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• A finite element model was developed to explain the causes of the rupture behavior transition of the Wenchuan earthquake. • The auxiliary stress regime transitioned from S H >S h >S v in the southwestern segment to S H >S v >S h in the northeastern segment. • The stress change associated with the significant first subevent reached 1.0 MPa. • Initial stress and stress change jointly controlled the northeastward rupture of the Mw7.9 Wenchuan earthquake. The rupture process of the Wenchuan earthquake demonstrated a transition from thrust-dominated slip to northeastward strike-slip motion along the Longmen Shan Fault Zone. The initial stress has been reported as playing a critical role in this process; however, the stress changes, especially those caused by the significant first subevent of the Wenchuan earthquake are not well understood. Here, we employ a three-dimensional finite element model of the Sichuan-Yunnan region to analyze the stress change caused by the significant first subevent and explore the possible influence on the following ruptures. The results indicate that the auxiliary maximum principal compressive stress (S H) associated with the significant first subevent was horizontal and that the auxiliary stress regime was S H >S h >S v , supporting the ongoing regional thrust motion near the southwestern segment of the rupture plane. However, in the northeastern segment, the auxiliary stress regime transitioned to S H >S v >S h , demonstrating that the stress changes promoted the transition of the rupture behavior from predominantly thrust motion in the southwest to right-lateral strike-slip motion in the northeastern segment, which was also supported by the dominant shear stress change and the subtle normal stress change along the fault plane in the northeastern segment. In addition, our modeled results also indicate that the orientation of the maximum principal compressive stress changed from SEE to northeastward NEE along the strike of Longmen Shan Fault Zone. This anticlockwise rotation hastened the rupture behavior transition, suggesting that both the initial stress and the stress changes associated with the first subevent jointly controlled the following northeastward rupture of the Mw7.9 Wenchuan earthquake. [ABSTRACT FROM AUTHOR]

Abstract: Strong earthquakes that occurred in the Chinese continent are usually characterized by grouped activity, long-distance jumping migration, and different main activity areas formed in different times. In the present study, a three-dimensional (3-D) finite element model was set up for the Chinese continent involving surface topography, major active fault zones and initial stress field to study the mechanism of the long-distance jumping migration of main active areas for the strong earthquakes that occurred in the Chinese continent. A number of numerical simulation experiments have been made by introducing the birth and death of element groups under different boundary loading configurations. Our results show that (1) in an environment where always exists an initial stress field in the Earth’s crust, the areas where strong earthquakes have occurred (taken as a killed element in the model) have no longer abilities to concentrate the stress largely enough and can cause the stress adjustment by an order of MPa in a large area, which may be one of the main factors affecting the long-distance jumping migration of the follow-up strong earthquakes, and (2) it is hard to accurately predict where the follow-up strong-earthquakes will occur because it could be affected by many factors such as the loading manner, geological structure, active fault zones, initial stress field, or stress field adjustment induced by strong earthquakes, but in an active period of earthquakes, the major activity area migration of strong earthquakes may be affected largely by the boundary loading configurations. These results suggest that it is helpful to predict a trend of the strong earthquake migration by investigating various kinds of boundary loading manners and the relationship between stress field adjustment induced by strong earthquakes and the regions where the strong earthquakes occurred. [Copyright &y& Elsevier]

• We analyze the Coulomb stress changes on the main active faults in the southwestern Yunnan region due to co-, post-, and inter-seismic effects. • Co-seismic Coulomb stress change between the two 1988 Lancang-Gengma earthquakes is 265 kPa, indicating the triggering effect. • Inter-seismic stressing rate is estimated using the numerical model and GPS data, and the maximum value reaches 2 kPa.yr−1. • The greatest total stress build-up occurs on the middle section of the Nantinghe Fault and the middle section of the Longling-Lancang Fault. Southwestern Yunnan is expressed as a seismically active region bordering the southeastern Tibetan Plateau. However, the recent seismicity in this region has been relatively quiet. Considering some highly-populated cities nearby and the historical record in this region, understanding the stress changes on the main active faults is crucial for seismic hazard analysis. In this study, we calculated the co- and post-seismic Coulomb failure stress change (Δ CFS) on these faults caused by historical earthquakes in and around the region. Then, by integrating the inter-seismic tectonic loading estimated using a geodynamic model and GPS data, we analyzed the total stress change on all of these faults. The results reveal that the Δ CFS patterns caused by events on different segments of the Sagaing Fault vary; the combined Δ CFS increased on the middle segment of the Longling-Lancang Fault, the Heihe Fault, and the Nantinghe Fault, among which the maximum change reached 3.5 kPa. The great 1950 Assam earthquake in the eastern Himalayan syntax increased the Δ CFS on almost all faults, and the maximum change reached 20 kPa. More importantly, the Δ CFS caused by earthquakes in the southwestern Yunnan region was significantly larger than that caused by earthquakes in the surrounding region because of the geographical proximity. The inter-seismic tectonic stress loading rate reached 2 kPa.yr−1. A significant total stress build-up occurred on five segments distributed among four faults, namely, the Nantinghe Fault, the Longling-Lancang Fault, the Heihe Fault, and the Menglian Fault, making them possible candidates for the next major earthquake in this region. [ABSTRACT FROM AUTHOR]