JpGU-AGU Joint Meeting 2020

Presentation information

[J] Oral

S (Solid Earth Sciences ) » S-SS Seismology

[S-SS16] Active faults and paleoseismology

convener:Mamoru Koarai(Earth Science course, College of Science, Ibaraki University), Hisao Kondo(Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology), Takashi OGAMI(National Institute of Advanced Industrial Science and Technology), Yoshiki Sato(Advanced Industrial Science and Technology, Geological Survey of Japan)

[SSS16-08] Paleo-seismological survey on the Futagawa fault at Futa, Nishihara Village, Kumamoto prefecture

*Daisuke Ishimura1, Yoshiya Iwasa2, Naoya Takahashi3, Ryuji Tadokoro4, Ryuhei Oda1, Kosumo Kajii5, Jun Matsukaze1, Takashi Ishizawa6, Hiroyuki Tsutsumi5 (1.Department of Geography, Tokyo Metropolitan University, 2.Graduate School of Education, Hiroshima University, 3.Department of EarthScience, Tohoku University, 4.Graduate School of Science, Chiba University, 5.Department of Environmental Systems Science, Faculty of Science and Engineering, Doshisha University, 6.International Research Institute of Disaster Science, Tohoku University)

Keywords:2016 Kumamoto earthquake, Futagawa fault, Paleo-seismological survey, K-Ah tephra

After the 2016 Kumamoto earthquake, paleo-seismological surveys were conducted not only on the Futagawa fault but also on the subsidiary surface ruptures. Our group conducted the paleo-seismic trenching on the subsidiary surface ruptures, focusing on the coincidence between the primary and subsidiary faults. We already excavated two trenches: Miyaji trench (Ishimura et al., 2018) on the small surface ruptures 10 km far from the eastern end of the primary fault and Komori Farm trench (Ishimura et al., 2019) on the Idenokuchi fault running parallel to the primary fault. These paleo-seismic trenching revealed that these faults moved repeatedly in the past and their recurrence intervals were estimated to be 2-3 thousand years, indicating that the activity of these subsidiary surface ruptures coincided with that of the Futagawa fault as same as the 2016 Kumamoto earthquake. On the other hand, paleo-seismic event age of the Futagawa fault is not well constrained although many trenches were excavated on it (Kumahara et al., 2017; Iwasa et al., 2018; Shirahama et al., 2018; Tsutsumi et al., 2018; Ueta et al., 2018; Toda et al., 2019). Thus, we conducted the trench survey at Futa, central part of the Futagawa fault to reveal paleo-seismic age, especially younger than K-Ah tephra (7.3 ka; Machida and Arai, 2003) and to compare the event ages between the primary and subsidiary faults.

We conducted the paleo-seismic trenching at Futa where the Futagawa fault crosses the Futagawa river. At the outcrop along the Futagawa river, Ishimura (2019) identified the main fault of the Futagawa fault displacing the lava and sediments from the Late Pleistocene to Holocene and subsidiary faults showing smaller displacement than the main fault. The trench site is ca. 50 m east from this outcrop. We identified two surface ruptures on the trench site: the north strand shows 30-40 cm north-up normal faulting and the south strand shows left-stepping, en echelon cracks. We totally excavated five trenches on two surface ruptures.

As a result, we identified at least three events including the 2016 event after K-Ah tephra consistent with the previous studies. At present, we are conducting radiocarbon dating and thus will show the details of faulting events and event ages in the presentation.