Journal of Geophysical Research: Solid Earth最新文献

Understanding volcanic eruption triggers is critical toward anticipating future activity. While internal magma dynamics typically receive more attention, the influence of external processes remains less understood. In this context, we explore the relationship between seasonal snow cycles and eruptive activity at Ruapehu, New Zealand. This is motivated by apparent seasonality in the eruptive record, where a higher than expected proportion of eruptions (post-1960) occur in spring (including the two previous eruptions of 2006 and 2007). Employing recent advancements in passive seismic interferometry, we compute sub-surface seismic velocity changes between 2005 and 2009 using the cross-wavelet transform approach. Stations on the volcano record a higher velocity in winter, closely correlated with the presence of snow. Inverting for depth suggests these changes occur within the upper 300 m. Notably, we observe that the timing of the previous two eruptions coincides with a period associated with an earlier velocity decrease at approximately 200–300 m depth relative to the surface. Reduced water infiltration (as precipitation falls as snow) is considered a likely control of seasonal velocities, while modeling also points to a contribution from snow-loading. We hypothesize that this latter process may play a role toward explaining seasonality in the eruptive record. Our findings shed light on the complex interactions between volcanoes and external environmental processes, highlighting the need for more focused research in this area. Pursuing this line of inquiry has significant implications toward improved risk and hazard assessments at not just Ruapehu, but also other volcanoes globally that experience seasonal snow cover.

To examine the microstructural evolution that occurs during transient creep, we deformed samples of polycrystalline olivine to different strains that spanned the initial transient deformation. Two sets of samples with different initial grain sizes of 5 μm and 20 μm were deformed in torsion at T = 1,523 K, P = 300 MPa, and a constant shear strain rate of 1.5 × 10⁻⁴ s⁻¹, during which both sets of samples experienced strain hardening. We characterized the microstructures at the end of each experiment using high-angular resolution electron backscatter diffraction (HR-EBSD) and dislocation decoration. In the coarse-grained samples, dislocation density increased from 1.5 × 10¹¹ m⁻² to 3.6 × 10¹² m⁻² with strain. Although the same final dislocation density was reached in the fine-grained samples, it did not vary significantly at small strains, potentially due to concurrent grain growth during deformation. In both sets of samples, HR-EBSD analysis revealed that intragranular stress heterogeneity increased in magnitude with strain and that elevated stresses are associated with regions of high geometrically necessary dislocation density. Further analysis of the stresses and their probability distributions indicate that the stresses are imparted by dislocations and cause long-range elastic interactions among them. These characteristics indicate that dislocation interactions were the primary cause of strain hardening during transient creep in our samples. A comparison of the results to the predictions of three recent models reveals that the models do not correctly predict the evolution in stress and dislocation density with strain in our experiments due to a lack of previous such data in their calibrations.

In tight crystalline rocks faults are known to be substantially more hydraulically conductive than the rock matrix. However, most of our knowledge relies on static measurements, or before/after failure data sets. The spatio-temporal evolution of the permeability field during faulting remains unknown. Here, we determine at which stage of faulting permeability changes most, and the degree of permeability heterogeneity along shear faults. We conducted triaxial deformation experiments in intact Westerly granite, where faulting was stabilized by monitoring acoustic emission rate. At repeated stages during deformation and faulting we paused deformation and imposed macroscopic fluid flow to characterize the overall permeability of the material. The pore pressure distribution was measured along the prospective fault to estimate apparent hydraulic transmissivity, and propagation of the macroscopic shear fault was monitored by locating acoustic emissions. We find that average permeability increases dramatically (by two orders of magnitude) with increasing deformation up to peak stress, where the fault is not yet through-going. Post-peak stress, overall permeability increases by a factor of three. However, at this stage we observed local heterogeneities in permeability by up to factors of eight, ascribed to a partially connected fracture network. This heterogeneity decreases with fault completion at residual shear stress. With further slip on the newly formed fault, the average hydraulic transmissivity remains mostly stable. Our results show that permeability enhancement during shear rupture mostly occurs ahead of the rupture tip, and that strongly heterogeneous permeability patterns are generated in the fault cohesive zone due to partial fracture connectivity.

Teleseismic waveforms contain abundant interpretable information about Earth's properties. They can be used to explore the refined structure of Earth's interior, especially in the regions with imbalanced spatial distribution of seismic activity. However, it's technically infeasible to numerically simulate high-frequency (>1 Hz) teleseismic wave propagation within a whole domain iteratively in full-waveform inversion due to its vast computational costs. We develop a 3D FK-LTSOS (Frequency-Wavenumber, Layered Time-Space Optimized Symplectic) hybrid method and then apply it to teleseismic full-waveform tomography to tackle this computational challenge. The 3D FK-LTSOS hybrid method combines the semi-analytical solution computed by the FK method rapidly in a 1D background model and the numerical solution calculated by the 3D LTSOS method accurately in 3D localized heterogeneous media with topography to simulate teleseismic wave propagation efficiently and accurately. The comparison of synthetic seismograms shows its accuracy and stability when simulating wave propagation in the topographic model. Based on this hybrid method, the teleseismic full-waveform tomographic method is developed to efficiently resolve the detailed structure of the local research domain utilizing high-frequency teleseismic data. The essential contents of full-waveform tomography are presented, including misfit function, Fréchet kernels, smoothing strategy, and nonlinear conjugate gradient method. Synthetic data application for spherical anomaly models with planer surface and Gaussian topography, and observed data application for the crust-upper mantle structure beneath eastern Tibet confirm the validity of the teleseismic full-waveform tomography and demonstrate that our tomographic method can image the localized structures speedily from full-waveform information.

