THE PRESEISMIC STRESS FIELD DUE TO A COMPRESSIONAL TORSIONAL FAULT AND STRESS VARIATION

For studying the geomechanical process of the development and occurrence of shallow tectonic earthquakes, this paper proposes a model for the earthquake fault before the shock, consisting of three stages: the firststress accumulates in the relatively stable locked zone; the secondthe slipping zone extends and the locked zonecontracts in which high concentration of stress takes place; the thirdresiduallocked zone contracts to a certain degree causing violent displacement of the fault.Based on this variation of mechanical state at the earthquake fault, the additional stress given rise on its surroundings and the stress change of this process have been computed and analyzed. The results show that this model agrees with observations and experiences in practice and that it explains certain observed facts before the 1975 Haicheng earthquake.