Journal of Geophysical Research: Solid Earth最新文献

Seismic and geodetic observations show that slow slip events (SSEs) in subduction zones can happen at all temporal and spatial scales and propagate at various velocities. Observation of rapid tremor reversals indicates back-propagating fronts traveling much faster than the main rupture front. Heterogeneity of fault properties, such as fault roughness, is a ubiquitous feature often invoked to explain this complex behavior, but how roughness affects SSEs is poorly understood. Here we use quasi-dynamic seismic cycle simulations to model SSEs on a rough fault, using normal stress perturbations as a proxy for roughness and assuming rate-and-state friction, with velocity-weakening friction at low slip rate and velocity-strengthening at high slip rate. SSEs exhibit temporal clustering, large variations in rupture length and propagation speed, and back-propagating fronts at different scales. We identify a mechanism for back propagation: as ruptures propagate through low-normal stress regions, a rapid increase in slip velocity combined with rate-strengthening friction induces stress oscillations at the rupture tip, and the subsequent “delayed stress drop” induces secondary back-propagating fronts. Moreover, on rough faults with fractal elevation profiles, the transition from pulse to crack can also lead to the re-rupture of SSEs due to local variations in the level of heterogeneity. Our study provides a possible mechanism for the complex evolution of SSEs inferred from geophysical observations and its link to fault roughness.

The Epidemic Type Aftershock Sequence (ETAS) model is the most widely used and powerful statistical model for aftershock forecasting. While the distribution of aftershocks around the mainshock is anisotropic, the spatial probability density function of the ETAS model is commonly assumed to be isotropic due to insufficient information. In addition, its parameter estimation can be highly biased due to catalog incompleteness after the mainshock. Thus, we extended the recently developed 2D temporal ETASI, which accounts for short-term incompleteness, to 2D and 3D spatiotemporal ETASI, considering additional spatial occurrence probabilities in the framework of ETAS and ETASI to improve aftershock forecasting. We replaced the isotropic spatial kernel with anisotropic kernels estimated by a spatial probability map of stress scalars, including Coulomb stress changes on master fault orientation (MAS), Coulomb stress changes on variable mechanisms (VM), maximum shear (MS), and von Mises stress (VMS), and the nearest distance to the ruptured fault of the mainshock (R). The fit to six prominent mainshock-aftershock sequences in California demonstrates that the ETASI model outperforms the standard ETAS model. Furthermore, positive information gains indicate that using stress calculations as additional input information can improve the parameter fit. This improvement is weaker in 3D, which is likely related to greater positional uncertainty in the depth domain. However, incorporating the probability map calculated as a function of the nearest distance to the mainshock rupture leads to the best performance in all model variants.

We use the full waveform inversion method to study the crustal-mantle seismic structure beneath Central Asia. By combining earthquake waveforms and ambient noise cross-correlations, we construct a 3D model of Vp and Vs down to a depth of 220 km. This model reveals a complex Indian-Asian plate configuration and interaction, resulting from the plate subduction, indentation, and break-off. Beneath the Hindu Kush, the marginal Indian slab with its lower crust is successfully imaged, the latter of which hosts vigorous intermediate-depth seismicity. The subducted marginal Indian slab can be traced further east to the Kohistan Arc, which is a previously undetected structure. We first imaged a flat cratonic Indian plate beneath the Pamir. The indentation of the cratonic Indian plate forces the Asian plate to delaminate, indicated by the south-eastwards dipping high-velocity anomalies, atop which a south-dipping low-velocity zone is observed with higher resolution than previous studies, which we interpret as the delaminated Asian lower crust. In addition, a sharp velocity transition at lithospheric depth is newly discovered and coincides with the Talas-Ferghana fault, delineating the boundary of the Ferghana basin with the Central Tian Shan. Low-velocity anomalies mainly focus beneath the south and northern part of the Central Tian Shan with deep Moho, indicating the lithosphere is possibly delaminated and the deformation of the Central Tian Shan is probably concentrated at the north and south margins by the Tarim basin and Kazakh Shield, respectively. In contrast, West Tian Shan displays a simpler lithospheric structure with a single deep Moho.

