JGR–Solid Earth

Gypsum, an anisotropic hydrous mineral, localizes strain through dehydration-induced embrittlement and hydrodynamic lubrication, affecting the deformation in active tectonic settings. Here, we examine the micromechanical status of syn- and post-dehydration gypsum-phases, crucial for understanding intra-crystalline deformation. We micro-indented the (010) planes of natural gypsum crystals from 30 to 140°C at various strain-rates to track mechanical changes during progressive dehydration and phase transition. Thermogravimetric analyses reveal that gypsum dehydrates between 100 and 140°C following a sigmoidal curve. Micromechanical data show, at 30°C, the mean hardness of gypsum is 2.42 GPa, reducing to 0.5 GPa at 140°C, after complete dehydration. Following the sigmoidal dehydration trend, elastic modulus, fracture toughness, and yield stress likewise drops with progressive dehydration. The Gypsum-Hemihydrate phase exhibits strain-rate sensitive elastoplastic deformation between 30 and 110°C, as hardness decreases with increasing strain-rate. We propose that delamination of the layers and extensive tensile cracking along [010] cause this strain-rate sensitive behavior, displaying Indentation Size Effect. Dehydrated phases (Hemihydrate and γ-Anhydrite) are formed as exfoliated pseudomorphic flakes through topotactic transformation along [010] and show two sets of oppositely dipping slip systems. During the late dehydration stage (120–140°C), plastic strain is accommodated by the sliding of these flakes along with inelastic pore compaction, resulting in strain-rate insensitive plastic deformation of the system.

Dynamic perturbations reveal unconventional nonlinear behavior in rocks, as evidenced by field and laboratory studies. During the passage of seismic waves, rocks exhibit a decrease in elastic moduli, slowly recovering after. Yet, comprehensive physical models describing these moduli alterations remain sparse and insufficiently validated against observations. Here, we demonstrate the applicability of two physical damage models—the internal variable model (IVM) and the continuum damage model (CDM)—to provide quantitative descriptions of nonlinear co-seismic elastic wave propagation observations. While the IVM uses one internal variable to describe the evolution of elastic material moduli, the CDM damage variable is a mathematical representation of microscopic defects. We recast the IVM and CDM models as nonlinear hyperbolic partial differential equations and implement 1D and 2D numerical simulations using an arbitrary high-order discontinuous Galerkin method. We verify the modeling results with co-propagating acousto-elastic experimental measurements. Subsequently, we infer the parameters for these nonlinear models from laboratory experiments using probabilistic Bayesian inversion and 2D simulations. By adopting the Adaptive Metropolis Markov chain Monte Carlo method, we quantify the uncertainties of inferred parameters for both physical models, investigating their interplay in 70,000 simulations. We find that the damage variables can trade off with the stress-strain nonlinearity in discernible ways. We discuss physical interpretations of both damage models and that our CDM quantitatively captures an observed damage increase with perturbation frequency. Our results contribute to a more holistic understanding of co-seismic damage and post-seismic recovery after earthquakes bridging the worlds of theoretical analysis and laboratory findings.

Surface hotspot motions are approximately a factor of two faster in the Pacific than the Indo-Atlantic, and the Indo-Atlantic large low shear velocity province (LLSVP) appears to be significantly taller than the Pacific LLSVP. Hypothesizing that surface hotspot motions are correlated with the motion of plume sources on the upper surface of chemically distinct, intrinsically dense LLSVPs, we use 3D spherical mantle convection models to compute the velocity of plume sources and compare with observed surface hotspot motions. No contrast in the mean speed of Pacific and Indo-Atlantic hotspots is predicted if the LLSVPs are treated as purely thermal anomalies and plume sources move laterally across the core-mantle boundary. However, when LLSVP topography is included in the model, the predicted hotspot speeds are, on average, faster in the Pacific than the Indo-Atlantic, even when modest topography is assigned to both LLSVPs (e.g., 100–300 km). The difference in mean hotspot speed increases to a factor of two for larger and laterally variable LLSVP topography estimated from seismic tomographic model S40RTS (up to 1,100–1,500 km for the Indo-Atlantic region vs. 700–1,400 km for the Pacific region) and our results also broadly reproduce the convergence of Pacific hotspots toward the center of the Pacific LLSVP. These largescale features of global hotspot motions are only reproduced when ambient mantle material flows over large, relatively stable topographical features, suggesting that LLSVPs are chemically distinct and intrinsically dense relative to ambient mantle material.

