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Determining the mechanisms responsible for intraplate volcanism ‐ such as slab devolatilization melting versus active mantle plumes ‐ remains a challenge. The greater South China Sea (SCS) region has experienced extensive intraplate Cenozoic volcanism across areas including Hainan, Southeast Indochina, northern Borneo, the northern SCS, and the post‐spreading SCS basin. The prevalence of volcanism distributed widely across this region prompts fundamental questions about the key geodynamic processes driving such diverse magmatic activities. In this study, we elucidate the mantle transition zone (MTZ) discontinuities in this region using SS precursors, which helps to overcome the sparse seismic coverage due to its predominantly oceanic setting. We collected over 16,000 high‐quality seismograms that sample the upper mantle and MTZ beneath this region from global earthquakes and stations. After correcting for the effects of shallow crustal variations and upper mantle heterogeneity on traveltimes of SS phases and their precursors, we unveil lateral variations in the MTZ boundaries (d410 and d660) and intricate features of the mid‐MTZ reflectors (S520S). Significant MTZ thinning and normal S520S waveforms beneath Hainan provide compelling evidence for mantle upwelling through the MTZ. Conversely, the evident splitting of S520S beneath the northern SCS, Southeast Indochina, and northern Borneo, all characterized by stagnant subducted slabs, indicates that the volcanism in these regions likely originated from a mechanism distinct from the active upwelling beneath Hainan. Dehydration melting attributed to devolatilizing stagnant slabs in the MTZ is a potential cause for Cenozoic volcanism in these regions. Plain Language Summary: Determining the causes of volcanic activity far from tectonic plate boundaries remains a challenge. The greater South China Sea region experienced widespread volcanism during the Cenozoic era, prompting questions about the driving geological processes behind this phenomenon. This study investigated the boundaries of the mantle transition zone (MTZ) beneath the region using seismic wave analysis of over 16,000 seismic recordings. By analyzing the travel times of signals reflected from the MTZ boundaries and internal reflectors, lateral variations in the structure of the MTZ boundaries and internal features were revealed. Beneath Hainan, thinning of the MTZ and normal waveforms of the internal MTZ reflector suggest upwelling of hot mantle material through the MTZ, likely causing the surface volcanism. In contrast, beneath the northern South China Sea, Southeast Indochina, and northern Borneo, the distorted waveforms of signals from the internal MTZ reflector indicate a different mechanism. These latter regions contain ancient, subducted slabs present in their MTZs. Dehydration melting, caused by the release of water from these stagnant slabs within the MTZ, is a potential cause of the Cenozoic volcanism in these areas, distinct from the mantle upwelling observed beneath Hainan. Key Points: SS precursors from the mantle transition zone (MTZ), notably the S520S, provide insights into mantle dynamics beneath the greater South China Sea (SCS) regionEvidence of MTZ thinning and normal S520S waveform beneath Hainan suggests active mantle upwelling through the MTZThe S520S splitting beneath areas with stagnant subducted slabs indicates a possible magmatic origin through flux melting [ABSTRACT FROM AUTHOR]

Megathrust geometric properties exhibit some of the strongest correlations with maximum earthquake magnitude in global surveys of large subduction zone earthquakes, but the mechanisms through which fault geometry influences subduction earthquake cycle dynamics remain unresolved. Here, we develop 39 models of sequences of earthquakes and aseismic slip (SEAS) on variably‐dipping planar and variably‐curved nonplanar megathrusts using the volumetric, high‐order accurate code tandem to account for fault curvature. We vary the dip, downdip curvature and width of the seismogenic zone to examine how slab geometry mechanically influences megathrust seismic cycles, including the size, variability, and interevent timing of earthquakes. Dip and curvature control characteristic slip styles primarily through their influence on seismogenic zone width: wider seismogenic zones allow shallowly‐dipping megathrusts to host larger earthquakes than steeply‐dipping ones. Under elevated pore pressure and less strongly velocity‐weakening friction, all modeled fault geometries host uniform periodic ruptures. In contrast, shallowly‐dipping and sharply‐curved megathrusts host multi‐period supercycles of slow‐to‐fast, small‐to‐large slip events under higher effective stresses and more strongly velocity‐weakening friction. We discuss how subduction zones' maximum earthquake magnitudes may be primarily controlled by the dip and dimensions of the seismogenic zone, while second‐order effects from structurally‐derived mechanical heterogeneity modulate the recurrence frequency and timing of these events. Our results suggest that enhanced co‐ and interseismic strength and stress variability along the megathrust, such as induced near areas of high or heterogeneous fault curvature, limits how frequently large ruptures occur and may explain curved faults' tendency to host more frequent, smaller earthquakes than flat faults. Plain Language Summary: Subduction zones, where one tectonic plate dives beneath another, generate the largest earthquakes worldwide. Our study investigates how the shape and tilt of these large offshore underground faults, termed "megathrusts," may determine the size of large earthquakes, how often they happen, and how similar or different subsequent events are. By creating computer simulations of earthquakes in subduction zones, we found that the angles and dimensions of the megathrust may set a limit on how big an earthquake can get. We also find that the presence of bends or curves along these faults can make earthquakes more unpredictable, sometimes leading to more variable series of smaller quakes before the biggest one hits. Our findings may help explain why some areas near subduction zones are prone to larger or more frequent earthquakes than others. Understanding these patterns can improve our ability to prepare for these natural disasters, potentially reducing their damaging effects on nearby communities and infrastructure. Key Points: Systematic earthquake cycle simulations reveal how subduction zone geometry controls megathrust earthquake size and timingDip and curvature affect characteristic slip style: periodic uniform ruptures versus supercycles of variably‐sized slow‐to‐fast eventsSeismogenic zone dip and dimensions limit maximum earthquake size; curvature‐linked stress and strength heterogeneity modulates recurrence [ABSTRACT FROM AUTHOR]

The Mesozoic subduction zone over the Dongsha Waters (DSWs) of the South China Sea (SCS) is a part of the westward subduction of the ancient Pacific plate. Based on the comprehensive interpretation of deep reflection seismic profile data and polar magnetic anomaly data, and the zircon dating results of igneous rocks drilled from well LF35-1-1, the Mesozoic subduction zone in the northeast SCS is accurately identified, and a Mesozoic subduction model is proposed. The accretion wedges, trenches, and igneous rock zones together form the Mesozoic subduction zone. The evolution of the Mesozoic subduction zone can be divided into two stages: continental subduction during the Late Jurassic and continental collision during the late Cretaceous. The Mesozoic subduction zone controlled the structural pattern and evolution of the Chaoshan depression (CSD) during the Mesozoic and Neogene eras. The gas source of the hydrate comes from thermogenic gas, which is accompanied by mud diapir activity and migrates along the fault. The gas accumulates to form gas hydrates at the bottom of the stable domain; BSR can be seen above the mud diapir structure; that is, hydrate deposits are formed under the influence of mud diapir structures, belonging to a typical leakage type genesis model. [ABSTRACT FROM AUTHOR]

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Abstract The Allalin gabbro of the Zermatt-Saas meta-ophiolite consists of variably metamorphosed Mg- to Fe-Ti-gabbros, troctolites, and anorthosites, which are crosscut by basaltic dykes. Field relationships of the various rock types and petrographic studies together with bulk rock and mineral chemical composition data allow the reconstruction of the complete geological history of the Allalin gabbro. With increasing magmatic differentiation, the incompatible element content in clinopyroxene increases (e.g., REEs and Zr by a factor of 5), whereas the Mg# decreases (from 86.4 to 74.6) as do the compatible element contents (e.g., Cr and Ni by factors of 3.5 and 5, respectively). Exhumation to shallower depths led to subsolidus ductile deformation and cooling of the gabbro followed by the intrusion of fine-grained basaltic dykes, which display chilled margins. Bulk rock data of these dykes reveal strong similarities in fluid-immobile trace element patterns to tholeiitic pillow basalts of the Zermatt-Saas and nearby meta-ophiolites. The recalculated REE patterns of the melt in equilibrium with igneous clinopyroxene is very similar to the REE patterns of the mafic dykes, indicating a cogenetic origin of pillow basalts, dykes, and gabbros. Together with the previously determined Jurassic intrusion age of the gabbro, our observations demonstrate that the Allalin gabbro intruded as a tholeiitic magma in a slow spreading MOR environment of the Piemonte-Ligurian ocean of the Alpine Tethys. Subduction of the Allalin gabbro resulted in different eclogitization extents of the Mg-gabbros as a function of variable hydration degrees. Metagabbros with low extents of hydration record incomplete eclogitization; the magmatic mineralogy (olivine + clinopyroxene + plagioclase) is preserved together with disequilibrium textures in the form of reaction coronae surrounding mineral boundaries. Metagabbros with high extents of hydration are completely eclogitized and display pseudomorphic replacement textures of magmatic minerals by eclogite-facies mineral assemblages, which required significant major to trace element transport across mineral domains. The locally variable extents of hydration took place near the sea floor, as recorded by the presence of Cl-apatite (6.28 wt% Cl), and an increase in B concentrations of minerals pseudomorphically replacing olivine (e.g., chlorite with 0.20–0.31 µg/g B and omphacite with 0.22–0.25 µg/g B) compared to magmatic olivine (0.12–0.16 µg/g B). Moreover, the chemical zonation pattern of metamorphic garnet coronae is different in completely eclogitized gabbros and gabbros with relic igneous minerals, in agreement with a main hydration event prior to subduction. The Allalin gabbro therefore represents a classical example of an oceanic gabbro formed in a slow spreading setting in the mid Jurassic that experienced heterogeneous hydration near the sea floor. Paleogene subduction of the gabbro to 70–80 km depth produced variably equilibrated gabbroic eclogites. In eclogite-facies Mg-gabbros, the water-rich minerals chlorite, talc, and chloritoid pseudomorphing magmatic olivine remained stable to these depths, revealing the potential relevance of hydrated Mg-gabbros as a fluid source at subarc depths in subduction zones.

