Earthquake physics (1)

The ability to forecast earthquake timing would represent the most significant breakthrough in geohazard mitigation for our societies. Recent large-scale seismological observations in Baja California have shed light on the occurrence of strain localization and foreshock migration towards the epicenter of large earthquakes, years before the actual rupture. However, this process that could serve as an early warning is still largely unknown. To this day, rock mechanics experiments have focused on strain localization on intact samples and friction experiments mostly overlooked volumetric processes that could occur in the rock surrounding the fault. In this study, we present state of the art friction experiments conducted in an oil-confined biaxial shear apparatus. We recorded rate-and-state friction parameters and off-fault deformation in bare surfaces of Carrara marble subjected to increasing load point velocity (from 10-6 to 10-2 m/s) under low and high stresses (Pc = 15 and 50 MPa). Our findings indicate that there is a direct link between stick-slip nucleation and off-fault deformation at low load point velocity when rock deformed inelastically, provided that the fault is conditionally unstable (a-b < 0). We suggest that off-fault inelastic deformation may trigger unstable slip by decreasing the stiffness of the surrounding rock volume. Our study suggests that inelastic off-fault deformation favors earthquake nucleation, and the localized to ductile transition may partially control the minimum depth of the seismogenic zone. Furthermore, we also present the first laboratory observation of unequivocal precursory strain localization around a fault during stick-slip cycles.

Faults and fractures are important geological features that need special attention in many engineering tasks. These discontinuities in the rock mass are key elements in seal integrity evaluations for subsurface CO2 storage. Stress changes associated with CO2 injection into reservoir formations can potentially trigger movement along pre-existing fractures. It is therefore necessary to evaluate the capability of the rock mass to withstand such movement which can lead to CO2 migration out of the confinement or unwanted microseismicity. In this study, the aim is to increase the understanding of fractured rock mass strength through the comparison between experiments performed in direct shear box and triaxial apparatus. The direct shear box has traditionally been the preferred method of quantifying the resistance of a rock mass to slip along pre-existing weakness planes. However, from subsurface sampling depths of kilometres to testing in the laboratory, a fracture will undergo stress relief that is likely to have effect on the rock mass cohesion. This is underlined by the fact that the two faces of a fractured rock mass are usually taken apart from each other during sample preparation. In triaxial tests, the purpose is often to fracture an intact sample of the rock. Further movement along this newly created fracture should therefore have many similarities with the shear box experiment, except for the beforementioned challenges with stress relief and possible fracture mismatch. Yet, to our knowledge, little work has been done trying to compare the residual strength measured in the triaxial with that measured in the direct shear box. In this experimental study, samples of the Berea sandstone are tested in both the direct shear box and triaxial apparatus. Results from direct shear box tests on pre-cut samples are compared with results from triaxial tests with shear mobilization on newly created fractures. In addition, experiments on pre-cut cylindrical samples tested using the inverted shear end caps in the triaxial apparatus are included to further bridge the gap between results obtained in the direct shear box and triaxial tests. Tests are performed under a range of normal stresses and using both air, brine and oil as pore fluid to identify potential sensitivities of the various test protocols. Results show how the residual strength in the triaxial test is connected to the slip resistance in the direct shear box, thus providing important information needed in the evaluation of typical experimental rock mechanics studies.