A proposal of surveying and evaluating system of active faults for earthquake assessment

1. Paleoseismology of the Itoigawa-Shizuoka Tectonic Line active fault system: We investigated co-seismic faulting activity of the Itoigawa-Shizuoka Tectonic Line active fault system (ISTL) to clarify behavioral segmentation of long and massive faults. Geomorphologic and geologic surveys, trench excavation, and seismic reflection survey in the southern to central parts of ISTL revealed paleoseismologic faulting events occurred in the last thousands years and characteristics of geometric, structural, and geomorphologic segments. Each paleoseismic event, co-seismic displacement of deposit, average slip rate, and recurrence intervals suggest that the latest paleo-earthquake occurred in 1700 cal y BP and involved multiple segments in the Okaya to the Shimotsuburai faults. The estimated surface rupture length for this event is up to 77 km or possibly up to 94 km long. The another latest event occurred after 1200 cal y BP at the Ichinose fault and adjacent active faults. In addition, ca. 1200 cal y BP event at the Gofukuji fault occurred and involved multiple segments in the northern ISTL. Behavioral boundaries of these distinctive paleoseismic events were present in segment boundaries of geometric characters and slip rate variation. In the ISTL, geometric segmentation and slip-rate variation likely coincide with the estimated behavioral segmentation. Therefore, this finding suggests that geometric segment and slip-rate variation play an important role to determine the size of the maximum behavioral segment. 2. Active fault study on the 1999 Taiwan Chichi Earthquake area: The earthquake fault was appeared along the Chelungpu Fault while the 1999 Chichi Earthquake has occurred. The N-S striking fault has been recognized as an active fault, however E-W direction earthquake fault has not been described before the earthquake as an active fault. The later fault appeared just beneath the Shihkang Dam and the dam was destroyed by the fault. This study revealed that the E-W direction fault had been found as an active fault through the airphoto study before the earthquake. Also the recurrence interval of the active of the Chelungpu Fault near the Shihkang Dam where the fault bends to the E-W direction, is estimated to be in the order of 1,000 to 2,000 years from age data acquired from the fault exposure. 3. Active fault study on the 2000 Western Tottori Earthquake area: In order to evaluate the fault activity associated the 2000 Western Tottori Earthquake, we carried out aerial photograph interpretations of fault landforms, geological investigation and trenching study in the epicentral area. As the results of the aerial photograph interpretation, we found some northwest-southeast trending lineaments with sinistral offsets of valleys and ridges in the epicentral area. Above lineaments are sub-parallel and the total length of the lineaments is about 23 km. The fracture zones with fault gouge and horizontal striations on sharp fault planes were found along the lineaments, the fact also suggests that the lineaments coincide with the active fault. According to the outcrop exposure of the fault and trench excavation at Kusumi, we found the evidence of the recent surface-ruptured event that had occurred sometime between 770-1260 A. D. Hence there is possibility to estimate the magnitude 7 class earthquake before the 2000 Western Tottori Earthquake by the lineament length of about 23 km. 4. A Proposal of Earthquake Hazard Assessment System by Fault Segmentation: Procedure to evaluate the earthquake magnitude by fault segmentation is proposed as follows, (1) Fault segmentation with rupture length of historical earthquake by literature survey (Earthquake segment). (2) Fault segmentation with geomorphic and geologic investigation (Geometric segment and Structural geologic segment). (3) Investigation on the fault segmentation and fault behavior (behavior segment). (4) Evaluation of the fault magnitude. (author)