A STUDY OF CERTAIN FEATURES OF A FORESHOCK SEQUENCE AND THAT OF AN ORDINARY EARTHQUAKE SWARM

To study certain features of the seismic activity of a foreshock sequence and their difference from that of an ordinary earthquake swarm will be of value to the study of the process of development of the main shock at its focus and to earthquake prediction. It is for this purpose that the detailed data, of 17 of the larger earthquakes that occurred within the last ten years or so in China, have been processed and analysed. The results show that the activity of the foreshocks begins at a comparatively early date before the main shock, that there secms to be some correlation between the foreshock activity and changes in the wave velocity ratio. If appropriate time intervals are chosen before the main shock, the number of foreshocks in an interval generally increases with time exponentially. Furthermore, a foreshock with magnitude over 4 of a large earthquake differs in the rate of strain release from that of an earthquake of the same magnitude of an ordinary earthquake swarm.