Journal of Geophysical Research: Solid Earth最新文献

Because moderate magnitude earthquakes generate significant local ground shaking, but are poorly preserved in the geologic record, they present a major challenge for characterizing seismic hazards along orogenic fronts. The Mw 6.0 Jishishan earthquake and most aftershocks were located to the east of the west-dipping main thrust fault of the Laji Shan, NE Tibetan Plateau, and the earthquake produced serious disaster and surface ruptures, which provides an opportunity to study the rupture patterns and seismogenic mechanisms of moderate earthquakes within the orogenic front. Here we found that it produced three oblique thrusting surface rupture zones with variable lateral components of slip through field investigations and photogrammetry, spanning a total length of ∼8.2 km. The maximum vertical and horizontal displacements of the earthquake were measured as ∼6 and ∼5 cm, respectively. The deep and shallow rupture geometry shows that the seismic faults dip to the east or northeast. Combined with the surveys of pre-existing structures and collected global earthquake cases, we believe that the earthquake is the first reported moderate event (M < 6.5) with multi-segment surface ruptures caused by antithetic thrusting at the front of an orogenic belt. Based upon the analysis of the rupture mechanisms of moderate magnitude earthquakes, we show that four seismogenic models can account for their occurrence along intracontinental orogenic belts, including the shallow thrust, backthrust, blind thrust, and antithetic thrust models. The small or secondary faults in orogenic fronts are worthy of attention in seismic hazard investigations and assessments.

Fractures can exhibit mixed modes of displacement, that is, combinations of displacements parallel and perpendicular to the fracture plane, that make a displacement-to-length (D_max/L) scaling analysis challenging. However, such analysis is important for understanding the propagation and nature of mixed-mode fracture populations. In this study, we investigate the D_max/L scaling relationship for fractures involving opening and shearing modes from field measurements at the spectacularly exposed Koa'e Fault Zone in Hawai'i Volcanoes National Park, Hawaii. Its major structures have prominent fault scarps and display openings of up to several meters. They are locally accompanied by monoclines and sheared and pure joints. Through structural mapping and detailed field observations, we identify a morphological continuum along the structures representing different stages in the evolution of the faults. Contrary to previous studies, our observations support that faults are formed by the downward propagation of joints that transition to faulting at depth, then creating the monoclinal flexure. Our measurements allow us to investigate the D_max/L scaling behavior for the total mixed-mode displacement and their individual vector components, that is, reliefs and openings. The D_max/L scaling relationships for all structure types, including pure joints, sheared joints, and faults, show a power-law relation with a near-linear dependence of maximum displacement and length. The joint apertures scale to length with a nearly linear scaling relationship, not following the widely observed square root scaling relationship that all opening-mode fractures are believed to have.

We developed a novel three-dimensional magnetotelluric adaptive inversion algorithm optimized to interpret field datasets collected in realistic geological environments. Using a newly designed data-driven indicator, it tends to enhance features in data-sensitive regions and generate a set of multiscale inversion models with gradually increased resolution. Additionally, utilizing the nested tetrahedral grids, it meets different mesh resolution requirements for forward modeling and inversion, which addresses the trade-off between modeling accuracy and computational load. Validation against synthetic data confirms the algorithm's ability to efficiently delineate subsurface structures, notably enhancing the interpretability of magnetotelluric data. We applied the proposed algorithm to reinterpret field magnetotelluric data collected in the North China Craton within complex geological settings. The resulting conductivity structures reveal consistent high conductivity anomalies in the western Ordos Basin and the North China Plain, reflecting younger geological conditions. Additionally, high resistivity characteristics are observed beneath mountains such as the Luliang and Taihang Mountains, and three common high-conductivity anomalies from the upper mantle are identified. Notably, we found a previously identified conductor at 20–70 km depth beneath the southern Bohai Bay Basin, previously interpreted as electrical conductivity anisotropy, is now positioned at a deeper depth near the lithosphere-asthenosphere boundary, suggesting it may represent upwelling asthenospheric material. This research highlights the proposed adaptive inversion algorithm's potential to enhance subsurface imaging in geophysical exploration, with future integrations with other geophysical methods and efficiency improvements poised to extend its applicability to more complex datasets, aiding resource exploration, geohazard assessment, and deep Earth studies.

