Analytical method for evaluating superficial fault displacements in volcanic air fall deposits: Case of the Hirayama Fault, south of Tanzawa Mountains, central Japan, since 21,500 years B.P.

Abstract

The establishment of an analytical method for evaluating superficial fault displacements must contribute greatly to the improvement of methods for earthquake prediction in the Quaternary volcanic areas where many volcanic air fall deposits of known age are widely traceable. However, because the air fall deposits of uniform thickness cover many surfaces of blocks separated by faults, cumulative displacements along faults cannot always be detected in the deposits. Then, the study of geomorphological developments along a fault becomes important in analysis of superficial faults in air fall deposits. The Hirayama fault, south of the Tanzawa mountains, central Japan, is analyzed as an optimal example for studying the methodology in air fall deposits. The most important result of the detailed analysis is that the displacement rate of the Hirayama fault has been decreasing since 21,500 years B.P.; that is, the Hirayama fault has nonuniform history of movement. The result consequently indicates that the assumption of uniform history in fault movement is not always safe in earthquake prediction.