The evolution of fluid injection-induced seismicity, generally characterized through the number of events or their seismic moment, depends on, among other factors, the injected fluid volume. Migration of seismicity is observed during those sequences and might be caused by a range of mechanisms: fluid pressure diffusion, fluid-induced aseismic slip propagating along a stimulated fault, interactions between earthquakes. Recent theoretical and observational developments underline the important effect on seismicity migration of structural parameters, like fault criticality, or injection parameters, like flow rate or pressurization rate. Here, we analyze two well-studied injection-induced seismic sequences at the Soultz-sous-Fôret and Basel geothermal sites, and find that the evolution of the seismicity front distance primarily depends on the injected fluid volume. Based on a fracture mechanics model, we develop new equations relating seismicity migration to injected fluid volume and frictional and structural properties of the fault. We find that the propagation of a fluid-induced aseismic slip front along the stimulated fault, triggering seismicity, explains well the observations made on the two sequences. This model allows us to constrain parameters describing the seismicity front evolution and explains the diversity of migration patterns observed in injection-induced and natural earthquake swarms.

3D forward gravity modeling combined with receiver function (RF) analysis characterizes the crustal structures of the southern part of the Mongolian collage. The seismic signals of the 48 stations of the MOBAL2003 and the IRIS-PASSCAL experiments were analyzed to get the RFs. This analysis revealed a significant difference between the crustal structures of the Hangay dome and the tectonic zones in the south. In addition, seismic stations south of the Hangay dome display significant signals related to the occurrence of a low-velocity zone at lower crustal level confirmed by the gravity anomalies. Finally, these seismic analysis inputs, the boundaries, the lithologies, and the density values from rock samples of the different tectonic zones constitute the starting points from the 3D forward gravity modeling. The resulting crustal density model indicates: (a) the likely absence of a Precambrian basement block beneath the Hangay dome, (b) an alternation of two low-velocity/low-density zones (LVLDZs) with high-density zones in the Baydrag microcontinent interpreted as fragments of early Tonian plutons, (c) the occurrence of an LVLDZ at the lower crustal level beneath the Lake zone, the Mongol-Altai Accretionary Wedge, and the Trans-Altai Zone. Therefore, the combination of the seismic RF with gravity analysis and modeling reveals new crustal structures of the Mongolian collage and enhances the occurrence and the extent of an LVLDZ at lower crustal level. These LVLDZ may demonstrate the existence of the relamination of a hydrous material in southern Mongolian collage.

We determine high-resolution tomographic models of isotropic P-wave velocity (Vp) and tilting-axis anisotropy of the Alaska subduction zone using a large number of local and teleseismic data recorded at many portable and permanent network stations in and around Alaska. We find a flat high-Vp slab in the mantle transition zone (410–670 km depths) beneath western Alaska, which is connected with the subducting Pacific slab at 0–410 km depths, suggesting that a big mantle wedge has formed under western Alaska. Our tilting-axis anisotropy model reveals complex mantle flows in the asthenosphere. Corner flow in the mantle wedge above the subducting Pacific slab and toroidal flow in the big mantle wedge are revealed, which may cause the Cenozoic intraplate volcanoes in western Alaska and the Bering Sea. In central Alaska, the mantle wedge beneath the Denali volcanic gap is characterized by high-Vp and subhorizontal fast velocity directions normal to the volcanic arc, which may reflect a remnant of the subducted Yakutat slab. In SE Alaska, the shallow subduction of the Wrangell slab is visible above 150 km depth, and hot mantle upwelling through the Wrangell-Yakutat slab gap may contribute to the Wrangell volcanic field.

Two-phase flow, a system where Stokes flow and Darcy flow are coupled, is of great importance in the Earth's interior, such as in subduction zones, mid-ocean ridges, and hotspots. However, it remains challenging to solve the two-phase equations accurately in the zero-porosity limit, for example, when melt is fully frozen below solidus temperature. Here we propose a new three-field formulation of the two-phase system, with solid velocity (v s ), total pressure (P t ), and fluid pressure (P f ) as unknowns, and present a robust finite-element implementation, which can be used to solve problems in which domains of both zero porosity and non-zero porosity are present. The reformulated equations include regularization to avoid singularities and exactly recover to the standard single-phase incompressible Stokes problem at zero porosity. We verify the correctness of our implementation using the method of manufactured solutions and analytic solutions and demonstrate that we can obtain the expected convergence rates in both space and time. Example experiments, such as self-compaction, falling block, and mid-ocean ridge spreading show that this formulation can robustly resolve zero- and non-zero-porosity domains simultaneously, and can be used for a large range of applications in various geodynamic settings.