Journal of Geophysical Research: Solid Earth最新文献

Induced shear failure is key for enhancing hot dry rock (HDR) resource exploitation, aiming to creating an extensive fracture network. Although the macroscopic shear behavior of HDR has been extensively investigated, its microscopic shear failure mechanisms remain unclear. In this study, microscale mode II shear fracture experiments were performed for the first time under various thermal cycling. Micro-scale double-edge notched cube specimens were fabricated from primary minerals and interfaces using micro-machining and subjected to in situ shear tests under a scanning electron microscope. The microscopic shear failure mechanisms and fracture parameters of the minerals and interfaces were investigated. The results showed that multiple thermal cycles reduced macroscopic shear strength by initiating thermal cracks rather than by reducing the strength of the microscale minerals. All three minerals exhibited L-shaped crack propagation below 300°C, with shear initiation and tensile failure. Biotite failed progressively, whereas feldspar and quartz failed catastrophically. High temperatures and multiple thermal cycles caused thermal voids and complex crack paths in feldspar and quartz, whereas biotite exhibited fiber-like plastic slip bands without thermal cracking. The orientation and strength of the mineral interfaces affected crack deflection and branching. Energy was dissipated by interlayer plastic slip in biotite and by micro-crack friction and slip in quartz and feldspar. The microscopic mode II fracture toughness and critical energy release rate were 0.5–3.3 MPa·m^0.5 and 0.01–0.11 kJ/m², respectively. This research provides novel insights into the microscale shear failure of HDR and a critical micromechanical basis for multiscale fracture modeling and macroscopic shear failure analysis.

Fluid transport properties at the base of the seismogenic zone exert a critical control on fault strength, slip behavior, and fluid circulation. However, quantitative constraints on permeability in deep fault rocks remain limited. We present new laboratory measurements of permeability, porosity, and specific storage on cataclasite- and pseudotachylyte-bearing mylonitic rocks along the Red River Fault (RRF), China. Experiments conducted at effective pressures up to 165 MPa reveal systematically low permeabilities, with mylonitized cataclasites and pseudotachylyte-bearing rocks exhibiting the lowest values (10⁻²²–10⁻²¹ m²), while mylonites display relatively higher permeabilities, up to 10⁻¹⁹ m². A key finding is the pronounced permeability anisotropy, where permeability parallel to foliation is up to two orders of magnitude greater than perpendicular values. This anisotropy reflects the microstructural alignment of phyllosilicate minerals and cavitation bands, which provides interconnected flow pathways along foliation while impeding cross-foliation transport. The hydraulic architecture of the RRF at the base of the seismogenic zone is thus characterized by overall low permeabilities with fluid flow facilitated along mylonitic foliation. Such fabric-controlled anisotropy persists even under mid-crustal pressure conditions, indicating that fault zones at the base of seismogenic depths can retain directional fluid pathways despite overall low permeabilities. Our results provide critical constraints on the hydraulic architecture of the RRF and offer broader insights into the role of anisotropy in controlling fluid flow and deformation at the base of the seismogenic zone.

The Okavango Rift Zone (ORZ) is an incipient continental rift in Botswana at the terminus of the Southwestern Branch of the East African Rift System. The lack of syn-rift magmatism and tectonic processes overprinting pre-rift structures provide an opportunity to investigate incipient-stage rift processes and the role of pre-existing structures in rift initiation and strain localization. We present SEISORZ, a ∼450-km-long wide-angle seismic transect across the ORZ and neighboring tectonic terranes. A 2.5-D V_P tomographic inversion reveals crustal thinning within a ∼130-km-wide section of the Damara Belt hosting the ORZ where Moho depth is 38.7 ± 3.4 km, shallower than in other Damara Belt terranes (46.3 ± 1.4 km) and the Kalahari Craton (45.6 ± 2.0 km). Mantle V_P is consistent with ultramafic lithologies without evidence for metasomatism, partial melt, or elevated temperatures. Crustal V_P is variable but consistent with geological information and with lower-crustal mafic lithologies. However, beneath the rifting region, the model shows low crustal velocities (ΔV_P = −0.26 ± 0.05 km/s) that we interpret as damage from rift-related faulting and deformation, and to a lesser extent elevated temperatures possibly from excess radiogenic heat production. Upper crustal heterogeneity correlates with known and newly detected intra-rift faults, suggesting that pre-existing structures promoted strain localization and establishment of the rift border fault system. Collectively, all these factors point to a rheologically weak section in the Ghanzi-Chobe zone which is more susceptible to deformation in response to far-field stresses than neighboring terranes, explaining why incipient rifting is localizing there and not across any other of the terranes that compose the Damara Belt.

