Mapping active faults and folds of the Nagamachi-Rifu line fault system based on high-resolution DEM and a borehole dataset

<p>　 Earthquake surface rupture appears not only along the principal fault that caused the earthquake but also pre-existing other faults or folds nearby. Although surface ruptures along subsidiary structures are often small and discontinuous, given its extensive occurrence, comprehensive mapping of active faults and folds is necessary for understating strain distribution within a fault zone to evaluate fault displacement hazards. The Nagamachi-Rifu line fault zone, consisting of sub-parallel reverse faults, runs across Sendai City, and recent studies suggest that there are several active structures not shown on existing active fault maps. However, it is difficult to obtain their convincing geomorphic evidence because their surface deformation is subtle and is easily obscured by erosion and artificial modification. To reevaluate active faults and folds of the fault zone, we studied shallow geologic structure across the fault zone using a borehole dataset and conducted a geomorphological analysis using 2 m DEM. Our results indicate that the fault surface of the Nagamachi-Rifu line does not reach the surface causing flexural deformation at the surface rather than brittle deformation, as the previous studies pointed out based on seismic reflection surveys. We also found that the deformation zone of the Dainenjiyama fault is broader than previously thought. This is probably because its fault scarp retreated by hundreds of meters due to erosion. To the northwest of the Dainenjiyama fault, there is an NNE-SSW trending zone where the base of the Quaternary deposit was elevated compared to its surroundings, which may correspond to the previously estimated active anticline. The ratio of vertical displacement of the Nagamachi-Rifu line, Dainenjiyama fault, and the anticline is estimated to be 5-6: 3: 1. We could not newly find other faults or folds, including those exposed at outcrops reported in previous studies, suggesting that there are several active structures of which cumulative displacement is too small to be observed in our analysis. Nonetheless, a shallow geologic structure revealed from a borehole dataset, coupled with detailed topographic analysis, can help to locate active structures with minor surface expression and evaluate seismic hazards.</p>