The subduction of oceanic plateaus significantly reshapes subduction systems, yet its impact on lithospheric hydration prior to subduction is not well understood. Here we investigate these processes as the Caroline Plateau approaches the southern Mariana Trench, using velocity models derived from wide-angle seismic lines. Our results from OBS2017-1 reveal that the crust of the subducting plate thickens from similar to 7.5 km beneath the trench axis to similar to 16.0 km at the outer rise, accompanied by reduced seismic velocities in the region between the toe of the plateau and the trench axis. These velocity reductions even exceed those observed beneath adjacent oceanic crust near Challenger Deep, indicating intensified hydration at the leading edge of the plateau, accompanied by a narrowing of the bending-related faulting zone. The simultaneous subduction of plateau and normal oceanic crust governs plate configuration, highlighting the role of oceanic plateaus in enhancing heterogeneous water percolation and modulating subduction dynamics.

Plate subduction creates unique topographic features in hadal trenches, yet their influence on microbial ecosystems and the global ocean remains unclear. Here, we conducted a topography-targeted investigation across 6-11 km of water depth within the Mariana Trench, integrating metagenomic, metaproteomic, and geochemical analyses. Coupled with high-resolution topographic mapping, our analyses reveal topography as an overlooked determinant of hadal geochemical and microbial heterogeneity. Convex areas exhibit classical sediment-depth-decay patterns with sparse, cooperative microbial communities. Conversely, concave features maintain higher biomass and activity as well as dense microbial interactions. Critically, slope concave sites incubate previously unrecognized microbial hotspots and may serve as interchange hubs, potentially facilitating genetic exchange and upward dispersal of microorganisms from Earth's deepest regions to the broader ocean. Our findings demonstrate that topographic features, rather than water depth, significantly correlate with organic carbon influx and its microbial turnover rates, enabling predictive modeling of hadal carbon cycling with global implications.

Intermediate-depth earthquakes (IDEs), i.e., earthquakes at depths of 70 to 300 km, have been observed in subduction zones globally and extensively investigated. However, the seismogenic mechanism of IDEs is still controversial, especially in the southern end of the Mariana Trench, where near-field observations are lacking. By using machine-learning-based methods in three sets of near-field Ocean Bottom Seismogram (OBS) network data, we detected and located more than 1,000 intraplate and interplate earthquakes. The seismogenic volumes in different regions of the subducted plate are different, showing the character of double seismogenic zones (DSZ) and single seismicity layer (SSZ). The seismicity features coincide well with the regional landform, development of outer-rise faults, and hydration scenarios, suggesting a dehydration-related mechanism for the generation of IDEs. The subducted slabs experience different degrees of slab hydration, leading to various seismic behaviors.

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.

The subduction of oceanic plateaus is a global phenomenon that reshapes the tectonic configuration of subduction systems and plays a crucial role in water cycling and volatile fluxes. While previous studies have primarily focused on the processes occurring after plateaus into subduction zones, but the effects of subducting oceanic plateaus on bending and hydration before subduction remain unclear. Using wide-angle seismic data perpendicular to the trench, we investigated these processes as the Caroline Plateau approaches the trench. The P-wave velocity structure shows a gradual increase in crustal thickness from ~7.5 km beneath the trench and outer trench slope, 9.0–12.0 km in the outer rise region, to 16.0–17.0 km beneath the plateau, indicating that the Caroline oceanic plateau is approaching the trench. Seismic velocities near trench axis are lower than those in other subduction zones and at Challenger Deep to the east, where the trench is far from the oceanic plateau. These low seismic velocities, combined with a narrower array of normal faults, suggest that the participation of the oceanic plateau in the subduction process reduces the width of bending fault zone parallel to the trench, while increasing fracturing, serpentinization in the lithosphere ahead of the plateau. Along the trench, the juxtaposed subduction of the oceanic plateau and oceanic crust influences the tectonic configuration of the overriding plate, highlighting the impact of incoming oceanic plateau on subduction dynamics.

The southern Mariana subduction zone, home to the Challenger Deep-the deepest known point on Earth-poses significant challenges for studying the hydration of the subducting plate due to its extreme depth. This study uses S-wave seismic tomography and V-p/V-s ratios to investigate hydration and serpentinization at the Challenger Deep. We observe a low V-p and V-s layer in the upper mantle with V-p/V-s ratios exceeding 1.8, reaching up to 1.95 at the Moho. These high ratios indicate a strong serpentinized layer (>15 vol%) with significant changes in the mechanical properties of the serpentinized peridotite. Additionally, V-p/V-s ratios in the crust and uppermost mantle increase from the outer rise to the trench axis, demonstrating that bending-related faulting and hydration intensify as the plate approaches the trench. Our results suggest extensive faulting, hydration, and mantle serpentinization at the Challenger Deep, making this region an extreme example of water cycling in subduction zones.

