豊橋技術科学大学では、1997.3.16.発生した愛知県東部地震を含み、今まで構築した地震観測システムTASSEMによって、いくつかの観測結果を得ました。これらを加えて、K-NET強震計データの収集も行い、東三河地域の地震動分布を整理した。なお、大学の位置しているこの地域には西南日本の地質を大きく二分するが地域中央を東北東から西南西に通過している。このの存在は同地域の地震動特性に大きな影響を及ぼすと考えられる。本研究では三河地域において地質構造の食い違いをなしているを対象にし、を挟んで観測された地震観測記録を分析することにより、同地域の地震動特性、およびが地震動の伝播に及ぼす影響について検討を行った。

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The Median Tectonic Line (MTL) is one of the most significant tectonic lines in Japan, spreading up to 1,000 km through the southwest part of Japan. Especially MTL stretching from the western Shikoku Island to the western Kii Peninsula includes many faults active in the late Quaternary.
The marine seismic reflection survey using Uniboom and Water gun as energy sources has been carried out in Tomogashima strait, where is a part of the MTL spreading between the Shikoku Island and the Kii Peninsula. The survey revealed the characteristics of faults in this area, which are similar to the fault distribution pattern pointed by the previous study (Maritime Safety Agency, 1978). These faults, however, contain different sense of displacement and also seem to have different activity age, namely some show the deformation of the Holocene layer, while others are limited in the estimated middle Pleistocene layer in their activities. It is estimated that MTL in this area is not the single continuous active fault, but is the combination of faults showing different type in its activity and age.

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The Quaternary activities along the Median Tectonic Line around Awa-Ikeda have been explained as thrust fault, right lateral slip fault, etc. Such explanations sound too much simple to interpret this biggest fault in Japan. The writer reexamined the activity of Median Tectonic Line around Awa-Ikeda and brought the following interpetations.
1) There are clearly recognized thrust fault and vertical fault with strike-slip, and the former is intersected by the latter.
2) As for the stages of fault activity, the former is thought to be at the Shobudani stage and the latter at the lower terrace stage or later.
3) These fault activies are expected to be found in the lower Yoshino River basin and in the Kino-kawa River basin.

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本論文では, 道路建設に伴う直上の断層破砕帯における2カ所の地すべり地の機構を調べ, 対策工の検討を行った. その結果, 直上の断層破砕帯の粘性土は, 近傍の地すべり地のすべり面の粘性土と比べてせん断強度が小さい上, 残留強度への低下が著しいことがわかった. これが直上の地すべりの対策を困難にしている理由の一つである.

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The Median Tectonic Line (MTL) active fault system in Shikoku, which is a 180km-long right-lateral strike-slip fault system, is composed of several active faults. The Iyo fault zone, which is composed of the Iyo, Kominato and Hongu faults, is distributed in the westernmost parts of Shikoku. We conducted a seismic reflection survey, arrayed boring researches and a trench study to reveal the fault structure and fault activity of the Iyo fault zone. The seismic reflection survey, which traverses the Kominato and Hongu faults, reveals that the Kominato fault is a south-dipping reverse fault, and the Hongu fault is a high-angle fault with upheaval on the northern side. The Kominato and Hongu faults merge about 600m below the surface. The Kominato fault may converge with the Iyo fault at a grater depth, because some previous research suggests that the Iyo fault may be a high-angle fault, based on geological and geomorphological data. In addition to this result, the Kominato and Hongu faults have no strike-slip component detectable on fault topography, and the average vertical-slip rate of the Kominato fault is roughly estimated to be 0.6mm/yr from the result of boring research. The fault mechanism of both of the Kominato and Hongu faults is different from that of the Iyo fault, because the Iyo fault is the same typical right-lateral fault as other members of the MTL active fault system. The Kominato and Hongu faults have formed a tectonic bulge through Pliocene to Quaternary compressional regimes in the northern area of the Iyo fault. These geophysical and geological facts indicate that the Iyo fault is a part of the main fault, and the Kominato and Hongu faults have developed as spray faults in the northern area of the Iyo fault. The developing process of the Iyo fault zone may be explained by tectonic setting such as a contractional bend or slip partitioning by the Iyo fault. On the other hand, we found evidence for two surface-faulting events on the trench walls at the Hongu fault. The most recent surface-faulting event of the Hongu fault is inferred to have occurred between 3, 660 and 2, 010 y. B. P. The timing of penultimate faulting event of the Hongu fault is constrained to have occurred between 7, 160 and 3, 580 y. B. P. It was proposed from the previous research that the most recent surface-faulting event of the Iyo fault occurred some time after the 14th century. The penultimate faulting event of the Iyo fault is assumed between 11, 000 and 7, 000 y. B. P. The Iyo fault zone, therefore, have occurred four earthquake events during the past 10, 000 years. But the faulting history does not coincide between the Iyo and Hongu faults under the present data. This fact suggests that the timing of faulting event is different between the main fault and spray fault.

