抄録
The Kasumi Hill, one of the eleven hills which occur parallel with each other located between the Kanto Plain and the Kanto Mountains, extends east-southeastwards. It is bounded on the north by the Aki River and on the south by the Kawaguchi River. The Yajigawa River flow dawn within the Kasumi Hill along the long axis of it, and separates it into the northern and southern areas. The Yajigawa Fault occurring along the Yajigawa River is located on the extension of the normal fault which bounds the Itsukaichi sedimentary basin of Early to Middle Miocene and extends east-southeastwards. The activity of the Yajigawa Fault was probably in related to the reactivation of the Itsukaichi-Kawakami Tectonic Line which also bounds the Itsukaichi Basin, therefore the Yajigawa Fault was initially formed during Early to Middle Miocene. It is estimated that the Yajigawa Fault revived between the ages of deposition of the Miocene formation and the Plio-Pleistocene Kasumi Gravel Formation. Moreover, as the Plio-Pleistocene sediments are cut by the Yajigawa Fault in this area and is flexed or its east extension, the fault again activated in or after Plio-Pleistocene. Topographic data show the amount of recent displacement of the fault in this area increases eastwards, and the leveling data show that both of the southern and northern areas are inclinating northward, suggesting that the Kasumi Hill is presently active.
