著者

宍倉 正展 越後 智雄 行谷 佑一

出版者

一般社団法人 日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.2020, no.53, pp.33-49, 2020-12-25 (Released:2021-07-11)

参考文献数

25

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Marine terraces (probably Holocene terraces) divided into three levels of L1 to L3, representing the activity of off-shore active faults, have been developed along the northern coast of the Noto Peninsula, central Japan. To evaluate the mean vertical displacement rate and recurrence interval of off-shore active faults, this study tries to estimate the emergence age of terraces using an idea of time predictable model with the rate of relative sea-level fall. In the Saruyama-oki segment off the northwestern coast, assuming the ages of the L1 terrace to be 6,000 or 3,500 years ago based on previously reported peak age of post-glacial transgression, it is inferred that the mean vertical displacement rate is 0.87 mm/year or 1.49 mm/year, and recurrence interval is 2,000 or 1,200 years. However, the emerged sessile assemblages indicate the shortest interval of 300 years because the height and age suggest that two uplift events of 0.7 m and 0.8 m occurred in the 9th century and 12th to 13th centuries, respectively. In the Wajima-oki and Suzu-oki segments off the north-central to northeastern coast, the height distribution of the lower terraces indicates undulating deformation with two peaks, which is consistent with the height distribution of the late Pleistocene terrace. This suggests the characteristic displacement and its accumulation due to the fault activity of these segments. Under the same assumptions for estimating the age of terraces, the mean vertical displacement rate and recurrence intervals are 0.67-0.72 mm/year or 1.14-1.23 mm/year and 900-1,400 years or 500-800 years respectively. The age of emerged sessile assemblage indicates that the latest event of the Wajima-oki segment can be correlated to the historical earthquake of AD 1729.

著者

岩佐 佳哉 濱 侃 中田 高 熊原 康博 後藤 秀昭 山中 蛍

出版者

一般社団法人 日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.2022, no.57, pp.1-13, 2022-12-26 (Released:2023-06-27)

参考文献数

19

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In order to evaluate the applicability of 3D scanners for field survey on surface ruptures, we examined the scanning accuracy, point cloud density, usability, and time efficiency of the instruments of three different SLAM methods, Avia for LiDAR SLAM, ZED 2 for Visual SLAM, and iPad Pro for Depth SLAM We conducted experimental surveys on the surface ruptures associated with the 2016 Kumamoto Earthquake at two locations. One is the surface rupture preserved as the earthquake heritage in the Aso field of Tokai University, while another is a normal fault rupture in the forested area at Miyayama, Nishihara Village, Kumamoto Prefecture. All the scanners obtained detailed point clouds, from which we successfully made digital surface models, cross-profiles and contour maps in a few tens of minutes. We came to know that Avia is most effective among the three scanners for wide-area mapping and that iPad Pro is a useful handy instrument for mapping limited areas. From our experimental survey, it is highly recommended to use Avia and iPad Pro together (in the field) in order to collect detailed geometric data of surface ruptures immediately after earthquake.

著者

寒川 旭

出版者

一般社団法人 日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.2011, no.35, pp.67-73, 2011-09-30 (Released:2015-12-09)

参考文献数

20

著者

岡田 篤正 植村 善博 東郷 正美 竹村 恵二 吉岡 敏和 堤 浩之 梅田 康弘 尾池 和夫 松井 和夫 杉森 辰次 杉山 直紀 園田 玉紀 梅田 孝行 松村 法行 山田 浩二 古澤 明

出版者

一般社団法人 日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.2005, no.25, pp.93-108, 2005-06-30 (Released:2012-11-13)