Several geological and geophysical studies suggest the small-scale presence of low viscosity zone (i.e., LVZ) beneath the Quaternary volcanoes of northeastern Japan. Before the 2011 M_w9.0 Tohoku-oki earthquake, scientists hypothesized that these LVZs cause localized crustal deformations around the Quaternary volcanoes. However, the deformation-signals related to these LVZs were too weak to properly understand the LVZ rheology. After the 2011 Tohoku-oki earthquake, InSAR and Global Navigation Satellite System (GNSS) observations reported significant ground movements around five Quaternary volcanoes including the Mt. Akitakoma, Mt. Kurikoma, Mt. Zao, Mt. Azuma, and Mt. Nasu. Using the early years of postseismic GNSS displacements, we extracted the short-wavelength components of strain-rate distribution, which clearly show the localized crustal contraction near the five volcanoes. To explain such spatial pattern of localized contraction, we propose a 3-D rheological model of LVZs near the five volcanoes, using power-law Burgers rheology. Most of our modeled LVZs have narrow tops (width of 20–40 km), wide roots (width of 80–100 km), limited arc-parallel dimensions (≤80 km), and are located at the depth range of 15–55 km in the lower crust-upper mantle. Based on the localization of postseismic strain rate, newly proposed 3-D LVZ models highlight an arc-parallel heterogeneity of subsurface rheology along the volcanic front of northeastern Japan, which is consistent with previously reported clusterization pattern of late Cenozoic calderas and high geothermal gradient near the five Quaternary volcanoes.

On 1 January 2024, a devastating M_J 7.6 earthquake occurred on the Noto Peninsula in Japan. When such a strong earthquake occurs, affected near-surface soil layers behave nonlinearly and may undergo some structural changes driven by Flow Liquefaction, Cyclic Mobility, or Slow Dynamics phenomena. The structural changes can be manifested by short-lasting coseismic and long-lasting postseismic site-response changes that are related to variations in near-surface shear-wave velocity. To examine this behavior, we perform a systematic analysis of Horizontal-to-Vertical (H/V) spectral ratios from regional earthquake waveforms recorded at 160 sites in the years 1996–2024. We identify significant H/V peaks and their directionality in the frequency range of 0.1–25 Hz separately for periods before and after the M_J 7.6 earthquake. This allows us to measure long-lasting relative changes in predominant frequency caused by the strong shaking, with maximum drops of −21% and a dependence on experienced ground motion levels. Next, the short-lasting changes during the M_J 7.6 earthquake reveal strongly nonstationary behavior. The frequency of spectral peaks decreases simultaneously and omnidirectionally with the strong shaking and then logarithmically recovers. The observed extreme short-lasting predominant frequency drops reach −93% relative to the initial value, and their occurrence time divides the nonstationary behavior into elastic softening and recovery phases. This behavior is physically related to temporal changes in near-surface shear-wave velocity as a consequence of changes in shear moduli. The introduced phenomenon of elastic softening and recovery may have a significant impact on a broad scale of geophysical research topics.

Tonalite–trondhjemite–granodiorite (TTG) gneisses are the dominant component of Archaean continental crust, with their parent magmas generally thought to have formed due to the partial melting of hydrated basalts; however, this process typically produces melts with a notably lower Mg^# than most natural TTGs. By contrast, ultramafic volcanic rocks commonly preserved in Archaean greenstone belts may represent an alternative source of TTG magma that has been largely overlooked. Here, we use petrological modelling to investigate anatexis of komatiites and komatiitic basalts from the Warrawoona Group of the Pilbara craton. In all cases, komatiite is refractory and generates no melt within the pressure-temperature range considered. Komatiitic basalts, however, could produce 20–25 vol. % of MgO-rich melts during greenstone belt sinking and hot subduction. Anatexis of komatiitic basalts generates melt fractions too depleted in large ion lithophile elements to represent natural TTGs; however, hybridization of melts produced by partial melting of tholeiitic basalts and komatiitic basalts during crustal overturn would generate magma that resembles natural TTGs. All calculated melts are felsic in composition, and TTGs with high Mg^# could have been generated entirely within the crust, with no requirement for the assimilation of mantle materials. By contrast, Archaean sanukitoids require some assimilation of mantle materials with crustal melts, indicating that the oldest sanukitoids preserved in each Archaean craton may record temporary and localized subduction on the early earth. The ubiquitous occurrence of sanukitoids worldwide by c. 2.7 Ga may provide a minimum age for the onset of global plate tectonics.

