Journal of Geophysical Research: Solid Earth最新文献

Hornblende amphibole is difficult to deform plastically in experiments due to its anisotropic nature and breakdown at relatively low temperatures (∼850°C). The lack of experimental analysis of hornblende plasticity hampers interpreting the deformation mechanisms of natural samples, which remain unresolved and debated. Here, we used strongly textured amphibolite, oriented for the activation of hornblende's cleavage and/or easy slip system, to investigate the interplay of brittle and plastic deformation mechanisms. Samples with the lineation oriented at 30° to the loading direction were deformed at a confining pressure of 1 GPa, strain rates of 10⁻⁵ to 10⁻⁴ s⁻¹, and temperatures of 400, 600, and 800°C. Deformed samples exhibit marked tilting of significant subvolumes manifested as kink bands. On the grain scale, deformation is accommodated by fracturing and dislocation mechanisms. A significant decrease in sample strength with temperature is accompanied by an increase in intragrain misorientations due to an increase in dislocation activity. The dominant orientation of the intragrain misorientation axis shifts from [001] at 400°C to [010] at 800°C. Nano-scale analysis revealed that at 800°C, intragrain misorientation occurs through a sequence in which dislocation structures develop first and then act as sites for fracture nucleation. The observed intragrain misorientation is corroborated by an example from the Javanahalli schist belt (India). We conclude that the experimentally observed transition in the dominant intragrain misorientation axis accompanying the transition from fracture to dislocation-mediated deformation can be used to interpret conditions experienced by naturally deformed samples.

Earthquake nucleation length, a critical parameter characterizing the transition from quasi-static propagation to dynamic rupture in the nucleation zone, has been observed to decrease with elevated shear stress loading rate. Recent laboratory experiments suggested that injection can also act as a loading condition, with the nucleation length shortening under high-rate injection. In this study, we perform numerical simulations to investigate how hydraulic diffusivity and injection rate affect the nucleation length of injection-induced seismicity on (aging) rate-and-state faults. Similar to tectonic earthquakes, the nucleation process of injection-induced seismicity falls into two distinct nucleation regimes—no-healing and constant-weakening—defined by the ratio of the weakening to healing rates at the center of the nucleation zone ($��{\Omega }_c�$). The nucleation length is generally much larger in the constant-weakening regime. Interestingly, we find that the effects of injection rate and hydraulic diffusivity on nucleation length depend on the nucleation regime. In the no-healing regime ($��{\Omega }_c�$ ≫ 1), the nucleation length decreases with increasing injection rate or decreasing hydraulic diffusivity. In contrast, within the constant-weakening regime ($��{\Omega }_c�\cong �1�$), the nucleation length displays an opposite trend in most cases. The contrasting behavior can be attributed to differences in the weakening processes within each regime and the timing of the transition from no-healing to constant-weakening. We discuss underlying mechanisms driving local fault response to changes in hydraulic diffusivity and injection rate, as well as the implications of these findings for field- and lab-scale injection-induced ruptures.

The D″ is a layer lying below a seismic discontinuity that occurs ∼250 km above the Core-Mantle boundary (CMB). A commonly proposed origin of this discontinuity is the transformation of bridgmanite (Bdm) to a post-perovskite structure (Bdm-ppv) but this generally produces results that do not match all the seismic observations. In this work we build a thermodynamic model of this transition using ab initio calculations incorporating the effects of Fe, Al, and Ca and predict that it is not one but two overlapping transitions. The second transition is the high temperature (>∼2,700 K) dissolution of Davemaoite (Dvm) phases into the Bdm phase to make a chemically mixed Bdm-Dvm phase. This reaction is controlled by Dvm and ferropericlase (fp) phases which means different mineralogical compositions will exhibit different phase behaviors and thus different seismic and dynamical behaviors near the CMB. We predict that assemblages with pyrolitic compositions match closest to the seismic observations of the D″ and that the presence of velocity increases near the top of the D″ followed by linked velocity increases near the CMB are both indicators of pyrolitic assemblages. Harzburgitic and basaltic mixtures are predicted to lack some of these features. In a pyrolitic mantle we then predict the existence of Bdm-Dvm at the CMB which will have different chemical, thermal and dynamical properties to Bdm + Dvm and could alter interactions between the mantle and the core.

