中央構造線活断層系・重信断層の変位地形と活動履歴 Holocene Faulting of the Shigenobu Fault, the Median Tectonic Line Active Fault System, West Shikoku, Japan

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中央構造線活断層系は,四国では活動度が高く,累積的な右横ずれ断層変位地形が連続的に認められるが,松山平野東部においてはこれまで活断層の存在が知られていなかった.このため,その東方の北方断層と西方の伊予断層との間には約10kmの断層線の不連続が存在し,ここがセグメント境界にあたると考えられてきた.筆者らはこの地域に長さ約6kmの明瞭な低断層崖地形が存在することを発見し,これを重信断層と命名した.本稿では活断層の存在を証明する地形学・地質学的な証拠を提示し,これに基づいた第四紀後期の運動様式および最近の活動履歴について考察する.重信断層は,N75~85°Eの走向を持ち,沖積面を南落ちに変位させる.直線的な低断層崖は重信町志津川から松山市高井町まで連続し,旧河道や侵食崖の屈曲から,これに沿って右ずれの変位が認められる.トレンチ掘削調査によって,地形から推定した位置に沖積層を変位させる断層が出現した.トレンチ壁面に出現した主断層は,走向N75~85°E,傾斜85~90°Sである.断層変位を受けた沖積層の年代から,5,550yrBP以降に少なくとも1回活動したと考えられる.

An active fault system extends for about 200 kilometers along the Median Tectonic Line (MTL) in Shikoku, Southwest Japan. Along this fault system, previous studies recognized a 10-km-long gap in the surface trace in the eastern part of the Matsuyama Plain. The gap is the largest so far found in Shi-koku. The authors newly identified very distinct strands of active faults in this gap by means of inter-pretation of high-resolution vertical aerial photographs and field work. The faults are called the Shige-nobu fault and the Shigenobu-kita fault. This paper aims: l) to depict the fault topography of the Shige-nobu and Shigenobu-kita faults; and 2) to examine the rupture history based on trench excavation across the western part of the Shigenobu fault at Takai, Matsuyama City. The results are summa-rized as follows:<br> (1) The trace of Shigenobu fault extends straight from Shitsukawa, Shigenobu Town, to Takai, Matsuyama City. The strike of the fault is N 75 to 85 degrees E. Alluvial fan surfaces are offset by this fault, and a clear fault scarplet is recognized continuously. The apparent vertical offset indicated by the height of the fault scarplet is 0.4 to 2.7 meters.<br> The Shigenobu-kita fault runs about 200 meters to the north of the Shigenobu fault. This fault, lo-cated on the margins of the plain, extends at least for 2 kilometers from Hommura to Ushibuchi, Shige-nobu Town. The M and M2 surfaces are deformed by this fault, but the L2 surface is not displaced.<br> (2) Distinct evidence of right-lateral movement is continually discernible along the Shigenobu fault. Several abandoned streams, located on the L2 surface and the L3 surface, are offset by 1.9 to 4 meters and pressure ridges occur on the northern side of the fault scarplet. The southern side of the Shigenobu fault is downthrown consistently.<br> (3) Active faults appear in a trench wall across the fault scarplet. These faults with shear plane in the trench are called F-1, F-2, and F-3. While F-2 and F-3 show no lateral offset, F-1 shows a compo-nent of lateral displacement. Therefore F-1 appears to be the main fault of the Shigenobu fault.<br> (4) The strike of F-1 is N75 to 85 degrees E, and the general dip is 85 to 90 degrees S. All horizons of the alluvial deposits in the trench are displaced by the main fault and are downthrown to the south.<br> (5) It is apparent that at least one faulting event of the Shigenobu fault occurred in the past 5, 550 years.

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  • 地理学評論. Ser. A  

    地理学評論. Ser. A 72(4), 267-279, 1999-04-01 

    The Association of Japanese Geographers

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各種コード

  • NII論文ID(NAID)
    10002223276
  • NII書誌ID(NCID)
    AN1016484X
  • 本文言語コード
    JPN
  • 資料種別
    SHO
  • ISSN
    00167444
  • NDL 記事登録ID
    4783486
  • NDL 雑誌分類
    ZG1(歴史・地理)
  • NDL 請求記号
    Z8-571
  • データ提供元
    CJP書誌  CJP引用  NDL  J-STAGE 
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