吉野川北岸における中央構造線活断層系の再検討

Active Faulting of the Median Tectonic Line along the Yoshino-gawa Valley, Shikoku, Southwest Japan

後藤 秀昭

第四紀研究

徳島県吉野川北岸地域の中央構造線において, 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. 吉野川北岸での中央構造線は, 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.

An active fault system extends for about 200 kilometers along the Median Tectonic Line (MTL), in Shikoku, southwest Japan. The author investigated fault topography along the Yoshino-gawa valley by means of interpretation of high-resolution vertical aerial photographs and fieldwork. The purposes of this paper are as follows : to examine precise surface fault geometry for the accurate recognition of gaps and continuity; to discuss structural characteristics of each fault ; and to discuss occurrence compressional and tensional structures in relation with the sinuosity of the master right-lateral strike-slip fault.

The author newly identified an active fault in the western part of Naruto fault and in the central part of Tsunden fault. A topographic depression that links Tsunden fault and Chichio fault was also recognized. Thus the active fault trace extends continuously from Naruto fault to Chichio fault for about 50 kilometers.

Reverse faults—more precisely, frontal reverse faults—are distributed in several localities on the south side of the strike-slip faults. These faults can be divided into two groups. One group of frontal faults consists of faults that run parallel to the strike-slip fault. In this case the strike of these frontal faults is N 60-65° E, while the general strike of the strike-slip fault without frontal reverse fault is N 75-85° E, so the right-lateral strike-slip faults accompanying a frontal reverse fault is oblique by about 10-20°counterclockwise to the strike of the strike-slip fault without a frontal reverse fault. The average slip rate of the frontal faults is 0.1-0.8 mm/yr., which is similar to the vertical component of the slip rate on the strike-slip faults without a frontal reverse fault. The vertical displacement of a right-lateral strike-slip fault accompanying a frontal reverse fault is smaller than that of a fault without a frontal reverse fault. It is suggested that the frontal reverse fault dips gently to the north. The other group has faults which run on the south side of the strike-slip fault, bending sharply. These frontal reverse faults are straight off the corner, and are represented by a south-facing flexural scarp and an antithetic fault which has a range-facing scarplet. The strike of these faults is oblique by about 15-20°counterclockwise to the strike of the strike-slip fault.

The higher the angle between the strike-slip fault and maximum horizontal principal stress, the larger the dip slip component is. Thus, frontal reverse faults catch up with the vertical slip component—or, in other words, shorten the component of the strike-slip faults.

中央構造線

活断層

変位地形

横ずれ断層

低角な逆断層

吉野川北岸

地理・地誌・紀行 [ 290 ]

日本語

日本第四紀学会

[ISSN] 0418-2642

[NCID] AN0034136X