参考文献数

36

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The Kameoka basin is located to the west of the Kyoto basin. On the northeast side of the basin, two faults trending the northwest to southeast direction exist along the foot and the former edge of a mountain, respectively. They compose of the Kameoka fault zone with the length of about 13km (Okada&Togo ed.,2000).To elucidate such characteristics as distribution, subsurface structure and activity of those faults, we have carried out seismic reflections (P-waves) and deep drilling surveys across the faults. Volcanic ash and pollen analysis were also performed using core samples obtained by drillings. In this paper, we report the results of these surveys, especially about the characteristics of the concealed faults related to basin formation.By these surveys, three faults were detected along the three sections by the seismic reflection crossing the eastern half of the Kameoka basin, named as Fl, F2 and F3 faults from west to east. All faults incline to the northeast to form the reverse fault type uplifting to the northeast side.The Fl fault is concealed under the alluvial plain of the Katsura River and is an active fault having remarkable displacement of vertical direction to a few hundreds of meters. An accumulation of the displacement in the vertical direction is plainly recognized on the topographical and geological sections.The F2 fault appears in the wide deformation zone on the hanging. wall of Fl fault and is thought to be a subordinate fault of the F1 fault. From the distribution, the F2 fault is corresponded to be an active fault described by Okada&Togo ed. (2000) and identified at former edge of a mountain in the Kameoka basin. In this paper, we will call the Fl fault and the F2 fault as“ the Kameoka fault within the basin”. It is surely distributed about 4.6 km from the Umaji to the Hozu settlements in the southeast direction.Of the Kawarabayashi reflection profile, one reflection layer C has vertical displacement of 65m resulted from the activity of“ the Kameoka fault within the basin”. A pure seam from core samples of the layer is confirmed as so-called Oda Volcanic ash at 420-450ka. Therefore, the average slip rate of the vertical displacement is estimated at 0.15m per thousand of years or less, during the last about 430,000 years.We also found a fault scarplet (relative height 1.5-2.5m) on a low terrace. It seems to be formed by the F2 faulting since about 20,000 years ago. Hence the faulting of“ the Kameoka fault within the basin” since the late Pleistocene is certain, and also there is a possibility of the activity in the Holocene from the existence of the reverse-inclined terrace II at Umaji.Judged from distribution, the F3 fault is corresponding to "the Kameoka fault in the foot of a mountain" described by Okada&Togo ed. (2000). There is no evidence of the F3 faulting during the late Quaternary.

著者

西村 卓也

出版者

一般社団法人 日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.2017, no.46, pp.33-39, 2017-03-31 (Released:2018-03-29)

参考文献数

26

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Many previous studies have revealed distribution of strain rates in the Japanese Islands using data of continuous GNSS station installed since mid 1990’s. They discovered “strain concentration zones” including the Niigata-Kobe Tectonic Zone and the Ou backbone Range in inland and a side of Sea of Japan away from major plate boundaries including the Nankai Trough and Japan Trench. We used GNSS data after the increase of GNSS stations in 2002 and examined distribution of site velocities and strain rates during 2005-2009 with higher spatial resolution. And then, we compared it with major active faults and found that many active faults locate in regions where maximum shear strain rates were high. We also removed elastic deformation due to interplate coupling on the subducting plate interface along the Nankai Trough and compared between distribution of the corrected strain rates and shallow seismicity. The comparison suggests a tendency that the higher maximum shear strain rates, the more frequent shallow M >_ 6 earthquakes occur. We, therefore, suggest that the GNSS data is incorporated into long-term evaluation of large inland earthquakes.

著者

岩佐 佳哉 熊原 康博 後藤 秀昭 石村 大輔 細矢 卓志

出版者

一般社団法人 日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.2022, no.56, pp.47-58, 2022-06-28 (Released:2022-12-28)

参考文献数

32

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The Futagawa fault, extending southwest from Aso caldera, is one of the major dextral strike-slip active faults in Kyushu, southwest Japan. On 16 April 2016, the Kumamoto earthquake (Mj 7.3) occurred, and ~31-km-long right-lateral surface ruptures appeared along the Futagawa fault. After the 2016 earthquake, several trenching surveys were conducted across surface ruptures to reveal the faulting history. However, no trenching survey has been carried out in the 15-km-long middle section from Dozon to Aso caldera. We conducted a trenching survey and an additional hand auger survey to reveal faulting history in Komori, Nishihara Village, in the middle of the section. Furthermore, we carried out a geomorphological survey for the detailed description of the surface ruptures around the trench site. At the trench site, a ~40-cm-deep graben was formed by the 2016 earthquake. A similar graben structure appeared on the trench wall units, which shows larger vertical deformation than that of the 2016 earthquake, indicating that similar types of deformation to the 2016 earthquake have repeatedly occurred at this site. Based on such deformational features of units, we identified at least four faulting events, including the 2016 earthquake, since about 11,500 cal BP. Also, the timing of the penultimate event was 2,240-1,910 cal BP and the calculated recurrence interval was 2,400-3,800 years. The penultimate event may have been simultaneous in the section from the northeastern part of the Aso caldera to the southwestern part of the fault zone, similar to the 2016 Kumamoto earthquake. If this idea is correct, based on the overlap among event dates from previous studies as well as our result, the timing of the preceding earthquake is about 2,000 cal BP.