Determining the water content in the lithospheric mantle is crucial for understanding its dynamic evolution. Because the electrical conductivity of mantle minerals is particularly sensitive to water, the magnetotelluric (MT) method becomes a vital tool to determine the water content in the lithospheric mantle. Here we used broadband and long-period MT data collected along a 600-km-long, NS-trending profile to obtain the electrical resistivity structure of the lithosphere across the southwestern South China Block. By combining the results of laboratory electrical conductivity measurements of mantle minerals, xenolith-derived composition, and geotherm information, we further estimated the water content of the lithospheric mantle. The results show that the Youjiang Basin has a relatively thin lithosphere segmented by zones of low-resistivity that spatially coincide with major faults. The relatively conductive mantle lithosphere could be explained by the combined effects of water in nominally anhydrous minerals, sulfide and phlogopite. Combined with regional tectonic context, we proposed that H₂O-rich fluids derived from the previously subducted slabs and related metasomatic processes lead to lithospheric hydration and thinning within the Youjiang Basin. Additionally, such processes, together with magmatic-hydrothermal activities, likely contribute to the formation of gold deposits within the basin. By contrast, the lithosphere beneath the Yangtze Craton is characterized by high resistivity extending to a depth of ∼200 km, representing a typical cratonic lithosphere that has not undergone significant tectonic modification and contains no or very little water.

One hypothesized mechanism that triggers deep-focus earthquakes in oceanic subducting slabs below ∼300 km depth is transformational faulting due to the olivine-to-spinel phase transition. This study uses finite element modeling to investigate phase transformation-induced stress redistribution and material weakening in olivine. A thermodynamically consistent constitutive model is developed to capture the evolution of phase transformation in olivine under different pressure and temperature conditions. The overall numerical model enables considering multiscale material features, including the polycrystalline structure, mesoscale heterogeneity, and various phases or variants of phases at the microscopic level, and accounts for viscoplastic behaviors with thermo-mechanical coupling effects. The model is validated with several benchmarks, including a phase diagram of phase transformation from olivine to spinel. The validated model is used to study the interactive behaviors between defects (heterogeneity) and phase transformation. The simulation results reveal that spinel formation under pressure initiates near inclusions and along the grain boundaries, consistent with experimental observations. At lower temperatures, the transformation leads to the formation of thin conjugate bands of spinel diagonal to the compression loading direction. Local stress analysis along these bands also suggests the initiation of faulting. In contrast, the numerical results at higher transformation rates show that significant spinel formation occurs over a larger area at elevated temperatures, leading to ductile behavior, which agrees with experimental findings. Numerical simulation of multiple inclusions under confined pressure also shows the formation of a network of spinel bands resembling phase-transformation patterns observed in the laboratory experiments. Additionally, stress softening patterns due to phase transformation are similar to experimental observations.

We present palaeomagnetic results from the Miocene section of the International Ocean Discovery Program (IODP) Site U1490. Detailed paleomagnetic investigations are crucial for providing a long-term record of Miocene relative paleointensity (RPI) variations, as well as the palaeoclimatic and paleoceanographic history of the Cenozoic Equatorial Pacific. These investigations also aim to enhance the database of Pacific magnetostratigraphy. Magnetic measurements were conducted at a 1 cm resolution on u-channel samples from the spliced section, with the goal of extracting a high-resolution magnetostratigraphic and RPI records. Stepwise demagnetization of the natural remanent magnetization yielded well-defined magnetostratigraphy over a time interval of approximately 9 million years, between the bottom boundaries of the Chron C5Dr.2r (18.066 Ma) and the Chron C4An (9.105 Ma), partially assisted by astronomically tuned isotope stratigraphy. The main magnetic carriers are both single-domain magnetofossils with equant octahedral morphology and pseudo-single-domain detrital magnetite. Our RPI data from the western equatorial Pacific are of the highest quality from 18 to 12 million years ago, comparable to the long-term RPI record from IODP Site U1336 in the eastern equatorial Pacific during the late Early to Middle Miocene with common fluctuations of 10⁴–10⁵ years timescale. The comparison also indicates that the method for Quaternary RPI-assisted chronostratigraphy can also be applied to older sediments to enhance the resolution of stratigraphic correlation.

The Apennines are a tectonically active belt that has experienced significant earthquakes (Mw$��>�$6). The largest events primarily occurred along the chain axis, where a complex system of normal faults accommodates 2–3 mm/yr of SW-NE oriented extension, as precisely measured by a dense Global Navigation Satellite System network. Geodetic strain rates are now frequently used in earthquake hazard models; however, the impact of using such estimates, computed through different methods, for seismic hazard assessments may be difficult to evaluate. This study explores the relationship between geodetic strain rates and seismicity rates in the Apennines using three distinct horizontal strain rate maps and an instrumental seismicity catalog. We find that the principal directions of geodetic strain rate are kinematically consistent with those of strain release. We estimate a spatially heterogeneous seismogenic thickness using the distribution of earthquake depths, and we isolate likely independent seismicity using three different declustering methods. We observe a correlation between independent seismicity rates and the magnitude of strain rate, which can be represented by either a linear or, more accurately, by a power-law relationship. The variability in the strain-seismicity relationship depends on the combination of independent seismic catalogs and strain rate maps. This relationship is primarily influenced by the declustering technique more than the choice of the strain rate map and, in particular, by the number of aftershocks excluded during declustering. Seismicity models derived from these combinations were used to estimate and compare the seismic moment release rate with the tectonic moment rate estimated from strain rate maps and seismogenic thickness. Findings indicate that the tectonic moment rate exceeds the seismic moment release rate. We highlight uncertainties and potential causes, one of which could be a possible aseismic release of part of the moment rate.

