FRACTURE OF ROCKS IN SOLID MEDIUM : A CONSIDERATION OF THE OCCURRENCE OF THE EARTHQUAKE SEQUENCES

Abstract

The fracture phenomena of rocks in the solid mediums was studied. A large quantity of residual stress was observed just after the major fracture. The ratio of stress drop to initial stress was large for the weak mediums and small for the strong mediums. The effect of existence of free surface on the strength of specimen, the stress drop ratio at the moment of fracture, and the stress relaxation after the major fracture was also quite remarkable. The three features of the earthquake occurrence was mentioned. (1) Aftershock sequences occur in accordance with Omori's formula. (2) Earthquakes occur generally in groups. (3) The earthquake group changes its type with focal depth from aftershock type to earthquake-cluster type, and finally to isolatedshock type. The above features were well explained with the results of the rock fracture in the solid mediums.