Abstract Slow earthquakes are slow fault slip events. Quantifying and monitoring slow earthquake activity characteristics are important, because they may change before large earthquakes occur. Statistical seismicity models are useful for quantifying seismicity characteristics. However, no standard statistical model exists for slow earthquake activity. This study used a high-quality catalog of low-frequency earthquakes (LFEs), a type of slow earthquake, in the Nankai subduction zone from April 2004 to August 2015 and conducted the first comparison of existing statistical LFE activity models to determine which model better describes LFE activity. Based on this comparison, this study proposes a new hybrid model that incorporates existing model features. The new model considers the LFE activity history in a manner similar to the epidemic-type aftershock sequence (ETAS) model and represents the LFE aftershock rate (subsequent LFE occurrence rate) with a small number of model parameters, as in the Omori–Utsu aftershock law for regular earthquakes. The results show that the proposed model outperforms other existing models. However, the new model cannot reproduce a feature of LFE activity: the sudden cessation of intense LFE bursts. This is because the new model superimposes multiple aftershock activities and predicts extremely high seismicity rates during and after the LFE bursts. I suggest that reproducing and successfully predicting the sudden cessation of intense LFE bursts is critical for the further improvement of statistical LFE activity models. In addition, the empirical equations formulated in this study for the LFE aftershock rates may be useful for future statistical and physical modeling of LFE activity. Graphical Abstract

The Allalin gabbro of the Zermatt-Saas meta-ophiolite consists of variably metamorphosed Mg- to Fe-Ti-gabbros, troctolites, and anorthosites, which are crosscut by basaltic dykes. Field relationships of the various rock types and petrographic studies together with bulk rock and mineral chemical composition data allow the reconstruction of the complete geological history of the Allalin gabbro. With increasing magmatic differentiation, the incompatible element content in clinopyroxene increases (e.g., REEs and Zr by a factor of 5), whereas the Mg# decreases (from 86.4 to 74.6) as do the compatible element contents (e.g., Cr and Ni by factors of 3.5 and 5, respectively). Exhumation to shallower depths led to subsolidus ductile deformation and cooling of the gabbro followed by the intrusion of fine-grained basaltic dykes, which display chilled margins. Bulk rock data of these dykes reveal strong similarities in fluid-immobile trace element patterns to tholeiitic pillow basalts of the Zermatt-Saas and nearby meta-ophiolites. The recalculated REE patterns of the melt in equilibrium with igneous clinopyroxene is very similar to the REE patterns of the mafic dykes, indicating a cogenetic origin of pillow basalts, dykes, and gabbros. Together with the previously determined Jurassic intrusion age of the gabbro, our observations demonstrate that the Allalin gabbro intruded as a tholeiitic magma in a slow spreading MOR environment of the Piemonte-Ligurian ocean of the Alpine Tethys. Subduction of the Allalin gabbro resulted in different eclogitization extents of the Mg-gabbros as a function of variable hydration degrees. Metagabbros with low extents of hydration record incomplete eclogitization; the magmatic mineralogy (olivine + clinopyroxene + plagioclase) is preserved together with disequilibrium textures in the form of reaction coronae surrounding mineral boundaries. Metagabbros with high extents of hydration are completely eclogitized and display pseudomorphic replacement textures of magmatic minerals by eclogite-facies mineral assemblages, which required significant major to trace element transport across mineral domains. The locally variable extents of hydration took place near the sea floor, as recorded by the presence of Cl-apatite (6.28 wt% Cl), and an increase in B concentrations of minerals pseudomorphically replacing olivine (e.g., chlorite with 0.20–0.31 µg/g B and omphacite with 0.22–0.25 µg/g B) compared to magmatic olivine (0.12–0.16 µg/g B). Moreover, the chemical zonation pattern of metamorphic garnet coronae is different in completely eclogitized gabbros and gabbros with relic igneous minerals, in agreement with a main hydration event prior to subduction. The Allalin gabbro therefore represents a classical example of an oceanic gabbro formed in a slow spreading setting in the mid Jurassic that experienced heterogeneous hydration near the sea floor. Paleogene subduction of the gabbro to 70–80 km depth produced variably equilibrated gabbroic eclogites. In eclogite-facies Mg-gabbros, the water-rich minerals chlorite, talc, and chloritoid pseudomorphing magmatic olivine remained stable to these depths, revealing the potential relevance of hydrated Mg-gabbros as a fluid source at subarc depths in subduction zones. [ABSTRACT FROM AUTHOR]

Earthquakes that rupture several faults occur frequently within the shallow lithosphere but are rarely observed for intermediate‐depth events (70–300 km). On 29 November 2007, the Mw7.4 Martinique earthquake struck the Lesser Antilles Island Arc near the deep end of the Wadati‐Benioff‐Zone. The sparse regional seismic network of 2007 previously hampered a detailed examination of this unusually complex event. Here, we combine seismic data from different studies with regional moment tensor inversion results and 3D full‐waveform modeling. We show that the earthquake is a doublet consisting of dip‐slip and strike‐slip motion along two oblique structures, both activated under extensional stress along the strike of the slab. Comparison with tectonic reconstructions suggests that the earthquake ruptured along a re‐activated ridge‐transform segment of the subducted Proto‐Caribbean spreading ridge. The unprecedented resolution of the source process highlights the influence of pre‐existing structures on localizing slab deformation also at intermediate‐depth. Plain Language Summary: Some earthquakes in continents and near the ocean floor are known to break multiple, differently oriented, faults. Such compound earthquakes are rarely observed in subducted plates in the intermediate‐depth region between 70 and 300 km. Intermediate‐depth earthquake mechanics and stress state are possibly different from shallower earthquakes, and maybe they hinder complex events. On 29 November 2007, the Mw7.4 Martinique earthquake occurred at a depth of ∼150 km, near the deep end of the regional seismic zone below the Lesser Antilles Arc. The regional seismic network in 2007 was relatively sparse; it revealed the earthquake had a complex mechanism but did not previously allow for an in‐depth study. In this study, we combine different types of data and methods, including full waveform information, based on a recently derived 3D regional velocity model. Our analyses shows that the Martinique earthquake consisted of at least two distinct sub‐events on perpendicular faults in the subducted plate—a source doublet. By comparing the orientations of the doublet faults with plate tectonic reconstructions, we infer that this intermediate‐depth earthquake broke along a fossil plate‐boundary. This indicates that such structures remain structural weaknesses even after subduction. Key Points: Moment tensor solutions, aftershock activity, back‐projection, and source‐time function suggest a complex rupture of the 29 November 2007, Mw7.4 Martinique earthquakeRegional Moment Tensor modeling and aftershock cross‐correlation identified the event as a source doubletEarthquake likely re‐activated a fossil ridge‐transform structure associated with the subducted Proto‐Caribbean spreading ridge [ABSTRACT FROM AUTHOR]

Subducted seamounts are recognized as structures that influence seismicity in subduction zones. Understanding the detailed structures of seamounts, including before and during subduction, is essential for a comprehensive grasp of their influence. Of particular importance is their competency and deformation history during subduction. To better understand seamount subduction and related processes, we analyzed seismic reflection profiles around the Daiichi-Kashima Seamount, the Katori Seamount, and a knoll situated on the oceanward slope. These three features are located at different distances from the trench axis but all fall within the hinge-line of the subducting plate. The Daiichi-Kashima Seamount is currently subducting at the junction between the Japan and northern Izu-Ogasawara trenches, while the Katori Seamount and the knoll have not yet reached the trench axis. A thick limestone layer capping the Daiichi-Kashima Seamount indicates that the core of the Seamount is at least partially intact. On the other hand, our work reveals a large number of trench-parallel or sub-parallel faults within each of the edifices. The seismic profiles also reveal sediments draping the flanks of the edifices, interpreted as turbidite and/or debris flow deposits originating from slope failures. Consequently, our findings show that seamounts and other topographic features begin brittle deformation and failure due to flexural bending of the incoming oceanic plate long before actual subduction, with implications for the strength and competency of seamounts during subduction. [ABSTRACT FROM AUTHOR]

Remnant fabrics inherited from mid‐ridge play a key role in variations of plate deformation, hydration and seismicity along subduction zones. However, the quantitative relationships between the orientation of these remnant faults, plate bending and seismic activity at subduction zones still remain unclear. Here, we investigated the flexure of the Alaska subducting plate and assessed the relations between plate flexural curvatures, trench–fabric angle and seismicity. The results show that the flexural curvature and seismicity have nearly linear relationships with the trench–fabric angle. We find that remnant faults are more likely to be reactivated by plate bending at low trench–fabric angle region, causing greater plate deformation, hydration, weakening and seismicity. Plate weakening in turn promotes flexural bending curvature, resulting in higher degree of fracture and more earthquakes. However, at the high trench–fabric angle region, it is inclined to form new trench‐parallel bending faults that have smaller bending curvature and less seismicity. [ABSTRACT FROM AUTHOR]

The Ramagiri greenstone terrane (RGT) of Central Dharwar craton, India, is recognized as remnants of Archean oceanic crust squeezed between ancient proto‐continental terranes. The granites and gneisses formed as a result of partial melting, resisted subduction and collided, sinking the intervening ocean basin. The geochemical characteristics of the bimodal metavolcanics of RGT were carefully assessed using suitable proxy. An insight into them suggests that the mafic rocks bear characteristics of island arc tholeiites belonging to suprasubduction zone setting, as corroborated by their low TiO2/Yb (avg 443) and high Th/Nb (avg 0.8). Their high V/Ti (avg 0.3) ratio points towards a mantle source that has been influenced by subduction, whereas low Ti/Yb echoes high degrees of shallow melting with little residual garnet. The felsic rocks on the other hand are metaluminous–peraluminous, mostly calc‐alkaline and geochemically akin to I‐ and S‐type syncollisional and volcanic arc granite emplaced during intercontinent collision. Their immobile element plot with low Nb + Y (avg 25) concentration also points towards a volcanic arc environment. [ABSTRACT FROM AUTHOR]

The 26 January 1700 CE Cascadia subduction zone earthquake ruptured much of the plate boundary and generated a tsunami that deposited sand in coastal marshes from northern California to Vancouver Island. Although the depositional record of tsunami inundation is extensive in some of these marshes, few sites have been investigated in enough detail to map the inland extent of sand deposition and depict variability in tsunami deposit thickness and grain size. We collected 129 cores in marshes of the Salmon River estuary in Oregon and reanalyzed 114 core logs from a 1987–88 study that mapped the inland extent of circa 1700 CE sandy tsunami deposits. The ca. 1700 CE tsunami deposit in the Salmon River estuary is easily recognized in cores ≤1 m deep in which a buried marsh peat is overlain by a well sorted sand bed with a sharp lower contact that thins and fines inland. We use tsunami deposit data and models of sandy tsunami sediment transport (using Delft3D‐FLOW) to test 15 rupture models that could represent a ca. 1700 CE earthquake. At least 12–16 m of slip offshore of the Salmon River, which results in 0.8–1.0 m of coastal coseismic subsidence, is required to match the ca. 1700 CE sand deposit's inland extent, which is consistent with models of heterogeneous megathrust slip in ca. 1700 CE. Our methods of detailed tsunami deposit mapping, combined with sediment transport modeling, can be used to test models of megathrust ruptures and their tsunamis to potentially improve earthquake and tsunami hazard assessments. Plain Language Summary: We rely on the geologic record of great earthquakes and tsunamis, often preserved in the stratigraphy of coastal estuaries, to determine the size, frequency, and location of prehistoric events. At the Cascadia subduction zone, the 1700 CE earthquake caused a meter or more of sudden coastal subsidence and a tsunami that not only inundated coastal areas in Cascadia but was also recorded in Japan. Computer modeling implies that the earthquake occurred around 9 p.m. on 26 January 1700. Although sandy tsunami deposits from the 1700 earthquake are widespread throughout Cascadia, few sites have been cored extensively enough to confidently identify the inland extent of sandy tsunami deposits. We used over 200 sediment cores from the Salmon River estuary in Oregon to map the extent of sandy tsunami deposits from 1700. We then ran numerical models of sediment transport driven by a variety of earthquake generation sources to determine that earthquakes causing at least 0.8 m of subsidence are needed to generate a modeled tsunami capable of recreating the 1700 tsunami deposits observed in cores. This study demonstrates that tsunami deposit mapping and modeling methods can be used to improve our understanding of the impacts of past great earthquakes and tsunamis. Key Points: Models of sediment transport test earthquake and tsunami sources by comparing the modeled and observed distribution of sandy depositsSandy deposits can define the minimum inundation of the last major tsunami, around 1700 CE, generated at the Cascadia subduction zoneTo match tsunami deposit data, models require an earthquake that caused ≥0.8 m of coastal coseismic subsidence at the Salmon River [ABSTRACT FROM AUTHOR]

