奈良県金剛断層系の構造と最新活動時期

Abstract

New geophysical and geologic findings show, for the first time, subsurface branching and Holocene activity in the Kongo fault system. The system extends 18km along the southwest side of the valley that contains the ancient capital city of Nara. It continues southward to the Median Tectonic Line, towards which it swings westward. In the north it contains three faults of parallel strike, called Yamaguchi, Kongo, and Yamada. Seismic reflection surveys show that the Yamada and Kongo faults merge about 300m below the surface. They displace a prominent reflector, interpreted as the contact between granite and Pleistocene deposits, by 240-350m. The displacement of Pleistocene beds and terraces implies vertical slip rates in the range of 0.1-0.6mm/year. The last major surface rupture produced faults seen in outcrop and in a trench. It probably occurred in the first few centuries AD, as judged from a dozen radiocarbon ages. The most closely limiting of these ages are 1900±60 14C yBP (the faulted strata) and 1780±50 14C yBP (overlying, unfaulted strata), or between 40 BC and AD 380. A minimum vertical displacement of 1.2m, measured in the trench, implies an earthquake magnitude close to 7, comparable to the 1995 Kobe earthquake. The average recurrence interval is probably in the range of 2000-12, 000 years. This range is broad because the slip rate is poorly known, and results probability estimates for the next earthquake, on the basis of the characteristic earthquake model, highly variable.