Stress development in heterogeneous lithosphere: insights into earthquake initiation in the Tan-Lu Fault Zone

Abstract

The Tan-Lu Fault Zone (TLFZ), East China, is the site of several historical M 6.5–8.0 earthquakes and remains seismically active today. Although differential kinematic and geomorphic features in the upper crust can well divide the TLFZ into several segments, many questions remain regarding the spatial distribution of the middle-large earthquakes along the active faults and what triggers earthquakes in the seismic zone. Namely, what is the relationship, if any, between the observed seismicity and the fault segmentation along the TLFZ? Here we present a three-dimensional viscoelastic finite element model to simulate the stress distribution along the TLFZ and better understand the relationship between the regional stress field and observed seismicity, with a particular emphasis on how the heterogeneous lithosphere influences earthquake initiation at different depths and along the various segments of the TLFZ. The global crustal model Crust 1.0 and structural information from regional and areal studies are utilized to infer the lithospheric viscosity structure and provide an important input to the numerical model. The model results are well supported by the geological evidence, in terms of both the observed stress field and distribution. Our results suggest ~ENE-oriented maximum principal compressive stress in East China, which are consistent with the local fault-plane solutions. The calculated maximum shear strain and rupture values at different crustal depths are consistent with the distribution of the large earthquakes, which suggest that these larger earthquakes are initiated by stress localization in the heterogeneous lithosphere along the TLFZ. Cross-sections of our calculated maximum shear strain and rupture values along the TLFZ support the initiation of large earthquakes in the middle crust, particularly along the Bohai Bozhong and Yishu segments. Furthermore, the “soft” low-velocity zone (fluid system) in the lower crust along the TLFZ provides the potential for earthquake stratification and initiation in the middle crust along the TLFZ. Our results provide strong evidence that the stress localization of the heterogeneous lithosphere in different segments of the TLFZ and the lower crustal fluid system initiate different magnitude of earthquakes in each seismic zone.