Mitsuhisa Watanabe’s research while affiliated with Toyo University and other places

In the northwestern part of the inner Aso caldera, distinctive fault rupture zones appeared at the time of the mainshock of the Kumamoto earthquake sequence, separated from the northern end of the Futagawa fault. They extend NE-SW for totally about 8 km in the seismically active Futae-toge zone of normal faulting (Sudo and Ikebe 2001) and the certain aftershocks of the earthquake were distributed in the fault zone. They consist of several parallel and sub-parallel sharp scarps by normal faulting as observed after the 1987 Edgecumbe earthquake in New Zealand. Along the fault traces, paddy fields were displaced vertically up to 2 m along with up to ~30-cm right-lateral horizontal displacement in places. The rupture zones develop on the paddy fields generally dipping less than 1.0 degrees on the fluvial surfaces from the central cones of the Aso volcano. It is noteworthy that the fault ruptures are distributed in the area with seismic intensity less than the JMA scale 5+. From the characteristics of the surface fault ruptures, we conclude that the ruptures were triggered by normal faulting with right-lateral slip during the mainshock, but not simple lateral spreading caused by shaking.KeywordsNormal fault rupturesAso calderaDrone photogrammetry

The surface ruptures with consistent right-lateral slip components appeared in the ENE-WSW direction intermittently along the western part of the Bungo Kaido road during the 2016 Kumamoto earthquake. The ruptures seem to have appeared in severely damaged zones, indicating that a strong ground motion was generated in a very shallow part.KeywordsBungo Kaido roadKumamoto earthquakeSurface ruptureRight-lateral slipStrong ground motion

During the 2016 Kumamoto earthquake, surface fault ruptures appeared not only along the well-known active Futagawa-Hinagu fault but also in the urbanized area of Mashiki Town, where no active faults have previously been identified. This chapter describes the distribution of the earthquake faults in Mashiki Town and evaluates their relationship with tectonic landforms and building damage based on Suzuki et al. (2018). We conclude that: (1) The earthquake faults were continuous for at least 5 km from the main trace of the Futagawa fault; (2) The fault diverged from east to west but maintains the total amount of displacement of approximately 70 cm; (3) The earthquake fault locations coincided with those of tectonic landforms, and (4) building damage tended to be severe near the earthquake fault.Keywords2016 Kumamoto earthquakeEarthquake faultActive faultBuilding damage

The largest foreshock (Mj 6.5, Mw 6.2) of 21:26 (JST), April 14, 2016, caused surface fault ruptures in the Takaki–Fukuhara area (including the Itera–Oike area), and the ruptures enlarged associated with the mainshock (Mj 7.3, Mw 7.0) of 01:25 (JST), April 16, 2016, as reported by previous studies. This indicates that the same fault slipped repeatedly during the largest foreshock and mainshock. We conducted additional investigations of surface ruptures related to the largest foreshock, and newly identified surface ruptures accompanied with the largest foreshock that became larger associated with the mainshock in Takigawa, Takaki, and other areas. As a result, the length of the surface ruptures accompanied with the largest foreshock is at least 7.5 km, as compared to 6 km reported by the previous study. In this chapter, we describe the surface ruptures from south to north, including those reported by previous studies, in order to understand the foreshock–mainshock sequence of the 2016 Kumamoto earthquake.KeywordsLargest foreshockRight-lateral strike-slip offsetRepeated slipForeshock–mainshock sequence

Destructive large earthquakes occur not only along major plate boundaries but also within the interior of plates. To establish appropriate safety measures, identifying intraplate active faults and the potential magnitude of associated earthquakes is essential before an earthquake occurs. This study was conducted to document the geomorphic expression of a previously unrecognized 50‐km‐long active fault in Ulaanbaatar, the capital of Mongolia. Mapping of the fault was accomplished using the Advanced Land Observation Satellite elevation dataset provided by Japan Aerospace Exploration Agency (JAXA), a stereo‐scope interpretation of CORONA satellite images, the emplacement of trenches across the fault trace, and field study. The Ulaanbaatar fault (UBF) is marked by fault scarps on the surface and left‐lateral stream deflections. The fault displaces late Pleistocene deposits and is thus considered to be active. Based on the length of the fault, the UBF is believed to be capable of causing earthquakes with magnitudes greater than M 7 and subsequent associated damage to buildings and heavy causalities within the metropolitan area. We strongly suggest that building resistance requirements in Ulaanbaatar should be revised to mitigate for the potential of extensive seismic damage. The results of this study can be used to revise the seismic hazard map and stipulate a new disaster prevention strategy to improve public safety in Ulaanbaatar. It is also possible that there may be other active faults in the vicinity of Ulaanbaatar, and these require investigation.

Correct identification of active faults that may cause earthquakes in the near future is essential for earthquake disaster prevention. A previously unrecognized active fault across Ulaanbaatar was discovered in 2018, and a joint survey between Japan and Mongolia has begun to study the detailed distribution and seismic activity of the fault. Recent earthquakes caused by active faults in Japan, such as the 1995 Kobe earthquake and the 2016 Kumamoto earthquake, have led to severe damage near the surface traces of active faults. Thus, it is necessary to strengthen disaster prevention measures near active faults. To enhance earthquake resilience, detailed mapping of active faults is important. Therefore, after the Kobe earthquake, Japan has begun the creation of detailed active fault maps in urban areas and is contemplating adequate countermeasures. Mongolia suffered several major earthquakes in the first half of the 20th century. Thus, it is important to obtain the location of active faults in the national land and to use this information in national land planning. Moreover, it is necessary to investigate the existence and detailed activity of active faults around populated city areas, and to conduct an assessment of earthquake resistance based on the information obtained. The discovery of the Ulaanbaatar Fault (UBF) requires international cooperative research with respect to seismic assessment as well as risk reduction.

