Gang Luo

Geomechanical modeling of stresses adjacent to salt bodies: Part 1—Uncoupled models

We compare four approaches to geomechanical modeling of stresses adjacent to salt bodies. These approaches are distinguished by their use of elastic or elastoplastic constitutive laws for sediments surrounding the salt, as well as their treatment of fluid pressures in modeling. We simulate total stress in an elastic medium and then subtract an assumed pore pressure after calculations are complete; simulate effective stress in an elastic medium and use assumed pore pressure during calculations; simulate total stress in an elastoplastic medium, either ignoring pore pressure or approximating its effects by decreasing the internal friction angle; and simulate effective stress in an elastoplastic medium and use assumed pore pressure during calculations. To evaluate these approaches, we compare stresses generated by viscoelastic stress relaxation of a salt sphere. In all cases, relaxation causes the salt sphere to shorten vertically and expand laterally, producing extensional strains above and below the sphere and shortening against the sphere flanks. Deviatoric stresses are highest when sediments are assumed to be elastic, whereas plastic yielding in elastoplastic models places an upper limit on deviatoric stresses that the rocks can support, so stress perturbations are smaller. These comparisons provide insights into stresses around salt bodies and give geoscientists a basis for evaluating and comparing stress predictions.

Geomechanical modeling of stresses adjacent to salt bodies: Part 2—Poroelastoplasticity and coupled overpressures

We use a fully coupled poroelastoplastic geomechanical model to study how stresses and pore pressures evolve in sediments bounding a spherical salt body. Drained analyses (pore pressures remain hydrostatic) demonstrate that sediments yield in response to loading by the salt, which leads to a redistribution of stresses and to deformations larger than predicted by poroelastic or solid Coulomb-plastic models. Undrained analyses (overpressures develop while no dissipation occurs) illustrate that salt loading induces pore pressures that extend kilometers away from the salt body. We also model the flow and consequent dissipation that occur in the sediments because of this undrained salt loading. We show that with time, the pressure field dissipates and expands. The dissipation process takes millions of years, which suggests that pore-pressure perturbations caused by salt loading should still be present in mudstones near many salt bodies. Under drained conditions, stress perturbations generate low minimum principal stresses above and below the salt, resulting in convergence of pore pressure and minimum principal stress at these locations. Such conditions are challenging to drill through. In undrained systems, sharp drops in pore pressure may occur above and below the salt, whereas both the pore pressure and the minimum principal stress rise next to the salt. In contrast to previous models that do not couple changes in stress to changes in pore pressure, the coupled approach presented here has the potential to predict in-situ stresses and pore pressures more accurately in a wide variety of geologic settings.

Multi-timescale mechanical coupling between the San Jacinto fault and the San Andreas fault, southern California

Fault interaction is believed to influence seismicity and crustal deformation, but the mechanics of fault interaction over various time scales remain poorly understood. We present here a numerical investigation of fault coupling and interaction over multiple time scales, using the San Andreas fault and the San Jacinto fault in southern California as an example. The San Andreas fault is the Pacific–North American plate boundary, but in southern California, a significant portion of the relative plate motion is accommodated by the subparallel San Jacinto fault. We developed a three-dimensional viscoelastoplastic finite-element model to study the ways in which these two faults may have interacted (1) during and following individual earthquakes, (2) over multiple seismic cycles, and (3) during long-term steady-state fault slip. Our results show that the cluster of nine moderate-sized earthquakes (M 6–7) on the San Jacinto fault since 1899 may have lowered the Coulomb stress on the southern San Andreas fault, delaying the “Big One,” an earthquake of magnitude 7.8 or greater that may result from rupture of much of the southern San Andreas fault. In addition to the static Coulomb stress changes associated with individual earthquakes, variations of seismicity over seismic cycles on one fault can influence the loading rate on the other fault. When the San Jacinto fault experiences clusters of earthquakes such as those in the past century, the loading rate on the San Andreas fault can be lowered by as much as ∼80%. Over longer time scales, these two faults share the slip needed to accommodate the relative plate motion. Hence, an increase in slip rate on one of these two faults causes complementary decrease on the other, which is consistent with geological observations.

The role of pore fluid overpressure in the substrates of advancing salt sheets, ice glaciers, and critical‐state wedges

Critical-state wedges, ice glaciers, and salt sheets have many geometric and mechanical similarities. Each has a tapering geometry and moves along a basal detachment. Their motions result from the combined effects of internal deformation and basal sliding. Wedge deformation and geometry, basal conditions, and overpressure (pore fluid pressure less hydrostatic pore fluid pressure) development within the substrate interact with each other in this mechanically coupled system. However, the nature of this interaction is poorly understood. In order to investigate this coupled system, we have developed two-dimensional poromechanical finite-element models with porous fluid flow in sediments. We have simulated the advance of a salt sheet wedge across poroelastic sediments in this study. We emphasize that our results have applications beyond salt wedges to both critical-state wedges and ice glaciers. Overpressure develops within the substrate over time during the advance of the wedge. The magnitude of the overpressure influences the wedge geometry and the wedge advance rate. Lower overpressure results in a thicker and steeper wedge geometry, and a slower advance rate, while higher overpressure favors a thinner, wider, and more flattened wedge geometry and a faster advance rate. This study provides key insights into the links between wedge geometry, basal shear stress, and overpressure in substrates.