Can spatial stress gradients explain optimal dip angles of active faults?

Thrusts are usually aligned along dip angles ranging in between 5°-30°; strike-slip-faulting earthquakes localize along steeply dipping faults (70°-90°), while normal-faulting events are nucleated along rifts in extensional regimes having intermediate dip (45°-65°). Structural, morphological, and geophysical differences are well-known in the three main tectonic settings. This empirical observation is usually explained by the orientation of the stress tensor and the slope of the yield envelope defined by the Mohr-Coulomb criterion (critical-stress theory) assuming a complete frictional control of faulting. However, why the slope has a given value? We suggest that the slope dip is constrained by the occurrence of the largest shear stress gradient along that inclination (Zaccagnino and Doglioni, 2023). We model the optimal dip angle in different tectonic settings as a function mechanical property of rocks and stress orientation and test our hypothesis using about three hundred dip angles of non-volcanic shallow (depth less than 30 km) global large (Mw > 7.0) natural seismic events from 1990 to 2021. Our model well reproduces observations. We also try to explain some peculiar cases such as low-angle normal faults in terms of local stress field rotation at shallow depths. Aseismic creep often localized along flat decollements is associated, in our framework, to elevated but spatially homogeneous stress both along thrusts and low angle normal faults, whereas along ramps larger shear stress gradients determine significant energy accumulation and stick-slip behaviour. Reference Zaccagnino, D., & Doglioni, C. (2023). Fault dip vs shear stress gradient. Geosystems and Geoenvironment, 100211.