A POSSIBLE TRIGGERING MECHANISM FOR EARTHQUAKE SWARMS

The spatial and temporal distribution and characteristics for the following earthquake swarms occurred after large events have been analyzed. They are: Xiong Yue swarm after the Haicheng Earthquake of 1975, Liang Cheng swarm after the Helinger Earthquake of 1976 and Baodi swarm after the Tangshan Earthquake of 1976. The additional stress (tensile) of magnitudes 3-6 bars, corresponding to the magnitudes of stress drops, calculated for the small earthquakes of the swarms have been obtained by using the dislocation model. Such additional stress may be explained as caused by the decrease in confining pressure. On the basis of results of triaxial experiments on rock samples under high pressure with different courses of loading and the studies of earthquakes induced by injecting water into the rockmass in-situ, it is pointed out that additional stress of such magnitude could trigger shallow earthquakes in an area with high stress level. Considering the character and function fo fluid diffusion at depth, the observed facts that high swarm activity lags behind the major earthquake could be understood. Hence the major earthquake might be able to trigger the swarms in the surroundings.