The mechanical and hydraulic behavior of faults in geothermal systems is strongly impacted by fluid-induced alteration. However, the effect of this alteration on fault properties in geothermal reservoirs is under documented. This affects our ability to model the properties of subsurface structures, both in reservoirs and caprocks, and potential hazards during geothermal exploitation. We investigated fault rocks from the caprock of a fossil hydrothermal system in the Apennines of Italy. We combined field structural observations with mineralogical and microstructural analyses of faults that guided the circulation of hydrothermal fluids and steered the caprock formation. We also performed friction experiments and permeability tests on representative fault rocks. We document fault weakening induced by the effect of hydrolytic alteration leading to the enrichment of clay minerals along the slip surfaces of major faults. Alunite-clay-rich rocks are much weaker (friction coefficient 0.26 < μ < 0.45) than the unaltered protolith (trachyte, μ = 0.55), favoring strain localization. The late-stage enrichment of clays along faults induces a local decrease in permeability of three orders of magnitude (1.62 × 10⁻¹⁹ m²) with respect to the surrounding rocks (1.96 × 10⁻¹⁶ m²) transforming faults from fluid conduits into barriers. The efficiency of this process is demonstrated by the cyclic development of fluid overpressure in the altered volcanic rocks, highlighted by chaotic breccias and hydrofracture networks. Permeability barriers also enhance the lateral flow of hydrothermal fluids, promoting the lateral growth of the caprock. Velocity-strengthening frictional behavior of alunite-clay-rich rocks suggests that hydrolytic alteration favors stable slip of faults at low temperature.

We study Sn-to-Lg conversion due to significant crustal-thickening, using Sn and Lg amplitude ratios (Sn/Lg) to identify continental mantle earthquakes. We perform 2.5D simulations up to 5 Hz and 2,000 km using an enhanced AxiSEM3D. Our simulations compare propagation in a constant-thickness crust from a source at three depths straddling the Moho, to 48 models of the same three sources propagating through Moho ramps of four different widths (dips) at four different distances from the source, so perturbations of Sn/Lg are only due to crustal-thickening. We compare our synthetics to data from 12 earthquakes recorded on the HiCLIMB array across Tibet, of which six previously studied events from northwestern Tibet traverse no major crustal-thickness variation, and six new events are located south of the Himalaya across a major Moho ramp. Our simulations show that Sn/Lg for mid-crustal earthquakes is consistently lower than for mantle earthquakes, especially for areas beyond the end of the ramp. Our observations on real data, with similar travel paths for each group of events (ramp-crossing and non-ramp-crossing), display bi-modal separation of Sn/Lg for both groups, identifying new mantle earthquakes in northern India. Sn-to-Lg converted waves may be readily detected by enhanced high-frequency Lg content, as suggested by previous mode-coupling studies, verified here with synthetics, and observed in data. Lastly, we observe higher frequency content in Lg from crustal earthquakes than from mantle earthquakes in simulations and in data, offering a new discriminant for continental mantle earthquakes based on frequency content of Lg waves alone.

The majority of geothermal energy is produced in tectonically active volcanic-arc regions due to their high geothermal gradients. Reservoirs in these settings are often stratified with smectite/kaolinite-, illite-, and chlorite-rich zones, in order of increasing depth and temperature. Eighteen andesitic core and surface samples were taken from five geothermal fields in the Lesser-Antilles and Cascade volcanic arcs. The collected samples have experienced various degrees of alteration and can be considered, in their ensemble, to be representative of the previously mentioned alteration zones. The influence of the alteration was assessed through biaxial rate-and-state friction experiments on prepared gouge. The samples were each tested at 10, 30, and 50 MPa normal stress in both nominally dry and nominally wet conditions. While significant water-induced frictional-strength reduction was observed, phyllosilicate content dominates frictional behavior, with increased phyllosilicate content reducing frictional strength, promoting velocity-strengthening behavior, and reducing frictional healing. Negative frictional healing is observed and likely related to the presence of expandable clays, leading to frictional weakness over long time periods. It is suggested that, by controlling frictional strength, phyllosilicate content influences the depth of onset of ductile shear zones, which often underlie these reservoirs and are critical for the horizontal advection and vertical sealing of geothermal fluid. Further, as these types of reservoirs are likely critically stressed, varying degrees of alteration within different reservoir zones can give rise to the formation of stress jumps. Overall, the frictional behavior depended to a first order on overall phyllosilicate content, potentially simplifying engineering studies.