Paleoseismological study along the Magome-toge fault, central Japan

Kiso-sanmyaku-seien fault zone, extending for 60 km along the Kiso River, consists of three echelon faults which trend in the N-S to NE-SW direction. We trenched the southernmost of these faults, the Magome-toge fault, at Kudaritani and Fukutochizawa. Though only 16 km apart along the same fault, these two sites differ greatly in their earthquake histories. In trench walls of Kudaritani, we recognized three faulting events: one in the Holocene and two in the Late Pleistocene. Another older event is also inferred from a prismatic gravelly clay bed deposited in front of the fault plain. Our precise sequential 14C dating of the humic soils revealed that the latest faulting event occurred 5,000 - 3,800 cal yr BP. The penultimate event occurred after the fall of AT tephra (25,000 - 27,000 cal yr BP) and before 11,000 cal yr BP, and the third recent event predated the fall of the AT tephra. Faulting intervals are therefore estimated to be around 10,000 to 23,000 years. At Fukutochizawa, geological evidence of two faulting events in the late Holocene was detected at an outcrop and four hand-dug pits. We conclude that the latest event occurred in or after 13th century because the youngest radiocarbon age of humus displaced by the latest faulting is 720-650 cal yr BP (AD 1230-1300). An earlier event was recognized by a colluvial wedge intercalated in the gravel beds. Since the layers above and below the colluvial wedge were dated to 1860-1690 cal yr BP and 2290-2270 / 2160-1990 cal yr BP, respectively, we estimate that the penultimate event occurred 1690-2290 cal yr BP. The interval between the two dated events is thus about 1000-2000 years. The northern part of the Magome-toge fault appears more active than the southern part, and a segment boundary probably exists somewhere between the two sites.