Mechanism of crustal deformation around the Lajishan-Jishishan Tectonic Belt, NE Tibet, and implications for occurrence of the 2023 Jishishan MS 6.2 earthquake

Abstract

The Lajishan-Jishishan Tectonic Belt (LJTB) experienced several strong earthquakes in recent decades despite its low slip rate and low strain rate. The latest 2023 Jishishan M_S 6.2 earthquake received special attention for its unusual secondary disasters, which raises the question about the mechanism of frequent strong earthquakes in the tectonic belt. In this study, we construct a three-dimensional (3D) viscoelastic finite element model to analyze the crustal deformation and stress–strain state around the LJTB, considering the effects of fault geometry and lateral rheological heterogeneity. The results show that with the uniform rheology in the mid-lower crust, there is a significant strike-slip component at both end segments and a thrust component in the middle segments of the tectonic belt, and strike-slip and weak thrust deformation in the north segments of the belt, indicating the important role of fault geometry in controlling the regional crustal deformation. The strengthening of rheology in the mid-lower crust benefits the fault slip but subtly influences crustal deformation. For the middle and south segments of the LJTB, the crust experiences intense horizontal compression, and the heterogeneous rheology in the mid-lower crust facilitates fault thrust motion, as indicated by the 2023 Jishishan M_S 6.2 earthquake. The crustal deformation around the tectonic belt is attributed to the hybrid crustal shortening and localized mid-lower crustal flow. Under the compressive tectonic stress field, the strain energy gradually increases on the Jishishan fault zone located above the mid-lower crust with high rheology, eventually generating the Jishishan M_S 6.2 earthquake.