抄録
The Negoro fault, a segment of the Median Tectonic Line(MTL) active fault system in the western part of Kii peninsula, marks a southern range front of the Izumi Range, where its late Quaternary faulting is well defined on the topography and geology. We have carried out several drillings and two trench excavations across this fault at Imataki(Nioh-dani valley), where the terrace riser of low terrace 2 surface formed at ca 20,000 years ago is right-laterally dislocated about 50m with vertival displacement (ca 6.5-7m) of northside uplifting component.
By these surveys clear fault contacts between the basement and the Quaternary deposits were observed on the trench walls and characteristics of the Negoro fault have been clarified as follows:
1)The fault shattered zone exposed on both sides of trench wall is composed of the Cretaceous Izumi sedimentary rocks, ryolitic rocks intruded to the fault zone in the Miocene and Sambagawa metamorphic rocks, arranging from the north to the south. This zone dipping to the north is thrust over Holocene sediments with angles of 25°∼60°. Therefore, this fault zone corresponds to the geologically defined MTL in the narrow sense. 2)The latest event of this fault had occurred during deposition of the G1 formation. This event is estimated to be after about 3,400yBP and before 1,750yBP on carbon-14age(calibrated calender years are BC173y and AD265y, respectively). 3)The vertical displacement at this event was about 1m, uplifting to the north-side. The faulting had associated the predominantly right-lateral displacement, comsidered from the lateral offset of the valley fill deposits across the fault and striations dipping with angle of 30°on the fault plane.4)The recurrence interval of this fault is estimated to be 2,000∼3,000 year. The right lateral displacement at the latest event was about 4∼10m, considered from the average slip rate and amount of vertical displacement at this event.5)Almost same type of late Quaternary faulting has been clarified by recent researches in the Tokushima plains. However, fault behaviors such as the time of latest event, cumulative displacement, amount of displacement at one event and average slip rate are fairly different.
