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The crustal structure beneath the Japanese islands, including depth distributions of the Conrad and Moho discontinuities, was estimated using a tomographic inversion of regional body wave arrival times. Depth distributions of the bottom of the surface layer, the Conrad, and the Moho were modeled with two‐dimensional B spline functions, while velocity distributions in layers were expressed by three‐dimensional B spline functions. The depth of the discontinuities and the velocity in the layers were estimated simultaneously by the least squares method. The velocity structure was sequentially estimated from shallower parts to deeper parts to avoid correlation between them. This sequential analysis provided improved depth resolution. The deepest region of the Moho discontinuity was located in central Honshu, reaching about 40 km. The Moho discontinuity was generally deep in the central part of the islands, whereas it was relatively shallow in the Kanto, southwestern Chubu, and Chugoku districts in Honshu and in northern Kyushu. Some of the shallow Moho regions would be related to graben formation due to tensile tectonic stress since the Miocene. The results were compared with those of seismic refraction surveys and receiver function analyses, and it was found that the obtained model was consistent with many of these studies.

The location and origin of Neoproterozoic‐Cambrian sutures provide keys to understand the formation and evolution of the supercontinent Gondwana. The Larsemann Hills is located near a major Neoproterozoic‐Cambrian suture zone in the Prydz Belt, but has not been examined locally by comprehensive geophysical studies. In this study, we analyzed data collected from a one‐dimensional (1D) joint seismic‐MT array deployed during the 36th Chinese National Antarctic Research Expedition. We found that a sharp Moho discontinuity offset of 6–8 km shows up in the stacked image of teleseismic P‐wave receiver function analysis; coinciding with the abrupt Moho offset, a near‐vertical channel with (a) low resistivity extending to the uppermost mantle depths, and (b) high crustal Poisson's ratio in the crust is identified. These findings provide evidence for the determination of the location and collisional nature of the Prydz belt or a portion of it. Plain Language Summary Our study seeks to unravel the history of a supercontinent called Gondwana. We do this by exploring ancient geological features known as sutures. These sutures are like stitches that hold the Earth's crust together, and they're crucial in understanding how continents were once connected. We specifically focused on a place in Antarctica called the Larsemann Hills, which is located near an important suture zone. This region hasn't received much attention from scientists until now. During the 36th Chinese National Antarctic Research Expedition, we made some exciting discoveries. We found a clear boundary in the Earth's crust, a bit like a seam in a piece of clothing. At the same time, we noticed a unique underground pathway. This pathway had special properties, suggesting that it reaches deep into the Earth's mantle. It's a bit like finding a hidden treasure beneath the Earth's surface. Our findings strongly suggest a connection between these underground discoveries and the ancient sutures in the Earth's crust. In other words, we're piecing together a puzzle that can help us learn more about the Earth's past and how continents have moved over millions of years. Key Points A distinct Moho discontinuity offset of 6–8 km is found in the stacked image obtained from P‐wave receiver function In conjunction with the abrupt Moho offset, a nearly vertical conduit with low resistivity and high Poisson's ratio is identified These geophysical results provide crucial evidence for determining the collisional nature and location of the Prydz orogenic belt

Oceanic lithosphere descends into Earth’s mantle at subduction zones and drives material exchange between Earth’s surface and its deep interior. The subduction process creates chemical and thermal heterogeneities in the mantle, with the strongest gradients located at the interfaces between subducted slabs and the surrounding mantle. Seismic imaging of slab interfaces is key to understanding slab compositional layering, deep-water cycling and melting, yet the existence of slab interfaces below 200 km remains unconfirmed. Here, we observe two sharp and slightly dipping seismic discontinuities within the mantle transition zone beneath the western Pacific subduction zone that coincide spatially with the upper and lower bounds of the high-velocity slab. Based on a multi-frequency receiver function waveform modelling, we found the upper discontinuity to be consistent with the Mohorovičić discontinuity of the subducted oceanic lithosphere in the mantle transition zone. The lower discontinuity could be caused by partial melting of sub-slab asthenosphere under hydrous conditions in the seaward portion of the slab. Our observations show distinct slab–mantle boundaries at depths between 410 and 660 km, deeper than previously observed, suggesting a compositionally layered slab and high water contents beneath the slab.Two seismic discontinuities in the mantle transition zone beneath the western Pacific represent subducted slab interfaces that could be the slab Moho and partially molten sub-slab asthenosphere, according to an analysis of seismic data.