We have developed a novel deconvolution‐based reverse time migration method to image lithospheric structures using teleseismic data recorded by dense seismic arrays. Unlike traditional approaches that rely on the retrieval of Green's functions (or receiver functions), the new method directly utilizes the recorded three‐component (3‐C) seismic waveforms to reconstruct subsurface wavefields, which has the advantage of avoiding the estimation and removal of source time function. Importantly, it enables the asymptotic estimation of P‐to‐S transmission conversion coefficients at the elastic discontinuities and enhances resolving power for the fine‐scale heterogeneities. Taking these improvements, we have obtained a full three‐dimensional (3‐D) high‐resolution image of the subduction zone beneath southwest Japan. This image provides a comprehensive configuration of the severely deformed subducting plate beneath the Kii Channel and Kinki Peninsula. Plain Language Summary: Wave equations based seismic imaging method theoretically possesses the most effective capacity for characterizing crustal and upper mantle strong laterally heterogeneous structures. However, its advantages have not been well manifested in the imaging of lithospheric structures, primarily due to the uncertainties introduced by the laborious data preprocessing to remove the complex source‐side effects associated with each earthquake event. To address these uncertainties, we propose a novel method to automatically eliminate source side effects by introducing a deconvolution imaging condition. This enables the direct use of three‐component seismic records only requiring some fundamental preprocessing, such as retrending, remeaning, tapering, and band‐pass filtering, to image the Earth's interior structures. Accordingly, the first attempt of full three‐dimensional elastic imaging beneath southwest Japan is intriguing, and unveils more detailed structures of the subduction zones compared to previous studies. Key Points: Deconvolution imaging condition is proposed for elastic reverse‐time migration of teleseismic waveforms with minimal data preconditioningComplex influence of source time function and long‐distance propagation of the incident wavefield is effectively handledThree‐dimensional elastic reverse time migration image beneath southwest Japan reveals more detailed structures of the Philipine Sea plate [ABSTRACT FROM AUTHOR]

Slow earthquakes are slow fault slip events. Quantifying and monitoring slow earthquake activity characteristics are important, because they may change before large earthquakes occur. Statistical seismicity models are useful for quantifying seismicity characteristics. However, no standard statistical model exists for slow earthquake activity. This study used a high-quality catalog of low-frequency earthquakes (LFEs), a type of slow earthquake, in the Nankai subduction zone from April 2004 to August 2015 and conducted the first comparison of existing statistical LFE activity models to determine which model better describes LFE activity. Based on this comparison, this study proposes a new hybrid model that incorporates existing model features. The new model considers the LFE activity history in a manner similar to the epidemic-type aftershock sequence (ETAS) model and represents the LFE aftershock rate (subsequent LFE occurrence rate) with a small number of model parameters, as in the Omori–Utsu aftershock law for regular earthquakes. The results show that the proposed model outperforms other existing models. However, the new model cannot reproduce a feature of LFE activity: the sudden cessation of intense LFE bursts. This is because the new model superimposes multiple aftershock activities and predicts extremely high seismicity rates during and after the LFE bursts. I suggest that reproducing and successfully predicting the sudden cessation of intense LFE bursts is critical for the further improvement of statistical LFE activity models. In addition, the empirical equations formulated in this study for the LFE aftershock rates may be useful for future statistical and physical modeling of LFE activity. [ABSTRACT FROM AUTHOR]

Seismic surveys along subduction zones have identified anomalously high ratio of P‐ to S‐wave velocity (VP/VS) in the subducting oceanic crust that are possibly due to the presence of pore water. Such interpretations postulate that the pore structure is homogeneous at the scale of the seismic wavelength. Here we present the first statistical evidence of a heterogeneous pore structure in oceanic crust at scales larger than laboratory samples. The spatial correlation of measured bulk density profiles of the crustal section of the Samail ophiolite suggests that the pore structure is heterogeneous at scales smaller than ∼1 m. Wave‐induced fluid flow cannot follow the loading during the seismic wave propagation at this estimated heterogeneity, which implies that fluid‐filled microscopic pores and cracks have a limited impact on the observed high VP/VS anomalies in the subducting oceanic crust. Large‐scale cracks may therefore play an important role in shaping these anomalies. Plain Language Summary: Seismic studies along subduction zones have identified unusually high ratios of P‐ to S‐wave velocity (VP/VS) in the subducting oceanic crust, which indicates the presence of water‐filled cracks and pores. The close link between pore water and local seismic activity highlights the importance of quantitatively interpreting these seismic anomalies in terms of pore characteristics. Previous interpretations have assumed that the microscopic pore structure is quite homogeneous, even at macroscopic scales as large as the seismic wavelength. However, our analysis of a bulk density profile of the crustal section of the Samail ophiolite, Oman, which is a fossilized oceanic plate preserved on land, indicates that the pore structure is more heterogeneous than previously assumed. This means that the fluid flow within the unit volume that represents the macroscopic physical properties of the rock cannot follow the wave‐induced loading during seismic wave propagations. This results in a relatively small impact of water on the seismic velocity, as inferred from theoretical models that predict the effective elastic properties of rock containing fluid‐filled cracks. Therefore, microscopic cracks may not have a large impact on the high VP/VS values of subducting oceanic crust, whereas large‐scale cracks may play a more significant role. Key Points: The bulk density of the crustal section of the Samail ophiolite is more spatially heterogeneous than previously assumedThe effect of fluid‐saturated microcracks on low‐frequency seismic velocities is modeled as an unrelaxed condition for this heterogeneityThe high VP/VS anomaly in the subducting oceanic crust can be explained by both microcracks and large‐scale cracks [ABSTRACT FROM AUTHOR]

The petrogenesis of basaltic arc lavas with heavy Mo isotopic compositions is controversial. Given that oceanic crust preferentially loses isotopically heavy Mo during subduction at forearc depth and the remaining Mo is isotopically light and locked in rutile, fluids derived from such crust at subarc depth cannot account for the heavy Mo isotopic compositions of basaltic arc lavas. Serpentinites can contain substantial Mo and volatiles and are a potential source of isotopically heavy Mo in basaltic arc lavas. However, the Mo isotopic composition of serpentinites and the Mo isotope behavior during serpentinite dehydration are poorly constrained. Here, we present Mo contents and isotope data for serpentinites from the South Sandwich Trench-Fracture Zone intersection, and deserpentinized peridotites and high-pressure meta-serpentinite veins from the western-central Alps and Betic Cordillera, Spain. Most of these samples have heavy Mo isotopic compositions (δ98/95Mo NIST 3134 , ranging from −0.26 ‰ to 1.84 ‰) with low Ce/Mo (0.10–23.98) and Ce/Pb (0.17–3.59) ratios, which are distinct from the compositions of eclogite-facies metabasalts and metapelites. In addition, no systematic variations in Mo contents and Mo isotopic compositions occur for the studied samples, indicating limited Mo isotope fractionation during serpentinite dehydration. The above results suggest that serpentinites and their derived fluids at subarc depths can serve as the main isotopically heavy Mo sources for basaltic arc lavas. The Mo isotope data with Ce/Mo and Ce/Pb systematics suggest that serpentinite-derived fluids or melting of serpentinite diapirs at subarc depths can explain the heavy Mo isotopic compositions of basaltic arc lavas. Furthermore, recycled oceanic slab (oceanic crust + serpentinized mantle) would has limited influence on the Mo systematics of the mantle, which explains why the mantle has maintained a nearly constant Mo isotopic composition since 3.5 Ga. [ABSTRACT FROM AUTHOR]

Weak serpentine minerals affect the mechanical behavior of serpentinized peridotites at depth, and may play a significant role in deformation localization within subduction zones, at local or regional scale. Mixtures of olivine with 5, 10, 20 and 50 vol. % fraction of antigorite, proxies for serpentinized peridotites, were deformed in axial shortening geometry under high pressures (ca. 2–5 GPa) and moderate temperatures (ca. 350°C), with in situ stress and strain measurements using synchrotron X‐rays. We evaluate the average partitioning of stresses at the grains scale within each phase (mineral) of the aggregate and compare with pure olivine aggregates in the same conditions. The in situ stress balance is different between low antigorite contents up to 10 vol. %, and higher contents above 20 vol. %. Microstructure and stress levels suggest the deformation mechanisms under these experimental conditions are akin to (semi)brittle and frictional processes. Unlike when close to dehydration temperatures, hardening of the aggregate is observed at low serpentine fractions, due to an increase in local stress concentrations. Below and above the 10–20 vol. % threshold, the stress state in the aggregate corresponds to friction laws already measured for pure olivine aggregates and pure antigorite aggregates respectively. As expected, the behavior of the two‐phase aggregate does not evolve as calculated from simple iso‐stress or iso‐strain bounds, and calls for more advanced physical models of two‐phase mixtures. Plain Language Summary: In subduction zones, a tectonic plate plunges beneath another one, and result in large mechanical stresses. These conditions can lead to earthquakes or ground displacements observable at human timescales. Measuring how viscous the rocks within subduction zones are, may help understand these events. Serpentinite are rocks from subduction zones which contain a variable amount of a weak minerals, including serpentine, and other stronger minerals. Using deformation experiments this study seeks to measure serpentinites viscosity, as a function of the amount of serpentine. At low serpentine content (at least up to 10%, but lower than 20 vol.%) and under temperature relevant for cold subduction zones, we found that the rock remains as hard as the strongest mineral which bears the whole load, and can even become harder because of grain‐scale stress concentrations. At 50% volume fraction of serpentine, the rock has the same viscosity as serpentine itself. These measurements may for instance help larger scale numerical models of interseismic processes that happen between earthquakes, in subduction zones. Key Points: In situ stresses within antigorite + olivine aggregates deformed under high pressures and temperaturesStress partitionning changes with a threshold between 10% and 20 % antigorite volume fractionOlivine controls stress before this threshold and the aggregate hardens relative to pure olivine [ABSTRACT FROM AUTHOR]