Accurate characterization of fracture geometry and reservoir structure is essential for the successful design and development of Enhanced Geothermal Systems (EGS). Conventional surface seismic imaging often lacks the resolution to delineate hydraulic fractures at depth due to strong attenuation and limited source frequency. This study presents high-resolution imaging of the Utah FORGE site using Distributed Acoustic Sensing (DAS) recorded microseismic data during the [16A(78)-32] injection activities. We developed an imaging technique that leverages microseismic events as imaging sources, applies prestack Kirchhoff migration to each individual source, then stacks hundreds of sources to generate a 3D reflectivity volume. The imaging workflow produces a high-resolution map of the regional granitoid contact and, more importantly, reveals internal structures within the heart of the geothermal reservoir that have not been previously described. By correlating well-log data and geological evidence, we identify a low-angle interface located just below and nearly parallel to the granitoid contact. Key findings also reveal two natural fractures near the stimulation zone, visible prior to stimulation, which may accommodate the injected fluid and affect the hydraulic fracturing efficiency. Those internal structures are difficult to observe by typical surface sources since the regional granite-alluvium interface is strong and not easily penetrated by seismic waves. Time-lapse imaging of the hydraulic fracture is conducted and integrated with low-frequency DAS to construct a more comprehensive fracture geometry. In conclusion, the 3D fracture volume produced by DAS microseismic reflection imaging deepens our understanding of geothermal reservoir dynamics, potentially enhancing geothermal characterization and exploitation strategies.

The 15 January 2022 submarine volcanic eruption of Hunga Tonga-Hunga Ha'apai released immense energy throughout the ocean, solid Earth, and atmosphere. We analyze mid-oceanic column acoustic pressure recordings from 24 freely drifting Mobile Earthquake Recorder in Marine Areas by Independent Divers sensors, and from 11 moored hydrophones in the International Monitoring System. We focus on the pulsed hydroacoustic phase which propagated horizontally through the ocean as a 30-min T wave with energy around 2.5–10 Hz. The records show high correlation between some receivers, significant variation among others, and varying amplitudes that cannot be explained by distance alone. We investigate the origin of this heterogeneity via the influence of bathymetric features that may block, or occlude, T-wave propagation, affecting both shape and amplitude of the records received. We count the number of seafloor obstacles within the horizontal plane of the first (ray-theoretical) Fresnel zone at a depth of 1,350 m, where the fundamental-mode T-wave eigenfunction is maximal. Adjusted for geometric spreading, the cross-correlations and sound pressure level differences between receivers systematically relate to differences in occlusion count. Our model of signal loss due to seafloor interactions predicts a 5.6 dB reduction in sound pressure level per logarithm of occlusion count, explaining 88% of the T-wave sound pressure variance across the ocean. Source characterization requires adequate path models. Our findings describe how to correct signal amplitudes for seafloor roughness. This is important for constraining volcanic or explosive yield estimates and earthquake magnitudes, and useful to model detectability through various oceanic corridors when designing hydroacoustic monitoring networks of the future.

Hydrologic observations and experimental studies indicate that inelastic dilation from coseismic fault damage can cause substantial pore pressure reduction, yet most near-fault hydromechanical models ignore such inelastic effects. Here, we present a 3-D groundwater flow model incorporating the effects of inelastic dilation based on an earthquake dynamic rupture model with inelastic off-fault deformation, both on pore pressure and permeability enhancement. Our results show that inelastic dilation causes mostly notable depressurization within ∼1 km off the fault at shallow depths (<3 km). We found agreement between our model predictions and recent field observations, namely that both sides of the fault can experience large-magnitude (∼tens of meters) water level drawdowns. For comparison, simulations considering only elastic strain produced smaller water level changes (∼several meters) and contrasting signs of water level change on either side of the fault. The models show that inelastic dilation is a mechanism for coseismic fault depressurization at shallow depths. While the inelastic dilation is a localized phenomenon which is most pronounced in the fault zone, the pressure gradients produced in the coseismic phase have a broader effect, increasing fluid migration back into the fault zone in the postseismic phase. We suggest field hydrologic measurements in the very-near-field (<1 km) of active faults could capture damage-related pore pressure signals produced by inelastic dilation, helping improve our understanding of fault mechanics and groundwater management near active faults.

