Journal of Geophysical Research: Solid Earth最新文献

Changbaishan volcanic field (CBVF) located on the border of China and the Democratic People's Republic of Korea (DPRK) is famous for intense volcanism in the Cenozoic. Many studies show evidence for partial melt beneath the volcano, but details on the structure of the magmatic system are lacking, due to a lack of data in the region. In this study, we obtained a high-resolution crust and upper mantle shear wave velocity (Vs) model beneath the CBVF by ambient noise tomography and receiver functions using a new dense seismic array. The absence of velocity anomalies beneath Wangtian'e and Namphothe volcanoes suggests a lack of magma within the crust. However, our models reveal two low Vs anomalies beneath Tianchi associated with magma reservoirs. The shallow low velocity anomaly (<4 km) overlaps with petrological estimates of the assembly depth of erupted rhyolite magma reservoir and the depth inferred for a hydrothermal reservoir from a recent MT study. The deeper one is located between 7 and 14 km depth with a lateral extent of ∼30 km, with a melt fraction of ∼6%–12%. Underlying the deeper low velocity zone in the lower crust is a region of faster velocity compared to the surrounding region. We interpret this as a low melt fraction crystal mush. This structure is similar to that seen at other large caldera volcanoes worldwide, suggesting a possible common mechanism underlying magmatism at these volcanoes.

Water is essential for the formation of granites and continental crust, whereas charnockite, being an important component of deep crust, is inferred to be formed in low-water environments. Charnockite is an orthopyroxene-bearing felsic rock, its origin, generation, and preservation remain hotly debated. Quantifying the magma water content of charnockite and further determining the orthopyroxene preservation mechanism is crucial to understanding the petrogenesis of charnockite. Here, we report a ca. 431 Ma peraluminous Gaozhou charnockite with granulitic enclaves in South China. The body displays A-type characteristics with crustal reworking zircon isotopic features (δ¹⁸O = 8.0–9.8 ‰; ε_Hf(t) = −11.5 to −3.4). The charnockite and its enclaves show identical mineral assemblages and comparable orthopyroxene chemical compositions. The two anhydrous minerals of orthopyroxene and garnet are identified as of peritectic and magmatic origins given their textural features and geochemical compositions. Moreover, petrographic observations and bulk geochemical data argue that the peritectic minerals were derived from the entrainment of their granulitic sources. Crystallization phase modeling indicates orthopyroxene would have been completely hydrated and formed biotite when water contents exceed ∼0.3 wt.% near the solidus. Water-in-zircon analysis and thermodynamic modeling indicate low magma water conditions (∼0.15 wt.%; 135 ppm, zircon water medians) for the Gaozhou charnockite from early crystallization to final solidification. CO₂-rich fluids flushed the charnockite reservoir further contributing to the stabilization of the orthopyroxene. Therefore, we propose a new entrainment model for the formation of charnockite that requires low-water environments achieved by high-temperature melting of dehydrated lower crust granulitic rocks.

Fragmentation plays a critical role in eruption explosivity by influencing the eruptive jet and plume dynamics that may initiate hazards such as pyroclastic flows. The mechanics and progression of fragmentation during an eruption are challenging to constrain observationally, limiting our understanding of this important process. In this work, we explore seismic radiation associated with unsteady fragmentation. Seismic force and moment tensor fluctuations from unsteady fragmentation arise from fluctuations in fragmentation depth and wall shear stress (e.g., from viscosity variations). We use unsteady conduit flow models to simulate perturbations to a steady-state eruption from injections of heterogeneous magma (specifically, variable magma viscosity due to crystal volume fraction variations). Changes in wall shear stress and pressure determine the seismic force and moment histories, which are used to calculate synthetic seismograms. We consider three heterogeneity profiles: Gaussian pulse, sinusoidal, and stochastic. Fragmentation of a high-crystallinity Gaussian pulse produces a distinct very-long-period seismic signature and associated reduction in mass eruption rate, suggesting joint use of seismic, infrasound, and plume monitoring data to identify this process. Simulations of sinusoidal injections quantify the relation between the frequency or length scale of heterogeneities passing through fragmentation and spectral peaks in seismograms, with velocity seismogram amplitudes increasing with frequency. Stochastic composition variations produce stochastic seismic signals similar to observed eruption tremor, though computational limitations restrict our study to frequencies less than 0.25 Hz. We suggest that stochastic fragmentation fluctuations could be a plausible eruption tremor source.