The crystallographic preferred orientation (CPO) of olivine, specifically the type‐B characterized by c‐axes aligned parallel to lineation and b‐axes concentrated perpendicular to foliation, is essential for explaining the trench‐parallel seismic anisotropy in the forearc regions of subduction zones. However, its origin remains a subject of ambiguity and controversy. In this study, we present experimental findings on the formation of a type‐B olivine CPO through the dehydration of foliated serpentinite under a compressive stress at a pressure of 300 MPa and temperature of 700–750°C. Our results reveal a progressive evolution of olivine CPO, transitioning from a type‐C fabric to a type‐B fabric, with increasing grain size and dehydration level. The type‐B CPO observed in coarse‐grained olivine within fully dehydrated samples primarily arises from mechanisms involving anisotropic growth, grain rotation, and oriented coalescence of newly formed, small olivine grains following the decomposition of antigorite under a compressive stress. This study provides the first experimental evidence for a novel, low‐temperature dynamic dehydration mechanism, in contrast to the mechanism of high‐temperature plastic flow, for explaining the development of type‐B olivine CPO in forearc regions. Hence, it contributes significantly to our understanding of the formation of olivine CPO with implications for seismic anisotropy in subduction zone forearcs. Plain Language Summary: This study focuses on understanding why rocks in the forearc regions near subduction zones, where one tectonic plate slides beneath another, have a specific type of olivine arrangement known as type‐B olivine fabric. This arrangement is responsible for explaining why seismic velocities have a certain pattern in those areas. However, there has been disagreement about how this arrangement of olivine comes about. To investigate this, we conducted experiments where we subjected rocks rich in antigorite to high temperatures, causing them to transform into olivine. During these experiments, we observed changes in the size and shape of olivine grains, and their arrangement shifted from type‐C to type‐B as we adjusted the temperature and duration of the experiments. We found a relationship between the shape of olivine grains and how they were arranged. This change in arrangement can be explained by factors like the grains growing in different directions, rotating, and coming together in a specific way. Our discovery suggests that this particular olivine arrangement can occur during the transformation from antigorite to olivine at relatively low temperatures. This arrangement plays a more significant role for the trench‐parallel seismic anisotropy in the subduction forearc regions than we previously understood. Key Points: Dynamic dehydration of antigorite triggers the evolution of olivine crystallographic preferred orientation (CPO) from type‐C to type‐BTransitional type‐C olivine CPO is induced by topotactic and anisotropic growth of olivine during antigorite dehydrationType‐B olivine CPO is the final form of antigorite dehydration generated by anisotropic growth, grain rotation, and oriented coalescence [ABSTRACT FROM AUTHOR]

The top of the subducting plate is a thick, complex zone where the heterogeneous structure likely controls earthquake rupture processes. Imaging this heterogeneous channel typically involves active‐source methods with limited depth penetration, or low‐resolution teleseismic methods. To access short wavelengths at greater depth, we use high‐frequency P‐to‐S (PS, 1–15 Hz) mode‐converted arrivals from nearby earthquakes >50 km deep to image the plate interface at vertical scales <1 km. We use 37 broadband stations in southcentral Alaska between 2007 and 2008 at 10–15 km spacing, spanning the great 1964 earthquake rupture zone and adjacent deeper slow slip and tremor regions. The central 21 stations record high‐amplitude PS arrivals converting from the megathrust region, at depths corresponding to the top of a prominent low‐velocity zone (LVZ) in receiver function images. The PS/P amplitude ratio (APS_P) varies along strike and with depth of the conversion point but is independent of earthquake location and varies slowly between adjacent stations. APS_P changes with slab depth, indicating changes in lithology or fluid content of the plate interface, consistent with transitions in slip behavior from locked to slow slip. High APS_P cannot be explained by a velocity step or a single low‐velocity, high Vp/Vs layer, but requires several alternating high and low‐velocity layers. These observations indicate that the LVZ is a highly heterogeneous channel at multiple scales, resembling a subduction channel or sheared zone of metasediment and altered crust as observed in many exhumed subduction zones. Plain Language Summary: The largest earthquakes in the world occur in subduction zones. The contact between the overriding and subduction plates, the megathrust, hosts great earthquakes and less hazardous slow slip and tremor events. The location of these depends on the fault's physical properties, but there is a discrepancy between the structures different seismic imaging techniques observe. To address this issue, our study uses a previously underutilized P‐to‐S (PS) signal from local in‐slab earthquakes, which start as a P wave and converts to an S wave at the plate interface. The conversion occurs in the megathrust fault zone, and the arrival amplitude depends on the properties across it. The high amplitudes of PS compared to P across the array favor a model that represents the interface as a broad shear zone, where the velocity and number of layers create variability in the amplitudes. Mapping the amplitudes shows high variability in the fault properties at a finer scale than typical passive seismic imaging techniques. Key Points: Southcentral Alaskan stations record high amplitude arrivals converting from P to S at the megathrust for local earthquakes >50 km deepThe timing of P‐to‐S conversions varies between stations requiring a conversion from top of the subducting plateArrival amplitudes change coherently across the megathrust, indicating lithology or fluid changes at tens of km spatial scale [ABSTRACT FROM AUTHOR]

Petrological and major-trace element mineral chemistry studies have been carried out on pyroxenites from the Pakkanadu alkaline-ultramafic complex from the southern India to understand their origin and nature of magma responsible for ultramafic magmatism in the area. Pyroxenites display cumulus texture and consist of clinopyroxenes (cpx) and amphiboles (amp) as dominant phases with a subordinate amount of apatite, biotite, ilmenite, magnetite, pyrite, sphene, and calcite. Mineral chemistry classifies cpx as augite and diopside, whereas amp falls under tremolite-actinolite and hornblende- actinolite fields. Cpx are alkaline to sub-alkaline in composition and Mg# -- Al2O3 compositions suggest their crystallization under high-pressure conditions. A negative correlation between Mg# and TiO2 in cpx suggests early crystallization of magnetite and pyrite; high Mg# (76-92) suggests its link with the Alaskan- type intrusions, which may be crystallized through fractionation-accumulation Processes. Tectonic discrimination diagrams for cpx argue for the magmatic emplacements underan arc-tholeiitic environment in a subduction zone setting. Amp mineral chemistry (high SiO2 and low TiO2) indicate as the products of hydrothermal alteration of clinopyroxenes. A Low Al/Si ratio in the cpx suggests their derivation from silica-oversaturated magma, whereas low-Ti contents reflect slow cooling rate of the magma. Positive Rb, Ba and U anomalies in the multi-element patterns of the cpx probably signifying varying degrees of hydrothermal alteration in the studied samples. However, consistent Nb-Ta depletion can also be attributed to an enriched mantle source of the magma from which pyroxenites were crystallized. Moreover, single-cpx geothermobarometry yielded a crystallization temperature of 905 to 911°C under moderate to high pressure of 3-9 kbar. [ABSTRACT FROM AUTHOR]

Seismic anisotropy constitutes a useful tool for imaging the structure along the plate interface in subduction zones, but the seismic properties of mafic blueschists, a common rock type in subduction zones, remain poorly constrained. We applied the technique of electron backscatter diffraction (EBSD) based petrofabric analysis to calculate the seismic anisotropies of 14 naturally deformed mafic blueschists at dry, ambient conditions. The ductilely deformed blueschists were collected from terranes with inferred peak P‐T conditions applicable to subducting slabs at or near the plate interface in active subduction zones. Epidote blueschists display the greatest P wave anisotropy range (AVp ∼7%–20%), while lawsonite blueschist AVp ranges from ∼2% to 10%. S wave anisotropies generate shear wave splitting delay times up to ∼0.1 s over a thickness of 5 km. AVp magnitude increases with glaucophane abundance (from areal EBSD measurements), decreases with increasing epidote or lawsonite abundance, and is enhanced by glaucophane crystallographic preferred orientation (CPO) strength. Two‐phase rock recipe models provide further evidence of the primary role of glaucophane, epidote, and lawsonite in generating blueschist seismic anisotropy. The symmetry of P wave velocity patterns reflects the deformation‐induced CPO type in glaucophane—an effect previously observed for hornblende on amphibolite P wave anisotropy. The distinctive seismic properties that distinguish blueschist from other subduction zone rock types and the strong correlation between anisotropy magnitude/symmetry and glaucophane CPO suggest that seismic anisotropy may be a useful tool in mapping the extent and deformation of blueschists along the interface, and the blueschist‐eclogite transition in active subduction zones. Plain Language Summary: The directional dependence of seismic wave speeds in the subsurface, or seismic anisotropy, can allow us to map the Earth's structure in subduction zones. To improve the interpretation of seismic data collected in active subduction zones, we characterized the range of seismic anisotropy created by blueschists (a common subduction zone rock‐type) that were returned to the surface after being deformed in ancient subduction margins. We calculate the anisotropy of each blueschist rock from mineral orientations collected in the lab combined with the elastic properties of these minerals. Trends in seismic anisotropy were compared to the changes in composition and preferred orientations of minerals (produced by deformation). We found that blueschists can generate a broad range of seismic anisotropy, and that this anisotropy is enhanced by increasing amounts of the mineral glaucophane. The seismic anisotropy is further increased when the glaucophane minerals are more uniformly oriented, as is typical in rocks that have experienced higher levels of deformation. The seismic anisotropy and seismic wave speeds of blueschists are distinctive from those of other common subduction zone rocks. Therefore, these results suggest blueschist seismic anisotropy can be used to improve our ability to map structure and deformation occurring in active subduction zones. Key Points: Calculated blueschist Vp anisotropy up to ∼20% with a plateau at ∼10%, with lineation‐parallel fast axis and foliation‐normal slow axisThe anisotropy magnitude increases with glaucophane modal abundance/crystallographic preferred orientation (CPO) strength and is diluted by epidote/lawsonite abundanceThe glaucophane CPO type correlates with the Vp pattern and increasing AVp magnitude in mafic blueschists [ABSTRACT FROM AUTHOR]

This study provides comprehensive geochemical and petrological data for the newly discovered high-magnesium andesites (HMAs) from the Shillong Group of rocks in the Proterozoic Shillong Basin of the Assam-Meghalaya Gneissic Complex (AMGC). It is the first report of HMAs from the easternmost part of the Indian shield region. The AMGC holds immense geodynamic significance because this region has recorded magmatism related to Columbia, Rodinia, and Pangea supercontinental events. The present work discusses geochemical signatures of the Shillong HMAs characterized by calc-alkaline traits with high-K contents and significant concentrations of TiO₂ (0.54–0.76 wt %) higher than boninites (TiO2 < 0.5 wt %), and Mg# (53–56). They exhibit high LILE/HFSE and LREE/HREE ratios, which are distinctive features of magmas generated in subduction zones. Tectonic discrimination diagrams clearly suggest that the Shillong HMAs were generated in a back-arc basin regime. We argue that the Shillong HMAs were derived by 5% to 10% partial melting of a depleted lherzolite mantle source within the spinel – garnet transition zone. We also argue that the Shillong HMAs may have formed as a result of the interaction between subducted sediment-derived melts and mantle. The Shillong HMAs have undergone low- to medium-grade greenschist facies metamorphism characterized by significant mineral alteration, however, they still preserve bulk original alteration-resistant trace element compositions. From a regional perspective, the synchronous nature of origin and geotectonic setting, associated geochronological ages of formation, and regional structural trends suggest that the Shillong Basin is the easternmost continuation of the Mahakoshal Mobile Belt and specifically, the Bathani volcano-sedimentary sequence of the Central Indian Tectonic Zone (CITZ); and together they form the northern fragments of the same suture zone consisting of the CITZ, the Chotanagpur Granite Gneiss Complex, and the AMGC, which demarcates the northern and southern Indian blocks of the Greater Indian Landmass. [ABSTRACT FROM AUTHOR]

