Complex interaction between brittle and ductile rheologies in slow laboratory earthquakes

Earthquakes originate from frictional instabilities that nucleate and propagate along faults cutting through a multilayered crust. Geological observations show that fault zones are complex structures often composed of heterogeneous mineral assemblages with contrasting frictional rheologies, whose interaction strongly influences slip behavior during fault reactivation. Two major earthquakes (Mmax > 6) that struck central Italy in the past 30 years–the 1997 Colfiorito and 2016 Norcia events–nucleated within Triassic Evaporites (TE) consisting of anhydrites and dolostones. Previous laboratory experiments on stick-slipping faults in TE gouges highlighted the key role of shear zone fabric in controlling breakdown processes and slip dynamics. However, the individual contribution of each lithology to frictional failure mode remains unclear.

Here we present preliminary results from laboratory friction experiments on powdered TE samples aimed at disentangling the role of anhydrite and dolostone in controlling fault slip behavior and dynamics in TE faults. We conducted double-direct shear experiments on three gouge compositions: 100% anhydrite, 50:50 anhydrite-dolostone, and 100% dolostone. The experimental procedure comprises two main stages: (i) a fabric development phase, in which the gouge is sheared under 50 MPa normal stress at a load-point velocity of 10 μm/s; and (ii) a fault reactivation phase at boundary conditions designed to enhance frictional instabilities that are: normal stress reduced to 30 MPa, a low-stiffness element inserted in series with the shear loading axis, and re-shearing at 1 μm/s. We also monitored the evolution of the fault physical properties from fabric development to stick-slip, via an ultrasonic system that continuously transmits and receives acoustic waves through the experimental fault.

During the fabric development stage, all three fault gouges display stable sliding with a friction coefficient μ of ~ 0.6. In the fault reactivation stage, anhydrite faults exhibit slow (v < 20 μm/s), repetitive stick-slip with small stress drops (Δ𝛕 < 0.1 MPa), whereas dolostone faults accommodate shear through stable sliding. Interestingly, the 50:50 anhydrite-dolostone mixture does not exhibit intermediate behavior between the two  end-members but instead develops larger (0.1 < Δ𝛕 < 0.4 MPa), yet generally slower (v < 10 μm/s), slip instabilities, indicating a nonlinear mechanical interaction between anhydrite and dolostone.

Post-experiment microstructural analyses reveal that single-component gouges deform via cataclastic flow and frictional sliding along boundary and Riedel shear bands. In contrast, the 50:50 mixture exhibits extremely localized boundary shear planes dominated by nanometric dolostone particles embedded within foliated anhydrite-dolostone S-C structures. These features suggest a significant contribution of ductile, distributed deformation to energy dissipation during slow frictional ruptures. Ongoing ultrasonic wave analyses aim to characterize the relationship between the evolution of the elastic properties of the fault gouge, its internal structure, and the resulting slip behavior.

Our results provide new insights into the complex interplay between different frictional rheologies within fault zones of the seismogenic layer of northern Apennines, and highlight the role of compositional heterogeneity in controlling fault slip dynamics and energy dissipation.