伊豆半島南西部のNNE-SSW走向の断層とテクトニック的考察

Abstract

Though active faults of the NW-SE strike are readily recognized in the southern part of the Izu peninsula, there exist conjugate faults or fault-like structures in the NNE-SSW strike. The branch aftershock, activity of the earthquake off the Izu peninsula, which extended to the Amagi area in the NNE direction, is considered to be closely related to the above mentioned structures in this direction. These two conjugate fault systems can be considered to have been formed before the bending of the Izu peninsula to the west, and at present to be active as weak plane systems. Therefore the stress field released as earthquakes due to activation of these fault systems can not be considered to directly reflect the present stress field in the area concerned. This is the reason why there is a significant difference between the stress field deduced from focal mechanisms of earthquakes and one deduced from the crustal movement. The reason why faults of N-W direction are liable to be active in comparison to the other conjugate fault systems can be reasonably explained on the basis of the generalized Coulomb-Mohr fracture theory.