Abstract
High-resolution sedimentological analysis of core samples from the Tokushima Plain enable us to identify evidence for multiple faulting of the Median Tectonic Line active fault system (MTL) during the Holocene period. Continuous and close-interval analyses of lithology (0.5 cm-order) and magnetic susceptibility (2 cm-interval) were carried out on three core samples from the hanging wall and footwall sides of the Naruto-minami fault, which is part of the MTL. In addition, analyses of grain size (10 cm-interval) and composition of very fine sand fraction (20 cm-interval) were carried out on core samples obtained at both ends of the fault deformation zone. Based on these sedimentological analyses, we correlated 34 horizons across the bore holes and divided the strata into 8 sedimentary units. We also identified 6 intervals that are thicker at the footwall side of the fault. These intervals appear to be related to movements of the MTL and the age of these paleoseismic events are estimated on the basis of radiocarbon ages. At least 6 paleoseismic events are detected for the last 15000 years : 15151700 cal BP, 28003200 cal BP, 34004200 cal BP, 73007600 cal BP, 970010100cal BP, 1300015000 cal BP. The average dip slip rate of the Naruto-minami fault is estimated at 1.2 mm/year.
