Tephrochronological study on the long-term activity of Tachikawa fault, Tokyo metropolitan area, central Japan

In order to clarify the activity of the Tachikawa fault during the Quaternary, four boring cores collected by boring surveys conducted at Mitsugi in Musashi-Murayama City and Hakonegasaki in Mizuho Town, the western part of the Musashino uplands, Tokyo metropolitan area, were reexamined. Also, a geological survey in the Sayama hills was carried out. The transection along the MTB1 Core on the downward side, Musashi-Murayama Core on the flexure scarp, and MTB2 Core on the upward side crosses the Tachikawa fault in Mitsugi district. Sediments, composed of gravels, sands, silt, and peat, correlative to the Kazusa Group (early Quartenary strata), are overlain by terrace deposits and the Sagami Group. These sediments contain many tephra layers. Several tephras are identified, as follows. MTB1-9 to MTB1-10L (MTB1 Core) and MM-8 to MM-8.2 (Musashi-Murayama Core) are correlative to SGO identified within the Sayama Formation in the west part of the Sayama hills. MTB1-22 and MM-12 are correlative to SYG (1.7 Ma) in the northwest part of the hills. A tephra group composed of three tephras (MM-21.1, -22, and -24 in Musashi-Murayama Core and MTB2-21.5, -22, and -24.5 in MTB2 Core) is found at the westernmost part of the hills, and is defined as the Hakonegasaki Tephra Group here. Two widespread tephras, Ebs-Fukuda (1.75 Ma) and Tmg-R4 (2.0 Ma), are identified in the MTB1 Core (MTB1-26) and MTB2 Core (MTB2-26), respectively. Kd44 (1.968-1.781 Ma) in the Kazusa Group, Boso Peninsula, is correlated with MM-18 in Musashi-Murayama Core and MH-3 in the Mizuho Core. Altitudes of SYG show northeast-side-up offset with an accumulated deformation of ca.126m. No cumulative deformation of the strata between SYG and the Hakonegasaki Tephra Group (ca.2.0 Ma) was detected, indicating that the fault was not active during the period from 2.0 Ma to 1.7 Ma. The timing of initiation of the Tachikawa fault (