We develop a simple explanation for Gutenberg-Richter (G-R) size scaling of earthquakes on a single fault. We discretize the fault and consider all possible contiguous ruptures at that level of discretization. In this static model, we assume that slip scales with rupture length, and that the rupture rates at each point along the fault are consistent with an a priori long-term slip rate. These simple assumptions define an (under-determined) non-negative least squares inverse problem. Each solution to this inverse problem is a set of earthquake rates that matches the slip-rate constraint. We use a Markov Chain Monte Carlo (MCMC) algorithm to uniformly sample the solution space assuming constant slip rates along the fault. At finer discretizations, deviations from G-R behavior decrease, which is consistent with an entropic pressure toward G-R solutions. When the fault is discretized into 10 or more segments, random solutions found by the MCMC algorithm have G-R size scaling, even though there are trivial solutions that, for example, have earthquakes of only one size. This is because there are simply far more solutions that have G-R scaling; as the problem size increases, the strong degeneracy of G-R solutions results in other solutions becoming improbably rare. Also, the entropically favored G-R distribution has a $��b�$-value of approximately 1, which agrees with measured $��b�$-values in real earthquake catalogs.

Establishing a quantitative framework to evaluate the spatiotemporal patterns of slow earthquakes and to detect their anomalous activities is essential for understanding diverse slip behaviors on plate boundaries and seismic hazard assessment. In this study, we focus on deep tectonic tremors, which are one manifestation of slow earthquakes and are detectable through seismological observations along the Nankai Trough. We develop a probabilistic model based on a multivariate Hawkes process to describe both the temporal and spatial characteristics of their activity. Our analysis shows that more than half of the tremors are attributable to interactions with neighboring regions, which underlines the importance of incorporating spatial interactions into the forecasting model. Along-dip and along-strike variations in background tremor seismicity and the effective durations of inter-tremor interactions revealed in our model are consistent with previous geophysical and geological observations and conceptual frameworks, including tremor migration, depth-dependent slip modes, partial overlap with slow slip events (SSEs), and along-strike variations in plate convergence rate. We further compare the model-predicted tremor activity with observations and quantify their differences in cumulative event counts using the Kolmogorov–Smirnov test to identify transient anomalies. The detected anomalies include both short-duration (∼0.1 days) and long-duration (∼100 days) activations and quiescences. Although their spatial extent is much smaller (25 km) than that of SSEs, half of the short-term anomalies partially correlate with geodetically detected short-term SSEs. These results demonstrate the potential of our approach to provide a robust framework for forecasting slow earthquake activity and detecting its changes.

The crystal structure of Earth's solid inner core is fundamental to understanding the chemical composition and dynamical evolution of the core. However, despite extensive research, the structure still remains controversial with competing hypotheses regarding the stability of various Fe phases (e.g., bcc, fcc, and hcp). In this paper, we review the studies on the crystal structure of Fe under inner core conditions, and find, in line with previous work, that the main challenges come from the small energy differences between these structures. This has led to a variety of different conclusions across varying theoretical methods and precision, including ab initio, force field and machine learning methods. To address this problem, we employ a Bain path thermodynamic integration approach to reach consistent conclusions among different methods; we find that bcc Fe is mechanically stable but thermodynamically less stable under inner core conditions. Using the energetics from the Bain path method as a benchmark, we establish the requirements for converging free energy calculations through a two-phase thermodynamic modeling approach. These calculations confirm that the hcp phase is the most stable, exhibiting the highest melting temperature regardless of the method used. This unified conclusion on the hcp phase as the stable crystal structure provides a robust foundation for future studies on Earth's core.