The correlation between subduction zone features and megathrust seismicity provides relevant clues on what controls the generation, location and clustering of mega-earthquakes (magnitudes M_w ≥ 8.0). Thus far, weak correlations are found between subduction zone parameters and seismicity through bivariate statistical analyses. Here, we used Explainable Artificial Intelligence (XAI) to assess the relevance of geophysical properties and tectonic motions along major subduction zones, paired with novel proxies of slab stress from calculations of buoyancy-driven subduction. The features derived from these data sets, describing the physical state, kinematics, and dynamics, served as inputs to a Fully Connected Network (FCN) trained to classify segments according to the largest earthquake magnitude that ruptured it. The subsequent use of Layer-wise Relevance Propagation, an XAI technique, on a trained FCN provides an estimate of the relevance of the input, identifying the features most relevant to the classification. The XAI procedure confirmed the importance of subduction interface curvature, sediment thickness, long wavelength bathymetric roughness, and free-air gravity anomalies, as previously proposed. Interestingly, our procedure revealed the importance of slabs extending to the upper mantle as well as the trench-parallel slab stress, showing how three-dimensional subduction forces may control large earthquakes. This suggests the preferential occurrence of large earthquakes on megathrust segments around slab steps and edges, where the slab depth measured along trench varies abruptly. At these steps, the trench-parallel forcing is maximized by the excess load of neighboring deeper slabs.

Traveltime tomography considering reflection arrivals is a promising approach for investigating interface topography and near-interface velocity heterogeneity. In this study, we formulate this inverse problem as an eikonal equation-constrained optimization problem, in which the traveltime field of the reflection wave is accurately described by a two-stage eikonal equation. The novelty lies in deriving the Fréchet derivative with respect to interface topography. By employing the coordinate transformation technique to convert an irregular physical domain with an undulating interface to a regular computational domain, we successfully encode the interface topography into the anisotropic parameters in the eikonal equation. This approach enables us to derive explicit forms of the Fréchet derivatives related to interface topography and velocity based on the adjoint-state method, which is not only computationally efficient but also avoids potential inaccuracy in ray tracing. Several numerical experiments are conducted to verify our new method. Finally, we apply this method to central California near Parkfield by inverting traveltimes of both first-P and Moho-reflected waves (named PmP). The low-velocity anomalies imaged in the lower crust are consistent with the along-strike variations of low-frequency earthquakes (LFEs) beneath the San Andreas Fault (SAF), suggesting the presence of fluids that may influence the occurrence of LFEs in this region.

Here we analyze the rupture process of the 29 December 2020 M_W6.4 Petrinja earthquake (Croatia), the largest event instrumentally recorded in this area characterized by a moderate strain-rate intraplate setting. We use foreshocks and aftershocks, recorded at more than 80 broadband stations located 70–420 km from the earthquake, as empirical Green's functions (EGFs) to separate source effects from propagation and local site effects. First, we deconvolve the mainshock P-wave time windows from the EGFs in the frequency domain to obtain the corner frequency (f_c). Spectral analysis based on the Brune's source model reveals a large stress drop of 24 MPa. Next, by deconvolving the Love waves in the time domain, we calculate the Apparent Source Time Functions (ASTFs). We find that the average duration of the source is ∼5 s, with no significant directivity effects, indicating a bilateral rupture. To extract physical rupture parameters such as rupture velocity, slip distribution and rise time, we deploy two techniques: (a) Bayesian inversion and (b) backprojection onto isochrones of ASTFs. Both techniques show a low rupture velocity (40%–50% of the shear wave velocity) and a rupture length of less than 10 km, that is, much less than would typically be expected for a magnitude 6.4 earthquake. This apparent anticorrelation between stress drop and rupture velocity may be attributed to the complex and segmented fault system characteristic of immature intraplate settings.

Understanding hydrogen dissolution mechanisms in bridgmanite (Bgm), the most abundant mineral in the lower mantle, is essential for understanding water storage and rheological and transport properties in the region. However, interpretations of O-H bands in Fourier transform infrared spectroscopy (FTIR) spectra of Bgm crystals remain uncertain. We conducted density functional theory (DFT) calculations on vibrational characteristics of O-H dipoles and performed polarized FTIR measurements to address this issue. DFT calculations for four substitution models—Mg vacancies, Si vacancies, Al³⁺ + H⁺ substitution for Si⁴⁺, and Al substitution with Mg vacancies—reveal distinct O-H bands with different polarizations. Deconvolution of polarized FTIR spectra on Mg_0.88Fe²⁺_0.035Fe³⁺_0.065Al_0.14Si_0.90O₃ and Mg_0.95Fe²⁺_0.033Fe³⁺_0.027Al_0.04Si_0.96O₃ crystals shows five major O-H bands with distinct polarizations along principal crystallographic axes. These experimental and calculated results attribute O-H bands centered at 3,463–3,480, 2,913–2,924, and 2,452–2,470 cm⁻¹ to Mg vacancies, Si vacancies, and Al³⁺ + H⁺ substitution for Si⁴⁺, respectively. The total absorbance coefficient of bridgmanite was calculated to be 82,702(6,217) L/mol/cm². Mg and Si vacancies account for 43%–74% of the total water content, making them dominant hydrogen dissolution mechanisms in Bgm. The band frequencies for the Mg and Si vacancies in Bgm are drastically different from those in olivine and ringwoodite, corresponding to the significant changes in O-H bond strengths and in the Si and Mg coordination environments from upper-mantle to lower-mantle minerals. These results highlight the need to incorporate hydrogen dissolution mechanisms in Bgm for understanding electrical conductivity and rheology of the lower mantle.

