Intermediate-depth earthquakes, i.e., earthquakes occurring at depths of 60 to 300 km, have been observed globally. However, the mechanisms underlying intermediate-depth earthquakes and their potential relationship with shallow subduction zone structures are still poorly understood. Utilizing newly obtained near-field Ocean Bottom Seismometer (OBS) data and a machine-learning-based method (EQTransformer), we have detected and located 613 earthquakes from 5 September 2018 to 22 October 2019. The observation identifies the variations in the distribution patterns of intermediate-depth earthquakes at the junction of the Pacific plate and the Caroline Plateau. Double seismic zones (DSZs) were observed in the Pacific segment, while a single seismic zone (SSZ) was found in the Caroline segment. The consistency between observed seismicity patterns, tectonic geomorphology, outer-rise faulting, and slab P-T modeling strongly suggests intermediate-depth earthquakes are likely related to the dehydration of hydrous minerals. We propose that the seismicity difference between the two segments is attributed to the subducted oceanic plateau, which restricts hydration of the subducting plate thereby suppressing the generation of intermediate-depth earthquakes. Our results emphasize the important influence of oceanic plateau subduction in the generation and distribution of intermediate-depth earthquakes.

海洋多道反射地震成像是研究大洋岩石圈结构、岩浆系统和热液活动等信息的重要手段.受限于缆长、水深和采集方式等因素，水听器拖缆接收到的用于速度建模的壳内折射震相通常仅出现在较远偏移距，同时仅利用走时信息反演获得的速度模型只含有长波长的结构信息，严重制约了速度模型分辨率与地震成像效果.本文在传统初至波走时层析成像方法的基础上，加入地震波场的向下延拓、全波形反演和逆时偏移成像，发展了一套能显著提高洋中脊浅部结构分辨率的地震数据处理、建模与成像流程，并成功应用到东太平洋北部洋中脊五条垂直于洋脊轴的代表测线中.速度模型显示上地壳2A和2B层分别表现为高和低速度梯度特征，上地壳速度结构呈现不连续的低速异常特征，且与断裂或热液活动信号具有较好的空间对应关系.同时，地震成像也显示了2A/2B层存在明显的非均质性，表明上地壳结构受到岩浆-构造-热液作用的共同影响.本研究不仅为建立快速扩张洋中脊三维岩浆-构造-热液地质模型提供了支撑，同时为这套流程在其他研究区的应用提供了方法基础.

The southern Mariana subduction zone features the Earth's deepest trench, named the Challenger Deep, and tectonic deformation is obvious in the overriding and subducting plates. We conducted a 230 kmlong wide-angle reflection/refraction survey across the Challenger Deep. Based on ocean bottom seismometer (OBS) data, we image the velocity structure using forward and inverse modeling. The velocity model shows that the Pacific plate entering the subduction zone has normal crustal thickness (6.5- 7.2 km) but lower velocity than mature Pacific oceanic crust. PnP seismic phases with large offsets and strong seismic reflectivity are observed at eight OBS seismic record sections, which constrain the thickness (4.0-6.5 km) and velocity (7.1-7.5 km/s) of the upper mantle low-velocity layer. We suggest that the observed velocity reduction in the crust and upper mantle of the subducting plate is caused by bendingrelated normal faults, which not only fracture the crust but also provide pathways for water infiltration and serpentinization of the dry mantle. The upper mantle reflector constrained by PnP seismic phases possibly indicates a rheological and fault-slip interface derived from rapid variations in the mechanical strength of peridotite. The inner trench slope (ITS) near the Challenger Deep is characterized by indented topography, where the upper crust is obviously thinned and the lower crust is uplifted; the isovelocity contour of 6.5 km/s is 1.5 km below the seafloor. This may be caused by the difference in the degree of subducting plate rollback on either side of the western boundary of the diffuse deformation zone. (c) 2023 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Two tectonic plates converge at subduction zones where the subducting plate bends. Flexural bending of a subducting plate at a trench results in pervasive normal faulting, providing conduits for plate hydration and influencing the water budget and seismic behavior of the plate interface. 2-D plate bending simulations have demonstrated high strains due to strong bending may result in loss of strength of the lithosphere and intraplate earthquakes near the trench axis. However 3-D plate bending deformation may affect the along-strike slab pull force, especially when the deflection changes along the trench. Here we simulated the 3-D plate bending deformation and calculated bending stresses, and brittle failure of the Pacific plate at the Mariana Trench. We find that both the plate deflection and the yield zone depth (similar to 16-20 km) increases from the northern to southern Mariana Trench. The water flux in the plate at the southern Mariana Trench is estimated to be about 15% greater than that of the northern Mariana Trench. By comparing with 2-D models, we further find that the 2-D approaches may undereastimate the yield zone depth as they ignored the along-trench effects of plate deflection. The new results provide a self-consistent framework for interpretation of the observed surface normal faults, extensional earthquakes, and the inferred hydration of the subducting plate as constrained by seismic velocity anomalies.