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Recent paleoseismological studies revealed that the entire length of the right-lateral Median Tectonic Line active fault zone in Shikoku ruptured after the 16th century A.D. Based on aerial photograph interpretation and field observations, we identified geomorphic and artificial features offset along the fault zone that record the amount of right-lateral slip associated with the most recent and previous earthquakes. The amount of offset during the most recent earthquake varies considerably among the active faults that comprise the fault zone: about 7m along the Chichio and Ikeda faults in eastern Shikoku and 2-3m along the Shigenobu and Iyo faults in western Shikoku. In general, surface slip associated with the latest event is greater than 5m between the Zunden and Okamura faults, and decreases gradually to the east and west. There is a correlation between horizontal slip rates of the individual faults and surface slip during the most recent event. This is consistent with the paleoseismic observation that the recurrence intervals of the individual faults do not vary considerably along the fault zone and are in a range of 1000-2000 years. Based on the surface offsets and segmentation model by Goto and Nakata (2000b), we are able to calculate moment magnitudes for scenario earthquakes from the fault zone. Separate ruptures of the western Shikoku, Okamura, and eastern and east-central Shikoku segments would produce earthquakes with magnitudes of M_w7.3-7.4, M_w7.2-7.4, and M_w7.7-7.8, respectively. These ruptures would be as large as or greater than the largest recorded historical earthquake, the 1891 M_w7.4 Nobi earthquake.

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西南日本の地体構造を考える上で

は欠くことのできない地質要素のひとつである（以下では，便宜的に地質境界のを表現する場合には「」を，活断層としてのを表現する場合には「活断層系」を，また両方の断層を包括して表現する場合には「断層帯」を用いる）．断層帯は長い活動史を持ち，白亜紀に西南日本内帯/外帯の地質境界として形成されてから，現在もその一部が活断層として活動している．しかし活断層系の地下深部構造については現在も議論の分かれているところである．一方で，全長400km以上にわたる横ずれ活断層の破壊過程には不明な点が多いため，地震防災上も注目され地質学的のみならず地震学的にも研究が進められている．特に1995年兵庫県南部地震以降，正確な断層分布の把握，最新活動時期，活動間隔あるいは変位量といった断層活動性評価に資する情報が急速に蓄積されてきた．さらには，その様な活動性情報の収集は，長大横ずれ断層である活断層系の断層セグメンテーションの検討を促進し，その結果，断層破壊過程あるいは発生する地震の規模予測の議論へと展開されている．

本巡検では，四国西部の

活断層系を時空間的に意識しながら断層露頭を訪れ，地質境界の産状および活断層地形を観察する．また，の活動で形成された第二瀬戸内層群である郡中層の産状についても観察し，様々なフェーズにおける断層帯の運動像に迫る．

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The Median Tectonic Line active fault system (MTL) is one of the most active and longest active faults on land in Japan, and has a potential source of destructive large earthquakes. However, paleoseismological data along the MTL are sparse in Shikoku. The history of surface-rupturing earthquakes, particularly the timing of the latest events, on each segments is key data for examining the interaction of neighboring faults and evaluating the long-term seismic risk.
To date the latest surface-faulting events on the MTL active fault system in Shikoku, we excavated mini-trenches on the Iyo fault and extracted vertical thin sections of unconsolidated soil layers by the Geoslicer on the Okamura and Ikeda faults. In this paper, we discuss the characteristics of active faulting along the MTL based on the results at the three sites and previously published data.
The latest event occurred after the 14th century at Ichiba on the Iyo fault. The slip associated with the event was 2.1-2.3m based on a series of right-laterally offset rice paddy dikes. Geoslicer studies at Kishinoshita on the Okamura fault and at Iyo-Mishima on the Ikeda fault suggest that the latest event occured after the 16th century and after the 13th century, respectively.
The youngest age of the latest event is not well defined by the geological studies. Since historical documents during the 18th century do not record any activity of the MTL on land, the timing of the most recent events on the fault is confined to between the 13th and 17th centuries. Therefore, timing of the latest events in Shikoku is concentrated within the past several hundred years. This suggests a temporal clustering of active faulting on the MTL, similar to that observed from other major fault worldwide.

