Fault Stability Across the Seismogenic Zone

Abstract

Across the seismogenic zone, the transition from brittle to plastic deformation corresponds to a semibrittle regime where brittle fracturing and plastic flow coexist at high strength conditions. Thorough experimental investigations on brittle‐plastic transition are crucial to understand why natural faults behave in stable or unstable ways at varying crustal depths and why large earthquakes generally nucleate at the base of the seismogenic zone. To investigate semibrittle deformation in carbonates and the conditions promoting it, we reported here the results of experiments performed on Carrara marble saw cut faults in triaxial conditions. We studied the influence of the confining pressure (ranging between 45 and 180 MPa), axial loading rates (0.01 and 1 μm s−1) and initial fault roughness (smooth and rough) on fault (in‐)stability across the brittle‐plastic transition. We conclude that laboratory earthquakes may nucleate on inherited fault interfaces at brittle‐plastic transition conditions. The occurrence of laboratory earthquakes associated with increasing plastic deformation is promoted at high confining pressure, on smooth fault interfaces, or when the loading rate is slow. In a rather counterintuitive manner, increasing initial roughness promotes stable sliding and a larger amount of plastic deformation. Furthermore, we show that stable sliding tends to produce mirror‐like surfaces, while stick‐slips are associated with matte surfaces, on which the size of the asperities grows with increasing confining pressure. Finally, our results seem to reveal the influence of asperity hardness and melt viscosity on fault weakening.

Reproducing laboratory earthquakes with a discrete-continuum model

“…For σ3=45 MPa, friction coefficient reaches 0.40 before the first stress drop and stabilizes around 0.33 during established seismic cycle; While for σ3=180 MPa, peak and average plateau friction coefficients are close to 0.32 and 0.25, respectively. This low friction is close to that observed in similar experimental conditions [1,2].…”

“…As for the experiments reported in [1,2], the numerical model reproduces the deformation of a saw-cut sample (Fig. 1).…”

“…Future work will be dedicated to overcome the current limitations of the model by increasing its size, by introducing a realistic roughness at the fault surface, and by implementing plasticity in the bulk rock continuum model, observed in [1,2]. More data about stress redistributions and fault damage evolution, as well as detailed statistics on acoustic and slip events, will also be gathered in order to analyse more deeply the influence of gouge emission on the fault rheology.…”

“…We present a numerical model which aims at reproducing triaxial experiments on saw-cut marble samples described in [1,2]. This model is in 2D and miniaturized when compared to its real-world counterpart, but includes relevant deformation mechanisms.…”

Brittle Faulting of Ductile Rock Induced by Pore Fluid Pressure Build‐Up

“…Following recent data analysis methods (see Aubry et al., 2020; Meyer et al., 2019), strain partitioning between brittle and ductile deformations can be obtained by comparing the strain measured with the LVDTs and with the strain gages. This method helps in the differentiation between bulk deformation and sliding on a fault plane.…”