The Focal Mechanism Solutions (FMSs) of small earthquakes provide valuable insights into crustal structure and stress conditions, while its reliability is limited by the accuracy of first-motion polarity determination, which remains challenging. In this study, we develop an FMS determination workflow based on a rule-based Polarity picker using Order Statistics and Entropy theory (POSE). The performance of POSE is compared with two representative deep learning-based approaches (CNN, Ross et al., 2018, https://doi.org/10.1029/2017jb015251; APP, L. Zhang et al., 2023, https://doi.org/10.1785/0220220247) in two tectonically diverse regions: Southern California and Southeastern Tibetan Plateau. In Southern California, we evaluate the polarity picking accuracy using manual labels as reference. All methods achieve comparable picking accuracy, while POSE and APP identify twice as many polarities previously labeled as “unknown,” indicating higher picking sensitivity. Moreover, POSE exhibits superior stability against noise levels and picking uncertainties. We then compute FMS from the polarities derived by each method. In Southern California, all algorithms produced similar number of FMSs with a comparable ratio of high-quality solutions. In more tectonically complex Southeastern Tibet, POSE yields approximately twice as many FMSs as APP and ∼40% more than CNN. Analysis of POSE-derived FMS catalog reveals fine-scale variations in faulting style and stress orientation along the Xianshuihe-Xiaojiang Fault Zone that are not resolved in previous FMS studies but consistent with GPS-constrained regional strain fields. These results highlight the effectiveness and cross-regional generalizability of POSE, demonstrating its potential to enhance the resolution and reliability of stress field characterization from small-magnitude earthquake data sets.

The peeling-off of oceanic crust from the downgoing slab at the top of the lower mantle is considered a key mechanism contributing to the formation of low-velocity anomalies in the mantle transition zone and seismic scatterers in the lower mantle. However, its controlling factors and geodynamic feasibility remain debated. In this study, we systematically investigate the influence of key parameters—including the viscosity structure within and surrounding the slab, the Clapeyron slope of the ringwoodite-bridgmanite phase transition, and the age of the subducting plate—on crust-mantle separation using two-dimensional geodynamic modeling. Results show that the weakening of the upper oceanic crust and hydrated slab mantle, as well as the presence of a low-viscosity “lower mantle wedge” formed by deep subduction, promotes the peeling off of crustal eclogite from the downgoing slab. Large-scale peeling off and accumulation of oceanic crust at and below 660 km depth occur only when both the lower mantle wedge (η = 10¹⁹ Pa·s, ∼1,000 times weaker than the ambient mantle) and the hydrated slab mantle (η = 10²² Pa·s, ∼100 times weaker than the cold slab mantle) are extremely weakened. By integrating rock physics and geophysical constraints, we demonstrate the physical feasibility of this process. Crust–mantle separation provides an alternative explanation for the observed weakening of the P660P seismic phase, the development of low-velocity zones atop the lower mantle, and the formation of seismic scatterers. This mechanism also offers new insights into global mid-ocean ridge basalt recycling and Large Low Shear Velocity Province evolution.

Olivine and ahrensite are the primary components of the interiors of Fe-rich terrestrial planets and meteorites, making their phase relations crucial for planetary science. Moreover, their phase relations can be used for calibrating large-volume high-pressure devices such as multi-anvil apparatus. Here we defined the olivine–ahrensite phase relations in the MgO-FeO-SiO₂ system at 7.5–12.0 GPa at 1,530 and 1,950 K using a multi-anvil apparatus. Combining the current results with our previously determined binary loop at 1,740 K, we re-estimated the shock parameters of several L5 and L6-types meteorites. Also, we determined the olivine-ahrensite phase ratio and compositions along cold and warm Mars aerotherms for Mg/(Mg + Fe) ratios of 0.75 and 0.80. Using this mineralogical model, we estimated and compared seismic wave velocity profiles in Mars' interior to data from the InSight geophysical mission.

