著者

渡辺 満久

出版者

日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.2016, no.44, pp.1-8, 2016-03-31 (Released:2016-10-21)

参考文献数

18

---

There are several marine terrace surfaces in the southeastern part of Shimokita peninsula, northeast Japan. They are classified into H1, H2, M1, M2 and M3 surfaces in descending order. The M1 and M2 surfaces are correlated with those formed in MIS 5e and in MIS 5a, respectively. The Rokkasho fault merging into the extensive submarine fault along shelf edge in the north has successively deformed these terrace surfaces, resulting in a 1-2 km wide flexural scarp tilting to the east on the M1 and M2 surfaces. Vertical offset of the M1 surface is over 30 m in the north and less than 20 m in the south. The flexural scarp extends at least 15 km to the south of the Takahoko Lake. The Detoseiho fault is a subsidiary branch fault of the Rokkasho fault developed in the Rokkasho flexural scarp. Because the M1 surface is bending toward the east (toward the coast), the sand layer composing of the M2 surface abutted on the monocline slope. Following movements of the Rokkasho fault have deformed the M2 surface in the same direction as the M1 surface, and consequently the M1 and M2 surfaces converged upon in the flexural scarp. This makes it very difficult to distinguish one from the other. Such geomorphic development should be strongly controlled by the activity of active fault differentiating tectonic relief and the width of flexural scarp reflecting the depth of tip of fault plane. Further examinations on active faults extending parallel to the coast lines are required to solve the problems.

著者

岡村 行信

出版者

日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.2008, no.28, pp.31-39, 2008-03-31 (Released:2012-11-13)

参考文献数

36

---

Method and result of offshore active fault survey were reviewed. Acoustic and seismic waves are widely used for offshore topographic and geologic surveys. Higher frequency acoustic waves have high-resolution but attenuate rapidly in water or sediments, thus they are mainly used for seafloor topographic survey or shallow high-resolution seismic profiling survey. Multi-narrow beam sounding provided evolutionary detailed seafloor topographic maps that clearly show fault traces. Lower frequency seismic waves are widely used for survey of deep sea and deep subsurface geology, but their resolution is generally too low to evaluate the activity of faults in late Pleistocene or Holocene time. Multi-channel seismic profiling survey and digital signal processing technology tremendously improved quality of seismic profiles. Offshore active fault maps around Japan were published in 1980's and 1990's based mainly on analyses of single channel seismic profiles. The events of active fault have been identified only in shallow bay areas using high-resolution seismic profiles and sediment cores. In contrast, it is generally difficult to determine events in open sea areas, because of low quality of seismic profiles. Multi-channel seismic profiling system using a high-frequency sound source made it possible to obtain high quality seismic profiles in the open shallow sea area and showed an active fault in the source area of the 2007 Noto-Hanto earthquake. In the deep sea, low-frequency seismic profiling system generally show clear geologic structure including active faults, but it is difficult to determine their activity in the late Pleistocene and Holocene period. Analyses of turbidites and dive surveys using submersibles have been conducted to determine the ancient events of fault activity in the deep-sea area. There is no enough data of offshore active faults, especially in very shallow marine area along coast.

著者

岡田 篤正

出版者

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.71-90, 1987-08-20 (Released:2012-11-13)

参考文献数

84

著者

林 豊 前田 憲二

出版者

Japanese Society for Active Fault Studies

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.2009, no.30, pp.27-36, 2009

