Title: How Strain Localization and Multi-Physical Couplings Affect Fault Mechanics

Abstract: Most earthquakes are induced by a frictional instability along a pre-existing fault. The knowledge of the friction (shear strength) evolution in a fault is therefore of major importance, as it allows extracting many characteristics and features of seismic slip. In particular, the decrease of the friction with increasing velocity or displacement (a process called frictional weakening) determines the possible nucleation of earthquakes. Due to the extreme loading conditions during seismic slip, many competing physical phenomena are occurring (like grain crushing, mineral decomposition, thermal pressurization, melting, among others) and influence greatly the friction evolution. Moreover, Seismic slip is accompanied by extreme shear strain localization into a thin zone of finite thickness, which is often called Principal Slip Zone.

The phenomenon of strain localization is strongly related to fault’s weakening. However, to capture strain localization and obtain a finite band thickness, we will show that we need to consider high order continua, such as Cosserat continua. This theory is particularly interesting because it also allows the size of the microstructure to be taken into account explicitly in the model and it has been shown that this parameter plays an important role in the behavior of faults. In this work we present numerical simulations and experimental results on the influence of the microstructure size on the shear band thickness, using a Digital Image Correlation analysis of triaxial shear tests.

Moreover, experimental results have revealed a drastic decrease of the friction coefficient for velocities close to the maximum seismic one (1m/s), independently of the material studied. These high velocities induce an important shear heating. As a result, the temperature increase can be very large and can induce phase change in the material. Here we show, in a second part of the presentation, that a large set of experimental data for different rocks can be described by such thermally activated mechanisms, combined with the production of weak phases. By taking into account the energy balance of all processes during fault movement, we present a framework that reconciles the data, and is capable of explaining the frictional behavior of faults, across the full range of slip velocities.