We have investigated from rift inception to break up, the tectonic structure, spatial segmentation, and temporal evolution along ∼600 km of the East-Ceará and Potiguar basins of the Brazil Equatorial Margin. We have determined the initial rift configuration, the onset of the formation of the continental margin, and its structural evolution into a Southern, Central and Northern Segments. A distinct tectonic structure and evolution, limited by abrupt boundaries, characterize each segment. The segmentation boundaries define linear trends in the basement that extend from under the continental shelf to the deep-water domain. The trend of the segment boundaries appears to delineate flow-lines of the opening direction. The geometry of the boundaries, and available fault and dike patterns, provide information on the orientation of paleo-stresses. We integrated the distribution, geometry and age of structures to produce an evolutionary model and related it to plate kinematics. The initial Phase 1 deformation occurred in a four-arms configuration, each with different opening direction. This configuration was modified during a kinematic reorientation, causing two arms to stop opening during Phase 2. Phase 2 extension focused along two arms, creating a margin-type structure, which readjusted internal deformation into a Central, Northern and Southern Segments. Rift extension continued adapting to a gradual kinematic change into a Phase 3, when the Southern Segment developed a transcurrent fault system. Margin extension ended during Phase 4 with the initiation of spreading cells with a distribution mimicking the main rift segmentation, which remained as segmentation giving rise to oceanic fracture zones.

Earthquake rupture directivity impacts ground motions and provides insights on fault zone properties and earthquake physics. However, measuring directivity of small earthquakes is challenging due to their compact rupture sizes and complex path and site effects at high frequencies. Here, we develop a new approach that deconvolves energy envelopes of the S wave trains to remove path and site effects and robustly resolve azimuthal variations in durations of apparent source-time functions. Our method benefits from the coherence of energy envelopes for high-frequency seismic data, which provides more stable directivity results than conventional waveform deconvolution methods. We validate our method using both synthetic tests and real observations. We apply the algorithm to determine rupture directivities of 58 magnitude 3.5–5.5 earthquakes during the 2019 Ridgecrest earthquake sequence. The rupture directivities suggest an orthogonal interlocking fault system consistent with aftershock locations. Additionally, the rupture directivity pattern appears to correlate with spatial heterogeneity in earthquake stress drops. Our energy envelope deconvolution method enables directivity measurements at smaller magnitudes than traditional approaches and has potential for constraining small earthquake rupture dynamics.

We measured the thermal conductivity of composites consisting of mid-ocean ridge basalt (MORB) and polycrystalline olivine, as an analog for partially molten systems, to investigate the influence of low degree melting on heat transport. Experiments were conducted at 1 GPa and temperatures up to 1,600 K, with MORB fractions ranging from 0.1 to 10 vol.%. Adding MORB to the olivine matrix significantly altered the composite's thermal conductivity. Prior to melting, composites containing 0.1 and 10 vol.% MORB showed the most pronounced increase in conductivity relative to pure olivine, while intermediate fractions (∼1–5 vol.%) exhibited a decrease, followed by a rise at higher MORB contents. We attribute this non-monotonic behavior to impurity–lattice interactions within the MORB-bearing olivine, which reduce lattice disorder and enhance heat transport. Upon melting MORB, the thermal conductivity of the composites decreased, with the largest reductions (∼35%) observed in the 0.1 and 10 vol.% MORB samples, indicating that the melt acts as a thermal insulator. Applied to planetary interiors, these results suggest that lateral variations in melt fraction within thermal boundary layers could generate heterogeneities in heat flow, potentially affecting mantle convection patterns and the formation or evolution of thermal plumes.

The Mutnovsky and Gorely volcanoes in Kamchatka, located 70–80 km southwest of Petropavlovsk-Kamchatsky, pose significant hazard due to their potential for explosive eruptions. Mutnovsky also hosts the Mutnovsky Geothermal Power Plant (MGPP). This study presents the first crustal-scale three-dimensional seismic velocity model derived from ambient noise tomography, utilizing data from a temporary 2023–2024 seismic network of 65 mixed broadband and short-period stations, in addition to four permanent stations. The model reveals multiple low-velocity zones: a low-velocity anomaly at 2–5 km depth below sea level is interpreted as a Mutnovsky magma chamber, while the other one at 2.5–5 km depth beneath the MGPP likely reflects an active magmatic intrusion sustaining the geothermal system. A shallow anomaly at 0.5–1 km depth beneath the MGPP is attributed to production intervals associated with geothermal boreholes. Furthermore, the model indicates hydrothermal connectivity between the Mutnovsky field and the Zhirovskoy Valley, with no apparent connection to the Vilyuchinsky Valley springs. Beneath the Gorely caldera, a wide low-velocity zone suggests the presence of unconsolidated sediments and an underlying magmatic intrusion at 2–4 km depth.