2004 Volume 2004 Issue 24 Pages 129-138
Buried wood fragments -2.8m below the surface were measured as 4,500-4,600yBP in the alluvial plain at the southwestern part of Yanagase fault, which extends about 28km with NNW-SSE direction at the NE side corner of Lake Biwa, central Japan. As the former river course from the Lake Yogo lowland to the Nakanogo windgap is adapted as the distinguished fault reference across the Yanagase fault, culumative vertical and leftlateral amounts of displacement during late Quaternary are roughly estimated to be about 300m (west side down) and 0.4- 0.5km, respectively. The approximate age of basal gravels below the Lake Yogo lowland and gravels at the Nakanogo windgap is inferred to be around 0.4 million of years, presuming that alluvial sediments in the lowland have constantly been deposited to the basement. Using this age and amounts of displacement, average vertical and left-lateral slip rates along the Yanagase fault are obtained to be in the order of 0.75-1m and 1.0-1.5m, respectively per thousand of years. The westside down vertical displacement along the Yanagase fault sharply becomes larger to the southward, approaching to the Lake Biwa. This westside subsidence along this fault might has been triggered by the appearance and migration to the present situation of Lake Biwa since the late Quaternary(0.3-0.4my). This left-lateral late Quaternary faulting has simultaneously reactivated diversifiing former topograpy and using the pre-existed fault shattered zone. Although structural, geologic and geometric segmentations along the Yanagase fault zone is proposed by the characteristics of each fault, more detailed informations for these faults are required for the recognition to the earthquake and behavioral segmentaions. And concealed active faults under the alluvial plain are estimated by the photo-interpretation, geomorphological and geologic examinations on the southward extension of the Yanagase fault.
