No abstract

Slip on unfavourably oriented faults with respect to a remotely applied stress is well documented and implies that faults such as the San Andreas fault and low-angle normal faults are weak when compared to laboratory-measured frictional strength. If high pore pressure within fault zones is the cause of such weakness, then stress reorientation within or close to a fault is necessary to allow sufficient fault weakening without the occurrence of hydrofracture. From field observations of a major tectonic fault, and using laboratory experiments and numerical modelling, here we show that stress rotation occurs within the fractured damage zone surrounding faults. In particular, we find that stress rotation is considerable for unfavourably oriented 'weak' faults. In the 'weak' fault case, the damage-induced change in elastic properties provides the necessary stress rotation to allow high pore pressure faulting without inducing hydrofracture.

[1] The characterization of time-dependent brittle rock deformation is fundamental to understanding the long-term evolution and dynamics of the Earth's crust. The chemical influence of pore water promotes time-dependent deformation through stress corrosion cracking that allows rocks to deform at stresses far below their short-term failure strength. Here, we report results from a study of time-dependent brittle creep in water-saturated samples of Darley Dale sandstone (initial porosity, 13%) under triaxial stress conditions. Results from conventional creep experiments show that axial strain rate is heavily dependent on the applied differential stress. A reduction of only 10% in differential stress results in a decrease in strain rate of more than two orders of magnitude. However, natural sample variability means that multiple experiments must be performed to yield consistent results. Hence we also demonstrate that the use of stress-stepping creep experiments can successfully overcome this issue. We have used the stress-stepping technique to investigate the influence of confining pressure at effective confining pressures of 10, 30, and 50 MPa (while maintaining a constant 20 MPa pore fluid pressure). Our results demonstrate that the stress corrosion process appears to be significantly inhibited at higher effective pressures, with the creep strain rate reduced by multiple orders of magnitude. The influence of doubling the pore fluid pressure, however, while maintaining a constant effective confining pressure, is shown to influence the rate of stress corrosion within the range expected from sample variability. We discuss these results in the context of microstructural analysis, acoustic emission hypocenter locations, and fits to proposed macroscopic creep laws.

The reliable assessment of volcanic unrest must rest on an understanding of the rocks that form the edifice. It is their microstructure that dictates their physical properties and mechanical behavior and thus the response of the edifice to stress perturbations during unrest. We evaluate the interplay between microstructure and rock properties for a suite of edifice-forming rocks from Volcán de Colima (Mexico). Microstructural analyses expose (1) a pervasive, isotropic microcrack network, (2) a high, subspherical vesicle density, and (3) a wide vesicle size distribution. This complex microstructure severely impacts their physical and mechanical properties. In detail, porosities are high and range from 8 to 29%. As a consequence, elastic wave velocities, Youngs moduli, and uniaxial compressive strengths are low, and permeabilities are high. All of the rock properties demonstrate a wide range. For example, strength decreases by a factor of 8 and permeability increases by 4 orders of magnitude over the porosity range. Below a porosity of 11-14%, the permeability-porosity trend follows a power law with a much higher exponent. Microstructurally, this represents a critical vesicle content that efficiently connects the microcrack population and permits a much more direct path through the sample, rather than restricting flow to long and tortuous microcracks. Values of tortuosity inferred from the Kozeny-Carman permeability model support this hypothesis. However, we find that the complex microstructure precludes a complete description of their mechanical behavior through micromechanical modeling. We urge that the findings of this study be considered in volcanic hazard assessments at andesitic stratovolcanoes.