Seismic Event in Edo/Tokyo Downtown

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The oriented 9m deep sections of sediments are recovered from the river bed of the Kyu-Edo river in downtown Tokyo, Japan by using a recently invented 'long geo-slicer", a sampler of columnar soft sediment layrs. Normal-fault rupture with 20cm vertical offset is discovered 30cm below the present river bed. This seimic event is dated as old as 1,800 y.B.P.or youger. An older event is inferred from 1.3m difference in the depth of layr interface at a depth of 7-8m between the two sample sections only a meter apart. This event is dated as old as 2,900-3,700 y.B.P.The normal-fault rupture is oriented in N67E and if this rupture is an echelon fault produced by the underlying active fault suggested by Sugiyama and Endo in 1993 running N100E,the motion of the fault should be left-lateral strike slip. This inference is consistent with the tectonics in this region. This active fault was thought to be relatively inactive and classified as lower C-class fault. The evidence of two seismis event in the past 4,000 years found in our study may seem to be inconsistent with the low activity suggested from relatively small vertical offset of late Quaternary layrs, but our inference that the fault is of strike-slip type would explain the small offset. The significance of the seismic events found in our study would be better assessed when the effect of 2km deep late-Tertiary-Quaternary sediments in Tokyo on surface seismic rupture.