Landslides during the earthquakes on May 26 and July 26, 2003 in Miyagi, Japan

We conducted a site investigation for two major landslides: Dateshita landslide, Tsukidate-cho during the earthquake on May 26, 2003; and Nishisaruta landslide, Kanan-cho during the earthquake on July 26, 2003. In addition, we examined physical and mechanical soil properties and performed preliminary numerical simulations. The subsurface soil of the gentle slope of Dateshita landslide with an angle of about 7 degrees was a fill with pyroclastic sediments. The structure of the fill was very loose, but the unsaturated soil remained stable with high suction. The landslide occurred during or immediately after the principle motion of the earthquake. The slide mass behaved as a mudflow. Then the collapsed soil easily fluidized with cyclic shear. It is likely that the saturated fill liquefied during the earthquake. Moreover, it is possible that the unsaturated fill fluidized, losing the initial shear strength during cyclic shear induced by the earthquake. The Nishisaruta landslide with similar magnitude and configuration to the Dateshita landslide occurred a few minutes after the principle motion of the main shock. Rainfall was an important feature that exacerbated the Nishisaruta landslide, whereas no rainfall had been observed for a week before the Dateshita landslide. The subsurface soil of slope of the Nishisaruta landslide was a fill with fine-graded sand, which originated from sandstone on the hill. The upper portion of the slope that lost shear strength because of liquefaction descended along the slope, and spread with high water content on the lower rice field. Numerical simulations have suggested that the saturated fill liquefied during the main shock. Residual excess pore pressure induced by the foreshock affected the slope's stability.