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Although the Median Tectonic Line (MTL) fault system in Shikoku, southwest Japan, is one of the most active faults during the late Quaternary in Japan, the present-day seismicity along the MTL is not so remarkable. However microseismic activities have recently been reported in some regions along the MTL in Shikoku. It has been pointed out that these activities are those at segment boundaries of the MTL fault system. We carried out a temporal microseismic observation in one of these regions, the region around borders between Ehime, Kagawa and Tokushima prefectures, in order to determine the earthquake hypocenters precisely so that we can discuss the hypocenter distribution in context of the tectonics of the MTL. The seismometer network consisted of six observation points and had its areal extent of about 10km by 10km. Surface trace of the Ikeda fault goes through the network with its strike of N75°E. About 10km to the west of the network, there is a segment boundary of the MTL fault system which divides the Ikeda fault and the Ishiduchi fault. Hypocenters of 82 earthquakes are located during November 1990-August 1991. The seismic activity is high beneath the network, and no hypocenter is located near the segment boundary of the Ikeda and Ishiduchi faults. Most events located in the southern side of the MTL are in a depth range of 5-8km, and the distribution is almost flat or slightly inclines toward south from the MTL. The distribution pattern of the hypocenters in the northern side of the MTL is widely different from that in the southern side. The hypocenter depth increases with being away northward from the surface trace of the MTL. Although the number of events located is too few to conclude that the distribution is planar, the dip angle of about 50° is measured from a cross-sectional view on a vertical plane perpendicular to the strike of the MTL. The gap in the hypocenter distribution just beneath the MTL dipping almost vertically to the north, which has been reported by KIMURA and OKANO (1992) as an evidence that the MTL is near vertical, is not found in the present study.

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徳島県吉野川北岸地域のにおいて,大縮尺空中写真判読と現地調査をもとに,活断層の分布と運動様式の再検討を行った.その結果,従来断層が連続しないと考えられていた地域をつなぐような断層が,新たに確認された.吉野川北岸でのは,鳴門断層の西端部付近に不連続が存在する可能性は残るものの,鳴門断層から父尾断層まで湾曲や屈曲を伴いながら,ひとつづきの断層として連続することが明らかになった.
また,横ずれ断層の南側に低角な逆断層(前縁逆断層)の認められる地域がある.前縁逆断層は,(1)主断層の走向がその一般走向に対してやや斜交する(10～20°反時計回り)部分で,その南側に平行して存在する場合と,(2)主断層がその一般走向に対して大きく屈曲する位置で,その南側に存在する場合とがある.(1)は,横ずれ断層の走向の変化によって生じた水平短縮量の増大によって生じ,(2)は,断層の屈曲に伴う局地的な圧縮応力によって生じたと考えられる.

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四国西部と中央部のに沿っては, 中期中新世の貫入流紋岩を中心にスメクタイトを主体とする熱水変質帯が形成されている。四国東部では, 流紋岩の貫入は認められないが, 変質粘土の細脈が生成され, 断層周辺にスメクタイトを主体とする熱水変質作用が及んでいる。熱水変質に伴う岩盤劣化は, 断層破砕と解釈されていたため, これまでの地質調査で見逃されていた。熱水変質作用によって形成されたスメクタイトないしスメクタイトの混合層鉱物を含む粘土はせん断強度が小さく, 沿いの切土のり面における地すべりの素因になっている。また, スメクタイトを含有する粘土は, 予想外の低角度のすべりを発生させること, 切土後に数年を経て遅れて変状が現れることがあり, 斜面安定の評価上重要である。

