Abstract
We report the result of a case study of the exploration using Spatial AutoCorrelation Method (SPAC) and Multi-channel Analysis of Surface Wave (MASW) that was conducted in a liquefied site due to the 2011 off the Pacific coast of Tohoku Earthquake (Mw9.0). The site Yoshino General Park is located at the west bank of the Kokai river that is the boundary between Tsukuba and Joso cities, Ibaraki Prefecture, Japan (36.0746N, 140.0000E). The estimated velocity structure has a Vs lower than 100m/s at shallower than about 7m from the ground surface; 100m/s<Vs<200m/s from about 7m to the engineering bedrock that appears at about 27m from the surface. The depth of the bedrock and Vs values for the layers deeper than about 7m are, in general sense, consistent with the Vs structure converted from N-values (SPT) measured boreholes located not exactly at, but near to the site. The Vs estimated by the exploration for the layers shallower than about 7 m are significantly smaller than those converted from N-values. Due to the lack of the exploration before the earthquake and of N-values measured exactly at the site, it is not possible to judge whether these low Vs mean that the ground was loosen due to liquefaction or not. It can be said, at least, that the ground of the site was as loose as Vs<100m/s when the exploration was conducted and then it may be liquefied if attacked by a similar strong ground motion again. On the other hand, we conducted the centerless circular array method (CCA) using the seismometers of which natural frequency is 2Hz and obtained the dispersion curve almost coincident to that of MASW in the frequency range from 5.0Hz to 9.7Hz. Then, it is proved that CCA using the relatively cheap seismometers are competent for exploration of shallow ground.
