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The Canadian Cordillera marks a transition region from the current plate boundary through the Phanerozoic Cordilleran orogen to the Precambrian cratons. Knowledge of the subsurface structure of western Canada has been greatly advanced by seismological investigations during the past two decades, pioneered by the Lithoprobe project and, more recently, by regional passive seismic arrays. In this study, we construct a new model (WCANM22) of crustal thickness and P‐ to S‐wave velocity ratio, or Vp/Vs, by compiling receiver function data from 473 stations and existing constraints from over 2,600‐km long active source experiments. Our model covers a broad swath (about 1/4) of the land area of North America (105°–140°W, 48°–72°N) and shows an overall flat Moho beneath the Cordillera with an average depth of ∼36 km and a standard deviation of 3 km across orogenic belts. This study provides a comprehensive catalog of Vp/Vs in western Canada and reveals a moderate correlation between Poisson's ratio and the age of crustal domains. The average Vp/Vs values are 1.72, 1.79, and 1.82 for the Phanerozoic Cordillera, Proterozoic Cratons, and Archean‐aged Medicine Hat Block, respectively, suggesting continued modifications to crustal composition through episodic tectonothermal events. This distinct trend in western Canada sheds new light on the debated role of secular changes in the composition of continental crust. Plain Language Summary: The crust of western Canada consists of complex tectonic domains that collided during a protracted geological history dating back to 4 billion years ago. To uncover crustal structures and the associated tectonic processes, we systematically determine the depth to the base of the crust (Moho depth) and compressional‐to‐shear seismic wave speed ratio (Vp/Vs) beneath 473 seismic stations integrated with the results from existing active source experiments. Along a northwest‐southeast profile, Vp/Vs ratios exhibit a gradually increasing trend with the age of the crust. This variation in Vp/Vs may be caused by modifications to the crustal composition during the geological evolution of western Canada. Key Points: We construct a new model of crustal thickness and Vp/Vs of western Canada from integrated active and passive seismic constraintsWe propose that increased Vp/Vs with the age of tectonic domains may reflect a secular change in crustal compositionWe highlight key tectonic boundaries at crustal depth formed during the collision and assembly of cratons [ABSTRACT FROM AUTHOR]

Fluids in subduction zones can have major effects on subduction dynamics. However, geophysical constraints on the scale and impact of fluid flow during continental subduction are still limited. Here we analyze the VP/VS ratios in the Western Alpine region, hosting one of the best-preserved fossil continental subduction zones worldwide, to investigate the impact of fluid flow during continental subduction. We found a belt of high VP/VS ratios >1.9 on the upper-plate side of the subduction zone, consistent with a partially serpentinized upper-plate mantle, and a belt of unusually low VP/VS ratios <1.7 on the lower-plate side, at depths shallower than 30 km. We propose that these low VP/VS ratios result from a widespread network of silica-rich veins, indicating past fluid flow along the continental subduction interface. Our results suggest that past fluid flow may have reduced the effective stress along the subduction interface thus favoring continental subduction. [ABSTRACT FROM AUTHOR]

The Indo-Gangetic Plain (IGP) is characterized by thick sediments, predominantly comprising alluvial deposits, which can amplify seismic waves generated by earthquakes in the Himalayan region located to the north of the plain. The presence of loose sediments can indeed pose significant seismic hazards, mainly due to phenomena like soil liquefaction. These sediments pose a threat to densely populated Delhi and NCR regions, which are 200 km away from the plate boundary of India and the Eurasian plate. Scientists are concerned about people's safety in mitigating damage caused by high-rise buildings and loose sediments in the IGP region. Reliable knowledge of the sedimentary layer's thickness and velocity structure is crucial for investigating buried active faults, understanding significant destruction, and risk assessment. Sedimentary basins are also crucial for geo-resources such as hydrocarbon and geothermal energy. This research estimated the structure of the sedimentary layer beneath four stations in the Chandigarh–Ambala region in IGP using the high-frequency receiver function (PRF) technique. The study found that the sedimentary layer thickness varies significantly, with values from 2.0 to 3.0 km beneath the IGP and increasing northward. Shallow shear velocity (Sv) in the column of sediments below the Siwalik Himalaya ranges from 2.8 to 2.9 km/s, which can be utilized for assessing earthquake ground-motion sites. The study provides new perceptions of the geodynamic processes and seismotectonic structure of the Himalayan region, allowing for better identification of the earthquake hypocenter and assessment of seismic hazards. The shear wave velocity models estimated from this research can also be beneficial for assessing seismic hazards and earthquake-resistant construction. Estimates of the crustal thickness values from waveform inversion of the PRF at individual stations reveal that the Moho depth varies between 44 and 50 km in the Indo-Gangetic Plain. From Siwalik Himalaya to the higher Himalaya, it ranges from 44 to 65 km. The depth of Moho increases from the Indo-Gangetic plain towards the lesser Himalaya. [ABSTRACT FROM AUTHOR]

Abstract Anisotropic structures within the crust are frequently perceived to originate from stress-induced cracks, which have been mainly estimated on land through different wave speeds of orthogonally polarized S waves propagating in the anisotropic media. However, such estimations of crustal anisotropic structures in ocean areas, particularly for subduction zones around trenches, have not been investigated in detail due to the lack of long-term ocean bottom observations. In this study, we used ocean bottom seismometers of a permanent network deployed across the Japan Trench and the southern part of the Kuril Trench and applied the shear-wave splitting analysis to P-to-s converted waves extracted by receiver function analyses using teleseismic events. We estimated the anisotropic structures in marine sediments and oceanic crust for the incoming Pacific Plate and marine sediments for the overriding North American Plate. The obtained fast polarization directions for the incoming plate are mainly oriented to be parallel to the trench axis for the marine sediment and oceanic crust, which are formed by normal faults and cracks due to the upward plate bending in the outer-rise region, whereas results for marine sediments at the northern part of the Japan Trench are obliquely aligned to the trench axis. The oblique direction is consistent with the magnetic lineations of the incoming plate, indicating that ancient faults within the plate, which were formed in the shallow part of the crust during the creation of the oceanic plate at the ridge, are reactivated by the plate flexure. For the overriding plate, the fast polarization directions in the northern and southern parts of the study area are nearly normal to the trench axis. The central part shows two distinct features: the fast polarization directions parallel to the trench axis and small degrees of anisotropy. These patterns may reflect crack alignments associated with the lateral variation in postseismic crustal deformation after the 2011 Tohoku-Oki earthquake. Our results suggest substantial lateral variations in the stress field at the tip of the overriding plate along the strike direction. Graphical Abstract

Anisotropic structures within the crust are frequently perceived to originate from stress-induced cracks, which have been mainly estimated on land through different wave speeds of orthogonally polarized S waves propagating in the anisotropic media. However, such estimations of crustal anisotropic structures in ocean areas, particularly for subduction zones around trenches, have not been investigated in detail due to the lack of long-term ocean bottom observations. In this study, we used ocean bottom seismometers of a permanent network deployed across the Japan Trench and the southern part of the Kuril Trench and applied the shear-wave splitting analysis to P-to-s converted waves extracted by receiver function analyses using teleseismic events. We estimated the anisotropic structures in marine sediments and oceanic crust for the incoming Pacific Plate and marine sediments for the overriding North American Plate. The obtained fast polarization directions for the incoming plate are mainly oriented to be parallel to the trench axis for the marine sediment and oceanic crust, which are formed by normal faults and cracks due to the upward plate bending in the outer-rise region, whereas results for marine sediments at the northern part of the Japan Trench are obliquely aligned to the trench axis. The oblique direction is consistent with the magnetic lineations of the incoming plate, indicating that ancient faults within the plate, which were formed in the shallow part of the crust during the creation of the oceanic plate at the ridge, are reactivated by the plate flexure. For the overriding plate, the fast polarization directions in the northern and southern parts of the study area are nearly normal to the trench axis. The central part shows two distinct features: the fast polarization directions parallel to the trench axis and small degrees of anisotropy. These patterns may reflect crack alignments associated with the lateral variation in postseismic crustal deformation after the 2011 Tohoku-Oki earthquake. Our results suggest substantial lateral variations in the stress field at the tip of the overriding plate along the strike direction. [ABSTRACT FROM AUTHOR]