---

Six active fault zones have been selected on the basis of the reports on the long-term evaluation of active faults published until 2008 by the Earthquake Research Committee, Headquarters of Earthquake Research Promotion (ERC/HERP); the paleoseismic activity data of these zones reveal three or more earthquake recurrence intervals. Using the maximum likelihood method, seven probability density functions of a renewal process model are compared in order to determine the function that best fit the paleoseismic activity data of these active fault zones.<br>The exponential distribution model obtained by using the maximum likelihood method does not clearly reveal the earthquakes recurrence intervals. In contrast, the results obtained by using six other statistical models, i.e., Brownian passage time (BPT) distribution, lognormal distribution, gamma distribution, Weibull distribution, double-exponential distribution, and normal distribution, reveal the earthquake recurrence intervals. Thus, the new paleoseismic activity data of major active zones in Japan confirm the provisional conclusion of ERC/HERP, i.e., the exponential distribution does not clearly show the earthquake recurrence intervals. On the other hand, differences among the goodness of fit of the six models excluding the exponential distribution are small.<br>In 2001, ERC/HERP stated that when renewal process model with the BPT distribution is applied to the data of the occurrence intervals of earthquakes in the inland active fault zones in Japan, the aperiodicity parameter of the distribution should be set to 0.24 as a value common to all active faults. The aperiodicity parameter obtained by applying the same method to the data of the six active fault zones is equal to 0.44. Although the aperiodicity parameters, obtained by using the maximum likelihood method, reported in the ERC/HERP's report range between 0.17 and 0.29, those obtained in this study range between 0.09 and 0.66. Thus it is inappropriate to assume the same aperiodicity parameter for all the inland active fault zones in Japan.

著者

鈴木 康弘 岡田 篤正 竹村 恵二 慶 在福 金 幸隆 廣内 大助 伊藤 愛 大石 超 中村 洋介 成瀬 敏郎 北川 浩之 渡辺 満久

出版者

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.147-152, 2005

---

The Ulsan fault extends for 50 km along the NNW-SSE direction in the southeastern part of the Korean Peninsula; this is one of the most important active faults in Korea. Its paleoseismicity has recently attracted considerable attention. With the support of KOSEF (Korean Science and Engineering Foundation), excavation studies of this fault were conducted in 1999 as a part of the Korea-Japan cooperative research at Kalgok-ri in Kyongju city. The results obtained are summarized as follows. (1) The Ulsan fault plane has an eastward dip of approximately 30 degrees and exhibits typical reverse faulting. (2) It was reactivated three times in the past 30,000 years, in particular, twice after the age of AT tephra (approximately 25,000 years BP). (3) A vertical displacement of approximately 0.8 m occurred during the fault event, and the amount of net slip along the fault plane is calculated to be 1.6 m.

著者

浅田 敏

出版者

日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.1991, no.9, pp.1-3, 1991-10-31 (Released:2012-11-13)

著者

山口 和雄 加野 直巳 横倉 隆伸 木口 努 田中 明子 佐藤 比呂志

出版者

日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.1998, no.17, pp.54-64, 1998-12-29 (Released:2012-11-13)

参考文献数

32

---

A seismic survey was conducted across the Tachikawa active fault in the western suburbs of Tokyo metropolitan area. The deep structure and movement of the fault are discussed based on the CMP stacked seismic section together with the geological data published previously. The results are as follows:(1) A flexure, about 150 meters in width, underlies the flexure scarp of the fault and continues vertically down to 1000 meters in depth.(2) The displacement of the fa u lt is about 100 meters upthrow of the northeast side between 300meters and 600 meters in depth, while the displacement at basement depth is about 100 meters downthrow of the northeast side.(3) The northeast side o f the fault had subsided relatively to the southwest side in the past. The fault movement was stopped for a while. Then the fault movement was reversed and the northeast side has been upheaved up to now. This is an inversion tectonics of the fault movement.

著者

東郷 正美 佐藤 比呂志 池田 安隆 松多 信尚 増淵 和夫 高野 繁昭

出版者

日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.1996, no.15, pp.1-8, 1996-11-29 (Released:2012-11-13)

参考文献数

16

---

By excavation of the Ochikawa-Ichinomiya remain located on the flood plain along Tama River near the boundary between Tama City and Hino City, Tokyo metropolitan area, a fault was found in young alluvium. This fault is regarded as the continuation of Tachikawa fault, a major active fault existing in the left bank area of Tama River, because it is located on the southeastern extension of Tachikawa fault line, and its strike is almost parallel to the Tachikawa fault.Detailed investigation of the fault outcrop made it clear that the last faulting event on the Tachikawa fault had occurred after A. D.1020-1158, the mid-Heian period. At this place, the last faulting event was dominantly strike slip with horizontal shortening of about 0.6 m.