A M6.8 earthquake struck the High Atlas Mountains in Morocco on 8 September 2023, ending a 63‐year seismic silence. We herein attempt to clarify the seismogenic fault and explore the underlying mechanism for this seismic event based on multiple data sets. Utilizing probabilistic Bayesian inversion on interferometric radar data, we determine a seismogenic fault plane centered at a depth of 26 km, striking 251° and dipping 72°, closely aligned with the Tizi n’Test fault system. Given a hypocenter at the Moho depth, the joint inversion of radar and teleseismic data reveals that the rupture concentrates between depths of 12 and 36 km, offsetting the Mohorovičić discontinuity (Moho) at ∼32 km. Considering a strong link between magma activity and failure in lower crust, we propose that the triggering of the earthquake possibly was mantle upwelling that also supports the high topography. Plain Language Summary On 8 September 2023, a devastating earthquake with a magnitude of 6.8, struck the High Atlas Mountains in Morocco, breaking a 63‐year seismic silence. In this work we utilized geodetic, seismic, and seismicity data to investigate the earthquake, pinpointing its origin within the Tizi n’Test fault system at 12–36 km depths and causing displacement of the Mohorovičić discontinuity (Moho), where Earth's crust meets the mantle. Highlighting the impact of the mantle upwelling, this event underscores its significance in shaping regional topography and driving crustal deformation along established faults, leading to unusual, significant seismic activity in areas of minimal plate convergence. This earthquake highlights the importance of incorporating deep crustal dynamics into seismic hazard evaluations, challenging traditional risk models with its implications on earthquake genesis far from tectonic boundaries. Key Points Interferometric radar data, Bayesian inversion techniques and teleseismic data reveal the seismogenic fault and underlying mechanisms The seismogenic fault centered at a depth of 26 km, striking 251° and dipping 72°, closely aligned with the Tizi n’Test fault system We propose that the 2023 M6.8 earthquake in Morocco was possibly triggered by mantle upwelling on a steep fault that offsets the Moho

The water cycle at subduction zones remains poorly understood, although subduction is the only mechanism for water transport deep into Earth. Previous estimates of water flux 1 – 3 exhibit large variations in the amount of water that is subducted deeper than 100 kilometres. The main source of uncertainty in these calculations is the initial water content of the subducting uppermost mantle. Previous active-source seismic studies suggest that the subducting slab may be pervasively hydrated in the plate-bending region near the oceanic trench 4 – 7 . However, these studies do not constrain the depth extent of hydration and most investigate young incoming plates, leaving subduction-zone water budgets for old subducting plates uncertain. Here we present seismic images of the crust and uppermost mantle around the central Mariana trench derived from Rayleigh-wave analysis of broadband ocean-bottom seismic data. These images show that the low mantle velocities that result from mantle hydration extend roughly 24 kilometres beneath the Moho discontinuity. Combined with estimates of subducting crustal water, these results indicate that at least 4.3 times more water subducts than previously calculated for this region 3 . If other old, cold subducting slabs contain correspondingly thick layers of hydrous mantle, as suggested by the similarity of incoming plate faulting across old, cold subducting slabs, then estimates of the global water flux into the mantle at depths greater than 100 kilometres must be increased by a factor of about three compared to previous estimates 3 . Because a long-term net influx of water to the deep interior of Earth is inconsistent with the geological record 8 , estimates of water expelled at volcanic arcs and backarc basins probably also need to be revised upwards 9 . Seismic images of Earth’s crust and uppermost mantle around the Mariana trench show widespread serpentinization, suggesting that much more water is subducted than previously thought.

Forecast of eruptive activity is a core challenge in volcanology. Here, we present an actual forecast for the end of the 2021 La Palma eruption. Using continuous GNSS data, we identified a co‐eruptive quasi‐exponential deflation trend. Assuming mass conservation, magma upflow from an overpressurized reservoir drives the eruptive process. The forecast was carried out during the eruption, however there was uncertainty in the key percentage of drop in driving pressure necessary to stop this eruption. In hindcast, we explore how forecast uncertainty reduces with increase in ingested near‐real time data. We conclude that precise forecasts could have been possible, but only after twice a characteristic exponential decay time‐scale, providing error estimates of 45% of the actual duration. We verify the mass conservation assumption using eruptive material volumes and propose that the eruption dynamics was controlled by a main reservoir at a depth close to Moho discontinuity beneath Cumbre Vieja volcano. Plain Language Summary The forecast, an actual prediction of the temporal or spatial characteristics of a future event, of when a volcanic eruption will end is challenging. This is reflected in the scarcity of literature about it. During the 2021 eruption of La Palma, we used GNSS geodetic data that tracks the change in shape of a volcano to make such a forecast. We exploited a temporal decaying pattern of this data and some basic assumptions that could be interpreted as when the eruption would stop. The forecast was made before the eruption ended, although it was too uncertain to have practical implications. With the eruption already over, we looked back at the data and found that the more information we could have analyzed, the more accurate the forecast could have been. We conclude that accurate forecasts could be possible after twice the characteristic time of the decay process has passed. Key Points Using deformation data, the end of La Palma eruption, although uncertain, was possible to forecast Extensive end of eruption hindcasts to understand bounds on method applicability for La Palma eruptions Real‐time ground deformation interpretation could represent a simple and powerful tool for La Palma volcano monitoring

