Tectonic stress field and crustal strength of the central-south Tanlu Fault Zone

The accurate estimation of crustal strength, which is the capacity of the lithosphere to resist tectonic deformation, is fundamental to both seismic hazard assessment and geodynamic studies. This study integrates borehole logging data and focal mechanism solutions from the central-south Tan-Lu Fault Zone to analyze the characteristics of the tectonic stress field. The results indicate that the stress states in the shallow and deep crust are generally consistent, with a predominantly strike-slip stress regime and the maximum horizontal principal stress oriented preferentially ENE-WSW. The regional fault friction coefficient is approximately 0.3, significantly lower than the 0.6–1.0 range suggested by Byerlee's law, indicating a moderate level of fault frictional strength. Furthermore, constrained by these findings, a crustal strength profile was established for the central-south Tanlu Fault Zone. The profile reveals a relatively strong upper and middle crust underlain by an extremely weak lower crust, with regional tectonic forces primarily transmitted through the upper and middle crust. This extremely weak lower crust is closely linked to the destruction of the North China Craton since the Mesozoic, likely serving as both a consequence and a facilitating mechanism of the deep deformation processes that led to lithospheric thinning.