In this study, alternatively to the megathrust, we identify upper plate normal faults orthogonal to the trench as a possible tsunami source along the Lesser Antilles subduction zone. We study the Morne Piton Fault system, a trench-perpendicular upper crustal fault affecting the Lesser Antilles forearc at the latitude of Guadeloupe. By the means of seismic reflection, high resolution bathymetry, Remotely Operated Vehicle images and dating, we reassess the slip rate of the Morne Piton Fault at 0.2 mm.yr-1 since fault inception (i.e. 7 Ma), dividing by five previous estimations and thus increasing the earthquake time recurrence and lowering the associated hazard. We evidence a metric scarp with striae at the toe of the Morne Piton Fault system suggesting a recent fault rupture. We estimate a fault rupture area of ~ 450-675 km2 and then a magnitude range for the seismic event around Mw 6.5 ± 0.5. We present results from a multi-segment tsunami model representative for the worst-case scenario which gives an overview of what could happen in terms of tsunami generation if the whole identified Morne Piton Fault segments ruptured together. Our model illustrates the potential impact of local tsunamis on the surrounding coastal area as well as local bathymetric controls on tsunami propagation as (i) shallow water plateaus act as secondary sources and are responsible for a wrapping of the tsunami waves around the island of Marie-Galante, (ii) canyons are focusing and enhancing the wave height in front of the most touristic and populated town of the island, (iii) a resonance phenomenon is observed within Les Saintes archipelago showing that the waves' frequency content is able to perturbate the sea-level during many hours after the seismic rupture. [ABSTRACT FROM AUTHOR]

Detections of slow slip events (SSEs) are now common along most plate boundary fault systems at the global scale. However, no such event has been described in the south Peru - north Chile subduction zone so far, except for the early preparatory phase of the 2014 Iquique earthquake. We use geodetic template matching on GNSS-derived time series of surface motion in Northern Chile to extract SSEs hidden within the geodetic noise. We detect 33 events with durations ranging from 9 to 40 days and magnitudes from Mw 5.6 to 6.2. The moment released by these aseismic events seems to scale with the cube of their duration, suggesting a dynamic comparable to that of earthquakes. We compare the distribution of SSEs with the distribution of coupling along the megathrust derived using Bayesian inference on GNSS- and InSAR-derived interseismic velocities. From this comparison, we obtain that most SSEs occur in regions of intermediate coupling where the megathrust transitions from locked to creeping or where geometrical complexities of the interplate region have been proposed. We finally discuss the potential role of fluids as a triggering mechanism for SSEs in the area.

Abstract Earthquakes that rupture several faults occur frequently within the shallow lithosphere but are rarely observed for intermediate‐depth events (70–300 km). On 29 November 2007, the Mw7.4 Martinique earthquake struck the Lesser Antilles Island Arc near the deep end of the Wadati‐Benioff‐Zone. The sparse regional seismic network of 2007 previously hampered a detailed examination of this unusually complex event. Here, we combine seismic data from different studies with regional moment tensor inversion results and 3D full‐waveform modeling. We show that the earthquake is a doublet consisting of dip‐slip and strike‐slip motion along two oblique structures, both activated under extensional stress along the strike of the slab. Comparison with tectonic reconstructions suggests that the earthquake ruptured along a re‐activated ridge‐transform segment of the subducted Proto‐Caribbean spreading ridge. The unprecedented resolution of the source process highlights the influence of pre‐existing structures on localizing slab deformation also at intermediate‐depth.

Accurately modeling time-dependent coseismic crustal deformation as observed on high-rate Global Navigation Satellite System (HR-GNSS) lends insight into earthquake source processes and improves local earthquake and tsunami early warning algorithms. Currently, time-dependent crustal deformation modeling relies most frequently on simplified 1D radially symmetric Earth models. However, for shallow subduction zone earthquakes, even low-frequency shaking is likely affected by the many strongly heterogeneous structures such as the subducting slab, mantle wedge, and the overlying crustal structure. We demonstrate that including 3D structure improves the estimation of key features of coseismic HR-GNSS time series, such as the peak ground displacement (PGD), the time to PGD (tPGD), static displacements (SD), and waveform cross-correlation values. We computed synthetic 1D and 3D, 0.25 Hz and 0.5 Hz waveforms at HR-GNSS stations for four M7.3+ earthquakes in Japan using MudPy and SW4, respectively. From these synthetics, we computed intensity-measure residuals between the synthetic and observed GNSS waveforms. Comparing 1D and 3D residuals, we observed that the 3D simulations show better fits to the PGD and SD in the observed waveforms than the 1D simulations for both 0.25 Hz and 0.5 Hz simulations. We find that the reduction in PGD residuals in the 3D simulations is a combined effect of both shallow and deep 3D structures; hence incorporating only the upper 30 km of 3D structure will still improve the fit to the observed PGD values. Our results demonstrate that 3D simulations significantly improve models of GNSS waveform characteristics and will not only help understand the underlying processes, but also improve local tsunami warning.

Abstract We have developed a novel deconvolution‐based reverse time migration method to image lithospheric structures using teleseismic data recorded by dense seismic arrays. Unlike traditional approaches that rely on the retrieval of Green's functions (or receiver functions), the new method directly utilizes the recorded three‐component (3‐C) seismic waveforms to reconstruct subsurface wavefields, which has the advantage of avoiding the estimation and removal of source time function. Importantly, it enables the asymptotic estimation of P‐to‐S transmission conversion coefficients at the elastic discontinuities and enhances resolving power for the fine‐scale heterogeneities. Taking these improvements, we have obtained a full three‐dimensional (3‐D) high‐resolution image of the subduction zone beneath southwest Japan. This image provides a comprehensive configuration of the severely deformed subducting plate beneath the Kii Channel and Kinki Peninsula.

Abstract A detailed 3-D tomographic model of the whole mantle beneath the northern hemisphere (north of ~ 30°N latitude) is obtained by inverting a large amount of P-wave arrival time data (P, pP, and PP) to investigate transition of subducted slabs beneath Eurasia–Arctic–North America. We apply an updated global tomographic method that can investigate the whole mantle 3-D structure beneath a target area with high resolution comparable to that of regional tomography. The final tomographic model is obtained by performing independent calculations for 12 different target areas and stitching together the results. Our model clearly shows the subducted Izanagi and Farallon slabs penetrating into the lower mantle beneath Eurasia and North America, respectively. In the region from Canada to Greenland, a stagnant slab lying below the 660-km discontinuity is revealed. Because this slab has a texture that seems to be due to subducted oceanic ridges, the slab might be composed of the Farallon and Kula slabs that had subducted during ~60–50 Ma. During that period, a complex rift system represented by division between Canada and Greenland was developed. The oceanic ridge subduction and hot upwelling in the big mantle wedge above the stagnant slab caused a tensional stress field, which might have induced these complex tectonic events.

The Mesozoic subduction zone over the Dongsha Waters (DSWs) of the South China Sea (SCS) is a part of the westward subduction of the ancient Pacific plate. Based on the comprehensive interpretation of deep reflection seismic profile data and polar magnetic anomaly data, and the zircon dating results of igneous rocks drilled from well LF35-1-1, the Mesozoic subduction zone in the northeast SCS is accurately identified, and a Mesozoic subduction model is proposed. The accretion wedges, trenches, and igneous rock zones together form the Mesozoic subduction zone. The evolution of the Mesozoic subduction zone can be divided into two stages: continental subduction during the Late Jurassic and continental collision during the late Cretaceous. The Mesozoic subduction zone controlled the structural pattern and evolution of the Chaoshan depression (CSD) during the Mesozoic and Neogene eras. The gas source of the hydrate comes from thermogenic gas, which is accompanied by mud diapir activity and migrates along the fault. The gas accumulates to form gas hydrates at the bottom of the stable domain; BSR can be seen above the mud diapir structure; that is, hydrate deposits are formed under the influence of mud diapir structures, belonging to a typical leakage type genesis model.

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. [ABSTRACT FROM AUTHOR]

The architecture (geometry, fault network, and stacking pattern of accreted thrust sheets) of accretionary wedges influences subduction zone processes. However, it remains challenging to constrain the architectural evolution in natural accretionary wedges over geological timescales. In this study, we undertook sandbox analog modeling, with quantitative analysis of the wedge geometry and digital image correlation‐based kinematics, to delineate the wedge growth history with four décollement settings (single or double and continuous or discontinuous). The results show that the wedge is formed by repeated episodic frontal accretion with a constant cycle (i.e., the accretion cycle), and the degree of coupling between the base of the wedge and subducting plate interface appears to depend on the relative strengths of the wedge and detachment. An interbedded décollement layer in the incoming sediment facilitated wedge segmentation and rearrangement of the internal fault network, which weakened the wedge strength. A combination of a detachable high‐friction patch in the basal décollement and a continuous interbedded weak layer enabled underplating of underthrusted sediment beneath the inner wedge, which involved a low‐angle, long‐lived forethrust and multiple cycles of frontal accretion on short‐lived forethrusts at the deformation front. Our findings suggest that décollement configuration is a key factor in controlling the accretion cycle, strain distribution, fault network, and wedge strength on timescales of ∼105 yr in natural accretionary systems. This result should be considered when investigating modern subduction zones. Plain Language Summary: Accretionary wedges, which are one of the key components of subduction zones, comprise sediments scraped off from a downgoing plate. However, understanding the spatial and temporal changes of their growth and deformation patterns over geological timescales is challenging. In this study, we conducted laboratory experiments using different types of sands to quantify the deformation processes during wedge growth. We tested various layering conditions and, in particular, how single or double and continuous or discontinuous weak layers affect wedge growth. Our results show that wedge growth is achieved by repetition of a frontal accretion cycle, but the detailed nature of the cycle depends on the properties of the weak layers. In particular, an additional weak layer in the subducting sediment is critical in modifying the accretion cycle, strain distribution, and fault activity during wedge growth. Our findings suggest that weak layers are key in determining the stress–strain state in natural wedges and on the plate boundary as the wedge grows on a timescale of ∼105 yr. Key Points: An interbedded weak layer reduced the effective internal friction angle of the wedge, weakened the wedge, and formed a dense fault networkSubduction of a frictional barrier in the basal décollement resulted in out‐of‐sequence thrusting and facilitated sediment underplatingOur results contribute to an understanding of the growth processes, architectural evolution, and kinematics of natural accretionary wedges [ABSTRACT FROM AUTHOR]