Interactions between Earth's mantle and crust have shaped the planet's evolution through deep time. Hafnium (Hf) isotopes provide a unique fingerprint of magma sources, enabling the tracking of the crucial interaction zone in the upper mantle evolution through more than four billion years of Earth's history. However, previous studies have relied on a combination of evolved and juvenile zircons, making it challenging to distinguish the genuine evolution of mantle properties. Here, we present a global compilation of Hf isotopic analyses of zircons from juvenile crust to track the upper mantle's evolution throughout Earth's history. By employing Singular Spectrum Analysis and Wavelet Analysis for time series, we decompose the complex Hf isotopic evolution curves and determine the respective periods and interpretations of each component. Our analysis reveals a complex and dynamic evolution of the upper mantle, with distinct periods of stability and upheaval. We show that the upper mantle has undergone periodic perturbations through mixing with crustal materials since Earth's formation, primarily caused by plate subduction and weakly influenced by mantle convective cycles. Hf isotopes reveal vigorous mantle convection that propelled plate tectonics during the Hadean, along with numerous supercontinent cycles that originated in the early Mesoarchean and a notable shift in subduction modes during the Neoproterozoic. This Hf isotope survey provides new insights into Earth's tectonic machinery, advancing our understanding of the planet's geological history.

Submarine permafrost in the Canadian Beaufort Sea is relict terrestrial permafrost, which is continuously degrading since the change of thermal conditions induced by a marine transgression that followed the last glaciation. Permafrost degradation has a crucial socio-ecological significance because its thawing can result in geohazards like landslides or an increase in greenhouse gas emissions. These consequences are mostly regulated by the state of ice in permafrost. In this study, we use marine multichannel seismic data to apply a diving wave tomographic inversion on the outer 50 km of the Canadian Beaufort Shelf. Due to the close relationship between seismic velocity and ice content, we are able to infer detailed information about the present submarine permafrost condition. We find a clear variability of permafrost occurrences between the inner and outer Canadian Beaufort Shelf. At the inner shelf, discontinuous ice-bonding permafrost occurs extensively close to the seafloor but is interrupted by taliks. Within the outer ∼27 km of the shelf, ice-bonding permafrost is absent in the upper sediments and its top has plunged to >200 m below sea level. These findings add new details to the current state of the degrading permafrost. In addition, we observe seismic anisotropy in the frozen permafrost sediments.

During closure of an ocean through subduction and continental collision, bathymetric highs such as microcontinents can accrete, collide, or partially or completely subduct. Such interaction of future allochthonous terranes (FATs) with the overriding continent will modify the dynamics of the subduction zone, affecting its length and frictional resistance, and thus the force balance of the subduction system. Accreted microcontinents and microcontinental fragments are preserved in backarcs and collisional orogens, demonstrating that multiple terranes can accrete during a single Wilson-cycle, in what is termed accretionary orogenesis. In this study, we use thermo-mechanical numerical experiments of microcontinent-continent collision events to investigate parameters that influence whether microcontinents accrete, subduct, or collide. Our results indicate that multiple accretionary episodes are possible, but that a weak basal detachment layer within each FAT is paramount for such a scenario to occur. The introduction of a microcontinent, or FAT, in the subduction zone will affect the balance between slab-pull, far-field forces, and the subduction interface resistance. The strength (and rheological stratification) of the microcontinent determines the evolution of the subduction interface resistance throughout the collision event, exerting a first order control on the resulting geodynamic scenario. Collision with a strong microcontinent significantly increases the subduction interface resistance promoting terrane subduction and localization of deformation away from the subduction interface. In turn, collision with a weak microcontinent increases subduction interface resistance only mildly, allowing for multiple accretion events.