根尾村水鳥および中付近における根尾谷断層の第四紀後期の活動性

Abstract

The Neodani fault to the north of Nagoya, central Japan, is a principal strand of the Nobi fault system. The fault moved primarily in a left-lateral sense during the great Nobi earthquake of 1891 (Magnitude 8.0). The maximum vertical and horizontal offsets are 6 m at Midori and 7-8 m at Naka, respectively. After reviewing previous works, we discuss new geomorphological and geological data bearing on the 1891 and earlier paleoearthquakes in the latest Quaternary: specifically we find that1) The 1891 vertical offset was about 5 m at the western slope of Terayama which is located on the northwestern extension of the well-known Midori fault scarp.2) The subsurface data show that the vertical offset of the base of the alluvial deposits beneath the Midori scarp is about same as that of the ground surface at the scarp, implying that the 1891 event was the first faulting event since the beginning of the deposition of the allu-vium. The radiocarbon date suggests that the alluvium is not older than 1, 000 years in age.3) The lower river terrace, ca. 14, 000 years old, is displaced as 14 m vertically at Terayama in Midori where about 5 m vertical displacement occurred in 1891, and a stream incising the lower terrace on the fault line at Naka offsets ca. 28 m laterally where 7-8 m lateral displacement occurred in 1891. If the fault is characterized by the repeated occurrence of earthquakes of the same size, both the 14 m vertical offset at Terayama and 28 m lateral offset at Naka may be interpreted to be the result of 3 to 4 events occurring at average intervals of about 3, 000-4, 000 years since the formation of the lower terrace. The observation also indicates that the Neodani fault, at least in the central segment including Midori and Naka, has averaged 2m per one thousand years in the left-lateral slip rate in that period.