ON SPATIAL DISTRIBUTION OF LARGE AFTERSHOCKS OF THE SEQUENCE OF A MAJOR EARTHQUAKE AND PRELIMINARY THEORETICAL EXPLANATION

Sequences of 11 major earthquakes occurred in the continental area of China have been analysed for the characteristics of their spatial distribution of large aftershocks. It is found that:1. The large aftershocks are located in three areas: in the vicinities of the ends of the earthquake fault and the epicenter of the main shock.2. The characteristics of spatial migration of the large aftershocks are as follows: (1) The migration distance of the large aftershocks corresponds to the rupture length of the main shock, and increases with time; (2) The distance and direction of migration are related to the rupture pattern. For stronger main shocks with unilateral rupture, the migration of large aftershocks relative to the epicenter of the main shock will be unilateral; for an earthquake with bilateral rupture, the migration of the large aftershocks will be some what similar to relaxation oscillation from one end of the earthquake fault to the other relative to the epicenter of the main shock. For a twin earthquake (two main shocks), the large aftershocks migrate usually in a unilateral manner relative to the first main shock while in a bilateral manner relative to the second main shock.3. The vertical distribution of the large aftershocks in a section is within the fault plane and mostly are located on the front of the fault.A preliminary theoretical explanation for the above observational results has been attempted based upon the principles of rheology and fracture mechanics.