著者

岡田 篤正

出版者

一般社団法人 日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.2011, no.35, pp.1-13, 2011-09-30 (Released:2015-12-09)

参考文献数

73

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The 1586 Tensho earthquake occurred around the western part of central Japan. The magnitude is estimated to be around 8, as large as the 1891 Nohbi earthquake, mainly based on the intensity distribution map. However, historical documents concerning the precise damage and detailed crustal deformation etc. for this event are fairly limited as the occurrence time was during the Warring State Period.The Shokawa fault zone, Atera fault zone and Yoro-Kuwana-Yokkaichi fault zone, typical leading active faults having high activity and fresh topographical expression in central Japan, have been considered to be major candidates for the active faults which produced this great earthquake. The mysterious Tensho earthquake was selected as the main theme of a symposium held in Japanese Society for Active Fault Studies 2010 Fall Meeting, and was discussed in the oral session, based on the recent various data obtained by the investigations for these fault zones after the 1995 Hyogo-ken Nanbu (Kobe) earthquake (M7.3).

著者

松浦 律子

出版者

一般社団法人 日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.2011, no.35, pp.29-39, 2011-09-30 (Released:2015-12-09)

参考文献数

31

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Tensho earthquake is one of the most famous large inland earthquakes during the medieval times of Japan. It occurred on Jan. 18, 1586 in Chubu district. It was almost the equal size to Nobi earthquake in 1891, and we estimated its size is around M7.8-8.0. Many major active faults in Chubu district had been assigned as the source faults of this earthquake. However, the careful examination of the historical materials, and the precise analysis of the distribution of estimated reliable seismic intensities revealed that it is impossible to cause the damage of whole Tensho earthquake by a single earthquake of M8-class. The main shock occurred in the south-western part of Nobi basin. The source faults are narrowed down to some faults near Yoro and Suzuka mountains, and Ise Bay, or southern part of Yanagase, Sekigahara, and nearby faults, when we compare the intensity distribution with expected ones for several major fault groups in Chubu district. The famous destruction of Uchigashima Family in Kiun Castle and the crushing death of Mr. and Mrs. Ukon Maeda in Kifune Castle were caused by the different earthquake, which occurred on Jan 16, in some faults near Shokawa River, and its size was around M7.0 ±0.2. In order to reveal source faults of medieval destructive earthquakes, which usually have too few historical materials, we have to be careful to the credibility of information. Those written in later years may be modified through folklore transmission. Even in the primary historical sources such as a diary of an aristocrat or a letter by a missionary, hearsay information in remote area was left with true damages they really knew. Since there are much more destructive earthquakes than those known in the catalogue we have now, the trench results should not be restricted to select the candidate of earthquakes only from the current earthquake catalogue. For example, July 31, 1585, a very strong earthquake was felt in Mikawa, Ise, and Kyoto, and no destructive damage was known in Chukyo area. This must be the first candidate for the latest activity of Atera fault.

著者

萩原 幸男

出版者

一般社団法人 日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.1987, no.4, pp.9-17, 1987-08-20 (Released:2012-11-13)

参考文献数

11

著者

久保田 勲 浅川 一郎 平川 一臣 今泉 俊文

出版者

一般社団法人 日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.1989, no.7, pp.43-48, 1989-11-11 (Released:2012-11-13)

参考文献数

5

著者

岡田 篤正 堤 浩之

出版者

一般社団法人 日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.1990, no.8, pp.31-47, 1990-09-25 (Released:2012-11-13)

参考文献数

12

出版者

一般社団法人 日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.1988, no.5, pp.77-81, 1988-07-30 (Released:2013-03-22)

参考文献数

6

著者

岩佐 佳哉 熊原 康博 後藤 秀昭 中田 高

出版者

一般社団法人 日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.2020, no.52, pp.1-8, 2020-06-25 (Released:2020-12-25)