We investigate spatiotemporal changes of intermediate‐depth earthquakes in the double seismic zone beneath Central and Northeastern Japan before and after the 2011 magnitude 9 Tohoku earthquake. We build a template‐matching catalog 1 year before and 1 year after the Tohoku earthquake using Hi‐net recordings. The new catalog has a six‐fold increase in earthquakes compared to the Japan Meteorological Agency catalog. Our results show no significant change in the intermediate‐depth earthquake rate prior to the Tohoku earthquake, but a clear increase in both planes following the Tohoku earthquake. The regions with increased intermediate‐depth earthquake activity and the post‐seismic slips following the Tohoku earthquake are spatially separate and complementary with each other. Aftershock productivity of intermediate‐depth earthquakes increased in both planes following the Tohoku earthquake. Overall, aftershock productivity of the upper plane is higher than the lower plane, likely indicating that stress environments and physical mechanisms of intermediate‐depth earthquakes in the two planes are distinct. Plain Language Summary: Intermediate‐depth earthquakes occur at 70–350 km depth below the Earth's surface. Because of the high pressure and temperature at such depths, the physical mechanism of intermediate‐depth earthquakes is still poorly understood. In this study, we try to obtain insights into this problem by investigating the behaviors of intermediate‐depth earthquakes before and after the 2011 magnitude 9 Tohoku earthquake. We build an intermediate‐depth earthquake catalog in Central and Northeastern Japan around the Tohoku earthquake. This new catalog contains six times more earthquakes and is more complete than the standard catalog used in previous studies. After analyzing the earthquakes from the new catalog, we find no significant precursory acceleration of intermediate‐depth earthquake activities prior to the Tohoku earthquake. However, we observe a clear increase in intermediate‐depth earthquake rate following the Tohoku earthquake. We also observe that the aftershock productivity of intermediate‐depth earthquakes increased following the Tohoku earthquake. The subduction slab beneath our studied area is characterized by a double seismic zone, and we find that aftershock productivity in the upper plane of seismicity is higher than that in the lower plane. This phenomenon may be due to differences in the environments or physical mechanisms in the two planes of intermediate‐depth earthquakes. Key Points: We build a new catalog of intermediate‐depth earthquakes in Japan 1 year before and 1 year after the 2011 M9 Tohoku earthquakeThere is no significant increase in intermediate‐depth earthquake activities prior to the Tohoku earthquake but a clear increase after itAftershock productivities of double seismic zones increase after the Tohoku earthquake, and it in upper plane is higher than in lower plane [ABSTRACT FROM AUTHOR]

An Mw 6.2 earthquake occurred in Suzu, northeastern Noto Peninsula, Japan, on 5 May 2023, followed by many aftershocks. Before this mainshock‐aftershock sequence, an intense earthquake swarm lasted in the vicinity for 2.5 years. Here, we estimated the rupture process of the Mw 6.2 mainshock and relocated >20,000 surrounding small earthquakes. The results show that systematic upward migration occurred via a complex network of faults in the preceding swarm period and that the mainshock rupture was initiated near the shallow end of the swarm earthquakes. The mainshock rupture propagated farther updip, followed by many aftershocks in the shallow extension. Upward fluid movement likely caused systematic upward earthquake migration from a depth of 18–5 km. The present results indicate the importance of monitoring swarm events since large (M > 6) and dangerous earthquakes can occur during such swarms. Plain Language Summary: Growing evidence suggests that fluid movements at depth can cause earthquake swarms in the overriding plates of subduction zones. However, there is less evidence of fluid involvement in the occurrence of large (M > 6) earthquakes, and it has been reported that the generation environments of large earthquakes and swarm earthquakes tend to be different. During intense swarm activity in the northeastern Noto Peninsula, Japan, starting at the end of 2020, an M6.2 earthquake occurred on 5 May 2023, followed by many aftershocks. To clarify the occurrence mechanism of the M6.2 mainshock, we relocated >20,000 small earthquakes and estimated the rupture process. The results showed a systematic upward migration of earthquakes in the preceding swarm period on the deep side of the mainshock fault (depth from 20 to 12 km). The mainshock rupture initiated near the shallow end of the swarm activity and propagated updip (depth from 12 to 9 km). Many aftershocks followed in shallower part of the same fault (depth from 10 to 5 km). Upward fluid migration likely caused the observed systematic upward earthquake migration throughout the sequence. The results indicate that earthquakes as large as M > 6 can be triggered by fluid migration during enhanced seismicity in swarms. Key Points: An Mw 6.2 event initiated near the shallow end of a fault where the preceding earthquake swarm showed systematic upward migrationThe Mw 6.2 rupture propagated farther updip, followed by many shallow aftershocks on the faultUpward fluid migration can trigger earthquakes as large as Mw > 6 [ABSTRACT FROM AUTHOR]

The Manawatu region experiences deep tremor and long‐term slow slip events (SSEs); however, tremor is adjacent to, and not co‐located with, long‐term SSEs. Observations of episodic tremor and slip (ETS) elsewhere suggest it is possible smaller short‐term SSEs below the current detection threshold occur where tremor is observed. Therefore, we sought to determine if small SSEs occurred with Manawatu tremor. We decomposed GNSS data using times of tremor to assess average surface displacements and performed a static slip inversion to model the displacement during tremor. The slip inversion suggested small slow slip partially coincided with tremor and long‐term SSEs may influence these small SSEs by increasing slip rates. We suggest that the interface below deep long‐term SSEs may slip often, in small ETS‐like SSEs that are not individually detectable geodetically. The question remains as to the nature of the strong variability in SSE behavior with depth and duration in the southern Hikurangi margin. Plain Language Summary: In between the Earth's tectonic plates, energy builds over time and can be released along faults suddenly (seconds‐minutes; i.e., earthquakes) or slowly (weeks‐years; i.e., slow slip events). Slow slip often happens with low‐frequency earthquakes (i.e., tectonic tremor). The North Island, New Zealand features two colliding tectonic plates with the potential to generate large earthquakes. The interface between these plates has both deep tectonic tremor and large, long‐lasting slow slip, but the tectonic tremor is deeper on the fault than the large slow slip. Studies have suggested small, short‐lasting slow slip, usually not able to be detected, occur where tectonic tremor is found. In this study we tried a different approach to find the small slow slip. While small slow slip are not detected by themselves, we were able to detect their cumulative effect in the tectonic tremor area. We modeled small slow slip during tectonic tremor to find the mean sliding rate on the fault that is between the tectonic plates. The large long‐lasting slow slip may drive this smaller slow slip by making them slip faster. The question remains as to the cause of the many types of slow slip in New Zealand. Key Points: GNSS data indicates newly detected short‐term slow slip in region of deep tremorLong‐term slow slip events may influence these small slow slip events by increasing slip ratesThree types of slow slip overlap along‐strike in Hikurangi margin: shallow, short‐term; deep, long‐term; deeper, short‐term with tremor [ABSTRACT FROM AUTHOR]

A detailed 3-D tomographic model of the whole mantle beneath the northern hemisphere (north of ~ 30°N latitude) is obtained by inverting a large amount of P-wave arrival time data (P, pP, and PP) to investigate transition of subducted slabs beneath Eurasia–Arctic–North America. We apply an updated global tomographic method that can investigate the whole mantle 3-D structure beneath a target area with high resolution comparable to that of regional tomography. The final tomographic model is obtained by performing independent calculations for 12 different target areas and stitching together the results. Our model clearly shows the subducted Izanagi and Farallon slabs penetrating into the lower mantle beneath Eurasia and North America, respectively. In the region from Canada to Greenland, a stagnant slab lying below the 660-km discontinuity is revealed. Because this slab has a texture that seems to be due to subducted oceanic ridges, the slab might be composed of the Farallon and Kula slabs that had subducted during ~60–50 Ma. During that period, a complex rift system represented by division between Canada and Greenland was developed. The oceanic ridge subduction and hot upwelling in the big mantle wedge above the stagnant slab caused a tensional stress field, which might have induced these complex tectonic events. [ABSTRACT FROM AUTHOR]

This article discusses the basic elements of the crustal structure of the oceans and the transition zones to continents as suggested by the concept of plate tectonics (PT). Most of them are described briefly, without drawing on a detailed argumentation, since the author has already happened to refer to them and give them critical assessments in previous publications. The available information on the fate of the oceanic crust, as perceived by the PT to be submerging in a subduction zone, is discussed in more detail. This is due to the large role this phenomenon supposedly plays in the origin of the Earth's main (in terms of the volume of material erupted) volcanic process of the Pacific Ring of Fire and Tethys and also in transporting water to the upper mantle and the transition zone to the lower mantle. The first event rules out the second, but plate tectonics considers both to be real. According to the author, the crust-mantle exchange is provided by other processes leading to different results, although they may be close in time and place. This side of the issue is not analyzed in the article. The focus is on demonstrating the impossibility of the oceanic crust submersion into the mantle and into the continental crust, although the emergence of the latter scenario has no factual basis and in principle may not be considered. However, a comparison was made of the average density and velocity of seismic waves in closely spaced fragments of both types of crust in the transition zones of the Pacific. The continental one turned out to be noticeably denser than the oceanic one, as, naturally, the sub-crustal mantle in the same regions. [ABSTRACT FROM AUTHOR]

Quantitative reconstruction of thermal history recorded in mantle xenoliths is imperative for understanding the temporal change of thermal state and dynamics of the lithosphere‐asthenosphere boundary (LAB). We challenged this problem in the arc settings by examining nine spinel peridotite xenoliths from Ichinomegata maar in the back‐arc side of Northeast Japan Arc. Extensive mineral chemical analyses combined with the derivation depths of the xenoliths revealed a depth‐dependent variation of chemical zoning patterns in olivine and pyroxenes. The depth variation of thermal histories of the Ichinomegata xenoliths was decoded by applying diffusion‐controlled reaction modeling to reproduce the zoning patterns. The decoded thermal events in the order of occurrence are (a) ∼14 million years of cooling causing lithosphere thickening up to ∼55 km depth, (b) subsequent ∼12 thousand years of heating from the underlying asthenosphere resulting in lithosphere thinning up to the depths of ∼40 km, and (c) 1–68 days of heating during xenolith transportation by the host magma. The duration of the lithosphere thickening is consistently explained by the period of the Japan Sea opening. On the other hand, the timescale of the lithosphere thinning is too short to be explained by heat conduction through the ∼15 km thick LAB and requires a more effective heat transportation mechanism such as direct magma injection into the LAB or significant viscosity reduction of the mantle peridotite aided by the pervasive permeable flow of silicate melt. Plain Language Summary: Evolution of the lithosphere (tectonic "Plate" in the framework of plate tectonics) accompanies dynamic change of the depth of the lithosphere‐asthenosphere boundary (bottom of the "Plate"), which causes the temporal change of thermal structure in the mantle. We challenged quantitative estimation for the timescale of such temporal change in the arc settings by examining the chemical heterogeneities recorded in olivine and pyroxene of the mantle xenoliths (fragments of mantle materials brought up by magma) from Ichinomegata maar in the back‐arc side of the Northeast Japan Arc. The decoded thermal events in the order of occurrence are (a) ∼14 million years of cooling causing thickening of the lithosphere up to ∼55 km depth, (b) subsequent ∼12 thousand years of heating from the underlying asthenosphere resulting in thinning of the lithosphere up to the depths of ∼40 km, and (c) 1–68 days of heating during xenolith transportation by the host magma. The lithosphere thickening is consistently explained by the long cooling event since the back‐arc spreading event in the Japan Sea. On the other hand, the timescale of the lithosphere thinning requires a more effective heat transportation mechanism such as direct magma injection or significant viscosity reduction of the mantle. Key Points: A depth‐dependent thermal history in the arc settings was decoded from chemical zonings in olivine and pyroxene of Ichinomegata xenolithsThe lithosphere beneath Ichinomegata experienced ∼14 million years of thickening and subsequent ∼12 thousand years of thinningThe duration of xenolith transportation by the host magma took 1–68 days and might suggest increasing ascent velocity at shallower depths [ABSTRACT FROM AUTHOR]

