Resurvey of Possible Seismic Fissures in the Old-Edo River in Tokyo

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1 Bull. Earthq. Res. Inst. Univ. Tokyo Vol. 2.,**3 pp.,,3,.* * +, -. +, -. Resurvey of Possible Seismic Fissures in the Old-Edo River in Tokyo Kunihiko Shimazaki *, Tsuyoshi Haraguchi, Takeo Ishibe +, -. +, -. Association for Earthquake Disaster Prevention Graduate School of Science, Osaka City University Earthquake Research Institute, University of Tokyo Faculty of Environmental Studies, Hiroshima Institute of Technology and Takashi Nakata Abstract Geo-slicer sampling of riverbed strata, boring sampling of riverside strata, single-channel seismic reflection survey, and electric survey were carried out to understand the causes of ground fissures and ruptures with a vertical dislocation of,/ cm found by Shimazaki and others beneath the Old-Edo River in Tokyo in We concluded that fissures and ruptures were caused by gravity slumping due to liquefaction taking place near a paleo-valley, which was found by the reflection survey to run across the Old-Edo River. In the +331 survey, at least two liquefaction and slumping events, probably corresponding to two large earthquakes, were identified from a Geoslicer sample down to about 2 m in depth. Their ages were reported as after +2** ybp and,3** -1** ybp, respectively. Dating results for new samples obtained in this study are consistent with previous estimates. Key words: Geo-Slicer, Liquefaction, slumping traces, the Old-Edo River + +***,** * +332,** ; * nikosh@eri.u-tokyo.ac.jp +*22.+. /,0,* 229

2 B,**2,**3 GS- -, GS- -,,./ GS--, - +33, C ; +331 ; ; +331; D m, +2**,3** -1** E GS--, 0, GS--, CD DE +332, - m,* -* cm +*, * m A-B +* m +** m - 3* m * m E+ / 1 +33/ +** +332 m * m A 2 2 BE +33/

3 Fig. +. Cross section of the Geo-slicer core in the survey of +332 (from Shimazaki et al., +332). 231

4 Fig.,. Plan of survey points and datum. The blue and green dashed line indicate strike of fissure or fault by the Geo-slicer survey in +332, and fault strike from Sugiyama and Endo ( +330), respectively. Fig. -. Locations of Geo-slicer survey at riverbed of the Old-Edo River and boring survey at the Edogawa primary school. 232

5 Fig... Photograph of Geo-slicer samples. 233

6 Fig. /. Estimated cross section at the riverbed of the Old-Edo River based on the Geo-slicer survey. ***. *2,. m *2,.,/3. m,/3. /2.. m /2.. 1,2. m 1,2. +***. m +332 A GS--, / +* +* BC m /2.. m C 3 + D 1,2. m 234

7 Fig. 0. Cross section due to the single-channel seismic profiling survey. A ***. /2.. m **,3** -1** B C C BC /2.. m,,.* -* ybp. /22. m,*-* -* ybp.,*-* -* ybp +332 A Fig. 1. Lines of multi-channel seismic profiling survey (A-B and C-D) and location of graben structure. E+ -E- and N+ -N- indicate the location of faults (from Sugiyama and Endo, +33-)..,/ cm 235

8 Fig. 2. Result of electric survey. 0* m +** m3* m +* m +* m m +2** ybp.,3** -1** ybp. +3,* +3,3 ; +31. ;,**.,**+ 236

9 Fig. 3. Photograph and sketch of boring core samples at the Edogawa primary school. 237

10 Table +. Characterics of boring core samples at the Edogawa primary school. 238

11 Fig. +*. Comparison between boring core at the Edogawa primary school and the Geo-slicer sample at the riverbed of the Old-Edo River. 239

12 ,**. +, No.,, C 0*.,0 pp B, // pp ,+1,,* / * /*-..0,,., ,/3.1 //. +331,**+ //.,+1,, No.,, P ++,. +3,3,** ,**2 -/ -0. /* +,+.,**3,**3,* pp.,,** ,. +* ,,1, ,**-.+0,**+ 1***, 0*/ pp / +2, ,-+,.0. Geo-slicer +*0 /303. (Received November +0,,**3) +33, (Accepted February,-,,*+*),/+ pp. 240