参考文献数

11

---

On the 16 April Kumamoto earthquake (Mj7.3), ~31km-long right-lateral surface ruptures appeared along the previously mapped Futagawa and Hinagu faults. The surface ruptures appeared in Dozon, Mashiki Town, recording 2.2m of right-lateral displacement which is the maximum strike-slip displacement of these surface ruptures. Small surface deformations such as flexure of cultivated land and deformation of the waterway and left-lateral conjugated fault also appeared in this area. In order to reveal distribution and amount of small surface deformations, we created a digital surface model (DSM) based on photographs taken by unmanned aerial vehicle (UAV) and RTK-GPS survey and conducted a field survey. As a result, small and conjugated surface ruptures were observed about 100m northwest of the main trace of the strike-slip fault, and amount of these deformations are each about 5―30cm of north-down displacement. The amount of vertical offset of just above the main trace is 25―30cm of south-down offset but the total vertical offset in Dozon is a north-down vertical offset rather than a south-down when summing the vertical offset of the secondary trace and the main trace. We also conducted a trenching survey across the conjugated fault to reveal surface faulting history. While the vertical offset caused by the 2016 earthquake was 20cm down on the south, older strata exposed on the trench walls were offset more than 40cm. Based on the deformational features of exposed strata, we identified at least four faulting events including the 2016 earthquake. The timing of the event before the 2016 earthquake is 500―10,600yrsBP. It indicates that the conjugated fault is also cumulative. It is likely that the conjugated fault and small surface ruptures have repeatedly ruptured simultaneously with the main trace, because the conjugated fault follows the small surface ruptures and is consistent with the timing of events in the main trace.

著者

高橋 直也 遠田 晋次

出版者

一般社団法人 日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.2022, no.56, pp.1-12, 2022-06-28 (Released:2022-12-28)

参考文献数

46

---

Earthquake surface rupture appears not only along the principal fault that caused the earthquake but also pre-existing other faults or folds nearby. Although surface ruptures along subsidiary structures are often small and discontinuous, given its extensive occurrence, comprehensive mapping of active faults and folds is necessary for understating strain distribution within a fault zone to evaluate fault displacement hazards. The Nagamachi-Rifu line fault zone, consisting of sub-parallel reverse faults, runs across Sendai City, and recent studies suggest that there are several active structures not shown on existing active fault maps. However, it is difficult to obtain their convincing geomorphic evidence because their surface deformation is subtle and is easily obscured by erosion and artificial modification. To reevaluate active faults and folds of the fault zone, we studied shallow geologic structure across the fault zone using a borehole dataset and conducted a geomorphological analysis using 2 m DEM. Our results indicate that the fault surface of the Nagamachi-Rifu line does not reach the surface causing flexural deformation at the surface rather than brittle deformation, as the previous studies pointed out based on seismic reflection surveys. We also found that the deformation zone of the Dainenjiyama fault is broader than previously thought. This is probably because its fault scarp retreated by hundreds of meters due to erosion. To the northwest of the Dainenjiyama fault, there is an NNE-SSW trending zone where the base of the Quaternary deposit was elevated compared to its surroundings, which may correspond to the previously estimated active anticline. The ratio of vertical displacement of the Nagamachi-Rifu line, Dainenjiyama fault, and the anticline is estimated to be 5-6: 3: 1. We could not newly find other faults or folds, including those exposed at outcrops reported in previous studies, suggesting that there are several active structures of which cumulative displacement is too small to be observed in our analysis. Nonetheless, a shallow geologic structure revealed from a borehole dataset, coupled with detailed topographic analysis, can help to locate active structures with minor surface expression and evaluate seismic hazards.

