers on the trench walls.
Maximum surface displacement accompanied with the 2016 earthquake along the Futagawa fault zone was ca. 2 m in the central part of this fault. It is consist with active fault evaluation by HERP. But amount of slip at Tanaka site caused by the 2016 Kumamoto eartquke was 0.5 m in the right-lateral component. Therefore amount of slip was over-estimate at this site, whereas recurrence of this fault was underestimated. If 0.5 m of displacement was recognized as a parameters of the characteristic deformation before occurrence of the 2016 Kumamoto earthquake, the recurrence interval would be rather short and probability of occurrence of earthquake would be rather high.
Futagawa fault zone is located in Kumamoto prefecture in Central Kyushu. HERP published Longterm evaluation of this fault zone in 2002 and 2013. Paleoseismological surveys were conducted by Kumamoto prefecture in 1995 and by AIST in 2007 and 2015. Paleoseismological trenching survey at Tanaka site by Kumamoto prefecture showed that the accumulated offset of sediments including AT volcanic ash (28 ka) was 5.2 m. They estimated average interval as 0.25 m/ka. And they also interpreted this offset was formed by 2 or 3 faulting events based on the observations of deformation of geological layers on the trench walls.
Maximum surface displacement accompanied with the 2016 earthquake along the Futagawa fault zone was ca. 2 m in the central part of this fault. It is consist with active fault evaluation by HERP. But amount of slip at Tanaka site caused by the 2016 Kumamoto eartquke was 0.5 m in the right-lateral component. Therefore amount of slip was over-estimate at this site, whereas recurrence of this fault was underestimated. If 0.5 m of displacement was recognized as a parameters of the characteristic deformation before occurrence of the 2016 Kumamoto earthquake, the recurrence interval would be rather short and probability of occurrence of earthquake would be rather high.