Intraplate earthquakes induced by anthropogenic fluid injection present unexpected seismic risk to previously quiescent or low seismicity-rate regions. Despite many studies of induced seismicity, there are relatively few with detailed openly accessible constraints on the interaction between seismic sources and subsurface structures. In this study of the Raton Basin, we refine source observations from a dense nodal array and constrain basin structure using teleseismic receiver functions. The cross-correlation-based relocated hypocentres and a new set of focal mechanisms light up active fault segments and show clear spatiotemporal patterns. The geometric complexity of reactivated fault clusters appears greatest near higher rate injection wells. Simpler normal fault structure is found farther from injection wells and near abrupt structural transitions suggested by receiver functions. While less induced seismicity in the crystalline basement is expected when injection is >1 km from the top of the basement (like Raton), our receiver function analysis identified a basin thickness ~3 km beneath the nodal array and lateral variations in sedimentary structures. Our results explain potential fluid connectivity between the injection depths focused at ~1–1.5 km below the surface and basement fault activity that begins at ~3 km and reaches peak activity at ~4–8 km depths. This article is part of the theme issue 'Induced seismicity in coupled subsurface systems'. [ABSTRACT FROM AUTHOR]

The Tanlu Fault Zone (TLFZ) and Chifeng‐Kaiyuan Fault (CKF) serve as tectonic boundaries between the North China Craton (NCC) and the Central Asian Orogenic Belt (CAOB). Clarifying the refined structure of these tectonic boundaries is crucial for understanding the relationships between the tectonic units and the heterogeneity in the destruction of the NCC. In this study, two linear seismic arrays were deployed across these tectonic boundaries. Based on the phase velocity dispersion and receiver functions extracted from the seismic arrays, the Hamiltonian Monte Carlo algorithm was employed for the joint inversion of the S‐wave velocity (Vs) in the crust and uppermost mantle. The Vs model was then used to correct the time differences in common conversion point (CCP) stacking. The CCP stacking results indicate that the boundary faults TLFZ and CKF are both whole‐crustal faults that separate the NCC and CAOB. The Vs structure showed a significant low‐velocity anomaly in the mantle beneath the NCC, with intense seismic activity within the crust. This suggests that the NCC was affected by the subduction of the Western Pacific, leading to crustal and mantle destruction. In contrast, the CAOB exhibited a clear high‐velocity anomaly with relatively stable crustal structures. We believe that the NCC and CAOB have undergone structural modification and destruction due to the closure of the Paleo‐Asian Ocean and the activities of the TLFZ since the Late Mesozoic. During the Cenozoic, the region east of the TLFZ experienced more significant destruction in the NCC than the other adjacent tectonic units. Plain Language Summary: The joint inversion of Rayleigh wave phase velocities and receiver functions provides a reliable method for obtaining high‐precision S‐wave velocity (Vs) models. Here, we applied a joint inversion based on the Hamiltonian Monte Carlo algorithm to acquire detailed crust‐mantle Vs models in Northeast China and utilized this velocity structure to constrain the common conversion point (CCP) stacking to obtain a more reliable Moho depth. A comprehensive high‐resolution seismological model was established by integrating the Vs model, Moho depth, Vp/Vs ratio, and seismic activity. We then revealed the differential structural features and seismic heterogeneity on both sides of the tectonic boundaries between the Central Asian Orogenic Belt (CAOB) and the North China Craton (NCC). Our research results offer new constraints on the crust and upper mantle structure in Northeast China, contributing to the understanding of the geodynamic context underlying the differential structures between the CAOB and the NCC and the characteristics of their boundaries. Key Points: We obtained the fine crust‐mantle structure of tectonic boundaries in NE China using Rayleigh wave phase velocities and receiver functionsThe tectonic boundaries Chifeng‐Kaiyuan Fault and Tan‐Lu Fault are both whole‐crustal faults with strong seismic activityThe upper mantle of North China Craton exhibits low S‐wave velocity with strong seismic activity [ABSTRACT FROM AUTHOR]

We investigated velocity and anisotropic structure of the crust beneath the eastern margin of the Tibetan Plateau to better understand its deformation and evolution mechanism. We performed H‐κ and Pms anisotropy analyses to obtain crustal thickness, Vp/Vs ratio, fast polarization direction, and splitting time from 711 stations, and further conducted quality control using slowness, harmonic and statistical analyses. The Songpan‐Ganzi Block has a large splitting time and a fast polarization direction roughly parallel to the GPS motion and SKS fast direction. It also shows an overall high but complex distribution of Vp/Vs ratio, and large variations in crustal thickness, indicating that crustal deformation is likely caused by crustal shortening and lower crustal flow. The northern Sichuan‐Yunnan Rhombic Block (SYRB) is featured by a thick crust and high Vp/Vs ratio, suggesting that the crust is likely inflated by partial melting lower crustal rocks. The subblock also exhibits a strong azimuthal anisotropy with a splitting time greater than 0.6 s. The fast polarization direction aligns with the nearly N‐S extended direction and rotates clockwise in front of the Emeishan Large Igneous Province (ELIP). The observed anisotropy agrees with aligned amphibole minerals under a simple shear condition, supporting a southward lower crust flow being diverted by the ELIP. Anisotropy measurements on the southern SYRB are less robust and widely scattered, suggesting a deformation mechanism different from the northern SYRB. In addition, the southeastern margin of the Sichuan Basin shows a systematic pattern of crustal anisotropy consistent with a pure shear deformation mechanism. Plain Language Summary: We studied the crustal structure and deformation beneath the eastern edge of the Tibetan Plateau. By analyzing teleseismic data from 727 stations, we measured crust thickness, Vp/Vs ratio, and azimuthal anisotropy using receiver functions. Our findings show a complex crustal behavior pattern in the eastern Tibet margin. In the Songpan‐Ganzi Block, crustal shortening and lower crust flow seem to be causing deformation. The northern Sichuan‐Yunnan Rhombic Block, with its thick crust and high Vp/Vs ratio, appears to be influenced by partial melting of lower crust rocks. And a southward lower crust flow being diverted by the Emeishan Large Igneous Province. The eastern and southern part of this block shows a different pattern, indicating a unique deformation mechanism. Additionally, the southeastern margin of the Sichuan Basin appears to undergo a distinct type of crustal deformation. Key Points: A new crustal model with much more reliable and denser Pms anisotropic measurements is built in the eastern Tibetan MarginSongpan‐Ganzi Block is likely deformed by crustal shortening and lower crustal flowChannel flow may exist in the west low velocity zone (LVZ) while absent at the east LVZ [ABSTRACT FROM AUTHOR]

Kalimantan and Sulawesi are located within the complex tectonic setting of central Indonesia. The tectonic evolution process during the Mesozoic and Cenozoic led to the formation of this region. Studies of Moho depth variation beneath this region are still limited due to the lack of local to regional scale seismic stations covering the entire Kalimantan and Sulawesi region. The availability of seismic data has doubled and tripled recently, here, we conduct receiver function study using reliable dense seismic data from BMKG seismic network (IA) to obtain more detail Moho depth variation in Kalimatan and Sulawesi. We analyzed P–waveforms recorded at 60 seismic stations of BMKG seismic network (IA) distributed in Kalimantan and Sulawesi from more than 150 earthquakes with M ≥ 6 distributed within the epicentral distance range of 30o – 90o. The receiver function signals of each seismic station were computed using the iterative time-domain deconvolution method, then the Moho depths and bulk composition (Vp/Vs ratio) were computed using the modified H-κ stacking method which consider the sediment thickness. Our results show that the crustal thickness in Kalimantan varies from ~25 to ~37 km and Vp/Vs ratio varies from ~1.61 to ~1.96, which reflect a characteristic of stable Mesozoic-Cenozoic regions, while Sulawesi is observed within broad thickness range of ~22 – 50 km and Vp/Vs ratio of ~1.56 – 1.97. The extremely thick crust in Sulawesi reflects the rapid uplift and exhumation zones achieved by tectonic process sequences that had been occurred in this region. [ABSTRACT FROM AUTHOR]