著者

吉岡 敏和 水野 清秀 榊原 信夫

出版者

日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.1997, no.16, pp.87-94, 1997-08-30 (Released:2012-11-13)

参考文献数

9

---

The Senzan fault is an active reverse fault directing N-S to NNE-SSW in the central Awaji Island. During the 1995 Hyogoken-nanbu earthquake, a characteristic surface break appeared along the Nojima fault located at the western side of the Awaji Island, however, no surface break had occurred along the Senzan fault. We excavated an exploratory trench on the Senzan fault to detect the age of the faulting events, and we confirmed a reverse fault making a contact between slope deposits and a granitic rock. The youngest age of the hurried soil deformed by the fault is 1161±67 AD. This means that the last faulting event of the Senzan fault occurred after the 12th Century. This event may correspond to the historical Keicho Fushimi earthquake in 1596 AD.

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

---

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

---

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

金田 平太郎 岡田 篤正

出版者

日本活断層学会

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.2002, no.21, pp.73-91, 2002-06-30 (Released:2012-11-13)

参考文献数

35

---

Surface rupture with maximum right-lateral displacement of 150cm and length of about 1 lkm was observed during the 1943 Tottori earthquake of Mw: 7.0, which struck the eastern Tottori prefecture, southwest Japan. We compiled all the previous reports and papers regarding the surface rupture associated with the earthquake as precise location maps (Appendix 1) and a data table (Appendix 2). We also showed the slip distribution along the surface rupture (Fig.3).The trace of the 1943 surface rupture exhibited the large step and quite sinuous geometry compared with other strike-slip surface breaks. This observation probably means that the causative fault for Tottori earthquake is at the infant stage on its way of evolution based on the Wesnousky's fault evolution model (Wesnousky,1988). Most of the surface rupture during the earthquake occurred along the geomorphologically detectable active fault (Shikano-Yoshioka active fault system). Thus, we can recognize the possible surface faulting event around the epicentral area based on recent understanding of tectonic geomorpholgy. However, the length of the subsurface seismogenic fault is underestimated by the presently proposed method (Odagiri and Shimazaki,2000) and this sh ould lead to crucial underestimation of the seismic moment and also strong ground motions. The method to estimate the lengt h Df the seismogenic fault must be reestablished.

著者

金田 平太郎 岡田 篤正

出版者

Japanese Society for Active Fault Studies

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.2002, no.21, pp.73-91, 2002

---

Surface rupture with maximum right-lateral displacement of 150cm and length of about 1 lkm was observed during the 1943 Tottori earthquake of Mw: 7.0, which struck the eastern Tottori prefecture, southwest Japan. We compiled all the previous reports and papers regarding the surface rupture associated with the earthquake as precise location maps (Appendix 1) and a data table (Appendix 2). We also showed the slip distribution along the surface rupture (Fig.3).<BR>The trace of the 1943 surface rupture exhibited the large step and quite sinuous geometry compared with other strike-slip surface breaks. This observation probably means that the causative fault for Tottori earthquake is at the infant stage on its way of evolution based on the Wesnousky's fault evolution model (Wesnousky,1988). Most of the surface rupture during the earthquake occurred along the geomorphologically detectable active fault (Shikano-Yoshioka active fault system). Thus, we can recognize the possible surface faulting event around the epicentral area based on recent understanding of tectonic geomorpholgy. However, the length of the subsurface seismogenic fault is underestimated by the presently proposed method (Odagiri and Shimazaki,2000) and this sh ould lead to crucial underestimation of the seismic moment and also strong ground motions. The method to estimate the lengt h Df the seismogenic fault must be reestablished.

