断層の幾何学的バリヤと破壊プロセス

Abstract

Geometric patterns of surface earthquake faults are closely related to rupture propagation processes especially in the case of strike-slip faults. In this paper, the author examined geometric features of four strike-slip earthquake faults in Japan, that is, the Kita-Izu earthquake fault system of 1930, Izu-Oshima Kinkai earthquake fault system of 1978, the Nobi earthquake fault system of 1891 and the Kita-Tango earthquake fault system of 1927. Bends or jogs along a fault can be called as geometric barriers, which are classified into two categories as extensional and compressional, from a view point of volumetric change (Fig. 1). Studies of earthquake surface faults of recent inland large earthquakes in Japan show that ruptures were initiated from compressional barriers and terminated at extensional barriers. A compressional barrier in the course of rupture propagation could be the position of next rupture nucleation. Compressional barriers can be commonly described as “fragmentation barriers” (King, 1987) because short faults of various orientation are very popular in and around compressional barriers. The location of severe damage area is also closely related to that of barrier.Study of geometric characteristics of active faults will become very important because they give us a key idea on rupture initiation and termination as in the case of surface earthquake faults. Recognition of barriers along an active fault system from geometrical investigation will provide us some locations of detailed observation for geodetical, geophysical and geochemical anomalies along precautious faults.