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We analyzed 49,592 teleseismic receiver functions (RFs) recorded by 278 CEArray stations to image the mantle transition zone (MTZ) beneath the South China Block to understand the origins of deep velocity anomalies and their potential links to subduction and intraplate volcanism. We employed a fast‐marching method and a high‐resolution 3‐D velocity model (FWEA18) derived from full waveform inversion in computing P‐to‐S conversion times to better image the 410‐ and 660‐km discontinuities. Our results indicate that the common‐conversion‐point stacking of RFs using 3‐D conversion times yielded better migration images of the two discontinuities. The images revealed a slightly depressed 410‐km with a few small uplifted patches, and showed that the 660‐km beneath the western Yangtze Craton is depressed by 10–25 km, which is likely caused by the stagnant Paleo‐Pacific slab. The 660‐km beneath the southern Cathaysia Block has a 5–15 km high plateau with a topographic low at its central part. The lateral dimension of the topographic low is ∼150 km and is located beneath the central Pearl River Mount Basin near Hong Kong. We speculate that the topographic low occurs within the Hainan plume with a temperature excess of ∼300–400 K and is caused by the garnet phase transition. The displaced deep plume enters the MTZ and spreads nearly horizontally at the base. The plume evolves into two channels with a minor one toward the northeast and a major one toward the southwest, which keep moving upward to the 410‐km. The southwest channel is likely the source that feeds the Hainan volcanoes. Plain Language Summary: Using data from seismic stations in the South China Block, we investigated the mantle transition zone (MTZ) to understand the origins of deep velocity anomalies and their associations with subduction and intraplate volcanism. By applying advanced techniques and a ground‐truth reference model, we obtained clearer images of the 410‐ and 660‐km discontinuities. The images showed that the 410‐km discontinuity is slightly depressed with some small uplifted areas. Additionally, the 660‐km discontinuity beneath the western Yangtze Craton is depressed due to the presence of a stagnant slab from the ancient Pacific Ocean. In contrast, beneath the southern Cathaysia Block, the 660‐km discontinuity forms a high plateau with a central low area. This area located near Hong Kong may be related to a plume originating from the lower mantle. The plume, with elevated temperatures, enters the MTZ and spreads horizontally. It then evolves into two channels, with one moving toward the northeast and the other toward the southwest. The southwest channel likely supplies magma to the volcanoes in Hainan. These findings provide insights into the complex processes occurring deep within the Earth's mantle in the South China region. Key Points: FWEA18 is used in migrating CEArray receiver functions to image the mantle transition zone beneath the South China BlockThe 660‐km is depressed ∼10–25 km by stagnant slabs beneath the northwestern part of the blockThe Hainan volcanoes are fed by a displaced lower mantle plume beneath the central Pearl River Mouth Basin [ABSTRACT FROM AUTHOR]

Geometrical heterogeneity of the subducting Indian continental crust along the Himalaya‐Tibet collision zone remains enigmatic. Mass budget estimates describing shortening across the orogen are partly derived from observations made from seismic imaging of the deep earth. Here, using data from 38 broadband seismic stations covering Sikkim Himalaya, we produce high resolution seismic images in order to fill crucial gaps in our understanding of the evolution of Himalayan collision zone. We used 12,288 high quality receiver functions computed using waveforms of earthquakes having magnitude >5.5 in the distance range of 30°–100°. Our results reveal a highly imbricated and heterogeneous crust beneath Sikkim Himalaya. The Main Himalayan Thrust (MHT) responsible for large scale earthquakes in the Himalayan collision zone is not so distinct in the migrated images, but intermittent. A dominant cluster of earthquakes at shallower depths is associated with the MHT, marked by negative amplitude arrivals. The Moho is found to be gently dipping, reaching depths of ∼60 km beneath the Higher Himalaya compared to ∼40 km in the Himalayan foredeep. Interestingly, the Moho in this part of Himalaya has offsets and overlapping segments, indicating crustal imbrication in response to active shortening. Clusters of lower crustal earthquakes coincide with the junction of offsets in the Moho. Key Points: Receiver functions computed using new data from a network deployed in Sikkim HimalayaIntermittent arrivals at mid‐crustal depths signify a complex nature of the décollement surfaceEvidence for imbrication indicating brittle nature of the lower crust accommodating active shortening [ABSTRACT FROM AUTHOR]

The article presents a study of the Earth’s crust and upper mantle in the Pechenga ore region, as well as areas adjacent to it in the northwestern part of the Kola region. Applying the receiver function technique to data acquired by three broadband seismic stations, we obtained one-dimensional seismic velocity distribution models to a depth of 300 km. The stations are located in the northern parts of Finland and Norway, as well as in the Pechenga region of the Russian Federation. Despite the stations being in relatively close proximity (within 100 km of each other), the velocity models turned out to be significantly different, which indicates structural discontinuity within the lithosphere. Thus, Finland station data set revealed a gradient crust-mantle transition, which is not present in the other two models. At depths of about 150 km, a low-velocity zone was discovered, associated with mid-lithospheric discontinuity, which was not found beneath the Pechenga ore region. Furthermore, the crustal structure of the Pechenga region has an anomalously high Vp/Vs ratio to a depth of about 20 km. Considering the fact that the Pechenga (Nikel) seismic station was installed in close proximity to major copper-nickel deposits, this anomaly can be interpreted as a relic of Proterozoic plume activity.

The phase of secular evolution of continents is manifested as the degree of compositional differentiation, modification and maturation of continental crusts, which is vital in understanding the mechanism of continental evolution but is difficult to quantify. Here we use integrated passive- and active-source seismic profiling to conduct joint analysis and inversion and derive Vs and Vp/Vs section models across the North China Craton (NCC) to southeastern Altaids boundary zone that bears a tectonic transition from a reworked ancient craton margin to a Phanerozoic accretionary orogen. We systematically exploited the imaged multiple physical properties as precise and delicate proxies to constrain the compositional architecture in the crust across this important tectonic transition subject to various crustal evolutional phases. Our Vs and Vp/Vs imaging, together with the existing isotopic data, characterizes the Yin Shan–Yan Shan belt as the northern NCC margin with layered homogeneous compositions that point to an evolved crust. However, the lower-crustal low-Vs/high-Vp/Vs signature that overlaps the shallowly dipping to horizontal reflective fabrics suggests that the crust of the northern NCC margin has undergone considerable reworking through lower-crustal-stretching-assisted melt migration and mixing since the late Paleozoic to Mesozoic eras. The process probably involved crust–mantle interaction and thus resulted in a compositionally modified ancient crustal basement. On the contrary, the southeastern Altaids domain manifests crustal complexity in compositions and structures inferred to be indicative of a juvenile crust of the Phanerozoic accretionary orogen. Our results provide deep physical-property constraints that shed new light on the crustal evolution of a complex craton margin. [ABSTRACT FROM AUTHOR]

The Canary and Madeira Islands are two distinct hotspots in the Central‐East Atlantic that are close to each other. Their volcanism is generally attributed to underlying mantle plumes, but the detailed structure of these plumes is still not well understood. The thermal and compositional structure of the plume introduces complexities in the phase transitions of the mantle, which impact the depth and magnitude of seismic discontinuities. We use 1,268 high‐quality receiver functions from stations located at the two hotspots to detect P‐to‐s converted phases through a common‐conversion point stacking approach and conduct a detailed analysis of mantle seismic discontinuities. The results show that both hotspots are characterized by a thin mantle transition zone (MTZ), with sharp 410 and 660 discontinuities at depths of 428–421 km and 647–664 km, beneath the Canaries and Madeira respectively. The results indicate that the Canary plume crosses the MTZ, whereas the Madeira plume mainly influences the upper portion of the MTZ. Furthermore, we find reliable detections of a sharp X discontinuity beneath the Canaries at 287 km. Its presence suggests the accumulation of silica‐rich recycled eclogite at these depths. We also use the amplitudes of P410s and PXs to derive velocity jumps at corresponding discontinuities. Based on these measurements, we estimate that the basalt proportion is 60%–80%, with accumulation being more significant in the Canaries than in Madeira. The MTZ thickness, the presence of the X discontinuity, and the high basalt proportion provide compelling evidence for a deep‐rooted thermochemical plume beneath the study area. Plain Language Summary: We examine the Canary and Madeira Islands, two separate groups of islands in the Atlantic Ocean, close to each other. While it is generally accepted that volcanic activity in these regions is driven by underlying mantle plumes, the precise characteristics of these plumes remain relatively unknown. Tomography studies have revealed distinct structures of the plumes, with the Canary plume extending vertically throughout the upper mantle, while the Madeira plume is visible up to a depth of 300 km, beyond which the resolution of the images deteriorates. We use receiver functions, derived from the analysis of seismic waveforms, particularly the conversion of compressional waves to shear waves at seismic discontinuities within the Earth. Receiver functions offer a higher resolution compared to tomographic images, primarily due to their ability to directly measure seismic wave conversions at specific depths, reflecting where mantle minerals change their crystal structure. This analysis provides insights into the variations in temperature and mineral chemistry in the mantle, as both factors influence the depths of crystal rearrangements. Our research provides evidence supporting the existence of a deep‐seated thermochemical plume beneath the study area. However, beneath Madeira, the plume may be less vigorous or presently disconnected from its deep source. Key Points: We investigate the upper mantle discontinuities below Canary and Madeira using P receiver functionsDeeper 410 in both, shallower 660 in Canaries, normal in Madeira: suggests Canary plume crosses MTZ, Madeira plume only affects upper MTZDetection of X discontinuity suggests a deep‐rooted thermochemical plume [ABSTRACT FROM AUTHOR]

