This paper presents the results of geological investigation of newly excavated Hiji and Mizoguchi-kita outcrops along the Median Tectonic Line in Ina City, Nagano Prefecture. From the excavation, terrace deposit unconformably overlying fractured basement rocks along the MTL dividing of the Sanbagawa belt and the Ryoke belt clearly appear. Collapse of terrace deposit gravel into the basement rocks along newly reactivated fault can be observed in the Mizoguchi-kita outcrop, and upward extension of one of the MTL faults crosscut into the terrace deposit in the Hiji outcrop. Although organic material or tephra deposit cannot be found in the terrace deposit, we found very small amount of volcanic glass shards in silt and sand layer intercalated within the gravel. Bulk chemical composition of the glass fragments clearly show that they can be correlated with the volcanic glass of Ontake Daiichi Tephra (On-Pm1) which was erupted at c. 100 ka. Accordingly, the terrace deposit of the Hiji outcrop was deposited after c. 100 ka and the fault along the MTL is probably active fault (the fault activated after 120―130 ka in a narrow definition) even though little topographic evidence of dextral displacement can be identified in the Ina area. The fault gouge zone along this fault in the Hiji outcrop clearly show dextral shear sense which is concordant with the evidence of topographic displacement along the MTL to the south of the Ina area.
The 2016 Kumamoto earthquake sequence produced extensive surface ruptures not only along the Futagawa section of the Futagawa fault zone and the Takano-Shirahata section of the Hinagu fault zone, but along the Kita-amagi fault comprising the Uto section of the Futagawa fault zone. Although a large number of researches on paleoseismology have been carried out on the Futagawa section and the Takano-Shirahata section so far, no paleoseismic investigations have taken place on the Uto section. In order to unravel recent behavior of the Kita-amagi fault, including the 2016 earthquake, we performed detailed topographic survey and paleoseismic trenching of the southern trace of the Kita-amagi fault that bounds the northwestern margin of the Kita-amagi Uplands. A 20-cm-grid high resolution DEM (digital elevation model) and topographic profiles, both of which are based on total station survey, allowed us to document morphological features of the surface rupture zone in greater detail regardless of its slight horizontal and vertical displacements. A paleoseismic trench at a site immediately adjacent to the surface rupture exposed no clear evidence for the 2016 earthquake except for the ambiguous, small near-surface fissures. On the contrary, unlike the 2016 earthquake, evidence for a paleoseismic event with a distinct deformation of strata, whose structural relief is consistent with the long-term tectonic landform, was identified. Although the age of this event horizon was poorly constrained and number of events within the horizon could not be determined, at least one event that contributed the topographic development occurred sometime after ～16,000 cal BP but before ～3,000 cal BP or ～10,000 cal BP (after ～17,000 cal BP but before ～2,000 cal BP or ～9,000 cal BP, taking into account the possibility that the 14C ages of the organic sediments bracketing the event timing could be 1,000 years older or younger at the maximum than their actual depositional ages).
During the 2016 Kumamoto earthquake (Mw7.0), the Futagawa segment of the Futagawa fault zone and a part of Takano-Shirahata segment of the Hinagu fault zone ruptured and a 30-km-long surface rupture with right-lateral strike slip appeared. Although surface rupture distribution, co-seismic displacement and paleoseisimicity have been revealed after the 2016 event, the slip rate in the central part of the Futagawa segment is not estimated. In order to reveal vertical and horizontal slip rates, we conducted geomorphic analysis and field survey in and around Futa, Nishihara Village, Kumamoto prefecture. Co-seismic vertical displacement was estimated by elevation difference between pre- and post-earthquake digital elevation models (DEMs) created from LiDAR survey data and accumulated vertical and horizontal displacements of geomorphic features were measured by using those DEMs. Additionally, we investigated an outcrop to reveal the shallow fault structure and to estimate an age of geomorphic surface. From accumulated displacement and ages of geomorphic surface and Takayubaru lava, vertical and horizontal slip rates of the Futagawa fault were estimated to be 1 mm/yr or larger. Moreover, recurrence interval was estimated to be a thousand to two thousand years by using estimated slip rates and co-seismic displacements of the 2016 event, consistent with results of recent paleoseisimic surveys on the Futagawa segment. These results, slip rate and recurrence interval of the Futagawa fault, are higher than previous estimation and suggests that the Futagawa fault is one of the most active faults in Kyushu region.
In the 1920’s and 1930’s, three moderate to large inland earthquakes occurred in Honshu Island, Japan, namely the 1925 Kita-Tajima earthquake (M6.8), the 1927 Kita-Tango earthquake (M7.3), and the 1930 Kita-Izu earthquake (M7.3). Imperial Geological Survey of Japan (GSJ) sent geologists to these earthquake disaster areas soon after the earthquakes to investigate the geological effects including surface faulting, landslides, and their relation to damage. Their findings at the dawn of active fault research in Japan have made a great contribution to the subsequent earthquake and active fault studies. Curious to say, the geologists who were dispatched to the investigation are different every earthquake (Tsutomu Ogura for the 1925 Kita-Tajima earthquake; Kyukichi Watanabe and Hokoto Sato for the 1927 Kita-Tango earthquake; Keinosuke Ihara and Kiyohiko Ishii for the 1930 Kita-Izu earthquake). What are the reasons why those five geologists were sent to the investigations and why the geologists who engaged in the field investigation were different even though each of earthquakes occurred in a quite short period? In order to unravel these questions, the author has collected and investigated the materials that document the detail of annual activity of the geologists, their personal history, and the state of work at GSJ. According to the materials, they all experienced the disaster investigation of the 1923 Great Kanto earthquake and survived from massive dismissal by administrative reorganization due to the recovery of the earthquake in 1923 and 1924. Among them, the geologists who did not have engaged in an ordinary geological work such as Geological Sheet Map 1:75,000 by chance at the time of the earthquakes seem to have been dispatched to the earthquake disaster investigation. Furthermore, they were all not only the excellent and hardworking geologists but also the men who have manliness and were well-liked by their subordinates.