立川断層とその第四紀後期の運動

Abstract

The Musashino Upland, a dissected fan of the Tama River in the western suburbs of Tokyo, is displaced by the Tachikawa fault, which runs in a NW-SE direction with long-continued flexure scarps.Using the tephrochronological method and data of water well logs, the author investigated geomorphic features and movement history of the fault in detail. It is thus revealed that the fault has displaced the terrace surfaces of various ages, including the Tc3 surface of 14, 000 years B.P. and the Holocene old stream channels (Fig. 1, Fig. 10). The Plio-Pleistocene sediments, buried under the Musashino Upland, have been vertically displaced about 70m at Hakonegasaki (Fig. 5, Fig. 6). This fault has only vertical displacement with no strike-slip component, and the upthrown side of the fault is northest. The long-term average slip-rate, derived from the displacement of terrace surfaces, is the largest in the central part of the fault and gradually decreases toward the both ends. The maximum average slip-rate is 36cm/1000 years.The total length of the fault is 21km. If examined in detail, it may be divided into two segments from discontinuity of the geomorphic expression and difference in the long-term slip-rate (Fig. 11). No sign of fault movement has been found from precise levelling across this fault for 40 years. So it is thought that fault movements had been recurred on this fault with large earthquakes in the late Quaternary period. The fault displacement in an earthquake, the earthquake magnitude, and its recurrence interval are estimated at about 1.8m, M. 7.1, and 5, 000 years, respectively.There are two types of surface deformations along the Tachikawa fault. One is a flexure scarp of 100-300 meters in width (Fig. 8). This flexure scarp is thought to have been formed by the existence of thick unconsolidated fluvial gravel deposits overlying the faulted Plio-Pleistocene sediments. The other is a broad undulation of a few kilometers in a half-wave length across the fault trace (Fig. 12). This undulation seems to reflect the elastic dislocation of the crust due to the movements of the Tachikawa fault.