The receiver function technique is widely used to image crustal structure using P‐to‐S converted phases at the Moho discontinuity. However, the presence of sedimentary layer generates additional P‐to‐S conversions and reverberations, which can overprint the Moho phases and pose problems in imaging crustal structure with standard receiver function techniques. We introduce a robust two‐step method that uses H‐κ stacking to determine average thickness and Vp/Vs of the sedimentary layer, followed by waveform‐fitting of the observed receiver function to constrain the average crustal thickness and sub‐sediment Vp/Vs. We tested the method using both synthetic data and real‐data from stations located on sedimentary layers in the Netherlands and USA. We show that the new method outperforms other common approaches in retrieving accurate Moho depth and sub‐sediment Vp/Vs estimates, even in cases where the Moho phases are completely overprinted by large‐amplitude phases related to sedimentary layers. Plain Language Summary: We propose a two‐step method for modeling crustal structure using receiver functions from stations overlying sedimentary layers. In the first step, the sediment layer thickness and velocity ratio (Vp/Vs) are derived from depth versus Vp/Vs stacking of receiver functions. In the second step, the crust‐mantle interface (Moho) depth and sub‐sediment Vp/Vs are derived from the waveform‐fitting grid search of the observed receiver functions with synthetic ones. We tested this method using synthetic receiver functions and real‐data from stations in the Netherlands and USA. The new method works well in retrieving Moho depth and sub‐sediment Vp/Vs, even in cases where the Moho signals are completely obscured by signals from the sediment layer. Key Points: New approach for modeling receiver functions for stations overlying a sedimentary layerThe approach is effective even when Moho phases are completely overprinted by positive or negative amplitudes from the sedimentary layerSynthetic tests and real‐data applications show the advantages of the developed approach over common methods for addressing sediment impact [ABSTRACT FROM AUTHOR]

To advance our comprehension of the complex geological history and mantle dynamics in the North Atlantic region, we employ all available broadband seismic data recorded in Greenland to reveal an abnormal mantle transition zone (MTZ) structure. Central and eastern Greenland exhibits depressed 410 and 660 km discontinuities (d410 and d660, respectively) bordering the MTZ, indicative of a substantial thermal anomaly associated with an underlying plume, surpassing the 1,800°C threshold for post‐garnet phase transitions at the d660. Variations in MTZ thickness across Greenland stem from differing temperature anomalies at the d410 and d660, possibly linked to a tilted plume within the MTZ. These findings corroborate geodynamic models, elucidating the interaction between post‐garnet phase transitions and upwelling plumes. The results shed light on the origin of the enigmatic Icelandic hotspot track and its influence on the thermal and lithospheric structures beneath Greenland. Plain Language Summary: The physical and chemical processes responsible for producing the magnificent volcanic eruptions in Iceland and the North Atlantic region are debated issues in the geoscientific community despite numerous field and laboratory studies over the past several decades. One of the hypotheses for the formation of the volcanoes is that they originate from a column of hot rocks rising from the deep mantle, that is, a mantle plume. The geometry, depth extent, and temperature anomaly of the plume are not well defined but can be constrained using the topography of two phase‐transition boundaries approximately at the globally averaged depths of 410 and 660 km, respectively. By analyzing P‐to‐S converted phases from the two discontinuities recorded by seismic stations, our results support the existence of a mantle plume that is broader than most other plumes on Earth beneath east Greenland. In addition, the results suggest that the nature of the phase transition across the 660 km discontinuity is different between the hottest core of the plume stem and the colder peripheral areas. The existence of the particular type of phase transition in the plume center can explain the large dimension of the plume, the excessive volcanism and other observations in the area. Key Points: The central and east Greenland shows abnormal mantle transition zone (MTZ) thickness, indicating a deep‐seated mantle plumeSeismic data unveil the impact of post‐garnet phase transition at the 660‐km discontinuity on mantle dynamicsVariations in MTZ thickness suggest the existence of a tilted plume and offer insights into the plume behavior [ABSTRACT FROM AUTHOR]

The receiver function (RF) method is the most widely adopted method for imaging crustal structures using earthquake data. Through attenuation during long‐distance propagation, high‐frequency components are scarce in teleseismic waveforms, resulting in low‐frequency RFs and low‐resolution crustal images. The Pn‐wave contains more high‐frequency components because of the short epicentral distance. To improve the resolution of crustal structure studies, we propose the Pn‐wave receiver function (PnRF) method. Unlike other near‐earthquake phases, the Pn‐wave can be considered a plane wave in the crust beneath seismic stations, and interference from other phases can be avoided at epicentral distances of 5–15°. PnRFs calculated from both numerical synthetic data and observational data at broadband seismic stations show that all converted waves are present in PnRFs at the predicted time according to the theory of plane waves. PnRFs calculated by observational data of a dense nodal array clearly show not only the converted wave from the Moho but also the converted wave from the crustal interface, which is too weak to be observed in tele‐RFs because the Pn‐wave has a larger incident angle and higher frequency than the teleseismic P‐wave. When used in conjunction with dense nodal array observations, the PnRF method has the potential to image crustal structures with a high resolution close to that of the deep seismic reflection method. Plain Language Summary: As an economical seismic method, the receiver function method is widely adopted to image crustal structures using earthquake data. However, its resolution is very low because teleseismic waveforms retain only low‐frequency components and lose high‐frequency components due to attenuation during propagation over epicentral distances greater than 3,000 km. In this study, we propose the Pn‐wave receiver function (PnRF) method. Unlike other regional seismic phases, the Pn‐wave can be taken as a plane wave after returning to the crust, and interference from other phases can be avoided in the epicentral distance range of 500–1,600 km. Our numerical simulation tests and observational data show that the amplitudes of converted waves in PnRFs are 2–3 times larger than those in teleseismic receiver functions (tele‐RFs), which implies that PnRFs have a greater sensitivity to detect weak interfaces than tele‐RFs. The most important advantage of the PnRF method is that it can image crustal structure with far higher resolution than the tele‐RF method, even close to that of the expensive deep seismic reflection method, because Pn‐waves contain more high‐frequency components. Key Points: The Pn‐wave as a regional seismic phase is proposed to calculate the receiver function for imaging crustal structureConverted wave amplitudes in the Pn‐wave receiver function are 2–3 times larger than those in the teleseismic P‐wave receiver functionIt is a high‐resolution method to study crustal structure because Pn‐wave is richer in high‐frequency components than teleseismic P‐wave [ABSTRACT FROM AUTHOR]

A shallow crustal velocity structure (above 10 km depth) is essential for understanding the crustal structures and deformation and assessing the exploration prospect of natural resources, and also provides priori information for imaging deeper crustal and mantle structure. Passive-source seismic methods are cost-effective and advantageous for regional-scale imaging of shallow crustal structures compared to active-source methods. Among these passive methods, techniques utilizing receiver function waveforms and/or body-wave amplitude ratios have recently gained prominence due to their relatively high spatial resolution. However, in basin regions, reverberations caused by near-surface unconsolidated sedimentary layers often introduce strong non-uniqueness and uncertainty, limiting the applicability of such methods. To address these challenges, we propose a two-step inversion method that uses multi-frequency P-RF waveforms and P-RF horizontal-to-vertical amplitude ratios. Synthetic tests indicate that our two-step inversion method can mitigate the non-uniqueness of the inversion and enhance the stability of the results. Applying this method to teleseismic data from a linear seismic array across the sedimentary basins in Northeast China, we obtain a high-resolution image of the shallow crustal S-wave velocity structure along the array. Our results reveal significant differences between the basins and mountains. The identification of low-velocity anomalies (≤2.8 km s−1) at depths less than 1.0 km beneath the Erlian Basin and less than 2.5 km beneath the Songliao Basin suggests the existence of sedimentary layers. Moreover, the high-velocity anomalies (∼3.4–3.8 km s−1) occurring at depths greater than 7 km in the Songliao Basin may reflect mafic intrusions emplaced during the Early Cretaceous. Velocity anomaly distribution in our imaging result is consistent with the location of the major faults, uplifts, and sedimentary depressions, as well as active-source seismic results. This application further validates the effectiveness of our method in constraining the depth-dependent characteristics of the S-wave velocity in basins with unconsolidated sedimentary cover. [ABSTRACT FROM AUTHOR]

Abstract We analyzed 49,592 teleseismic receiver functions (RFs) recorded by 278 CEArray stations to image the mantle transition zone (MTZ) beneath the South China Block to understand the origins of deep velocity anomalies and their potential links to subduction and intraplate volcanism. We employed a fast‐marching method and a high‐resolution 3‐D velocity model (FWEA18) derived from full waveform inversion in computing P‐to‐S conversion times to better image the 410‐ and 660‐km discontinuities. Our results indicate that the common‐conversion‐point stacking of RFs using 3‐D conversion times yielded better migration images of the two discontinuities. The images revealed a slightly depressed 410‐km with a few small uplifted patches, and showed that the 660‐km beneath the western Yangtze Craton is depressed by 10–25 km, which is likely caused by the stagnant Paleo‐Pacific slab. The 660‐km beneath the southern Cathaysia Block has a 5–15 km high plateau with a topographic low at its central part. The lateral dimension of the topographic low is ∼150 km and is located beneath the central Pearl River Mount Basin near Hong Kong. We speculate that the topographic low occurs within the Hainan plume with a temperature excess of ∼300–400 K and is caused by the garnet phase transition. The displaced deep plume enters the MTZ and spreads nearly horizontally at the base. The plume evolves into two channels with a minor one toward the northeast and a major one toward the southwest, which keep moving upward to the 410‐km. The southwest channel is likely the source that feeds the Hainan volcanoes.

