活断層はどこから割れ始めるのか?-活断層の分岐形態と破壊伝播方向- Fault Branching and Directivity of Rupture Propagasion
This paper proposes a method to identify the directivity of rupture propagation based on the branching features of active fault traces.<BR>Direction of ruptre propagation is closely related to strong ground motions and resulting earthquake damage. Therefore, predicting rupture directivity is crucial in predicting strong motions to mitigate earthquake damage. However, the directions of fault ruptures were ascertained only after earthquakes from the observed seismological records and not before the earthquakes.<BR>We found an interdependent correlation between the branching direction of the surface ruptures and the direction of their propagation as shown in Fig. 1, from an investigation of recent earthquake fault ruptures such as the 1995 Northern Sakhalin earthquake, the 1995 Hyogoken-nambu earthquake, the 1992 Landers earthquake, the 1990 Luzon earthequake, the 1979 Imperial Valley earthequake, and the 1930 Kita-Izu earthequake. The branching of faults during rupture propagation is regarded as an effective energy dissipation process and could result in final rupture termination.<BR>Because patterns of surface traces of active faults are the results of repeated earthquake faulting, the branching of active faults leads us to suggest that the direction of rupture propagation is also predictable before the active faults generate earthquakes in the future.<BR>Several active faults with well-defined branching such as the active faults of the strike -sliptype in the Kobe-Osaka area, those in California, and the active fault sysytem in the northern Luzon, Philippines are examined. Branching of the reverse faults in the foot -hills of Darjeeling Himalaya is also shown as an example of active faults of the dip -slip type. This test clearly shows that the direction of rupture propagation, and in some cases the epicenter location, can be deduced from the branching features on the basis of our proposed method.