Blast testing is able to cause liquefaction phenomenon in situ artificially. This testing is effective to examine the method of countermeasure of liquefaction and evaluate the influence of geological and artificial structure to the liquefaction. Repeated electrical survey is thought to be an effective method to estimate the situation of soil that affected by the liquefaction using resistivity changes. A blast testing and repeated electrical survey was carried out in an experimental site. The resistivity change before and after blast testing was clear, and the obvious structural change was confirmed between natural and improved ground. This change may concern with the situation of discharge of pore-water. These result shows this technique will be an effective evaluation method of countermeasure technique against liquefaction. Recently, the high speed electrical survey-meter has developed, so the 2D or 3D resistivity monitoring will be applied in blast testing in near future. This technique will contribute to understanding of liquefaction phenomenon and development of the countermeasure technique against liquefaction.

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 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 (M_w9.0). 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 V_s lower than 100m/s at shallower than about 7m from the ground surface; 100m/s<V_s<200m/s from about 7m to the engineering bedrock that appears at about 27m from the surface. The depth of the bedrock and V_s values for the layers deeper than about 7m are, in general sense, consistent with the V_s structure converted from N-values (SPT) measured boreholes located not exactly at, but near to the site. The V_s 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 V_s 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 V_s<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.

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 Soil liquefaction induces severe damage to infrastructure, buildings and residential houses. It is important to detect liquefaction area for planning of recovery scheme or future liquefaction prediction. However, wide range of the occurrence of soil liquefaction or liquefaction at restricted area sometimes prevents us to map all of a damaged area due to earthquake. Recently, as new analyzed techniques to detect damaged areas, the use of synthetic aperture radar (SAR) images has been proposed. Interferometric correlation analysis is one of the most effective methods especially to detect damaged area due to earthquake. This technique makes use of the difference of correlations calculated from SAR datasets acquired at co-seismic and pre-seismic period. Although the effectiveness of this technique has been shown previously, there is a disadvantage of its application to areas where scattering characteristics is rather variable (e.g., sub-urban and rural area). To discriminate liquefaction area by using correlation change, we have proposed an improved correlation analysis to use multi-temporal SAR images acquired before an earthquake and successfully detected liquefaction area at Kanto district due to the 2011 Tohoku earthquake. In this paper, we show the application results of newly-developed technique to the 2011 Tohoku earthquake as well as the 2011 Christchurch earthquake.

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 Mihama-ku of Chiba city, which consists entirely of reclaimed ground, suffered from extensive liquefaction damage due to March 11, 2011 Tohoku earthquake. The damage map, however, showed non-uniform distribution by a great deal. This paper examines the cause of this non-uniformity by conducting earthquake response analyses based on a newly constructed soil model for entire Mihama-ku, microtremor measurements and investigation of the land reclamation process. It was found from the study, that the non-uniform liquefaction damage distribution was resulted from non-uniform ground conditions which were created through the land reclamation process of dredging.

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 Combined geological and geophysical investigations were conducted at Makuhari-Kaihin Park, Chiba Prefecture, where outstanding sand boiling from fissures was observed just after the main shock of the 2011 off the Pacific coast of Tohoku Earthquake. The site is lawn covered reclaimed land using dredged fine materials, developed just after the 1987 East off Chiba Prefecture Earthquake. The surface of the park is composed of embanked clayey soils and underlain by reclaimed fills up to 5 m in thickness. Natural marine sediments mainly composed of medium to coarse sand underlie the artificial layers. Because the boiling sand was characterized by fine-grained sand including large amount of shell fragments, it was presumed to be originated from the reclaimed fill layers. However limited information was available on the near-surface geology around the park and mechanism of the liquefaction was still uncertain. We then conducted high-resolution near-surface geophysical surveying at the park to clarify the near-surface structure and its geophysical properties. We adopted high-resolution S-wave reflection and high-resolution surface wave survey using the Land Streamer. Additional CPT, SCPT, and HPT were conducted at 7 points in the park. Suspension PS logging was carried out in a 30m deep hole. We also applied detailed sedimentological analysis to the cores sampled from 4 holes in the park to identify the liquefied layers and successfully discriminated the liquefied horizon. As a result, a low S-wave velocity layer was identified in the dredged layer about 3 to 5 m in depths, where fine grained silts were predominant but the deposition structure was strongly disturbed, or fluidized. The horizon was also characterized as high attenuation in S-waves. Liquefied layers were featured by negative pore water pressures and significantly low water injection pressures in CPT and HPT profiles respectively, indicating the existence of high permeability zone. The survey results showed that it is capable to delineate liquefied layers as geophysical anomaly zones.

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[in Japanese]

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