The left‐lateral Altyn Tagh Fault (ATF) system is the northern boundary of the Tibetan Plateau resulted from the India–Eurasia continental collision. How intracontinental deformation across the central ATF responds to the distal collision remains elusive, primarily due to unclear crustal structure. We obtained detailed crustal structure across the central ATF using receiver functions recorded by ∼NW–SE oriented linear dense array. The images reveal the Tarim lower crust is underthrusting beneath the Tibetan Plateau and reaches to a maximum depth of ∼75 km and undergoing partial eclogitization. The two south‐dipping interfaces imaged beneath the Altyn Tagh Range (ATR) represent the thrusting Northern Altyn Fault and its branch fault. Oblique convergent forces extruded upper crustal materials along the thrust faults, creating the pop‐up structure of ATR, supported by low Vp/Vs ratios. Our balanced cross‐section for the Moho suggests intracontinental deformation in the ATR has accelerated since the late Miocene. Plain Language Summary: The Altyn Tagh Fault (ATF), serving as the northern boundary of the Tibetan Plateau, demarcates the Tarim Basin from the Qaidam Basin. Understanding how intracontinental deformation across the boundary region would better inform the uplift and expansion of the plateau. This study reveals the fine crustal structure by analyzing seismic data from a ∼NW–SE oriented linear dense array across the central ATF. Combined with fault slip rates, we propose that the Tarim lower crust is underthrusting beneath the Tibetan Plateau, leading to the extrusion of upper crustal materials and the rapid uplift of the Altyn Tagh Range since the late Miocene, which provides insight into the lateral growth of the plateau. Key Points: Detailed crustal structure beneath the central Altyn Tagh Fault was imaged by receiver functions of a dense 2‐D seismic arrayThe Tarim lower crust is underthrusting to ∼75 km depth beneath the Tibetan PlateauThe Altyn Tagh range was uplifted rapidly since late Miocene through the thickening of the upper crust [ABSTRACT FROM AUTHOR]

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

The Ailaoshan Orogenic Belt (AOB), located at the southeastern boundary of the Tibetan Plateau, is an ideal place for investigating the mechanisms of lateral growth of Tibet. Using the data recorded by a dense seismic array across the Ailaoshan belt, we investigate the detailed lateral variations of crustal anisotropy on the basis of Pms phase of receiver functions. Remarkable crustal anisotropy is observed throughout this study region with a mean delay time of 0.33 ± 0.19 s, indicating the anisotropy primarily originates in the middle-lower crust. The fast directions beneath the AOB including the Ailaoshan-Red River shear zone (ARRSZ) and its western low-grade metamorphic unit generally align with the NW-SE strike of ARRSZ. The weak anisotropy in the South China Block (SCB) argues that the block is relatively stable, with limited internal deformation. Meanwhile, the anisotropy beneath the western boundary of the SCB is strong, and the N-S oriented fast direction is influenced by both the crustal stress and Xiaojiang Fault. Combining the high Vp/Vs and significant lateral variations of crustal anisotropy parameters, we suggest that the strike-slip motion along the ARRSZ induces the partial melting and pronounced anisotropy in the middle-lower crust of AOB, without the presence of crustal flow. The differences between crustal and mantle anisotropy indicate crust-mantle decoupling deformation of the AOB, supporting the block extrusion model occurring only in the crustal scale as the primary deformation pattern. [ABSTRACT FROM AUTHOR]

We collected high-quality teleseismic events recorded by 12 broadband seismographs deployed in the Anyuan Coal Mine and its adjacent areas in Pingxiang City, Jiangxi Province for nearly two years. The H-κ-c stacking method was employed to obtain the crustal thickness and Poisson's ratio distribution, then the characteristics of crustal structure below the stations were obtained by using the time-domain linear inversion method. The crustal thickness in the Anyuan Coal Mine and its adjacent areas ranges from approximately 32 - 35 km, with an average thickness of 33 km, which is consistent with the crustal thickness results in South China from previous studies using the receiver function method. The average Poisson's ratio of the crustal bulk composition in the study area varies between 0.22 and 0.25, which is lower than the global value with a 0.27 average, indicating a predominantly intermediate-acidic or felsic crustal composition. There is a weak negative correlation between Poisson's ratio and crustal thickness estimates in the Anyuan Coal Mine and its adjacent areas, suggesting that the absence of mafic-ultramafic materials in the lower crust is associated with the process of crustal delamination. The velocity inversion results indicate that the crustal structure including three velocity discontinuity interfaces, with the first at a depth of approximately 1.5 km, the second at about 10 - 15 km, and the third being the Moho. The study also indicates that the results obtained by the H-κ-c stacking method are significantly better than those obtained by the H-κ method, effectively reducing the standard deviation and dispersion of crustal thickness and vP/vS ratio. [ABSTRACT FROM AUTHOR]

A combination of receiver function (RF) analysis and shear wave splitting can reveal the anisotropic properties of the Earth's velocity structure. The RF profile exhibits harmonic patterns partially generated from the discontinuities of anisotropic media near the receiver. The anisotropic properties are determined using splitting parameters, which include the fast polarization direction (FPD) and split time. We utilized the seismograms recorded by stations on Shikoku Island, Japan. Also, we applied the Bayesian information criterion to control the spike count of each RF, which consists of a series of spikes generated by time‐domain iterative deconvolution. Anisotropic differences arose between northern and southern regions along the 30‐km iso‐depth line of the top surface of the Philippine Sea Plate. In the southern part of Shikoku Island, the FPDs are sub‐perpendicular to the plunge of the subducting slab, while in the northern part, they are sub‐parallel to the plunge of the slab. Our results indicate that anisotropic strengths are weaker in the tectonic‐tremor band in northwestern Shikoku and stronger in the northern part of central and eastern Shikoku around the no tectonic‐tremor area. These anisotropic variations may characterize Shikoku Island's geological structure. Plain Language Summary: Receiver function (RF) analysis is sensitive to subsurface discontinuities in seismic velocity using shear waves on seismograms. This study estimated a seismic velocity anisotropy by combining RF and shear wave splitting analysis to determine the polarization direction and delay time, characterizing the anisotropic medium. The observed fast direction of anisotropy aligns with the strike of the subducting Philippine Sea Plate in southern Shikoku, while in the northern region, it becomes sub‐parallel to the subducting plate's plunge (N‒S). Additionally, the delay time between the split fast and slow shear waves indicates the strength of anisotropy. In the tectonic‐tremor‐clustered area in northwestern Shikoku, the strengths of anisotropy are regionally weaker than in other areas. Moreover, stronger anisotropy was measured in the deeper northern part of central and eastern Shikoku, where there is a lack of tectonic‐tremor activity. Although several factors contribute to the anisotropy, we concluded that the anisotropy mainly reflects the structure, including the subducting slab, continental crust, and even the mantle wedge. Key Points: Fine‐resolution anisotropic properties have been calculated above slab Moho in Shikoku Island, southwest Japan, using receiver function analysisWeaker anisotropic strength was observed in tectonic‐tremor‐clustered areas with serpentinized mantle wedge, especially in northwestern ShikokuSerpentinization of the mantle and metasomatic reactions lead to hydrofracturing and relatively low anisotropic strength [ABSTRACT FROM AUTHOR]

