Liquefaction and its Characteristics during the 2011 Tohoku Earthquake

Abstract

The Tohoku Earthquake of March 11, 2011 was associated with widespread liquefaction-induced damage in the eastern part of Japan. Our continuing surveys nearly five years from soon after the earthquake revealed that the earthquake induced liquefaction at a significant number of locations in a 650-km long zone extending in the eastern part of Japan. This paper summarized the characteristics of liquefaction during the earthquake. Approximately 8600 grid cells identified to contain liquefied sites; the size of each grid cell was approximately 250 m × 250 m. The intensities of the ground motion of the liquefied grid cells were examined, and approximately 95 % of liquefied grid cells were found to be distributed in the areas where the I_JMA exceeded 5 upper during the main shock. Approximately 98% and 99% of liquefied grid cells were distributed in the areas where PGAs exceed 140cm/s² or where PGVs exceed 15cm/s, respectively. The liquefaction occurrence ratios (number of liquefaction grid cells divided by total number of grid cells) of different geomorphological unit were calculated where I_JMA exceed 5 upper. Those for artificial fills, sand dune, former river channel are large in this order. There was much difference between Tohoku and Kanto regions both in number of liquefaction and liquefaction occurrence ratios. To reveal the cause of the differences between two regions, the factor of safety (F_L) against liquefaction was evaluated at 231 sites where liquefaction occurred during the 2011 earthquake. Furthermore, likelihood of liquefaction-induced damage was evaluated based on the combination of liquefaction potential index P_L and thickness of surface un-liquefiable layer, H₁ overlying liquefiable layers, which were determined from the result of the F_L analysis. Comparing the result of these analyses of two regions, it was found that the ground of Kanto region is much more likely to liquefy than Tohoku region.