鹿野断層の発掘調査とその地形・地質・地震学的考察

Abstract

Three trenches were excavated across the Shikano fault which ruptured at the 1943 Tottoriearthquake (M=7.4). Materials exposed in the trenches included silts, sands, gravels of mostlyalluvial deposits and soils. Faulting was expressed in the trenches as conspicious flexures ofgeologic beds, abrupt termination of geologic units, and open tunnels that possibly formed duringfaulting. The latest faulting before the 1943 event was found in trenches A and B at Horakuji.The faulting before 1943 was inferred to have occurred between 1, 490 and 7, 970 B. P. years.The recurrence intervals for earthquakes like the 1943 Tottori earthquake along the Shikano fault isestimated to be 4, 000-8, 000 years.The average displacement rates for the past recent times were estimated by tectonic profiles alongthe Shikano fault. The offset of streams, ridges and geologic units suggest that the Shikano faulthas slipped 40 m right-laterally for the past 300, 000 (±100, 000) years, indicating an average rightslip rate of 0.1 mm/year. The maximum stream offset along the Shikano fault is 130 m. If weassume the constancy of the slip rate over the past time, the Shikano fault is inferred to haveinitiated its movement about 1 million years ago, and has a relatively low slip-rate compared withthose for active faults in the central and northern Japan. At four localities outcrops of the Shikanofault in bed rocks and overling vocalnic gravel beds of late Pliocene were found and discribed in thetext.Seismicity in and around the Shikano fault has been monitored since 1964 by the Tottori Micro-earthquake Observatory. The recent activity (1976-1980) shows that a zone of earthquakes (M>1.5) extends about 70 km along the Shikano and Yoshioka faults and their extensions to the eastand west. A detailed examination of the seismicity reveals that there are two seismic gaps that maycorrespond to the possible eastern and western ends of the Shikano-Yoshioka fault. Based on theseismicity, the Shikano-Yoshioka fault is 25 km long and 10 km deep shallowing towards the bothends.