During the 2004 Niigata-Chuetsu Earthquake, the Kanto Plain (Tokyo metropolitan area and environs) also experienced strong, persistent motion. The soft, thick sedimentary strata, beneath the Kanto plain are a cause of the amplification of the ground motion of the earthquake.

The geologic history of the Kanto Plain is extremely old and can be traced back to the opening of the Japan Sea, which occurred roughly 16.5 million years ago (A). At this time, the hard basement rocks were tensioned horizontally, resulting in the formation of normal faults and numerous grabens (B1, B2). Subsided blocks cut by normal faults on both sides are called grabens, while the blocks inclined to one side, following the fault on that side, are termed half-grabens (B2). These grabens were then completely filled with sediments. Subsequently, the entire Kanto region underwent a second subsidence in which sedimentary strata were deposited in a substantially uniform manner. Thus, it is thought that the subsurface sedimentary strata in the Kanto Plain formed a 3-stage units comprising a thick, uniform young sedimentary stratum (c1), an older sedimentary stratum (c2), and a still older sedimentary stratum which filled the grabens (c3) or half-grabens (c4) underlying the younger strata (C). In particular, the geological profile of the half-graben fill stratum displays a distinctive fan shape (C).

Based on this viewpoint, we attempted an interpretation of the seismic record (D1) for the area between Asaka and Konosu in the northwestern part of the Kanto Plain. Although the deep reflection surface is obscure, the deep geology can be interpreted as containing two half-grabens which are inclined to the south. Furthermore, it was found that the basement rock does not have a comparatively gentle profile (model A in D1), as had been thought until now, but rather, has structure consisting of deep concavities and prominent rises (model B in D1).

Ground motion amplification differs if the basement topography is different. In a simulation of ground motion modeling the Chuetsu Earthquake, it was found that, in particular, the 4s period component was greater than that estimated from the conventional model (D2). Because the motion of this 4s period affects the motion of super-high rise (about 200m class) buildings, it is necessary to consider the deep geological structure when constructing buildings of this height in the Asaka-Konosu area. Moreover, as the Tokyo metropolitan area contains many building of the same class, early investigation of the existence of grabens and half-grabens in the deep geology of the Tokyo area would also appear to be necessary.