Seismic Event in Edo/Tokyo Downtown

利用最近发明的“长地质切片机”，从日本东京市中心的九江道河床上采集到定向9米深的沉积物，这是一种柱状软沉淀层的取样器。在现今河床以下30 cm处发现了垂直错距为20 cm的正断层破裂。这一地震事件可以追溯到1800年前或更早。从两个相距仅一米的样品剖面之间7-8m深度的地层界面深度相差1.3m推断出一个较老的事件。该事件的年龄为2900-3700yB.P.，正断层断裂方向为N67E，如果该断裂是Sugiyama和Endo在1993年提出的走滑N100E的下伏活动断裂产生的梯形断裂，则该断裂的运动应为左旋走滑。这一推论与本区的构造相吻合。该活动断裂被认为是相对不活动的，属于低C级断裂。本研究中发现的近4000年来两次地震事件的证据，似乎与晚第四纪地层相对较小的垂直错距所显示的低活动性不一致，但我们的断裂带属于走滑型的推断可以解释这种较小的错动。当东京2公里深的晚第三纪-第四纪沉积物对地表地震破裂的影响时，我们的研究中发现的地震事件的意义将得到更好的评估。

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.