著者

遠田 晋次 鳥井 真之 小俣 雅志 三五 大輔 石澤 尭史

出版者

一般社団法人 日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.2021, no.54, pp.39-56, 2021 (Released:2022-01-19)

参考文献数

39

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Afterslip, mostly aseismic creep on and on the margins of the seismic source fault, occurs not only after a large subduction megathrust earthquake but also rarely on an onshore surface rupture such as the 1966 and 2004 Parkfield, California, earthquakes on the San Andreas fault. Here we present evidence for afterslip as post-seismic continuous creep along the part of the coseismic surface rupture of the 16 April 2016 Kumamoto earthquake of Mw 7.0 (MJMA 7.3). We describe evident surface afterslip on five sites along the northermost Hinagu fault that experienced 30-65 cm coseismic right-lateral slip. Even though the post-seismic tape measurements are not highly accurate and contains approximately±1-3 cm uncertainty depending on site condition, maximum right-lateral displacement have reached ~20 cm during the first one year after the mainshock. Additionally, up to 5-cm cumulative right-lateral slip of a wall newly built in February 2017 across the coseismic rupture proves that the post-seismic creep has been lasting for at least one year, probably longer than three years. Together with the field survey, we also mapped one-year postseismic ground displacement from analysis of interferometric synthetic aperture radar (InSAR) images. About 2.5 cm contraction across the Hinagu fault from InSAR is a little short but mostly consistent with ~3.5 cm contraction estimated from ~20 cm right-lateral post-seismic slip on the N10°E trending rupture zone. Furthermore, our field observation is also in agreement with the timeseries of horizontal movement of a GNSS station ~2 km southeast of the fault zone. To examine the contribution of local aftershocks to the afterslip, we examined the cumulative moment release of all the aftershocks that reached ~5.1 x 1024 dyne-cm. Assuming a 10 km by 10 km fault plane, one could expect ~16 cm slip that is roughly equivalent to the amount of measured surface afterslip. However, the numerous aftershocks are located probably off the subsurface Hinagu fault so that shallow aseismic creeping would play an essential role for surface afterslip. Although our field measurements, InSAR and aftershock analysis cannot simply judge that the afterslip has been continuously loading to the unruptured sections of the Hinagu fault zone, significant aftershock activity has been still occurring beyond the southern edge of the afterslip zone. The Hinagu afterslip gives us clues not only to understand the postseismic fault behavior but also how to prevent from postseismic damage of fault-crossing structures and better assess the timing of restoration.

著者

岡 孝雄

出版者

一般社団法人 日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.1985, no.1, pp.19-29, 1985 (Released:2012-11-13)

参考文献数

10

被引用文献数

1

著者

千田 昇 白木 守 松村 一良 松田 時彦 下山 正一

出版者

一般社団法人 日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.2005, no.25, pp.129-133, 2005-06-30 (Released:2012-11-13)

参考文献数

4

---

Trench investigation at Shindo relics excavated to clarify geological activity records of Senbonsugi fault which is one of the faults constituting westernmost part of Minoh fault system. Senbonsugi fault had actived at AD679, and wide cracks generated in the ground. These cracks had described in“Nihonshoki (Chronicles of Japan)”as width was about 7 meters.

著者

後藤 秀昭

出版者

一般社団法人 日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.2015, no.42, pp.73-83, 2015-03-31 (Released:2015-12-23)

参考文献数

19

被引用文献数

1

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Digital elevation model (DEM) have been indispensable data for identifying fault traces and measuring the amount of recent faulting. However, some of DEM produced by Air-borne / Terrestrial Light Detection and Ranging (LiDAR) are difficult to observe the tectonic geomorphology because of artificial modification, or because its density is too sparse. In this paper, we attempt to construct DSM by using SfM (Structure from Motion) - MVS (Multi-Video Stereo) with aerial photographs. We used old aerial photographs before artificially modified in Awa city, as well as aerial photographs by the 7-m-high pole camera (Hi-view) in Shikokuchuo city along the Median Tectonic Line active fault system in Shikoku, Southeast Japan. As a result, the 0.5m-mesh DSM and 0.05m-mesh DSM were generated from old aerial photographs scaled 1 to 8,000 and Hi-view photographs by compact digital camera, respectively. The relative height of the fault scarp based on the 0.5m-mesh DSM is almost same as that based on the 5m-mesh DEM of Geospatial Information Authority of Japan and 1m-mesh DEM of aerial photograph survey company. On the other hand, the shape of dense points based on the 0.05m-mesh DSM along the line is quite similar to the topographic profile based on the measurement by conventional total station method. These cases illustrate that SfM- MVS photogrammetry with Old / Hi-view aerial photography is quite useful new method for studying active tectonic geomorphology.

