地震の前兆現象発生のメカニズム

Abstract

The process of the occurrence of an earthquake is discussed from the mechanical point of view, and it is emphasized that the structural heterogeneity in and around the earthquake source region is one of the most important factors which cause the occurrence of precursory phenomena. Earthquake precursors are classified into the following two types. The one is the precursors which are caused by the increase in stress. In general, long-term precursors are those of this type. The gradual increase of earthquake swarm activity before the 1983 Japan Sea earthquake, the gradual increase in seismic activity in the surrounding region of the 1989 Loma Prieta earthquake before the main shock and the appearence of the seismic gaps of the second kind are discussed as examples of this type. The other is the precursors which are caused by a slowly progressive rupture process just before the sudden main fracture. Short-term precursors are mainly those of tis type. Foreshocks prior to the 1934 and 1966 Parkfield earthquakes are discussed as the typical examples of this type.