Objective seismic motion for liquefaction countermeasures in residential areas of Inashiki City Ibaraki, Japan

Appropriate countermeasures against residential damage induced by liquefaction under great earthquakes in Inashiki City, Ibaraki were investigated because residences sustained severe damage from the 2011 off the Pacific coast of Tohoku Earthquake of March 11, 2011. Metrics to ascertain the magnitude of objective seismic motions and the effectiveness of countermeasures were established, along with their targeted levels for securing safety. This paper presents outlines of damage features in Inashiki under actually measured seismic motion (259 Gal acceleration, 5-weak seismic intensity, M9) of the 2011 off the Pacific coast of Tohoku Earthquake. Simultaneously, the paper reports results of numerical simulations of liquefaction that damaged residences conducted to validate the model. To assess earthquake-related liquefaction, the repeatability of damage features observed from the great earthquake was improved as explained below. i) Age effects corresponding to the sedimentary time period of sandy deposits are considered when evaluating liquefaction factor FL. ii) Sandy deposits with plasticity index Ip of less than 25 are assumed for liquefiable layers. iii) Ground surface displacement Dcy corrected according to the magnitude of liquefaction factor PL. Subsequently, results from numerical analysis of liquefaction severity for cases with and without countermeasures were compared under the actually measured seismic motion. Then numerical analyses were conducted for cases under the seismic motion (200 Gal and M9.0), which are guidelines by the Ministry of Land, Infrastructure and Transport in Japan. Based on results from the numerical analysis stated above, setting of a fundamental policy was proposed for determination of countermeasures against liquefaction under a great earthquake. Countermeasures should be sought that enable attainment of rank A (H1 > 5 m , H1 is thickness of a non- liquefaction layer which is continuous from the ground surface) and rank B1 (H1 > 3 m and corrected Dcy≦10 cm ) under seismic motion of type 2 (medium shock by great quake: 200 Gal, M9.0) in all regions, except for areas ranked locally as B2 (H1 > 3 m and corrected Dcy > 10 cm).