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

澤 祥 坂上 寛之 隈元 崇 渡辺 満久 鈴木 康弘 田力 正好 谷口 薫 廣内 大助 松多 信尚 安藤 俊人 佐藤 善輝 石黒 聡士 内田 主税

出版者

Japanese Society for Active Fault Studies

雑誌

活断層研究 (ISSN:09181024)

巻号頁・発行日

vol.2006, no.26, pp.121-136, 2006

---

We conducted a tectonic geomorphological survey along the northern part of the Itoigawa-Shizuoka Tectonic Line (ISTL) with support from the Ministry of Education, Culture, Sports, Science and Technology of Japan as one of the intensive survey on ISTL fault system. This survey aims to clarify the detailed distribution of the slip rates of this fault system, which provides the essential data set to predict the coseismic behavior and to estimate the strong ground motion simulation. In order to achieve this purpose, the active fault traces are newly mapped along the northern part of the ISTL through interpretations of aerial photographs archived in the 1940s and 1960s at scales of 1: 10,000 and 1: 20,000, respectively. This aerial photo analysis was also supplemented and reinforced by field observations.<BR>One of the remarkable results by using this data set is a large number of, here 84, photogrammetrically measured landform transections to quantify the tectonic deformations. We could calculate vertical slip rates of the faults at 74 points, based on the estimated ages of terraces (H: 120 kyrs, M: 50-100 kyrs, Ll: 10-20 kyrs, L2: 4-7 kyrs, L3: 1-2 kyrs). The vertical slip rates distributed in the northern part of the study area show 0.2-5.5 mm/yr on the L terraces (less than 20 kyrs) and 0.05-0.9 mm/yr on the M and H terraces (more than 50 kyrs). The vertical slip rates of the faults located in the central and southern part of the study area are 0.2-3.1 mm/yr.

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

---

(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.

著者

小野 有五 斉藤 海三郎

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.2019, no.51, pp.27-52, 2019-12-25 (Released:2020-01-27)

参考文献数

48

---

Geomorphological and geological studies made clear the distribution and development of three major marine terrace surfaces, Iwanai Upland and Kyowa III surfaces (about 30m in eleveation), Kyowa II (about 40~50m ditto) and Kyowa I (about 60m ditto) in the Iwanai Plain, western Hokkaido. The Iwanai Upland surface only, covered by dune sand with Toya ash (115ka) at the top, gives the absolute age of the marine terrace formation, corresponding to MIS5e. A 100m-deep boring at the Iwanai Upland by Hokkaido Electric Power Company (HEPCO) shows two clear transgression-regression sequences: Sequence I, the uppermost 30m, corresponding to MIS5e, and II, between -33 and -65m, most possibly to MIS7. Base gravel beds, located at about -30m and -65m correspond to the river bed gravel deposited on the lower sea-levels, MIS6 and MIS8. This suggests a continuous tilting at the central part of the Iwanai Plain during the Middle Pleistocene. After MIS5e, this tilting changed to the uplift movement which elevated the Iwanai Upland surface. In the marginal area of the plain, the uplift movement continued to elevate the Kyowa II and I surfaces, also: the Kyowa II, covered by a thick dune sand with several paleosoils, is correlated to most probably MIS7, and the Kyowa I is characterized by a thick delta sequence in the Iwanai Plain, and a sand barrier or spit deposit on the site of Tomari Nuclear Power Station which sits at the base of Shakotan Peninsula. Both can be correlated by the top elevation of these sediments, higher than MIS7 terrace level, and also by their spatial distribution: barrier or spit at the bay mouth and the delta at the bay head. The Kyowa I can be correlated to MIS9, and a mean uplift rate seems to be 0.1~0.2mm/y after MIS9 to the present. Since the uppermost trace of F-1 fault attains to the sediments of MIS9 in the site of Tomari Nuclear Power Station, the fault movement occurred at least later than 330ka. Trace of dislocation by F-1 fault disappears gradually in the upper sediments which are covered by the slope deposits including the pumice grains of Spfa1 (42ka), Toya and one from Niseko volcano (around 220ka). As these pumice grains are largely moved by cryoturbation of MIS2, the coldest phase of the last glacial period in Hokkaido, it is evident that those slope deposits were finally settled around MIS2. This fact suggests the movement of F-1 later than 125ka cannot be deniable, which means the F-1 is regarded as the “active fault” according to the New Regulatory Requirements for Light-Water Nuclear Power Plants.