Seismogenic mechanism and the intermediate- and long-term earthquake risks of the 2008 Wenchuan and 2013 Lushan earthquakes

The 3D finite element method （FEM） and the discontinuous deformation analysis （DDA） were combined , with constraints from GPS data and focal mechanisms, to simulate numerically the movement, deformation and interaction of the tectonic blocks system of the Qinghai-Tibetan Plateau and its vicinity, and their correlation with the occurrence of the recent M_S≥7.0 earthquakes in this area. An " instability risk factor”, defined as the ratio of the normal stress to the friction strength, was introduced to characterize the degree of the instability risk on the boundary faults of the tectonic blocks in the studied region. The simulation shows the spatial coincidence between the segments of the boundary faults of the tectonic blocks with higher instability risk factors and the locations of the recent significant earthquakes in the studied areas. Particularly, the Longmenshan fault zone, where the 2008 Wenchuan and 2013 Lushan earthquakes occurred, is the fault zone with high instability risk factor. The calculation results indicate that the eastern boundary of the Qinghai-Tibetan Plateau is a fault zone of steepest strain-rate with strain-rate in the west side four times as high as that in the east side. Meanwhile, it is found that the eastern boundary of the Qinghai-Tibetan Plateau is also a high strain-energy zone with same trend and width as the Longmenshan fault zone. These results indicate that before the occurrence of the 2008 Wenchuan and 2013 Lushan earthquakes, relative high strain energy have already accumulated in the seismogenic fault of the 2008 Wenchuan earthquake, and these seismogenic faults have been in highly instability state mechanically. It is estimated that the occurrence of the 2001 western Kunlun Mountain Pass earthquake increased the Coulomb failure stress by about 0.16 MPa, a value corresponding to two years stress accumulation on the seismogenic fault, thus promoting the occurrence of the 2008 Wenchuan and 2013 Lushan earthquakes two years in advance. Based on the simulations after the 2008 Wenchuan earthquakes, it is found that the occurrence of the 2008 Wenchuan earthquake increased the Coulomb failure stress in the southwestern segment of the Longmenshan fault zone and southeastern segment of the eastern Kunlun fault zone, which indicates that the occurrence of the 2008 Wenchuan earthquake has promoted the occurrence of 2013 Lushan earthquake and the 2018 Jiuzhaigou earthquake.