Mechanisms of the different senses of fault slip in the north and south segments of the Huya Fault zone, eastern Tibetan Plateau: Constraints from numerical modeling

As one of the frontal faults of the eastward expansion of the Tibetan Plateau, the Huya Fault zone shows significant north-south differences in the senses of fault slip and types of strong earthquakes. The mechanism of different senses of fault slip is crucial for revealing the eastward expansion pattern of the plateau. In this study, we constructed a three-dimensional viscoelastic finite element model that embedded the Huya Fault zone, to investigate the effects of rheological heterogeneity and fault geometry on crustal deformation and three-dimensional fault slip rates. The results show that, with the continuous eastward extrusion of the Tibetan Plateau, in the south segment of the Huya Fault zone, the lateral rheological heterogeneity controls the thrust-dominated fault movement, and with the increase of the difference in rheological viscosity on both sides of the fault in the mid-lower crust, the horizontal movement on the west side of the fault is more likely to be transferred to the vertical movement. When a viscosity contrast between the two sides of the fault is about two orders of magnitude, the ratio between the dip-slip rate and the strike-slip rate reaches 3.3 similar to 4.0, showing a significant thrust motion, which is consistent with the sense of fault slip reflected by the strong SongpanPingwu earthquakes. On the contrary, in the north segment of the fault, even with the small lateral rheological heterogeneity in the mid-lower crust, the fault can show obvious thrust motion, which is in contrast to the strike-slip motion demonstrated by the M(S)7.0 Jiuzhaigou earthquake, suggesting that there may be no lateral rheological heterogeneity in the mid-lower crust in the north segment of the Huya Fault zone, and fault geometry is the main factor controlling the dominated strike-slip fault movement. This study provides a theoretical basis for understanding the spatial different senses of fault slip and seismogenic mechanism in the eastern margin of the Tibetan Plateau, and also the eastward expansion pattern of the plateau.