The 1995 Kobe earthquake and problems of evaluation of active faults in Japan

Abstract The Kobe earthquake ( M 7.2) of January 17, 1995, which was the most damaging earthquake in recent Japanese history, made manifest the need for reconsidering the method of evaluating active faults. An earthquake of this magnitude at this time was unexpected according to conventional evaluation, in which the potential magnitude of earthquakes at a certain site is estimated by considering the greatest earthquake in the past 400 years and the length of the active fault. The following characteristics of this earthquake made it appear unlikely by conventional understanding: (1) the Kobe earthquake involved several neighboring faults, which had been previously been identified as separate fault systems: (2) the surface rupture of about 10 km length was much shorter than the 50 km seismic faulting; (3) the interval of 400 years between the Kobe and penultimate Keicho earthquake of 1596 AD ( M 7.5), which has been revealed by historical documents and some excavations, is much shorter than the 2000 years estimated by calculating the average slip rate of displaced landforms. These shortcomings imply that active fault evaluation with the traditional characteristic earthquake model which deals with each fault separately, is not adequate for an area like Japan where active faults swarm. New concepts such as the block rotation model (Kanaori, 1990; Late Mesozoic-Cenozoic strike-slip and block rotation in the inner belt of Southwest Japan. Tectonophysics, 177: 381–399) considering the macroscopic tectonic framework for fault interactions are needed. Furthermore, fault dynamics cannot be ignored; physical and temporal parameters associated with faulting, such as moment release rate, must be considered for realistic and precise evaluation.