著者

岩佐 佳哉 熊原 康博 後藤 秀昭 中田 高

出版者

一般社団法人 日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.2020, no.52, pp.1-8, 2020

---

<p> On the 16 April Kumamoto earthquake (Mj7.3), ~31km-long right-lateral surface ruptures appeared along the previously mapped Futagawa and Hinagu faults. The surface ruptures appeared in Dozon, Mashiki Town, recording 2.2m of right-lateral displacement which is the maximum strike-slip displacement of these surface ruptures. Small surface deformations such as flexure of cultivated land and deformation of the waterway and left-lateral conjugated fault also appeared in this area. In order to reveal distribution and amount of small surface deformations, we created a digital surface model (DSM) based on photographs taken by unmanned aerial vehicle (UAV) and RTK-GPS survey and conducted a field survey. As a result, small and conjugated surface ruptures were observed about 100m northwest of the main trace of the strike-slip fault, and amount of these deformations are each about 5―30cm of north-down displacement. The amount of vertical offset of just above the main trace is 25―30cm of south-down offset but the total vertical offset in Dozon is a north-down vertical offset rather than a south-down when summing the vertical offset of the secondary trace and the main trace. We also conducted a trenching survey across the conjugated fault to reveal surface faulting history. While the vertical offset caused by the 2016 earthquake was 20cm down on the south, older strata exposed on the trench walls were offset more than 40cm. Based on the deformational features of exposed strata, we identified at least four faulting events including the 2016 earthquake. The timing of the event before the 2016 earthquake is 500―10,600yrsBP. It indicates that the conjugated fault is also cumulative. It is likely that the conjugated fault and small surface ruptures have repeatedly ruptured simultaneously with the main trace, because the conjugated fault follows the small surface ruptures and is consistent with the timing of events in the main trace.</p>

著者

近藤 久雄 勝部 亜矢 黒澤 英樹 佐々木 亮道 阿部 恒平 三輪 敦志

出版者

一般社団法人 日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.2015, no.43, pp.i-ii, 2015-09-30 (Released:2016-09-09)

被引用文献数

1

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(口絵1) トレンチ調査地および周辺の地形写真.2014年長野県北部の地震(Mj6.7)では,糸魚川-静岡構造線活断層系・神城断層の一部が活動し,約9kmの区間で地表地震断層が出現した.神城断層は糸魚川-静岡構造線の最北端を構成する断層で,いわゆる北部フォッサマグナの西端に位置する.写真は,糸静線の西側に分布する主として花崗岩類からなる北アルプス側から東へ向かって撮影したものである.写真中央の丘陵は,神城断層の東側に分布する鮮新-更新統の大峰帯からなり,さらに背後(東側)にみられる定高性を持つ山地は,フォッサマグナを充填する鮮新統・中新統からなる.丘陵の西縁と沖積低地の境界付近を神城断層が延びており,大局的には山地と盆地の境界付近を姫川が北流し,地表地震断層は姫川左岸側の沖積低地や河床に出現した.トレンチ用地は,堆積物の年代や層相を考慮し,写真中央付近の沖積低地面上で実施した.(口絵2) 地震直後のトレンチ掘削用地の写真.白馬村飯森地区に位置するトレンチ調査用地の水田では,地震に伴い0.5mの上下変位を伴う撓曲崖が出現した.写真は,地震から2日後の2014年11月24日に撮影したものである.周辺では約600mの区間にわたり地表地震断層が連続して出現したが,断層上盤側のトレンチ長さを十分に確保できること,米軍撮影の空中写真でみられる姫川の旧河道をできるだけ避けることを考慮して,掘削用地を選定した.(口絵3) トレンチ掘削後の全景写真.西へ向かって撮影.トレンチは,飯森地区の水田の災害復旧工事計画等を考慮して2015年3月に実施した.トレンチ掘削は,除雪後に地表地震断層の痕跡を消失しないよう慎重におこない,調査期間中の降雪に備えて屋根を設置するよう準備した.(口絵4) トレンチ掘削後の南壁面の写真.トレンチ壁面には,地表地震断層に連続する明瞭な断層が露出した.調査期間中はトレンチを覆い尽くす屋根を設置して,トレンチ内の作業や安全が確保できるように努めた.詳細は表紙説明を参照.