Detecting transition of stress and friction conditions by fault-slip analysis: Application to outcrop-scale faults in the Quaternary graben fill in the southwest Japan arc

Abstract

Conventional stress tensor inversion techniques detect multiple stress conditions by minimizing misfit angles between observed fault-slip directions and calculated shear stress directions. Although this strategy is effectual for analyzing reactivated faults, frictionless faults, and faults overpressured with crustal fluids, it tends to fall into a low detectability of stress tensors because of the weak constraint. This study incorporated the fault instability (friction condition) into the objective function of stress tensor inversion for the purposes of improving the detectability and determining the friction coefficients on faults. The modified function was found to be useful to detect multiple stress and friction conditions from heterogeneous fault-slip data especially when they have concentrations of fault plane orientations. The method was applied to outcrop-scale faults cutting the Quaternary strata which filled the Beppu-Shimabara graben in the southwest Japan arc. As a result, a stress transition from ENE-WSW tension to NNE-SSW tension at ca. 1 Ma was found. The friction coefficient on faults has changed from 0.5 to 0.9, which implies a hardening of the strata through diagenesis.