Accurate source parameters of global submarine earthquakes are essential for understanding earthquake mechanics and tectonic dynamics. Previous studies have demonstrated that teleseismic P coda waveform complexities due to near-source 3-D structures are highly sensitive to source parameters of marine earthquakes. Leveraging these sensitivities, we can improve the accuracy of source parameter inversion compared to traditional 1-D methods. However, modeling these intricate 3-D effects poses significant computational challenges. To address this issue, we propose a novel reciprocity-based hybrid method for computing 3-D teleseismic Green's functions. Based on this method, we develop a grid-search inversion workflow for determining reliable source parameters of moderate-sized submarine earthquakes. The method is tested and proven on five Mw5+ earthquakes at the Blanco oceanic transform fault (OTF) with ground truth locations resolved by a local ocean bottom seismometer array, using ambient noise correlation and surface-wave relocation techniques. Our results show that fitting P coda waveforms through 3-D Green's functions can effectively improve the source location accuracy, especially for the centroid depth. Our improved centroid depths indicate that all the five Mw5+ earthquakes on the Blanco transform fault ruptured mainly above the depth of 600 degrees C isotherm predicted by the half-space cooling model. This finding aligns with the hypothesis that the rupture zone of large earthquakes at OTFs is confined by the 600 degrees C isotherm. However, it is noted that the Blanco transform fault serves as a case study. Our 3-D source inversion method offers a promising tool for systematically investigating global oceanic earthquakes using teleseismic waves. Plain Language Summary Understanding where earthquakes happen and the geometric characteristics of the faults are crucial for studying earthquakes and the Earth's tectonics. Sufficient data collected by seismometers near the fault can help us figure out these details. However, usually only distant seismometers located on land are available for most underwater earthquakes in the vast oceans. Simulating seismic waves propagating from these remote earthquakes all the way to seismometers on land is computationally expensive. To save costs, traditional simulations typically use a depth-dependent Earth model. In other words, the ocean is excluded or is included with a flat seafloor. These simplifications cause significant error because the real non-flat seafloor results in much more complicated seismic waves. Here, we develop a computationally inexpensive method to accurately model how seismic waves propagate in an Earth model with non-flat seafloor. We demonstrate that this more accurate modeling can dramatically improve the earthquake source parameter estimation, especially the depth. We test this method using five moderate-sized earthquakes on the Blanco transform fault and confirm its high accuracy. The reliable depths indicate that all these earthquakes happened at depths with temperatures lower than 600 degrees C. This method can be applied globally to study submarine earthquakes.

Water is the most common volatile component inside the Earth. A substantial amount of water can be carried down to the interior of the Earth by subducting plates. However, how the subducted water evolves after the subducting slab breaks off remains poorly understood. Here we use the data from a passive seismic experiment using ocean bottom seismometers (OBSs) together with the land stations to determine the high-resolution, three-dimensional seismic structure of the Southwest Sub-basin (SWSB) of the South China Sea (SCS). At depths below 40 km, the mantle shear velocity (Vsv) beneath the northern side of the SWSB is similar to that of the conventional oceanic pyrolite mantle, but roughly 3% shear-velocity reduction is found beneath the southern side of the SWSB. Results of thermal dynamic modeling reveal that the observed shear-velocity reduction could be explained by the presence of 150-300 ppm of water and 5-10% of lower continental crust. The inferred high-water content at the southern side of the SWSB is consistent with a model in which the Proto-SCS plate subducted southward prior to and during the formation of the SCS basin, releasing water into the upper mantle of the SWSB.This study suggests that the observed shear-velocity reduction beneath the southern side of the Southwest Sub-basin (SWSB) of the South China Sea (SCS) may be due to the presence of 150-300 ppm of water and 5-10% of lower continental crust.

太平洋板块边界和内部均发育大量火山，是研究地球火山的天然实验场。综述了太平洋火山特征与深部成因机制，表明研究人员对地球不同环境下的火山（包括大洋中脊、俯冲带岛弧、板内地幔柱等）进行了系统性研究，分别构建了减压熔融、俯冲板片脱水与富水地幔楔熔融、地幔柱高温熔融的经典模式。但目前学界对于板内非地幔柱型火山的深部岩浆起源以及浅部喷发通道等重要科学问题仍缺乏清晰的认识。未来需要采用创新观测手段，开展多学科交叉研究以取得突破。

地球表层短周期的地表碳循环影响全球气候变化和地球宜居环境，而地球上90%以上的碳存在于地球内部，地球内部长周期的深部碳循环对地表碳循环产生重要影响。介绍了汇聚板块边界、离散板块边界、板块内部和新型海山等不同构造背景深部碳循环的研究现状，并阐述将来需要深入研究的科学问题，包括俯冲带脱碳机制及其效率、碳在地幔中的存在形式等。

Most rifted margins involve a certain degree of obliquity. Oblique extension has been well-studied in a magmapoor continental rift setting. However, few studies have addressed the structure and evolution of oblique rifting in a magma-rich continental rift setting. In this study, we used remote sensing and field measurements combined with geophysical data in order to study the tectono-magmatic evolution of seaward-dipping basalt sequences, known also as seaward dipping reflectors (SDRs), along the western Nuussuaq oblique volcanic passive margin. We calculated the directions and dips of lavas within onshore SDRs based on remote sensing and field data and analyzed the offshore SDRs through seismic reflection profiles to obtain a precise 3D structure of the inner SDRs along this margin. Our results show that the development of the inner SDRs can be divided into two stages. During the first stage (the late Paleocene), the oblique extension was partitioned into strike-slip and dip-slip components, and the basalt sequences were mainly bounded by N-S-trending continentward-dipping faults. During the second stage (the early Eocene), a local stress reorientation occurred in the western Nuussuaq and the basalt sequences were mainly bounded by NE-SW-trending continentward-dipping faults. The obliquity of the margin originated from both the reactivation of the inherited Itilli Fault and the magma intrusion into the crust, which weakened the crust and produced a pressure-induced extension orthogonal to the margin.