地学雑誌
Online ISSN : 1884-0884
Print ISSN : 0022-135X
ISSN-L : 0022-135X
論説
2008年岩手・宮城内陸地震の震源域における活構造評価
柳田 誠青柳 恭平下釜 耕太岡崎 和彦佐々木 俊法
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2020 年 129 巻 1 号 p. 89-122

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 The 2008 Iwate–Miyagi Nairiku Earthquake (Mj 7.2, MW 6.9) occurred in an area where no active fault had been indicated by maps and previous studies. A movement of the concealed source fault of the earthquake resulted in linear but fragmental surface ruptures over a short range. However, geodetical observations revealed a wide uplift at the mountainous side of the surface ruptures. A geomorphological and geological approach is discussed for estimating the active fault length in the area where the displacement accompanied by the active fault movement hardly appears on the ground surface. In order to estimate the length of the concealed fault, a series of geomorphological and geological observations are carried out focusing on the following points: 1) Wide uplift zone shown by relative heights of fluvial terrace surfaces. 2) Folded zone in Neogene strata with the eastern side steeply inclined. Both are presumed to be the results of surface deformation caused by the concealed fault. Additionally, the folded zone is sometimes accompanied by a flexural-slip fault that causes a displacement in the late Quaternary terrace gravel layer. As a result of integrated surveys on these points, it is suggested that the concealed fault is 30 kilometers long from Shitomae-gawa river to Sanhasama-gawa river with N–S strike and west dip. The fault length is equivalent to Mj = 7.3 from the empirical relationship between earthquake magnitude and fault length (Matsuda's formula), which is comparable to the magnitude of the 2008 Iwate–Miyagi Nairiku Earthquake.

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