Correct identification of active faults that may cause earthquakes in the near future is essential for earthquake disaster prevention. A previously unrecognized active fault across Ulaanbaatar was discovered in 2018, and a joint survey between Japan and Mongolia has begun to study the detailed distribution and seismic activity of the fault. Recent earthquakes caused by active faults in Japan, such as the 1995 Kobe earthquake and the 2016 Kumamoto earthquake, have led to severe damage near the surface traces of active faults. Thus, it is necessary to strengthen disaster prevention measures near active faults. To enhance earthquake resilience, detailed mapping of active faults is important. Therefore, after the Kobe earthquake, Japan has begun the creation of detailed active fault maps in urban areas and is contemplating adequate countermeasures. Mongolia suffered several major earthquakes in the first half of the 20th century. Thus, it is important to obtain the location of active faults in the national land and to use this information in national land planning. Moreover, it is necessary to investigate the existence and detailed activity of active faults around populated city areas, and to conduct an assessment of earthquake resistance based on the information obtained. The discovery of the Ulaanbaatar Fault (UBF) requires international cooperative research with respect to seismic assessment as well as risk reduction.

The Itoigawa–Shizuoka Tectonic Line (ISTL) fault zone is one of the longest and most active fault zones in Japan, extending for approximately 158 km from Otari Village to Hayakawa Town. The southern part of the ISTL fault zone is a ∼48 km long, east-vergent reverse fault zone, and has the potential for causing earthquakes larger than M 7. Slip rates need to be estimated, as well as the timing of faulting and amounts of coseismic displacement, to better understand seismic risk at the southern part of the ISTL fault zone. Geomorphic surveys and tephra analyses were conducted for the middle terrace surface in the Tsukuyama area, Minami-Alps City, Yamanashi Prefecture, to estimate the long-term slip rate. Based on the data obtained, the terrace-surface deposit is at least ∼40 m thick, and is directly covered by Ontake Daiichi tephra (On-Pm1), whose age is ∼100 ka. Thus, it is inferred: (1) the middle terrace in the Tsukuyama area is one of the depositional terrace surfaces developed during the 5d of the marine isotope stage (MIS); (2) dissection of the terrace surface started during the transition period from 5d to 5c of the MIS (∼100-110 ka); and (3) the vertical slip rate of the southern part of the ISTL fault zone is at least 0.9-1.0 mm/yr, because the vertical displacement is estimated at ∼100 m or larger.

Strike-slip faulting often develops en échelon faulting resulting from Riedel shears. When faulted basement rocks are covered by thick unconsolidated deposits, this en échelon faulting leads to the subsequent evolution of a series of pop-ups along the fault during cumulative displacement. Although previous sandbox experiments and numerical simulations have examined the evolutionary processes for Riedel-shear pop-up structure formation, few field examples have been reported until now concerning (1) the time-series of evolutionary processes of pop-up structure generation and (2) time periods necessary to reach the stage of a “throughgoing fault zone”. The Itoigawa-Shizuoka Tectonic Line (ISTL) fault zone is among the largest and fastest-slipping active fault zones in Japan, and a series of tectonic pop-ups related to left-lateral strike-slip displacement have developed in the Fujimi area in the central to southern part of the fault zone. This area is covered by thick (~820 m) unconsolidated deposits, which have resulted in Riedel-shear pop-up structures. We conducted a geomorphic analysis, an extensive drilling survey, and pit excavation survey on a pop-up in the Misayama-Godo area of Fujimi, and concluded that (1) the pop-up structures in the Misayama-Godo area have been built up by two marginal faults between ~100 ka and ~10–20 ka, and (2) the fault displacement at depth has been concentrated on the faults that run across the pop-up structures (corresponding to the “throughgoing fault zone”) since ~10–20 ka. One of the faults that run across the pop-up structures probably slipped during the most recent earthquake in the central to southern part of the ISTL fault zone. This study is presented as a robust field example of the evolution of Riedel-shear pop-up structures during cumulative strike-slip faulting.

In the vicinity of the earthquake fault in Mashiki downtown, catastrophic building damage occurred. The relationship between surface faulting and house damages suggests localized strong shaking along the surface fault ruptures. In order to analyze relation between surface faulting and house damage quantitatively, we mapped surface fault ruptures in detailed by field observation, and depicted damaged houses in the downtown by interpretation of aerial photograph taken by the Geospatial Information Authority of Japan on April 15 and 16. Most of the severely destroyed or collapsed houses are confined to the narrow zone less than 120m across along surface fault ruptures. 95 percent of collapsed houses are located within 120m from the fault traces. House damage ratio has clear relationship with the distance from the fault. The closer is distance from the fault rupture, the higher is ratio of house damage. Ratio of severely damaged houses including collapsed housed reaches 40 percent at maximum, and exceeds 30 percent within 55.85m from the fault. The house damages just on the fault are quantitatively analyzed, and nearly 50 percent houses were collapsed or severely destroyed. It is noteworthy that the damage ratio does not differ by the age of construction.