Seismic anisotropy beneath eastern North America, as expressed in shear wave splitting observations, has been attributed to plate motion‐parallel shear in the asthenosphere, resulting in fast axes aligned with the plate motion. However, deviations of fast axes from plate motion directions are observed near major tectonic boundaries of the Appalachians, indicating contributions from lithospheric anisotropy associated with past tectonic processes. In this study, we conduct anisotropic receiver function (RF) analysis using data from a dense seismic array traversing the New England Appalachians in Connecticut to examine anisotropic layers in the crust and upper mantle and correlate them with past tectonic processes as well as present‐day mantle flow. We use the harmonic decomposition method to separate directionally‐dependent variations of RFs and focus on features with the same harmonic signals observed across multiple stations. Within the crust, there are multiple features that may be correlated with stratification in the Hartford Basin, faults in the Taconic thrust belt, shear zones formed during Salinic/Acadian terrane accretion events, and orogen‐parallel crustal flow in the Acadian orogenic plateau. We apply a Bayesian inversion method to obtain quantitative constraints on the direction and strength of intra‐crustal anisotropy beneath the Hartford Basin. In the upper mantle, we identify a fossil shear zone possibly formed during oblique subduction of Rheic Ocean lithosphere. We also find evidence for a plate motion‐parallel flow zone in the asthenosphere that is likely disturbed by mantle upwelling near the southern margin of the Northern Appalachian Anomaly in the eastern part of the study area. Plain Language Summary: Seismic waves with different propagation and oscillation directions can exhibit different velocities when going through a medium with some directional properties; this phenomenon is called seismic anisotropy. Seismic anisotropy observed beneath eastern North America is often attributed to present‐day flow in the upper mantle. The mantle flow causes shear waves oscillating in the direction of flow (e.g., in the direction of North America plate motion) to travel faster than those that travel in other directions. However, this pattern does not hold true for some regions in the Appalachian Mountains, suggesting that past tectonic events can result in long‐lived, "frozen‐in" anisotropy in the lithosphere, which modifies the predicted anisotropic behavior beneath these regions. In this study, we investigate sources of seismic anisotropy beneath southern New England using a method based on directionally dependent variations of P‐wave to S‐wave conversions at interfaces with contrasts in anisotropy. This method can separate signals caused by different anisotropic features and constrain the depth distribution of anisotropy. We identify various anisotropic layers in the crust and upper mantle and correlate them with Appalachian mountain‐building events and complex present‐day flow in the upper mantle. Key Points: Anisotropic receiver function analysis reveals multiple anisotropic layers in the crust and upper mantle beneath southern New EnglandAnisotropic geometries at <10 km depth resolved by Bayesian inversion imply contributions from sediment layering and Paleozoic deformationArray‐wide anisotropic interfaces in the upper mantle show plate motion parallel flow distorted by localized mantle upwelling [ABSTRACT FROM AUTHOR]

Abstract To advance our comprehension of the complex geological history and mantle dynamics in the North Atlantic region, we employ all available broadband seismic data recorded in Greenland to reveal an abnormal mantle transition zone (MTZ) structure. Central and eastern Greenland exhibits depressed 410 and 660 km discontinuities (d410 and d660, respectively) bordering the MTZ, indicative of a substantial thermal anomaly associated with an underlying plume, surpassing the 1,800°C threshold for post‐garnet phase transitions at the d660. Variations in MTZ thickness across Greenland stem from differing temperature anomalies at the d410 and d660, possibly linked to a tilted plume within the MTZ. These findings corroborate geodynamic models, elucidating the interaction between post‐garnet phase transitions and upwelling plumes. The results shed light on the origin of the enigmatic Icelandic hotspot track and its influence on the thermal and lithospheric structures beneath Greenland.

Abstract The location and origin of Neoproterozoic‐Cambrian sutures provide keys to understand the formation and evolution of the supercontinent Gondwana. The Larsemann Hills is located near a major Neoproterozoic‐Cambrian suture zone in the Prydz Belt, but has not been examined locally by comprehensive geophysical studies. In this study, we analyzed data collected from a one‐dimensional (1D) joint seismic‐MT array deployed during the 36th Chinese National Antarctic Research Expedition. We found that a sharp Moho discontinuity offset of 6–8 km shows up in the stacked image of teleseismic P‐wave receiver function analysis; coinciding with the abrupt Moho offset, a near‐vertical channel with (a) low resistivity extending to the uppermost mantle depths, and (b) high crustal Poisson's ratio in the crust is identified. These findings provide evidence for the determination of the location and collisional nature of the Prydz belt or a portion of it.

Abstract The left‐lateral Altyn Tagh Fault (ATF) system is the northern boundary of the Tibetan Plateau resulted from the India–Eurasia continental collision. How intracontinental deformation across the central ATF responds to the distal collision remains elusive, primarily due to unclear crustal structure. We obtained detailed crustal structure across the central ATF using receiver functions recorded by ∼NW–SE oriented linear dense array. The images reveal the Tarim lower crust is underthrusting beneath the Tibetan Plateau and reaches to a maximum depth of ∼75 km and undergoing partial eclogitization. The two south‐dipping interfaces imaged beneath the Altyn Tagh Range (ATR) represent the thrusting Northern Altyn Fault and its branch fault. Oblique convergent forces extruded upper crustal materials along the thrust faults, creating the pop‐up structure of ATR, supported by low Vp/Vs ratios. Our balanced cross‐section for the Moho suggests intracontinental deformation in the ATR has accelerated since the late Miocene.

The Ailaoshan Orogenic Belt (AOB), located at the southeastern boundary of the Tibetan Plateau, is an ideal place for investigating the mechanisms of lateral growth of Tibet. Using the data recorded by a dense seismic array across the Ailaoshan belt, we investigate the detailed lateral variations of crustal anisotropy on the basis of Pms phase of receiver functions. Remarkable crustal anisotropy is observed throughout this study region with a mean delay time of 0.33 ± 0.19 s, indicating the anisotropy primarily originates in the middle-lower crust. The fast directions beneath the AOB including the Ailaoshan-Red River shear zone (ARRSZ) and its western low-grade metamorphic unit generally align with the NW-SE strike of ARRSZ. The weak anisotropy in the South China Block (SCB) argues that the block is relatively stable, with limited internal deformation. Meanwhile, the anisotropy beneath the western boundary of the SCB is strong, and the N-S oriented fast direction is influenced by both the crustal stress and Xiaojiang Fault. Combining the high Vp/Vs and significant lateral variations of crustal anisotropy parameters, we suggest that the strike-slip motion along the ARRSZ induces the partial melting and pronounced anisotropy in the middle-lower crust of AOB, without the presence of crustal flow. The differences between crustal and mantle anisotropy indicate crust-mantle decoupling deformation of the AOB, supporting the block extrusion model occurring only in the crustal scale as the primary deformation pattern.

Abstract Seismic anisotropy beneath eastern North America, as expressed in shear wave splitting observations, has been attributed to plate motion‐parallel shear in the asthenosphere, resulting in fast axes aligned with the plate motion. However, deviations of fast axes from plate motion directions are observed near major tectonic boundaries of the Appalachians, indicating contributions from lithospheric anisotropy associated with past tectonic processes. In this study, we conduct anisotropic receiver function (RF) analysis using data from a dense seismic array traversing the New England Appalachians in Connecticut to examine anisotropic layers in the crust and upper mantle and correlate them with past tectonic processes as well as present‐day mantle flow. We use the harmonic decomposition method to separate directionally‐dependent variations of RFs and focus on features with the same harmonic signals observed across multiple stations. Within the crust, there are multiple features that may be correlated with stratification in the Hartford Basin, faults in the Taconic thrust belt, shear zones formed during Salinic/Acadian terrane accretion events, and orogen‐parallel crustal flow in the Acadian orogenic plateau. We apply a Bayesian inversion method to obtain quantitative constraints on the direction and strength of intra‐crustal anisotropy beneath the Hartford Basin. In the upper mantle, we identify a fossil shear zone possibly formed during oblique subduction of Rheic Ocean lithosphere. We also find evidence for a plate motion‐parallel flow zone in the asthenosphere that is likely disturbed by mantle upwelling near the southern margin of the Northern Appalachian Anomaly in the eastern part of the study area.

