Mechanical Energy Dissipation During Seismic Dynamic Weakening in Calcite-Bearing Faults

Abstract

Earthquakes are frictional instabilities caused by the shear stress decrease, that is, dynamic weakening, of faults with slip and slip rate. During dynamic weakening, shear stress depends on slip, slip rate, and temperature, according to constitutive laws governing the earthquake rupture process. In the laboratory, technical limitations in measuring temperature during frictional instabilities inhibit the investigation and interpretation of shear stress evolution. Here we conduct high velocity friction experiments on calcite-bearing simulated faults, both on bare-rock and on gouge samples, at 20–30 MPa normal stress, 1–6 m/s slip rate and 1–20 m total slip. Seismic slip pulses are reproduced by imposing boxcar and regularized Yoffe slip rate functions. We measured, together with shear stress, slip, and slip rate, the temperature evolution on the fault by employing an innovative two-color fiber optic pyrometer. The comparison between modeled and measured temperature reveals that for calcite-bearing faults the heat sink caused by decarbonation reaction controls the temperature evolution. In bare-rocks, energy is dissipated as frictional heat, and temperature increase is buffered by the heat sink of the calcite decarbonation reaction. In gouges, energy is dissipated as frictional heat and for plastic deformation processes, balanced by the heat sink caused by the decarbonation reaction enhanced by the mechanochemical effect. Our results suggest that in calcite-bearing rocks, a common fault zone material for earthquake sources in the continental crust at shallow depth, the type of fault materials (bare-rocks vs. gouges) controls the energy dissipation during seismic slip.