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The Median Tectonic Line active fault system is an about 360-km-long right-lateral strike-slip fault system composed of several active segments. Several researchers have presented segmentation models on the basis of fault geometry and faulting history. However, very few studies so far been made to understand the genesis of segment boundary with a consideration of the 3-D fault geometry system of a long strike-slip fault system. We conducted precise gravity survey and seismic reflection survey to assess 3-D fault geometry and segmentation of the Median Tectonic Line active fault system. A prominent low gravity anomaly in the southwestern Matsuyama Plain is recognized. This low gravity anomaly region reflects a half -graben structure of the basement. Thick sediments filling the graben inferred from the seismic reflection survey across the low gravity anomaly region. The Iyo, Shigenobu, Kitakata, and Kawakami faults surround the low gravity anomaly region. The Iyo and Kawakami faults in these faults array depict an extensional right overstep. The Shigenobu and Kitakata faults within the overstep area may have faulted mainly with vertical displacement in the southern part, as reactivation of existing fault that occurred during Late Cretaceous to Paleogene. This fact indicates that these faults are secondary fault accompanied by the formation of pull-apart basin. We, presently, lack of data that connect activity of these active faults with formation process of the low gravity anomaly region, even though above mentioned facts demonstrate that these active faults have resulted faulting and constructed extensional right overstep structure controlled by the basement structure which have formed about Pliocene. On the basis of the above results, the Median Tectonic Line active fault system of the northwestern Shikoku is divided into two segments, the Iyo segment and the Kawakami segment, recognizing pull-apart basin as the segment boundary, since this pull-apart basin region in a long strike-slip fault causes heterogeneity of faulting.

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The Median Tectonic Line (MTL) active fault system is one of the most active intraplate faults in Japan. The fault system, which is more than 300 km long, is a right-lateral strikeslip fault with an average slip rate of 5-10 mm/y in east Shikoku. The 13.5-km-long Mino fault of the MTL active fault system is located at the western part of Tokushima Prefecture in east Shikoku. We carried out trench excavation surveys of the Mino fault at Ueno in Mino Town and Ikenoura in Mima Town. Both sites are situated at fault depressions formed on the middle and lower terrace surfaces by the activity of the Mino fault. Fault depression deposits consist of younger and finer grained layers with abundant 14C dating samples. We inferred the dates of faulting events from upward fault terminations on the trench walls.
At Ueno, it is recognized that the latest rupture event occurred between 1, 295-1, 390 cal A.D. and 1, 660-1, 950 cal A.D. (190 ± 50 years B.P.). Multiple faulting events are also suggested after K-Ah ash fall (about 5, 200 B.C.), because the ash layer steepens to near vertical. At Ikenoura, the latest rupture event is estimated to have occurred after 1, 525-1, 660 cal A.D.
Taking account of the previous reports on the eastern extension at Chichio and Zunden faults, we conclude that the MTL active fault system in east Shikoku ruptured in the 16th century A.D. or later. The liquefactions, which occurred between the latter half of the 16th century A.D. and the beginning of the 17th century A.D., are recognized at archaeological sites at Maruyama and Ogaki in the western part of Tokushima Prefecture. These sites are located immediately south of the Mino fault and the Ikeda fault to the west. This fact suggests that the liquefactions were caused by faulting of the MTL active fault system in east Shikoku.

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In Shikoku, earthquakes in the crust show a vertically steplike distribution across the Median Tectonic Line (MTL). Namely, the upper and lower boundaries of the distribution in the southern side of MTL are shallower than those in the northern side of the line. Since crustal microearthquakes mainly occur in the granitic layer (V_p=6.0km/s), the steplike distribution suggests a vertical dislocation in the granitic layer which was caused by uplifting of the southern part (Sanbagawa Metamorphic Belt) against MTL. On the other hand, most earthquakes in the mantle occur in the southern side of MTL in the central and eastern parts of Shikoku and occur in both the sides of MTL in the western part of Shikoku.

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（活）断層帯の右横ずれ断層運動，変位地形，変位速度など，1970年代の調査成果について，まず紹介した．トレンチ掘削調査による活動履歴や断層構造の解明が1980年頃からはじまり，やがて1回の変位量や最新活動時期などの究明が1990年代後半頃に行われた．こうした調査で得られた成果やその手法改善について述べた．1990年代に行われた詳細活断層図，地方自治体による活断層の総合的調査，さらに反射法地震探査について概略を紹介した．各種の調査から判明してきた断層運動の性質と履歴，四国域での最新活動時期（16世紀）と関連した歴史地震，これらに関与したと思われる活断層群（六甲・淡路島断層帯や有馬－高槻断層帯）の連動的な活動を指摘した．こうした経過で得られてきた諸成果をまとめた地震調査研究推進本部地震調査委員会（2003, 2011）による大地震の長期評価も紹介し，残された課題や展望について述べた．

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