No abstract

The 2008 Iwate⊖Miyagi Nairiku Earthquake (M j 7.2, M W 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 M j = 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.Key words： the Iwate⊖Miyagi Nairiku Earthquake, relative height of fluvial terrace surfaces, folded zone of Neogene stratum, flexural-slip fault, concealed fault assessment ：岩手・宮城内陸地震，TT 値，新第三系の褶曲帯，層面すべり断層，伏在断層の評価