Serpentinites play an important role in the delivery of water into subduction zones. In addition, serpentinites also contain ferric Fe and can transport significant redox potential. We present high-pressure and high-temperature experiments and Mössbauer spectroscopy measurements on natural lizardite and antigorite samples equilibrated at various oxygen fugacities in order to quantify the relationship between the oxygen fugacity f (O2) and the Fe3+/Fetot ratio in these two phases. In antigorite, Fe3+ partitions into the octahedral site and is charge balanced by tetrahedral Al. In lizardite, tetrahedral Fe3+ is observed only at low temperature as well as under high f (O2), whereas Fe3+ prefers the octahedral site at temperatures exceeding 500 °C and at 3 to 5 GPa. Although metastable, lizardite remains in redox equilibrium in our experiments at conditions above the lizardite to antigorite phase transformation at 300 °C and demonstrates a similar stability to antigorite. The Al concentration of lizardite is found to be temperature dependent, and it was possible to reequilibrate the Fe3+/Fetot ratio of lizardite from 0.1 to 0.9 by using redox buffers such as Fe metal, graphite, graphite–calcite, Re–ReO2 and Ru–RuO2. Our experiments on antigorite demonstrate that antigorite does not adjust its Al concentration on experimental time scales. Since Fe3+ is charge balanced by Al, it was also not possible to manipulate the Fe3+/Fetot ratio of antigorite. The coexisting phases, however, show chemical equilibration with this antigorite composition. We have retrieved the standard Gibbs energy for Fe3+- and Al-endmembers of antigorite and lizardite and calculated the metamorphic evolution of subducting serpentinites. The lizardite to antigorite transformation does not cause a decrease in the bulk Fe3+/Fetot ratio under f (O2) buffered conditions, in contrast to observations from some natural settings, but does result in the formation of additional magnetite due to antigorite having a lower Fe3+/Fetot ratio than lizardite at equilibrium. If the f (O2) of antigorite serpentinite is buffered during subduction, such as due to the presence of graphite and carbonate, the bulk Fe3+/Fetot ratio decreases progressively. On the other hand, in a closed system where the bulk serpentinite Fe3+/Fetot ratio remains constant, the f (O2) increases during subduction. In this scenario, the f (O2) of an antigorite serpentinite with a typical Fe3+/Fetot ratio of 0.4 increases from the fayalite–magnetite–quartz to the hematite–magnetite f (O2) buffer during dehydration. These f (O2) results confirm earlier inferences that fluids produced by antigorite dehydration may not contain sufficient oxidised sulphur species to oxidise the mantle wedge. Sufficiently high levels of f (O2) to mobilise oxidised sulphur species may be reached upon antigorite dehydration, however, if closed system behaviour maintains a high bulk redox potential across the lizardite to antigorite phase transformation. Alternatively, oxidation of the mantle wedge might be achieved by oxidising agents from sources in subducted oceanic crust and sediments. [ABSTRACT FROM AUTHOR]

Abstract Seismic surveys along subduction zones have identified anomalously high ratio of P‐ to S‐wave velocity (VP/VS) in the subducting oceanic crust that are possibly due to the presence of pore water. Such interpretations postulate that the pore structure is homogeneous at the scale of the seismic wavelength. Here we present the first statistical evidence of a heterogeneous pore structure in oceanic crust at scales larger than laboratory samples. The spatial correlation of measured bulk density profiles of the crustal section of the Samail ophiolite suggests that the pore structure is heterogeneous at scales smaller than ∼1 m. Wave‐induced fluid flow cannot follow the loading during the seismic wave propagation at this estimated heterogeneity, which implies that fluid‐filled microscopic pores and cracks have a limited impact on the observed high VP/VS anomalies in the subducting oceanic crust. Large‐scale cracks may therefore play an important role in shaping these anomalies.

A forest bed of tree stumps currently in the intertidal zone at Girdwood, south-central Alaska, records coseismic submergence during the penultimate great earthquake. Tree-ring samples from ten spruce stumps were crossdated to develop a 149-year-long ring-width chronology. Radiocarbon wiggle-matching found that single-ring ages from the chronology were offset 28 ± 7 years older than the IntCal20 calibration curve and that the last ring of the chronology dated as 1169 to 1189 CE (781–761 cal. yr. BP) at the 95% confidence level. Bark was observed on some stumps, six samples had the same year for the last growth ring, and so this wiggle-match date is also the best estimate of the date of the penultimate great earthquake. This date is in good agreement with a date for this event in a seismo-turbidite record from Skilak Lake but not with previous dates from Bayesian models of maximum- and minimum-limiting ages from coastal salt marshes. Reanalysis of the coastal salt marsh ages with the data grouped by area, context and material found that outer wood samples from stumps at coseismic submergence sites and a Bayesian limiting age model based on just herbaceous plant ages from Turnagain Arm and the Copper River area are both consistent with our wiggle-match date. Furthermore, coseismic emergence ages from Cape Suckling and Yakataga are older than the penultimate earthquake and so likely relate to an earlier uplift event in this eastern area. The rupture extent during the penultimate great earthquake appears to have been less than in the 1964 great earthquake and the interseismic interval between these two events was 785 ± 10 years.

Petrological and major-trace element mineral chemistry studies have been carried out on pyroxenites from the Pakkanadu alkaline-ultramafic complex from the southern India to understand their origin and nature of magma responsible for ultramafic magmatism in the area. Pyroxenites display cumulus texture and consist of clinopyroxenes (cpx) and amphiboles (amp) as dominant phases with a subordinate amount of apatite, biotite, ilmenite, magnetite, pyrite, sphene, and calcite. Mineral chemistry classifies cpx as augite and diopside, whereas amp falls under tremolite-actinolite and hornblende-actinolite fields. Cpx are alkaline to sub-alkaline in composition and Mg# - Al2O3 compositions suggest their crystallization under high-pressure conditions. A negative correlation between Mg# and TiO2 in cpx suggests early crystallization of magnetite and pyrite; high Mg# (76–92) suggests its link with the Alaskan-type intrusions, which may be crystallized through fractionation-accumulation Processes. Tectonic discrimination diagrams for cpx argue for the magmatic emplacements under an arc-tholeiitic environment in a subduction zone setting. Amp mineral chemistry (high SiO2 and low TiO2) indicate as the products of hydrothermal alteration of clinopyroxenes. A Low Al/Si ratio in the cpx suggests their derivation from silica-oversaturated magma, whereas low-Ti contents reflect slow cooling rate of the magma. Positive Rb, Ba and U anomalies in the multi-element patterns of the cpx probably signifying varying degrees of hydrothermal alteration in the studied samples. However, consistent Nb-Ta depletion can also be attributed to an enriched mantle source of the magma from which pyroxenites were crystallized. Moreover, single-cpx geothermobarometry yielded a crystallization temperature of 905 to 911°C under moderate to high pressure of 3–9 kbar.

Balazard prospecting area is located in eastern Iran, about 120 km south-west of Nehbandan in the Central part of Lut Block. This area consists of exposed Eocene volcanic rocks, intruded by several diorite and monzodiorite dykes and stocks. These intrusive rocks display porphyritic textures with mm-sized phenocrysts, most commonly of plagioclase, clinopyroxene and hornblende, embedded in a fine-grained groundmass with variable amounts of plagioclase, quartz, and opaque minerals. The assessment of geochemical properties of the major elements showed that these granitoids are metaluminous and of high-K calc-alkaline variety. The patterns of trace elements are identical, denoting enrichments in the light REE (LREE) to heavy REE (HREE). Moreover, LILE enrichment relative to HFSE and Nb, Ti, and P show negative anomalies. The Eu/Eu* ratios range from 0.96 to 0.76, which means that the plagioclase is a slag remnant in the origin of magma. The (87Sr/86Sr)i values of the assessed intrusive rocks range from 0.706 to 0.707, assuming an Oligocene age, while the εNdi values are between -1.9 to -3.2. These results manifest that the magmas were contaminated by continental crust. The contamination has probably occurred over the ascent of magma to crustal levels, and geochemical data verifies the preposition that the investigated intrusions were intruded in a volcanic belt resting on a subduction zone. Early magmas were created through the melting of mantle wedge peridotite, occasioning magma differentiation through crystal fractionation and crust contamination as they ascended to crustal levels. Introduction Balazard prospecting area is located 120 km of the south-west of Nehbandan in South Khorasan Province, Iran. The area is part of the volcanic-plutonic belt known as Lut Block, which owns an elongated shape stretched north-southwardly. Nehbandan Fault forms the eastern boundary of Lut Block. This is while it has been bordered by the Great Kavir Fault in the north, and by Nayband Fault in the west. The southern part of the block is likely defined by South Jazmourian Fault. Karimpour et al. (2012) argue that ~65% of the rocks cropped out in Lut Block are volcanic and plutonic. The magmatic activity of Lut Block initiated over the Middle Jurassic, particularly between 165-162 million years ago, which was associated with the intrusion of Shah-Kuh batholith (Esmaeily et al., 2005). This activity reached its highest level over the Tertiary period, particularly over the Middle Eocene (Arjmandzadeh & Santos, 2014). Over half of Lut Block is overlain by volcanic and subvolcanic rocks of Tertiary. These rocks show a thickness of up to 2000 m. They have been developed as a result of subduction before the collision between the Arabian and Asian plates (Berberian & King, 1981). There is a perceptible potential for a variety of mineralization in Eastern Iran and particularly in Lut Block, which is due to its past tectonism as a subduction zone that led to extensive magmatic activity. The Middle Eocene to the Early Oligocene (30-39 million years ago) is perceptible, particularly, in respect of magmatism and mineralization (Karimpour et al., 2012). Karimpour et al. (2012) believe that a great deal of the magmatism and mineralization events in eastern Iran have occurred over the Tertiary. However, mineralization associated with Cretaceous magmatism has also been identified, such as Sn-Cu mineralization observed in Cretaceous monzonitic rocks in Kalateh Ahani (Karimpour et al., 2012). Here we present new geochemical data (elemental and isotopic) from shallow intrusive rocks, with the aim of providing a more detailed understanding of the petrogenetic processes and geodynamic evolution of Lut Block. Material and methods A total of six samples from unaltered intrusive rocks at Balazard prospecting area and representing the main lithologies were grabbed to chemically analyze major and trace elements through petrographic assessment. Major and trace elements were determined using fused disks and a Philips PW 1410 spectrometer through wavelength-dispersive X-ray fluorescence (XRF) spectrometry. The chemical analysis was undertaken at Amethyst Laboratory in Mashhad, Iran. Inductively Coupled Plasma-mass Spectrometry (ICP-MS) was applied to analyze four of the samples for trace elements at Acme Laboratories in Vancouver (Canada). The samples were tested by a lithium metaborate/tetraborate fusion and total digestion with nitric acid prior to analysis. Four whole-rock samples of Balazard granitoid rocks were analyzed for their Sr and Nd isotopic compositions at the University of Aveiro's Laboratory of Isotope Geology in Portugal. The ground samples were treated with a HF/HNO3 solution in Teflon Parr acid digestion bombs, which were heated at 200°C for three days. The ultimate solution was evaporated and the samples were then dissolved in HCl (6.2 N) in acid digestion bombs and dried again. The elements were purified before being analyzed by means of a two-stage conventional ion chromatography technique: a) Sr and REE elements were separated in an ion exchange column by AG8 50W Bio-Rad cation exchange resin; b) Nd was purified from other lanthanides by means of columns with Ln Resin (ElChrom Technologies) cation exchange resin. All reagents that were applied to prepare the samples were sub-boiling distilled, and the water was produced using a Milli-Q Element (Millipore) apparatus. Sr was loaded onto a single Ta filament with H3PO4, while Nd was loaded onto a Ta outer side filament with HCl in a triple filament arrangement. The 87Sr/86Sr and 143Nd/144Nd isotopic ratios were determined using a Multi-Collector Thermal Ionization Mass Spectrometer (TIMS) VG Sector 54, with data acquired in dynamic mode and peak measurements at 1-2 V for 88Sr and 0.5-1.0 V for 144Nd. The Sr and Nd isotopic ratios were corrected for mass fractionation relative to 88Sr/86Sr = 0.1194 and 146Nd/144Nd = 0.7219. While the investigation was in progress, the SRM-987 standard yielded an average 87Sr/86Sr value of 0.710266 ± 14 (conf. lim 95%, N = 13) and a 143Nd/144Nd value of 0.5121019 ± 75 (conf. lim 95%, N = 12) in comparison to the JNdi-1 standard. Result and discussion Balazard intrusive rocks consist of diorite, quartz diorite, and quartz monzodiorite, and exhibit characteristics typical of high-K calc-alkaline rocks from a volcanic arc setting. The primitive mantle-normalized trace element spider diagrams display significant enrichment in LILE, including Rb, Sr, Ba, Zr, Cs, and Th, and depletion in some HFSE, such as P, Nb, and Y. Chondrite-normalized plots exhibit LREE enrichment and a significant La/Yb fractionation. Balazard granitoid rocks have (87Sr/86Sr)i values that vary between 0.7064 and 0.7066. In terms of isotopic compositions, Balazard granitoid rocks have εNdi between -1.9 and -3.2. The geochemical data are commensurate with the settlement of the investigated intrusions in a magmatic belt above a subduction zone suggesting contamination through being exposed to the continental crust during magma ascent to crustal levels. Balazard granitoid rocks display a range of (87Sr/86Sr)i values between 0.7064 and 0.7066. Additionally, the isotopic compositions of the rocks show εNdi values ranging from -1.9 to -3.2. These geochemical characteristics suggest that the investigated intrusions were emplaced in a magmatic belt above a subduction zone and were subsequently contaminated during magma ascent to continental crust.

