Abstract
The Yoro fault is an east vergent thrust that extends north-south for 30 km along the western margin of the Nobi Plain, central Japan. We conducted drilling investigation across the late Holocene fault-related landform at Shizu-Shobuhara site (Togo,2000) to define how this fault-related landform has developed during late Holocene time. Near-surface stratigraphic relations based on drillings of 21 cores up to 5 m long by Geoslicer and 15 cores up to 7 m long by percussion core sampler, age controls of strata based on radiocarbon dating, and structural analyses on stratal geometry indicate at least three episodes of active folding along the Yoro fault. Among them periods of two recent earthquakes postdate the deposition of A2-c (peat) layer (ca.2400-1200 yBP), and the most recent event occurred in the historical time.
Reliable restoration of deformed strata was constructed by using readily identifiable beds with highly constraints on those ages. Syntectonic strata, uninterruptedly deposited to bury the preexisting topography associated with the former deformational event(s), were deformed during the penultimate earthquake. Unconformity above the peat suggests truncation of the upper half of older folded deposits and breaching of the fold due to the following erosional event. Most recent earthquake produced topographic relief on the top of the subsequent infilling above unconformity and completed the approximate shape of the present-day fault-related landform at this site. These descriptions demonstrate that evolution of the late Holocene fault-related landform at this site can be explained as convolution of discrete events of deformation associated with active faulting and of erosion and sedimentation during interseismic periods.