The hydraulic fracturing process is a prominent example of fracture network evolution under stress. However, the interactions between hydraulic fractures and natural fracture networks, along with the connectivity evolution of the resultant fracture networks, require more research. This research incorporates discrete fracture networks to characterize subsurface structures and employs the Discrete Element - Lattice Boltzmann Method to simulate the hydraulic fracturing process. The dynamic evolution of subsurface structures, including the initiation of hydraulic fractures and their interaction with natural fractures, is systematically investigated. Results indicate that natural fractures significantly impact fracture initiation, propagation, and connectivity evolution, which in turn affects fluid production. Fracture strength is key for the interaction, and hydraulic fractures tend to propagate along weak natural fractures with low resistance. Fracture strength variability determines the final fracture networks, with low-strength fractures breaking due to the altered in-situ stress and forming local clusters. High injection rates and fluid viscosity result in a large pressure buildup and exaggerate the influential region. A multi-cluster system is thus formed during the hydraulic fracturing process, and its connectivity can be well quantified with a novel connectivity metric. In low-permeable reservoirs, fracture clusters connected to production wells can contribute instantly, while local clusters may contribute to production from a long-term perspective. Injection rate, fluid viscosity, fracture orientation, and clustering effects have consistent positive correlations with the total connectivity and production. Heterogeneity has a weak positive correlation with fluid production, while a moderate negative correlation with total connectivity.

Interactions between multiple-scale thermochemical heterogeneities in the lowermost mantle, specifically ultralow velocity zones (ULVZs) and large low shear velocity provinces (LLSVPs), are critical in lower mantle dynamics. However, the evolution of ULVZs formed outside LLSVPs has not been thoroughly explored. Here we perform two-dimensional numerical experiments to examine the evolution of highly dense ULVZs originating beneath cold downwellings and their interactions with the LLSVP. We find that ULVZs with an intrinsic density anomaly more than 500 kg/m³ compared with the ambient lowermost mantle cannot fully enter the LLSVP and would dwell at LLSVP margins for an indefinitely long time. This suggests that dense ULVZ within LLSVPs might have different sources from those outside LLSVPs. The buoyancy number and compositional viscosity of ULVZs are controlling factors on their dynamics and imprints on the core-mantle boundary (CMB), such as how much the ULVZ protrudes into the LLSVP and the CMB topography beneath the ULVZ. The excess density of ULVZs dictates their width but not their thickness. The oscillations of ULVZ morphology suggest that various types of plumes occur at the LLSVP margin. The mobility of ULVZ implies that the bottom margin of the LLSVP moves much faster than its center. Hot zones exist within the LLSVP near its margins, which may affect the evolution of ULVZs and subducted material nearby. The CMB topography under dense ULVZs are positive unless the buoyancy number of ULVZs exceeds 6.0. These results have intriguing implications for the distribution of ULVZs as well as thermochemical evolution in the lowermost mantle.

Scattering of elastic waves causes velocity dispersion, which increases uncertainty in seismic analysis. Understanding the sources of scattering and the degree of velocity dispersion are critical to improve subsurface imaging in efforts to locate resources and study subsurface processes. In addition to scattering, other mechanisms, such as the wave-induced fluid flow in saturated rocks cause velocity dispersion. To study the effect of scattering on velocity dispersion, we conducted laboratory measurements of ultrasonic velocities on dry rock samples and performed wave-propagation simulations on CT-scanned 3D volumes of those samples. The set of samples consists of homogeneous and heterogeneous carbonate rocks with porosities between 3% and 26%. Ultrasonic velocities were measured at frequencies between 0.3 and 1 MHz, and numerical wave propagation simulations on the digital volumes were performed using an elastic approximation and a finite-difference method. The homogeneous sample and the corresponding numerical simulations exhibit negligible velocity dispersion. On the other hand, heterogeneous samples exhibit significant dispersion, and the corresponding numerical simulations accurately reproduce the observed dispersion in terms of magnitude and frequency shift. We conclude that scattering has a first-order effect on the velocities of the elastic waves in heterogeneous samples. This effect should be considered in conjunction with laboratory measurements in heterogeneous carbonates similar to those studied here. Furthermore, we illustrate a method to characterize frequency-dependent ultrasonic velocities (i.e., dispersion) and show that finite-difference modeling can reproduce the laboratory-observed dispersion.

Geological interpretations, earthquake source inversions and ground motion modeling, among other applications, require models that jointly resolve crustal and mantle structure. With the second generation of the Collaborative Seismic Earth Model (CSEM2), we present a global multi-resolution tomographic Earth model that serves this purpose. The model evolves through successive regional- and global-scale refinements. While the first generation aggregated regional models, with this study, we ensure consistency between all individual submodels, resulting in a model that accurately explains wave propagation across scales. Recent regional tomographic models were incorporated, comprising continental-scale inversions for Asia and Africa, as well as regional inversions for the Western US, Central Andes, Iran, and Southeast Asia. Across all regional refinements, over 793,000 source-receiver pairs contributed. Moreover, the long-wavelength Earth model (LOWE) introduces large-scale structures outside of pre-existing local refinements. A full-waveform inversion for global anisotropic P-and S-wave speed structure over a total of 194 iterations with a minimum period of 50 s on a large data set of 1 hr of waveform data from 2,423 earthquakes and over 6 million source-receiver pairs ensures that regional updates in the crust and uppermost mantle translate into updates of deeper, global-scale structure. To test the performance of CSEM2, we evaluate waveform fits between observed and synthetic seismograms at 50 s for an independent data set on the global scale, and on the regional scale for lower periods. We accurately simulate waveforms within and across regional refinements, maintaining the original resolution of the submodels embedded in the global framework.