The crustal thickness and average Vp/Vs ratio are basic parameters to understand the current state and the tectonic processes in the Tibetan Plateau (TP). We gather receiver functions in the central‐southern TP and extract spatial variation of crustal thickness and Vp/Vs ratio using the H‐κ‐c method with careful quality control and stability analysis. Our results highlight the regions with high Vp/Vs ratios in the central‐southern Qiangtang block and two isolated patches around the Yarlung‐Zangbo suture, suggesting crustal partial melting. The disconnected high‐Vp/Vs patches imply a restrained pattern for present‐day crustal flow in the southern TP. Further correlation with mantle observations suggests that the high Vp/Vs region in the north can be attributed to mantle upwelling after lithospheric delamination, and the two isolated patches in the south may be related to the tearing of the Indian mantle lithosphere. Plain Language Summary: The Tibetan Plateau (TP), as the Third Pole, is known for its unusual characteristics and hot debate on how it uplifts and grows. To better understand this, we collect available receiver function data in the central‐southern part of the TP, and use an improved method to study the crustal thickness and the average Vp/Vs ratio. The crustal Vp/Vs ratio is a good indicator for partial melting in parts of the crust. In our results, the crust in the central‐southern Qiangtang block and two isolated regions near the Yarlung‐Zangbo suture has high Vp/Vs ratio, indicating the crust in these areas may have been partially melted. A partially melted crust is favorable for crustal flow to develop, and the crustal flow is considered to be an important mechanism of plateau growth. However, the high‐Vp/Vs patches in the southern TP are too disconnected to develop large‐scale crustal flow. By comparing with mantle observations, we think the crustal parting melting is related to mantle upwelling and heating, caused by lithosphere detachment in the northern TP and Indian slab tearing in the south, respectively. Key Points: H‐κ‐c results reveal two isolated patches with high crustal Vp/Vs in the southern Tibetan Plateau (TP), suggesting crustal partial meltingThe disconnection between the two patches does not support present‐day eastward crustal flow in the southern TPCorrelation with mantle observations indicates the two patches may be related to tearing of the Indian mantle lithosphere [ABSTRACT FROM AUTHOR]

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We provide a comprehensive image of the mantle transition zone (MTZ) beneath northeastern China by performing Variable Bin Radius Stacking of receiver functions. A massive seismic data set consisting of over 133 000 receiver functions recorded by 1208 broad-band stations is processed. Our results reveal fine-scale topography on the 410- and 660-km discontinuities defining the upper and lower bounds of the MTZ, lateral variations in the MTZ thickness and slab interfaces within the MTZ. In particular, unambiguous images of the slab interfaces provide direct evidence for the presence of the stagnant Pacific subducting slab below the eastern portion of the study area. A widespread deepening of the 410-km discontinuity is consistent with a hot and wet low-velocity upper mantle resulting from dehydration of the stagnant slab. Prominent depressions are evident in the depth to the 660-km discontinuity, with a thickened MTZ associated with the cold stagnating slab. Localized uplifts of the 660-km discontinuity are possibly caused by partial melt under the slab. These features attest to the influence of the Pacific plate on the MTZ. Additionally, a pronounced upwarp on the 660-km interface with a thin MTZ agrees with a previously hypothesized mantle upwelling through a slab window, possibly triggered by the sinking of the stagnant slab. Moreover, the western part of the study region is characterized by alternating ups and downs of the 410-km interface, while the topography of the 660-km discontinuity is relatively flat. We propose the western region is dominated by foundering of delaminated lithospheric blocks that induced upward mantle return flows upon entrance into the MTZ. [ABSTRACT FROM AUTHOR]

We constrained sedimentary basin structure using a nodal seismic array consisting of ten dense lines that overlie multiple basins in the northern Los Angeles area. The dense array consists of 758 seismic nodes, spaced ~250–300 m apart along linear transects, that recorded ground motions for 30–35 days. We applied the receiver function (RF) technique to 16 teleseismic events to investigate basin structure. Primary basin-converted phases were identified in the RFs. A shear wave velocity model produced in a separate study using the same dataset was incorporated to convert the basin time arrivals to depth. The deepest part of the San Bernardino basin was identified near the Loma Linda fault at a depth of 2.4 km. Basin depths identified at pierce points for separate events reveal lateral changes in basin depth across distances of ~2–3 km near individual stations. A significant change in basin depth was identified within a small distance of ~4 km near the San Jacinto fault. The San Gabriel basin exhibited the largest basin depths of all three basins, with a maximum depth of 4.2 km. The high lateral resolution from the dense array helped to reveal more continuous structures and reduce uncertainties in the RFs interpretation. We discovered a more complex basin structure than previously identified. Our findings show that the basins' core areas are not the deepest, and significant changes in basin depth were observed near some faults, including the San Jacinto fault, Fontana fault, Red Hill fault and Indian Hill fault. [ABSTRACT FROM AUTHOR]

The Hawaiian Islands represent the youngest portion of the long‐lived Hawaiian‐Emperor Seamount chain sourced from hotspot volcanism on the northwesterly moving Pacific plate. Despite being one of the best studied hotspots on Earth, our understanding of the lithospheric discontinuity structure of Hawaiʻi is inhibited by a lack of a long‐term distributed broadband seismic network and lithospheric‐scale imaging of other oceanic hotspots. In this study, we analyze 3,530 teleseismic waveforms from 1,130 events recorded at 47 stations to compute P‐wave radial receiver functions. We then incorporate an elevation correction to adaptive common conversion point stacking to create a 3D discontinuity volume of the lithospheric structure associated with an ocean island hotspot. We find a positive amplitude conversion consistently at ∼12 km below sea level, likely representing the crust‐mantle transition below much of the island. However, this conversion occurs at only ∼6 km below Kīlauea, where local tomography images a zone of high P‐wave velocities (>7 km/s) within the crust and deformation of olivine phenocrysts within a magmatic reservoir is thought to occur. At this location, seismicity also extends vertically to 35 km depth before merging with an interpreted sill complex at ∼40 km. We interpret this anomalously shallow high amplitude discontinuity as an olivine‐dominated magmatic reservoir with an estimated maximum 5% melt over the resolution of our data at crustal depths. This represents the shallow seismic signature of a much more extensive magma system extending to depth. These images provide a better understanding of the magmatic structure associated with oceanic hotspot volcanoes. Plain Language Summary: The Hawaiian Islands are the product of hotspot volcanism on the northwesterly moving Pacific plate. Our understanding on the formation of Hawaiʻi and other oceanic hotspot volcanoes is reliant on our ability to discern the process of magma formation and transportation through the mantle and up to the volcanic caldera. We have a thorough understanding of broad mantle characteristics and crustal structure beneath Hawaiʻi but lack any relatively higher resolved lithospheric seismic image given the instrumentation and data that are available. In this study, we derive a 3‐D discontinuity volume beneath Hawaiʻi. We find a discontinuity consistently at ∼12 km below sea level which we interpret to be the Moho. Beneath Kīlauea, the most active caldera on Hawaiʻi, this discontinuity shallows to ∼6 km and is unlikely to represent the Moho. These depths are consistent with high (mantle‐like) seismic velocities found in previous seismic studies. Seismicity surrounds this shallow discontinuity and extends down to ∼35 km depth possibly connecting to another magma reservoir. We interpret the shallow discontinuity beneath Kīlauea as ultramafic material formed within the crust by extensive differentiation in a low‐melt magma reservoir. These images provide a better understanding of magmatic structure associated with oceanic hotspot volcanoes. Key Points: Elevation‐corrected P‐to‐S receiver functions are used to create a 3‐D discontinuity model beneath the main Hawaiian islandA high‐amplitude positive arrival is imaged ∼6 km BSL at Kīlauea spatially coinciding with seismicity extending vertically to ∼35 km depthThis shallow arrival represents the uppermost portion of an olivine‐dominated magmatic reservoir with up to ∼5% melt [ABSTRACT FROM AUTHOR]

The southwestern part of the Sichuan Basin (SW-SCB) is adjacent to the eastern Himalayan syntaxis. Affected by the Indo-Eurasian collision and subsequent intrusion of the Indian plate into the Eurasian plate during the Cenozoic, this region is ideal for the study of the tectonic evolution of the intracontinental lithosphere and the dynamic processes of deep and shallow structures. In this study, we applied the receiver function technique to the data obtained from a recently deployed high-density broadband seismic array across the Sichuan Basin and Sichuan-Yunnan block (SCB-SYB). We conducted a multi-frequency and multi-model correction analysis to image the structure of the mantle transition zone beneath this region. The results showed the 660-km discontinuity gradually increasing in depth by 10–30 km beneath the western side of the Anninghe-Xiaojiang Fault, suggesting the presence of thermal anomalies caused by the subducted Indian plate from west to east. At the junction of the SCB-SYB, the 410-km discontinuity exhibited a slight uplift of 5–10 km, while the 660-km discontinuity showed a significant depression of ~30 km over a lateral range of ~150 km. Based on previous surface GPS observation and dynamic numerical simulation studies, we suggest that the sharp lateral small-scale topography of this 660-km discontinuity beneath the SW-SCB may have resulted from dripping delamination of the lithosphere within the strain localization area. Furthermore, the aggregation of delaminated lithospheric material at the base of the 660-km discontinuity determines the regional topography of mantle transition zone discontinuities. In this study, we provided seismological evidence for the challenging detection of small-scale intracontinental lithosphere dripping delamination. Moreover, it provides a new view for studying deep and shallow dynamic processes in intracontinental regions with stress concentration resulting from plate/continental subduction and collision. [ABSTRACT FROM AUTHOR]