The Azores Triple Junction offers a unique opportunity to investigate the interplay between volcanism and tectonic activity. After 60 years of quiescence in São Jorge Island, in March 2022, the island experienced a volcano-tectonic unrest, accompanied by widely felt earthquakes and surface deformation. We conducted an extensive study of this anomalous activity throughout 2022, through a purely automated analysis based on a deep-learning approach for seismic activity, combined with the processing and analysis of data from GNSS stations in the archipelago. The joint interpretation of ground deformation and seismicity suggests a failed magmatic eruption, which we have summarized in a four-stage conceptual model. The unrest began on 16 March 2022 with a period of vertical uplift that lasted for 2 days. On 19 March, the deformation reversed with a burst of seismicity that marked a rapid dike intrusion in the crust which abruptly stopped a few kilometers under the surface. Over the following weeks, the relocated seismicity suggests an intense overpressure near the Moho discontinuity and reveals, in great detail, a lateral magmatic expansion in a sill-like pattern. Finally, from the second week of April until the end of the year a decrease in seismic activity and a lack of deformation registered, indicates a decline and stabilization of the volcano-tectonic process.

More than a third of mid-ocean ridges have a spreading rate of less than 20 millimetres a year 1 . The lack of deep imaging data means that factors controlling melting and mantle upwelling 2 , 3 , the depth to the lithosphere–asthenosphere boundary (LAB) 4 , 5 , crustal thickness 6 – 9 and hydrothermal venting are not well understood for ultraslow-spreading ridges 10 , 11 . Modern electromagnetic data have greatly improved our understanding of fast-spreading ridges 12 , 13 , but have not been available for the ultraslow-spreading ridges. Here we present a detailed 120-kilometre-deep electromagnetic joint inversion model for the ultraslow-spreading Mohns Ridge, combining controlled source electromagnetic and magnetotelluric data. Inversion images show mantle upwelling focused along a narrow, oblique and strongly asymmetric zone coinciding with asymmetric surface uplift. Although the upwelling pattern shows several of the characteristics of a dynamic system 3 , 12 – 14 , it probably reflects passive upwelling controlled by slow and asymmetric plate movements instead. Upwelling asthenosphere and melt can be traced to the inferred depth of the Mohorovičić discontinuity and are enveloped by the resistivity (100 ohm metres) contour denoted the electrical LAB (eLAB). The eLAB may represent a rheological boundary defined by a minimum melt content. We also find that neither the melt-suppression model 7 nor the inhibited-migration model 15 , which explain the correlation between spreading rate and crustal thickness 6 , 16 – 19 , can explain the thin crust below the ridge. A model in which crustal thickness is directly controlled by the melt-producing rock volumes created by the separating plates is more likely. Active melt emplacement into oceanic crust about three kilometres thick culminates in an inferred crustal magma chamber draped by fluid convection cells emanating at the Loki’s Castle hydrothermal black smoker field. Fluid convection extends for long lateral distances, exploiting high porosity at mid-crustal levels. The magnitude and long-lived nature of such plumbing systems could promote venting at ultraslow-spreading ridges. An inversion model for the ultraslow-spreading Mohns Ridge, combining controlled source electromagnetic and magnetotelluric data, reveals passive mantle upwelling controlled by slow and asymmetric plate movements.

Within extreme continental extension areas, ductile middle crust is exhumed at the surface as metamorphic core complexes. Sophisticated quantitative models of extreme extension predicted upward transport of ductile middle-lower crust through time. Here we develop a general model for metamorphic core complexes formation and demonstrate that they result from the collapse of a mountain belt supported by a thickened crustal root. We show that gravitational body forces generated by topography and crustal root cause an upward flow pattern of the ductile lower-middle crust, facilitated by a detachment surface evolving into low-angle normal fault. This detachment surface acquires large amounts of finite strain, consistent with thick mylonite zones found in metamorphic core complexes. Isostatic rebound exposes the detachment in a domed upwarp, while the final Moho discontinuity across the extended region relaxes to a flat geometry. This work suggests that belts of metamorphic core complexes are a fossil signature of collapsed highlands. A long-standing controversy surrounds low-angle nature of observed detachment faults within metamorphic core complexes. Here, the authors show that post-orogenic collapse of mountain belts can create a low-angle detachment, resolving the controversy.

Maluku Islands is an area with relatively high seismic activity it is the meeting area of three plates: Indo-Australia, Eurasia, and the Philippines. One of the earthquake sources in Maluku is the Seram Trough subductions zone, located north of Seram and Buru Islands. This study proposes to determine the depth of the Moho discontinuity and slab subduction, the velocity model of P and S wave and the vp/vs ratio on Seram and Buru Islands using the receiver function from teleseismic earthquake P wave recorded by four broadband three components sensors of the BMKG network (Indonesia Agency for Meteorology, Climatology, and Geophysics). The depth of the moho discontinuity on Seram Island was identified at depth between 22-42 km, and on Buru Island at depth 30-40 km. Based on the cross section analysis of the results of the receiver function migration results into depth domain, the depth of the slab on Buru Island was identified at a depth of 40-100 km. The P and S wave velocity models on Seram and Buru Islands vary, with low velocity zone at the AAII station due to geothermal field activity in the east of Ambon Island.