A SIMPLER EXPLANATION FOR FAULT DIP ANGLES DISTRIBUTION?

Thrust faulting events, usually featured by dip angles ranging in between 5°- 30°, mostly take place along convergent plate boundaries and are caused by the elastic strain accumulation, which is released by multifaceted fault slip dynamics, varying from almost periodic silent events to mega-quakes. Strike-slip-faulting earthquakes localize along steeply dipping faults (70°- 90°) or transcurrent plate boundaries and transfer zones. Finally, normal faults develop along rift zones in extensional regimes having intermediate dip (45°- 65°), with a dominant gravitational contribution to their energy budget. Structural, morphological, and geophysical differences have been highlighted among the three main tectonic settings (Anderson, 1905). This empirical observation is usually explained by the orientation of stress components and the slope of the yield envelope defined by the Mohr-Coulomb criterion, often called critical-stress theory, assuming frictional properties of the crustal rocks (Sibson, 1974).

However, why the slope has a given value? In a recent paper (Zaccagnino and Doglioni, 2023) we suggest that the slope dip is constrained by the occurrence of the largest shear stress gradient along fault inclination. High homogeneous shear stress, i.e., without gradients, may generate aseismic creep as for example in flat decollements, both along thrusts and low angle normal faults, whereas along ramps larger shear stress gradients determine greater energy accumulation and stick-slip behaviour with larger sudden seismic energy release. Therefore, we set up a simple model and test it 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 correctly reproduces observations. Our idea complements previous knowledge on friction and faulting.

References

Anderson, E.M. (1905). The dynamics of faulting. Transactions of the Edinburgh Geological Society 8, 387–402.

Sibson, R.H. (1974). Frictional constraints on thrust, wrench and normal faults. Nature 249, 542–544.

Zaccagnino, D., & Doglioni, C. (2023). Fault dip vs shear stress gradient. Geosystems and Geoenvironment, 100211.