This study assesses the tsunami mitigation capacity of narrow fringing reefs (50–200 m width) under varying ecological health states using a high-resolution implementation of the COMCOT model. Satellite-derived reef bathymetry (20 m resolution) and non-uniform bottom friction coefficients were incorporated to simulate tsunami propagation from four ∼Mw 9 earthquake scenarios along the Manila Subduction zone. The results indicate that healthy reefs can reduce the maximum on-platform current velocities by nearly 80% and delay wave arrival by 5–10 min – providing a critical window for early evacuation. While these narrow coral reefs exert limited influence on initial tsunami wave crests, they significantly attenuate subsequent waves, with up to 50% reduction in wave heights when the peak arrives after the first crest. These findings highlight the disproportionate protective value of healthy narrow fringing reefs, framing them as essential natural infrastructure for tsunami risk reduction. This study offers quantitative thresholds for targeted reef conservation and supports the integration of ecological parameters into computational tsunami hazard modelling frameworks.

The data collected by ocean-bottom seismometers are unique compared to that of land seismic stations and require preprocessing before use. In this study, we focus on preprocessing seismic data obtained from a passive-source experiment in the Mariana subduction zone conducted by the Institute of Deep-Sea Science and Engineering, Chinese Academy of Sciences from 2018 to 2019. We outline the steps involved in preprocessing the data, including time correction using the ocean background noise cross-correlation function, tilt correction using a transfer function and orientation correction based on seismic wave polarization. Our results show that the maximum clock drifting was similar to 4 s on two stations (K06 and K20), whereas the rest of the station waveforms have clock drifting of less than 2 s. Among stations, only the K06 station is tilted, and the tilt angle is 19.82 degrees. We use the P-wave and Rayleigh wave polarization methods to determine the orientation. Our results indicate that the error in the result obtained using the P-wave polarization method is less than that of the Rayleigh wave polarization method. Additionally, we address the advantages and disadvantages of these preprocessing techniques. Our goal is to offer a comprehensive overview of the preprocessing process, to assist beginners in the field and to serve as a foundation for future research in passive-source ocean-bottom seismometers.

We employed seismic tomography to examine the velocity structure of the upper mantle in the Southernmost Mariana subduction zone. Our study focuses on data collected during a six-month experiment from 15 December 2016 to 12 June 2017, using 11 ocean bottom seismometers. By examining over 3700 local arrival times, we are able to determine the three-dimensional Vp and Vs structure. The subducting slab in this region displays a P- and S-wave velocity 2~6% higher than normal mantle and a lower Vp/Vs, with an average dip of 45° at depths ranging from 50 to 100 km. Additionally, our velocity images also shed new lights to the velocity anomalies of the mantle wedge region on top of the subducting slab, from the trench to the remnant arc. We observed slower velocity anomalies in the mantle wedge beneath the Southwest Mariana Rift, the West Mariana Ridge, and the forearc. In the outer forearc, a low-velocity anomaly is observed at depths shallower than 50 km, indicating mantle serpentinization and the presence of water. Additionally, a melt production region is observed beneath the central part of the forearc block at a depth of 40–60 km suggesting the possibility of melting processes in this region.

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.

The hadal zone, mostly comprising of deep trenches and constituting of the deepest part of the world’s oceans, represents the least explored habitat but one of the last frontiers on our planet. The present scientific understanding of the hadal environment is still relatively rudimentary, particularly in comparison with that of shallower marine environments. In the last 30 years, continuous efforts have been launched in deepening our knowledge regarding the ecology of the hadal trench. However, the geological and environmental processes that potentially affect the sedimentary, geochemical and biological processes in hadal trenches have received less attention. Here, we review recent advances in the geology, biology, and environment of hadal trenches and offer a perspective of the hadal science involved therein. For the first time, we release high-definition images taken by a new full-ocean-depth manned submersible Fendouzhe that reveal novel species with an unexpectedly high density, outcrops of mantle and basaltic rocks, and anthropogenic pollutants at the deepest point of the world’s ocean. We advocate that the hydration of the hadal lithosphere is a driving force that influences a variety of sedimentary, geochemical, and biological processes in the hadal trench. Hadal lithosphere might host the Earth’s deepest subsurface microbial ecosystem. Future research, combined with technological advances and international cooperation, should focus on establishing the intrinsic linkage of the geology, biology, and environment of the hadal trenches.

