Natural variability of fault friction and slip uncertainty exist in the Earth's crust. To what extent it influences crustal stress and its evolution is intriguing. We established a quasi-static, 2D model to simulate the stress evolution due to Coulomb frictional slips in the brittle crust. The model simply features randomly-oriented fractures with heterogeneous frictional coefficients. We emphasized the global stress response by summing the contribution of cascades of local frictional slip under specific boundary conditions. We illustrated that the decrease in stress difference manifests as a self-organized process that ultimately leads to frictional equilibrium. The model informs that the frictional equilibrium of a stochastic system can depart substantially from a deterministic estimation. Although the model quantitatively corroborates the notion of frictional equilibrium in places where fracture slip is the dominant mechanism for stress release, it reveals far more profound influence of system heterogeneity on the local and global stress evolution.

We characterized the dependence of ultrasonic velocities on confining pressure (Pc) and pore pressure (Pp) of six argon-saturated cores from three formations associated with the Bakken play in the Williston Basin (Lodgepole, Middle Bakken and Three Forks). We cycled Pc under constant Pp to simulate reservoir stress changes associated with depletion and injection. The ultrasonic velocities (Vp and Vs) in the axial direction were measured along the loading path, based on which the elastic moduli and effective stress coefficient were derived. Common to all specimens, both Vp and Vs under injection are consistently higher than under depletion at the same Pc and Pp. Derived elastic moduli assuming material isotropy qualitatively agree with logging data, but are consistently higher than those based on static measurements. We found the effective stress coefficient (α) with respect to Vp and Vs is close to unity when the simple effective stress is no more than 10 MPa, regardless of wave type, lithology and loading path. α for Vp and Vs generally increases for higher simple effective stress (Pc-Pp) and beyond unity, which is contrary to the trend obtained through static deformation and against theoretical expectations. It implies that Vp and Vs become more sensitive to Pp rather than Pc as (Pc-Pp) rises. This apparent increase of α with (Pc-Pp) is a priori unresolved, but can be plausibly attributed to the fact that the change of (Pc-Pp) altered the rock microstructure, which essentially rendered the pore pressure more effective. Submission Files Included in this PDF File Name [File Type] cover letter.docx [Cover Letter] Ma et al. IJRMMS.docx [Manuscript File] Ma et al. IJRMMS_figures.pdf [Figure] declaration-of-competing-interests_Ma.docx [Conflict of Interest] To view all the submission files, including those not included in the PDF, click on the manuscript title on your EVISE Homepage, then click ‘Download zip file’.

Subsurface engineering applications, such as the development of enhanced geothermal systems and unconventional oil and gas reservoirs, and the geologic disposal of CO 2 and waste water, involve fluid injection into the rock masses deep underground. Elevated fluid pressure can trigger shear slip(s) of pre-existing natural fractures, resulting in permeability changes. The mechanism of slip-induced permeability variation, however, is still not fully understood, and the permeability evolution associated with multiple slips is even more complex. We therefore performed laboratory experiments to investigate the fracture permeability evolution induced by shear slip in both saw-cut and natural fractures with rough surfaces. Our experiments show that the triggering fluid pressures required to induce slip in natural fractures are larger than in saw-cut fractures, likely due to the rougher surface of the natural fractures. We further observe that a critical shear displacement dominates the permeability evolution. When the accumulative shear displacement reaches the critical shear displacement, we find an initial permeability increase for natural fractures, followed by a perme-ability decrease after the most significant slip event. For the saw-cut fractures, the first slip shows the largest shear displacement and related permeability change, while further slips result in diminishing displacements. From the beginning to the end of all experiments, no consistent permeability increase or decrease is observed. Although the change of the fracture surface was not measured quantitatively, we found gouge material after rinsing each natural sample. tribute the slip-induced permeability evolution to the relationship between the damage of the main asperities, which decreases permeability, and the scale of the shear displacement, which increases permeability through shear dilation.

Four large hydropower stations have recently been built downstream the Jinsha River in Southwestern China with a strong regional tectonic activity background. There is widely felt seismicity since the impoundment of the Xiluodu and Xiangjiaba reservoirs, increasing the public concern in this region. We begin with a criticality analysis of the faults near these reservoirs to quantify their susceptibility to triggered seismicity. Then we focus on the Xiluodu reservoir to investigate the correlation between the impoundment and seismicity nearby. We analyze the spatio-temporal distribution of seismicity near the Xiluodu reservoir, and identify the plausible rapid and delayed seismic response due to the impoundment. According to the impoundment record, we explicitly model the hydro-mechanical changes due to diffusion and reservoir water load, i.e., in pore pressure, elastic stress, and the resulting Coulomb stress. Our results show that the pore pressure changes can reach a level that may trigger fault reactivation and consequently, seismicity nearby. The water load can also induce the positive Coulomb stress changes on faults, depending on the fault orientation, which is especially important for understanding the earthquakes that occurred shortly after the impoundment and at more than 10 km distance from the reservoir. The combination of these two effects can induce positive Coulomb stress change over a larger area, which overlaps the majority of the events after the impoundment. While the causal relationship between the impoundment and seismicity warrants further analysis, we hope to inform the regional seismic impact of impoundment with this timely study.