Prediction of coseismic fault behavior based on slip-rate and slip distributions for the northern part of the Itoigawa-Shizuoka Tectonic Line active fault zone, central Japan

Abstract

The Itoigawa-Shizuoka Tectonic Line active fault zone is one of the largest active faults in Japan, with a length of 150 km. The previous paleoseismological studies revealed a 14% possibility of a large earthquake in the next 30 years. The Japanese Ministry of Education, Culture, Sports, Science and Technology has initiated an intensive research project on this fault zone in 2005, in which we have conducted a tectonic geomorphological study on the northern part of the fault zone in order to predict its coseismic behavior. We have interpreted largescale aerial photographs taken not only in recent years but also in the 1940s and 1960s for examining faultrelated topography including those which have been already modified or destroyed, and then conducted a field survey to determine ages of geomorphic surfaces. We also carried out photogrammetric analyses which have resulted in our highly-dense offset data whose spatial intervals are of 500-1000 meters in average. Based on these surveys, we estimated a net-slip rate distribution along the northern part of the fault zone, and calculated coseismic slip distribution assuming that the fault zone follows the characteristic earthquake model. Largeroffset areas were identified in the Hakuba Village and the Ikeda Town. These peaks of surficial slip distribution would imply subsurface fault which generate strong ground motion. The maximum vertical offset during the last earthquake is estimated to be 5 - 6 meters. Therefore, the coseismic net slip would exeed 10 meters when we assume the fault-slip responds to the maximum shear stress. Our detailed coseismic slip distribution and the subsurface fault geometry predict a moment magnitude of 7.5 for the northern part of the fault zone.