The maximum ocean depth so far reported is about 11 000 m, and is located in the Mariana Trench in the Western Pacific Ocean. The hybrid unmanned underwater vehicle, Haidou, is developed to perform scientific survey at the deepest parts of the Earth oceans. For vehicles working at the full-ocean depth, acoustic positioning is the most effective and popular method. The 11 000 m class acoustic positioning system is relatively massive and complex, and it requires specialized research vessels equipped with compatible acoustic instruments. As a compact testbed platform, it is impractical for Haidou to carry an LBL/USBL beacon with its large volume and weight. During the descent to about 11 000 m, horizontal drift could not be eliminated because of the hydrodynamics and uncertain ocean currents in the sea trials. The maximum depth recorded by Haidou is 10 905 m, and determining the precise location of the deepest point is challenging. With the bathymetric map produced by a multibeam sonar, the terrain contour matching (TERCOM) method is adopted for terrain matching localization. TERCOM is stable in providing an accurate position because of its insensitivity to the initial position errors. The final matching results show the best estimate of location in the reference terrain map.

We investigate the spatiotemporal pattern of crustal anisotropy in the source area of the 2004 Niigata earthquake (M 6.8) that occurred in the northern segment of the Niigata-Kobe tectonic zone, central Japan, by measuring shear-wave splitting parameters from waveform data of local earthquakes. Our results show that the fast polarization directions in the upper crust have spatial variations across the region of the earthquake that are likely caused by both structural and stress field effects. The northwest-southeast direction near the northeastern end of the source zone (beneath station N.NGOH) and the east-west direction to the southwest (beneath station N.KWNH) are consistent with the spatial variation of the orientation of the maximum compression of the local stress field. Fast polarization directions at other stations tend to align in the directions of active faults and folds and thus are considered to be structure induced. These spatial patterns were unaffected by the earthquake. However, at two stations (N.NGOH and N.KWNH) we observe an increase in both the average and scatter of the normalized delay times (delta t) during the aftershock period. In addition, two stations (HIROKA and N.YNTH) that are located in the strike-normal direction east of the source area show an increase in the average of the normalized delta t and a rotation of up to 90 degrees of the fast direction immediately after the mainshock. We also notice that stations located very close to the source fault (DP.YMK and DP.OJK) show larger average delay times compared with stations farther away (HIROKA and N.YNTH) during the postseismic stage. To explain the temporal changes in the strength of the anisotropy, we speculate that spatiotemporal variations in microcrack development in and around the source area could be caused by static stress changes due to tectonic deformation and the earthquake rupture.

Due to the influence of ocean currents, Hadal landers deviate from the launch points during dives. The deviation offset and direction are difficult to be estimated. Moreover, the underwater acoustic positioning method is not suitable in the Hadal areas. Through the cross-line air-gun shooting operation carrying out on the sea surface, and by using the accurate position and timing information of each air-gun shot, as well as the arrival times of direct water waves recorded by the Ocean Bottom Seismometer (OBS), the seafloor positions of Hadal Landers can be measured precisely. In this method, the collected data are preprocessed and inversed for the actual landing positions of Wanquan Hadal Lander, using the Monte Carlo method combined with the least square method. The obtained position correction offsets of the three dives are 211 m, 178 m and 861 m respectively, with different deviation directions from each other. The measurement precision is +/- 20 m, which is much more accurate than the previous ones. In addition, this method determines precise locations of the three seafloor markers and provides reliable underwater reference coordinates for the future 10000 m-type unmanned or manned submersibles.

The potential tsunami hazard for atoll islands inside the South China Sea (SCS) has not been investigated previously. In this study, we assess the tsunami hazard for the Xisha Archipelago using a deterministic approach. The tsunami source considered in this study is hypothesized earthquakes in the Manila subduction zone. We employ COMCOT (COrnell Multi-grid Coupled Tsunami model) for tsunami simulations. Different with previous deterministic assessments, our study investigates the uncertainties caused by bathymetric resolution and rupture complexities (here refers to rupture speed and direction variations related to fault geometry complexity). Comparison between results of different resolution suggests that coarser grids may over/underestimate tsunami hazards around mid-ocean atoll islands and this depends on the surrounding bathymetry. But results of different datasets show that GEBCO may lead to a lower result than the multibeam data. We also investigate the uncertainties caused by different rupture speed and direction, which are highly affected by subducting seamounts/ridge. Different rupture direction will shift the hardest hit locations to areas, which are possibly underestimated in cases of instantaneous ruptures. Also, the distribution of slip and location of the rupture zone can affect the local tsunami wave height significantly. We suggest that near-island high-resolution bathymetric data and dynamic rupture process should be considered in future tsunami hazard assessment in locations with similar geological structures.