北陸地域南部の活断層 (2)

Abstract

We investigated the Sabae fault which runs from north to south in the central part of the Takefu·Sabae basin, Fukui Prefecture.The Sabae fault is an active fault because there were two large and disastrous earthquakes in 1639 and 1900 which have M over than 6.0. After the result observed by the Hokuriku Microearthquake Observatory, the seismic activity in recent years is very low in the area of the Sabae fault.The fault scarp can be observed along the east side of the Sabae plateau. It is inferred from the strike that the fault strike is NS or N10°W. Along the fault scarp, springs well or dry up. The alignment of these springs coincides with the fault trace. The length of the fault is estimated about 20km from the alignment of the springs and lineament of ground feature.For the confirmation in existence of the fault, γ-ray surveys were carried out along the several routes across the fault. The multichannel analyser with Na (TI) scintillation counter (called MCA) was used in the survey. Increase of γ radiation due to 214Bi is clearly attributed to the radioactive decay of gaseous 222Rn emanating through the fault zone.In the southern part of the Hokuriku district, large and disastrous earthquakes have been occurred in these 300 years such as the Nobi earthquake in 1891 and the Fukui earthquake in 1948. Therefore, the levelling surveys by the Geographycal Survey Institute were carried out several times along the route across the babae fault during a period from 1890 to 1948. The results of the surveys revealed that this basin is composed of three blocks. The block of the east side of the fault took place larger movement than the western block including the Sabae plateau. The different pattern between two blocks is due to existence of the Sabae fault.