Rupture models and viscoelastic stress changes of three Mw ~ 6.3 earthquakes along the southern front of the Qilian Mountains, northeastern Tibetan plateau

Abstract

Around the southern front of the Qilian Mountains, three Mw 6.3 strong earthquakes have occurred since 2003. The distances between the epicenters of these events range from 18 to 46 km. By collecting available datasets, including broadband waveforms, coseismic surface geodetic measurements and active fault mapping results, we invert rupture models for these strong earthquakes with a finite fault model approach in elastic half space. Constrained by these detailed rupture models, we then build a three-dimensional finite element model to calculate Coulomb stress change due to the coseismic displacement and postseismic viscoelastic relaxation. Combined with an effective friction coefficient of 0.2–0.8 and variable rheological characteristics, numerical results show that following the 2003 event, the Coulomb stress change along the 2008 rupture segment was positive with magnitudes between 1.8 × 103 Pa and 2.2 × 103 Pa. Among the stresses, the postseismic viscoelastic relaxation contributed 10.7% ~ 23.3%. Just prior to the occurrence of the 2009 earthquake, the total changes of the Coulomb stress around its hypocenter reached 2.71 × 104– 3.72 × 104 Pa, to which the 2003 event and the 2008 event contributed approximately 7 × 102– 1.5 × 103 Pa (2.58% ~ 4.03%) and 2.64 × 104– 3.57 × 104 Pa (95.97% ~ 97.42%), respectively. We believe that other postseismic behaviors, including afterslip and poroelastic rebound, could also yield extra changes in the Coulomb stress, which probably further enhance the triggering effect among the sequential earthquakes. The numerical results also show that the increased stress around the hypocenters of latter two events was mainly determined by the positive shear stress. After the three strong earthquakes, the Coulomb stress on some active faults increased and is still increasing, suggesting that great attention should be paid to the potential earthquake hazards in this region.