Shotorsang iron skarn is located in 60 km northwest of Neyshabour (Khorasan Razavi, Iran) in Quchan-Sabzevar magmatic belt. Subvolcanic intrusion rocks have intruded into Cretaceous limestones and created skarnization. These rocks are divided into syenite porphyry and granodiorite porphyry based on their geochemical characteristics. They are I type oxidizing, metaluminous, and tectonic. Setting of the subvolcanic rocks are the subduction zone of the continental margin (VAG). Comparing the mineralization potential of the subvolcanic rocks of this area based on the use of the graph of SiO2 against K2O, MgO, Na2O+K2O, and Ni-V shows that they are fertile in terms of the formation of Fe and Cu skarn. Syenite porphyry is the origin of this mineralization, and magnesium in skarn is taken from hydrothermal fluid. The diagram of Eu/Eu*, Ce/Ce*, (Pr/Yb)n ratios also confirms the presence of meteoric water in the formation of the skarn zone. The primary fluid, which has a positive anomaly of Ce/Ce* and Eu/Eu* had acidic and oxidant conditions and high temperature, and formed pyroxene skarn. A part of magnetite mineralization is formed in this zone, and in this condition, the highest amount of REE entered the pyroxene skarn zone and diluted the fluid in terms of REE. This issue has led to a sharp decrease in the amount of REE in the mineralization zone. Negative Ce/Ce* and Eu/Eu* anomalies indicate alkaline conditions with less concentrated REE content, consistent with chlorite skarn. The highest amount of Fe mineralization is formed in this zone. Introduction Shotorsang iron skarn is located in 60 km northwest of Neyshabour (Khorasan Razavi, Iran) in Quchan-Sabzevar magmatic belt. Subvolcanic intrusion rocks have intruded into Cretaceous limestones and created skarnization (Amini and Khannazer, 2000(. This study found that at least four subvolcanic intrusion rocks are present in this region: Granodiorite porphyry, biotite syenite porphyry, quartz syenite porphyry, and quartz diorite to monzodiorite porphyry. These are divided into syenite porphyry and granodiorite porphyry based on the geochemical characteristics. Material and methods About 90 samples were collected for laboratory investigations of petrogenesis studies. Moreover, 25 samples were selected to be analyzed using the XRF and ICP-MS methods. Laboratory studies were carried out in Ferdowsi University of Mashhad and samples were analyzed in Acme and Zarazma laboratories. Results Subvolcanic rocks are I-type oxidizing, metaluminous, and tectonic. Setting of the subvolcanic rocks are the subduction zone of the continental margin (VAG). Moreover, some samples have been placed at the border of syn-collision due to high Rb, which is the result of high potassium. Enrichment of LILE elements such as K, Cs, Ba, Rb and incompatible elements that behave similar to them like Th compared to HFSE elements is observed in all samples compared to the primitive mantle. Enrichment of LILE relative to HFSE indicates magma related to subduction zones. The Sr element shows opposite behavior compared to the LILE elements. This issue can be justified by the high amount of CaO in magnetite ore (from 0.3% to more than 3.5%), because the two elements are similar in terms of chemical properties. Comparing the mineralization potential of the subvolcanic rocks of this area using the graph of SiO2 against K2O, MgO, Na2O+K2O, and Ni-V shows that they are fertile in terms of the formation of Fe and Cu skarn Meinert, 1995(. For igneous rocks, it was confirmed that the amount of Rb increases during the fractionation and crystallization processes (Meinert, 1995 (. Granitoids with potential for iron skarn have lower Rb (39 ppm). This amount is 103 ppm for copper skarn and 69 ppm for gold skarn. The amount of Rb content for all granitoids of Shotorsang area is 80 ppm. Considering the connection of these granitoids with iron skarn in this area, the high Rb content can be justified by crustal contamination of these rocks (Martin-Izard et al., 2000). Moreover, it can be mixing of a mafic magma with a felsic magma at a shallow depth. The amounts of V and Ni in iron skarn deposits are the highest; that is, 152 and 35 ppm, respectively. Ni and V for Shotorsang syenite porphyry group are 16 and 82, respectively and for Shotorsang granodiorite porphyry are 20 and 48, respectively. These values are lower than the global average of iron skarn. In general, the high amount of Rb and the low amount of Ni and V confirm the hypothesis that the magma has been fractionated and contaminated with crust. If the amount of Rb as well as the amount of Ni and V increase, the mixing of a magma derived from the mantle or a mafic magma with a highly fractionated magma would seem to be a more acceptable hypothesis (Meinert, 1995). Therefore, according to what was stated about the tectonic setting and the cases mentioned above, as well as the process of changes in the spider diagram of the rare earth elements of the studied area, it can be expected that the fluid forming the iron skarn of this region has undergone magmatic fractionation and been contaminated with crust. (La/Yb)n, (La/Sm)n and (Gd/Yb)n ratios were used to evaluate the separation degree between REEs. (La/Yb)n determines the degree of separation between LREE and HREE (Aubert et al., 2001; Yusoff et al., 2013), while the other two ratios are used to determine the degree of separation between LREE and MREE, and between MREE and HREE, respectively (Yusoff et al., 2013). These ratios vary for (La/Yb)n from 1.12 to 1.69, for (La/Sm)n from 6.7 to 40.72, and for (Gd/Yb)n from 0.89 to 4.22. As it is known, during the skarnization process, the highest degree of separation has occurred between LREE and HREE (up to about 70 times) and the lowest degree of separation has also occurred between MREE and HREE. The highest value of these ratios is in the mineralization zone, which indicates that the highest amount of separation of REE elements has taken place in this zone. Syenite porphyry is the origin of this mineralization, and magnesium in skarn is taken from hydrothermal fluid. Discussion The diagram of Eu/Eu*, Ce/Ce*, (Pr/Yb)n ratios confirms the presence of meteoric water in the formation of the skarn zone (Kato, 1999). The primary fluid, which has a positive anomaly of Ce/Ce* and Eu/Eu*, had acidic and oxidant conditions and high temperature and formed pyroxene skarn. A part of magnetite mineralization is formed in this zone, and in this condition, the highest amount of REE entered in pyroxene skarn zone and diluted the fluid in terms of REE. This issue has led to a sharp decrease in the amount of REE in the mineralization zone. Negative Ce/Ce* and Eu/Eu* anomalies indicate alkaline conditions (Meinert, 1995) with less concentrated REE content, consistent with chlorite skarn. The highest amount of Fe mineralization is formed in this zone. Acknowledgements The authors are grateful for the cooperation of the employees of the Shotorsang iron ore mine, especially Mr. Qotbi.

Abstract Large variations in the maximum earthquake magnitude ( $$M_{{\text{max}}}$$ M max ) have been observed among the world’s subduction zones. There is still no universal relationship between $$M_{{\text{max}}}$$ M max and a given subduction-zone parameter, such as plate age, plate dip angle, or plate velocity, which suggests that multiple parameters control $$M_{{\text{max}}}$$ M max . Here, we conduct exhaustive variable selections that are based on three evaluation criteria; leave-one-out cross-validation errors (LOOCVE), Akaike information criterion (AIC), and Bayesian information criterion (BIC) to determine the combination of subduction-zone parameters that best explains $$M_{{\text{max}}}$$ M max . Multiple linear regression analyses are applied using 18 subduction-zone parameters as potential candidates for the explanatory variables of $$M_{{\text{max}}}$$ M max . The minimum BIC is obtained when five variables (trench sediment thickness, existence of an accretionary prism, upper-plate crustal thickness, bending radius of the subducting oceanic plate, and trench depth) are selected as explanatory variables; each variable contributes positively to $$M_{{\text{max}}}$$ M max . Minimum LOOCVE and AIC values are obtained when eight variables (the five parameters for BIC, plus the along-strike plate convergence rate, age of the subducting plate, and maximum depth of the subducting plate) are selected. Our selection of the trench sediment thickness and plate bending radius contributing to $$M_{{\text{max}}}$$ M max is consistent with previous studies. The results show that increasing upper-plate crustal thickness results in a large $$M_{{\text{max}}}$$ M max . In addition to smoothing the subducting-plate interface via subducted sediments, along-dip extension of the crustal area along the convergent plate boundary would be important for generating a large earthquake. Graphic Abstract