中央構造線活断層系の活動区の分割試案  [in Japanese] Proposal of the segmentation on the Median Tectonic Line active fault system  [in Japanese]

右横ずれが卓越した中央構造線活断層系は全体として直線状に延び, 地形的に明瞭に連続するが, 所々には間隙, ステップ, 屈曲部, 分岐部などの不連続部がある。この分岐のうち, とくに明瞭な横「し」の字状をなす構造については重視し, 上述の特徴に加えて, 地区毎の走向・変位速度・最新活動時期・活動間隔・地震との関係 などのMTL活断層系の諸性質を再検討して, 8セグメントと1つの不活動区域に分けた。活動的なセグメントはさらに2・3に細分される部分もあるが, こうした結果を第1図と第1表に示した。徳島県でのトレンチ調査によれば, MTL活断層系の最新活動は16世紀以降の歴史時代である。それは1596(慶長1)年大地震である可能性が大きく, 少なくともVaの部分が震源断層であったらしいが, その活動範囲はまだ詳しく判明していない。他のセグメントでも, 歴史時代の別の地震を起こした可能性が大きい。

The mapped trace of the Median Tectonic Line active fault system, which has been predominantly dislocated in the right-lateral sense, smoothly extends with many marked fault features. However, the faults are characterized at some localities by discontinuities such as gap, step, bending and branching. Especially branching off or sharp bending structure with large angle are generally considered to be a segment boundary. And such characteristics as strike, average slip-rate, late stage of fault rupture, recurrence interval and relation to the historical large earthquake or present-day seismicity are investigated to identify the segment of the MTL active fault system. The system was tentatively classified to the eight segments and partly subdivided into a few smaller ones. These results are shown on Fig. 1 and Table 1. Trench excavation surveys recently carried out in Tokushima Prefecture have clarified that the latest event on the MTL active fault system was after 16th Century. And it is highly probable for the candidate to be 1596 Keicho earthquake. The earthquake source fault was at least the segment Va on the MTL in the northeastern part of Shikoku. But the detailed distance is not made clear yet. The another segments in the central Honshu and Kyushu have probably moved as earthquake source fault in the historical time of Japan.