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In the western part of Tokyo, an active thrust called Tachikawa fault with NW strike and northeast side upheaval runs though the Musashino Upland, southwestern part of Kanto depression basin. This fault has cumulatively displaced fluvial terraces since the late Pleistocene, and the distribution of the vertical displacement and the slip rate along the fault has been obtained accurately. The total fault length is about 20km, the terrace surface that formed at 21ka has received 5.8m of displacement, and the long-term slip-rate is calculated at 0.28mm/year.
The Plio-Pleistocene Kazusa Group filling the Kanto Basin underlies beneath the Musashino Upland. Many drilling explorations and seismic reflection surveys of the Plio-Pleistocene sediments revealed that both of key horizons of about 2.4Ma and 3Ma have been vertically displaced about 100m by the Tachikawa fault. Because the accumulative displacement is not recognized between these two horizons, the major activity of the Tachikawa fault is estimated to start after 2.4Ma. Because the fault displacement of 100m is achieved from the fault slip rate at the late Pleistocene by 0.36 Ma if the speed is constant, or by 0.7 Ma if the speed increases gradually, the commencement of reactivation of the Tachikawa fault is presumed to be the early Middle Pleistocene. Seismic reflection data also reveal about 1km of vertical displacement of the Tertiary base with southwestern side upheaval along the trace of the Tachikawa fault. This fact indicates that the Tachikawa fault is the reactivated old geological structure by the inversion tectonics in the Quaternary.
In the northern part of the Yokohama city to the 15km south of the Tachikawa fault, a bed rock fault having the ENE strike and the length of 10 km or more displaces the Tertiary base by about 1km with south side upheaval. According to the seismic reflection, the flexural structure recognized in the Plio-Pleistocene sediment covering the basement fault shows 350m of vertical displacement in maximum. This fact shows that the reactivation of the fault occurred after the late Pliocene.
However, the time of the deformation is limited in the sedimentation period of Kazusa group, and the fault movement is not recognized since after the 1.4Ma key bed horizon.
By the way, it is known that the Izu Peninsula on the PHS has collided with EUR at about 1-0.7Ma. This collision might have caused the significant change in the stress field of the South Kanto region. The difference of the timing of reactivation between the two faults with similar geological condition except their strikes is interpreted to be the change of the reactivation place of old geological structure associated with the gradual change of the stress field according to the PHS plate collision with the EUR.
Information on the migration of such fault movements has a large significance for the assessment of a long-term stability of the faulting related to the HLW disposal etc.
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