Three-dimensional finite element modeling of stress evolution around the Xiaojiang fault system in the southeastern Tibetan plateau during the past ~ 500 years

Abstract

The left-lateral strike-slip Xiaojiang fault system is a main boundary between the southeastern Tibetan Plateau and the South China Block, and accumulates several large (M > 7) earthquakes in the past ~ 500 years and a dozen of moderate (5 < M < 7) earthquakes in the last ~ 50 years. We developed a three-dimensional viscoelastic finite element code to calculate stress evolution, by means of Coulomb failure stress changes, around this fault system over the past ~ 500 years due to regional tectonic loading, redistributions of static stress induced by the large earthquakes and postseismic viscoelastic relaxation associated with these large earthquakes. The regional tectonic loading is approximated by velocity boundaries from dense GPS data around the southeastern Tibetan plateau. Reasonable bounds and lateral variations are considered for the rheologies of the lower crust and upper mantle. The large (M > 7) earthquake ruptures occurred in the past ~ 500 years are simulated with split-node technique in seismogenic upper crust. We investigate the Coulomb failure stress changes mainly on vertical planes parallel to the different strands of the Xiaojiang fault system, and at the epicenters of large (M > 7) earthquakes in the past ~ 500 years and of moderate (5 < M < 7) earthquakes in the last ~ 50 years. Results show that stress evolution of the fault system is featured mainly with linear stress buildup from regional tectonic loading. This relatively simple stress evolution is significantly complicated by the large (M > 7) earthquakes that occurred in the past ~ 500 years both from their coseismic and postseismic effects, among which the postseismic effect results in the stressing rate being obviously accelerated near epicenters of the large (M > 7) earthquakes after their occurrence. Thus, the percentage of positive Coulomb failure stress changes of > 0.01 MPa at the epicenters increases from ~ 37% to ~ 50% for the large (M > 7) earthquakes, and from ~ 43% to ~ 64% for the moderate (5 < M < 7) earthquakes just before their occurrence, attesting that postseismic effect has a profound influence on the state of stress evolution around the Xiaojiang fault system due to viscoelastic deformation of the lower crust and upper mantle. Finally, we estimated the Coulomb failure stress changes along the individual strands of the fault system at present time, and find that with ~ 500 years' evolution, many patches of the fault system have received the Coulomb failure stress changes about + 0.2 MPa to + 1.5 MPa, among which the coseismic plus postseismic effects reach to ~ 50%–67%. This estimate highlights the importance that a great attention should be paid on potential hazard of large earthquake around the Xiaojiang fault system.