抄録
Fault activity in the Yoro–Kuwana–Yokkaichi fault zone is considered to have been the source of the 1586 Tensho Earthquake. The Geological Survey of Japan conducted array boring and trenching surveys at Hazawa and Niwada within the ~30-km-long Yoro fault system, which is the main component of this major fault zone. This paper reports the main results of these surveys, focusing on the most recent faulting events and with new paleoseismological data.
At Hazawa, the boundary between prodelta and delta-front sediments formed within the Kiso River system at around 4 ka has been displaced vertically across the fault by about 15 m, and the top of the deltaic sediments formed at around 1.7 ka by ~10 m, indicating repetitive fault activity since 4 ka. The floodplain on the up-thrown side of the fault has been terraced since about the 8th century; this probably reflects the penultimate faulting event. The last event occurred after the 8th century.
At Niwada, four offlapping sedimentary units separated by angular unconformities are capped by a buried soil or peat layer containing abundant plant fossils and charcoal fragments that have provided C-14 data to constrain the timing of faulting events. At least three episodes of fault activity have produced cumulative tilting during the period of stable sea level over the last 4 ka. The last two tilting events occurred after the 15th century and around the 8th century.
The most recent faulting events on the Kuwana and Yokkaichi faults occurred after the 13th century. Furthermore, the two most recent rises of relative sea-level probably reflect coseismic subsidence on the Nobi plain about 500 and 1200 years ago. It is highly likely that the Yoro–Kuwana–Yokkaichi fault zone produced both the 1586 Tensho and 745 Tenpyo earthquakes, although geological data alone is insufficient to determine the exact age of these events.
