Earthquake stress interaction in the northeastern Songpan-Garzê block and its implication for earthquake hazard

The Songpan-Garze block,located in Southwest China and the central segments of north-south seismic belt,lies to the west of the Yangtze block.It is one of most important tectonic units in the northern Tibet Plateau with several active faults in and around it.The seismic activity is highly active in this region,and several devastating earthquakes were recorded in this area.Based on earthquake stress triggering theory,we calculated the stress transfer and earthquake interaction process of the sequence of historical strong earthquakes in the northeastern Songpan-Garze block with elastic dislocation theory and the viscoelastic multilayered crustal model.The results show that Coulomb failure stress increases significantly in the epicenter of the Wudu earthquake,1879 and the M7.2 Songpan earthquake,August 23,1976.The increased stress encourages the occurrences of these two earthquakes.The accumulated stress is released on the epicenters of the M7.5 Diexi earthquake,1933 and the M6.5 Songpan earthquake,1973,whose occurrences are mainly delayed by previous earthquakes.The effective friction coefficient is very important in the study of the M6.7 Zhangla earthquake,1960,the M7.2,August 16 and the M6.7,August 22 Songpan earthquake,1976.If the value of 0.8 is chosen,the occurrences of three events are encouraged by previous earthquakes.The strong seismic activity in the northeastern Songpan-Ganzi block raises the stress accumulations on the West Qinling fault,the Maqin-Maqu segment of East Kunlun fault,the Kangding-Daofu segment of the Xianshuihe fault and the mid-segment of the Minjiang fault,and will encourage the occurrence of earthquakes on these faults.The historical earthquakes released the accumulated stress on the Longriba fault,on which the seismic activity will be depressed.Although the accumulated stress in the epicenter of the Wenchuan earthquake is released,the strong earthquakes in the northeastern Songpan-Ganzi block enhances the stress loading on the northeastern segment of the rupture plane of the Wenchuan earthquake,and will encourage it to rupture northeastwards.