Seismic anisotropy can provide valuable constraints for the study of subduction zone dynamics. This study presents a high-resolution 3-D azimuthally anisotropic shear wave velocity model down to 230 km beneath Alaska via ambient noise tomography and wave gradiometry method. The model reveals layered anisotropy patterns related to subduction tectonics. The shear wave's fast directions in the Aleutian fore-arc region exhibit trench-parallel, trench-normal, trench-parallel, and trench-normal variation relative to the trench trend with increasing depth. This anisotropic pattern may be attributed to the strike of fractures or faults in the overlying North American plate, subduction-driven mantle wedge corner flow, preexisting fabrics in the subducting Pacific Plate, and entrained flow in the sub-slab mantle, respectively. The depth-dependent anisotropy pattern in the back-arc mantle wedge reflects subduction-induced corner flow, altered by the subducting slab's changing geometry. Moreover, the model provides new insights into the anomalous magmatism in the Alaskan subduction system. The 3-D isosurface clearly shows the relatively high mantle wedge velocities beneath the Denali Volcanic Gap (DVG), suggesting a relatively dry and cold mantle wedge for the flat-slab subduction of Yakutat slab. The absence of a magma source likely caused the DVG. The Wrangell Volcanic Field (WVF) is characterized by a similar depth-dependent anisotropy pattern to the Aleutian-Pacific subduction system, providing additional evidence for the presence of Wrangell Slab. The formation of WVF may be the result of combined effects of toroidal mantle upwelling around the edge of the Wrangell Slab and melting due to dehydration of the Wrangell Slab.

The physical properties of the lithospheric and upper mantle's rock are determined by its composition and the in situ temperature and pressure conditions. Together, they have been referred to as the thermochemical structure. Information about the upper mantle's thermochemical structure could be obtained using methods from different disciplines of the earth sciences, in which the geophysical approaches show potential to map the 3D variations on both the regional and global scales. Thus, techniques for investigating the thermochemical structure in the spherical coordinates are needed, including forward modeling of the geophysical observables, calculating schemes of the thermophysical properties for the lithologies, and effective inversion algorithm, which is particularly important for large-scale applications. This paper first demonstrates an adaptive meshing architecture based on the tetrahedral mesh by the sophisticated constructions in a spherical shell. Techniques that enable rapid calculations of the thermophysical properties of the upper mantle's rocks are introduced in length. Methodologies for constructing 3D thermochemical models and forward modeling geophysical observations, including an inversion sub-routine that couples the lithostatic pressure and density variations to forward modeling, are introduced and examined in detail using synthetic data sets. We then introduce methods for determining 3D thermochemical structures of the upper mantle. The inverse problem is treated as a multi-task evaluation process and solved using advanced stochastic optimizing techniques. Estimated uncertainties of the resultant thermochemical models are obtained simultaneously for error analysis. The proposed forward modeling and inversion techniques are validated using synthetic data sets in both forward and inversion circumstances. Limitations and further developments are discussed in the subsequent concluding remarks.

In this work, we integrate seismic data recorded by nine coastal Mediterranean seismic stations and wave hindcast data for 1 January 1996 through 15 October 2023. We examine the relationships between the ocean wave-generated microseism signal (the most continuous and ubiquitous seismic signal on Earth) in terms of temporally varying spectral content, root mean square amplitude, and microseism power spectral density, with the main features of principal ocean wave attributes, specifically significant wave heights, wave period and wave power. To explore relationships between microseism and sea state, we performed a correlation analysis between seismic root square mean amplitude and significant wave height time series for the entire Mediterranean Sea for 1996–2023, including retrieving long-term trends for microseism energy and independently estimated wave power and calculating the Spearman correlation coefficient between the two trend time series. Despite the small number of stations available the analysis allows for a useful exploratory study on the microseism and its relationship with Mediterranean Sea state and wave power spanning 27 years. Given the recent increase in the number of regional seismic stations, the growth of data sharing, and the intensification of global warming and climate extreme events, the results and methods explored here can be further implemented and developed in coming years for coastal monitoring purposes in complement with other data sources.