Abstract
The South part of Eastern boundary fault System of the Shonai plain is divisible into two principal parts. The north segment called Matsuyama fault trending mainly N-S at the north side of the Mogami river has been studied by previous several works on recent activity, but no precise investigation performed at the south side of the river.
This paper focuses on the rate of slip and Holocene fault activity on the South part of the fault System of Shonai plain by detail drilling survey. Topography of surface deformation is also evident from study of large-scale aerial photograph interpretation.
Twenty-nine arrayed drilling holes were carried out about 400m long across the flexure scarp zone on the alluvial fan terrace. The drilling-hole data were collected as 20m intervals in the most intensive examined place with 5 to 10m in deep.
The upper layer mainly composed of sandy gravel and silt, and clay facies about 4m thick was recognized from the top to downward. Just below this layer, the authors found a respectable peat layer, which have probably deposited at about 5,500 to 6,000yrs. B. P. The deformation of the peat layer indicates the vertical offset of this layer is estimated approximately 3.7 m. According to vertical offset and possible determined age suggest that vertical slip rate of the fault is about 0.62-0.67mm/yr.