Machine learning methods were applied to reconsider the results of several passive seismic experiments in Finland. We created datasets from different stages of the receiver function technique and processed them with one of the basic machine learning algorithms. All the results were obtained uniformly with the k-nearest neighbors algorithm. The first result is the Moho depth map of the region. Another result is the delineation of the near-surface low S-wave velocity layer. There are three such areas in the Northern, Southern, and Central parts of the region. The low S-wave velocity in the Northern and Southern areas can be linked to the geological structure. However, we attribute the central low S-wave velocity area to a large number of water-saturated cracks in the upper 1–5 km of the crust. Analysis of the structure of this area leads us to the conclusion that macrofracturing was caused by the last deglaciation. [ABSTRACT FROM AUTHOR]

Near-surface seismic velocity structure plays a critical role in ground motion amplification during large earthquakes. In particular, the local Vp/Vs ratio strongly influences the amplitude of Rayleigh waves. Previous studies have separately imaged 3D seismic velocity and Vp/Vs ratio at seismogenic depth, but lack regional coverage and/or fail to constrain the shallowest structure. Here, we combine three datasets with complementary sensitivity in a Bayesian joint inversion for shallow crustal shear velocity and near-surface Vp/Vs ratio across Southern California. Receiver functions–including with an apparent delayed initial peak in sedimentary basins, and long considered a nuisance in receiver function imaging studies–highly correlate with short-period Rayleigh wave ellipticity measurements and require the inclusion of a Vp/Vs parameter. The updated model includes near-surface low shear velocity more in line with geotechnical layer estimates, and generally lower than expected Vp/Vs outside the basins suggesting widespread shallow fracturing and/or groundwater undersaturation.

Abstract: The Afar region is a tectonically distinct area useful for studying continental break-up and rifting. Various conflicting models have been suggested to explain the lateral variations of the anisotropy in this region. To address this issue, we investigated the tectonics of the Afar region using receiver function and shear-wave splitting measurements based on broadband seismic data from 227 stations in the region. Further, the receiver function results were inverted to obtain the crustal thickness and Vp/Vs ratio of the region. Our results reveal a thick African crust (thicker than 40 km) with typical Vp/Vs values for the continental crust, elongated down to 21 km along the rift system with very high Vp/Vs values near the fractured zones, suggesting crustal thinning near the fractured zones. Our shear-wave splitting measurements indicate a general fast axis orientation of N030E. However, substantial disparities in the fast anisotropy direction exist in the triple junction region, with some stations displaying a direction of N120E, which is perpendicular to the fast directions measured at the surrounding stations. In addition, many stations located close to the rifts and within the Arabian Plate provide mostly null measurements, indicating the presence of fluids or isotropic media. This study uses several methodologies to unravel the structure and evolution of the Afar region, providing valuable insight into the Afar, a tectonically distinct region, which will be useful for elucidating the mechanisms and characteristics of a continental break-up and the rifting process.

Abstract The crustal thickness and average Vp/Vs ratio are basic parameters to understand the current state and the tectonic processes in the Tibetan Plateau (TP). We gather receiver functions in the central‐southern TP and extract spatial variation of crustal thickness and Vp/Vs ratio using the H‐κ‐c method with careful quality control and stability analysis. Our results highlight the regions with high Vp/Vs ratios in the central‐southern Qiangtang block and two isolated patches around the Yarlung‐Zangbo suture, suggesting crustal partial melting. The disconnected high‐Vp/Vs patches imply a restrained pattern for present‐day crustal flow in the southern TP. Further correlation with mantle observations suggests that the high Vp/Vs region in the north can be attributed to mantle upwelling after lithospheric delamination, and the two isolated patches in the south may be related to the tearing of the Indian mantle lithosphere.

Abstract Alaska is located at the northernmost point of the interface between the Pacific plate and the North American continent. The subduction of the Pacific plate generates arc volcanoes along the Aleutian trench, which stops to the east at the Denali Volcanic Gap. This volcanic gap has been linked to the underthrusting of the Yakutat terrane, which might alter the thermal state of the mantle wedge and prevent melt formation. This implies that the limits of the volcanic activity should mirror the extent of the Yakutat subduction. However, the transition from the Pacific slab to the Yakutat terrane at depth is not fully understood. To investigate this issue, we processed a new composite seismic data set from six arrays deployed in the region from 2000 to 2018. We apply a multi‐mode 3D Kirchhoff migration to obtain high‐resolution 3D scattering images of the region. Our results highlight a sharp lateral boundary in the slab structure, with a 10 km Moho step, just offshore Anchorage, and a more gradual slab transition beneath the southern part of the Kenai peninsula. Our images from the Yakutat slab plunge down to 150 km depth are consistent with previous estimates of the Yakutat slab extent below the Alaska Range. Although the steeply dipping boundaries of the subducting Pacific lithospheres are not fully recovered, deep coherent signals from the Pacific slab are observed down to 150 km depth. These observations suggest that the crust is still partially uneclogitized at these depths in both slabs.

The collision between the Indian and Eurasian plates continues to drive significant deformation and uplift within the interior of the Tibetan Plateau, together with its outward expansion along the margins. In particular, the North Qilian Shan fold-thrust belt (NQLS) and the Hexi Corridor basins (HXBS) represent the northernmost region of the northeastern Tibetan Plateau. This area serves as a natural laboratory for deciphering mechanisms of crustal deformation and thickening along the plateau’s margins. Specifically, the northeastern Tibetan Plateau has been attributed to 1) southward underthrusting of the Asian lithosphere, 2) distributed shortening and crustal thickening, 3) vertical inflation of the Tibetan crust due to mid-lower crustal channel flow, and 4) intracontinental subduction facilitated by large-scale strike–slip faults. The exact mechanism underlying the most concentrated convergent stress in the western segment of NQLS–HXBS remains a subject of debate. To address this uncertainty, we gathered seismic data along a 130-km-long linear array that extends northward from NQLS, traversing the Jiuquan Basin and reaching the Huahai Basin. Our analysis, conducted through the receiver function method, reveals intriguing findings. The Moho depth deepens from 45–50 km beneath the Huahai Basin to 55–60 km beneath NQLS. Notably, a double Moho structure emerged, marked by a distinctive near-flat positive amplitude at a depth of 45–50 km beneath NQLS within a distance of 0–50 km. Our study presents a comprehensive analysis of the crust-scale deformation mechanism, shedding light on the following key aspects: 1) the development of a decollement at 12–20 km depth decoupling the upper and lower crust; 2) deformation of the upper crust occurring through south-dipping brittle thrust faults, while the lower crust features imbricate structures and duplexes; 3) evidence pointing to the underthrusting of the Beishan Block beneath NQLS, indicated by the double Moho beneath NQLS; and 4) the formation of a Moho ramp beneath the Jiuquan Basin, facilitating the transfer of shortening stress from beneath NQLS and HXBS to the north. In the context of the western segment of NQLS and HXBS, our speculation is that coupled distributed shortening and Beishan Block subduction beneath NQLS work in tandem to accommodate crustal deformation.

The north-south striking landform perpendicular to the dominant collision zone between the Indian and Eurasian plates was constructed in east-southern Tibet (EST) during the Late Miocene. The building processes remain ambiguous, partly owing to the lack of recognition of crustal-scale architecture. Here we deployed an east-west-oriented 120 km-long short-period dense array across the northwestern corner of EST. Results from P-wave receiver functions show a mid-crustal ductile detachment atop the crystalline basement of the eastern Lhasa terrane that has displaced the overlying crust including the upper part of the Yardong-Gulu Rift (YGR) eastward for about 100 km and an offset Moho geometry left behind beneath the Nyainqentanghla Range. Combined previous studies on the vertical heterogeneity in crustal strength beneath the YGR, a synthetic view implies a decoupled crustal architecture of EST to respond the dynamic interactions between the eastward extrusion of the Tibetan Plateau and the northward penetration of the Indian plate that constitutes the Eastern Himalayan Syntaxis. The integrated processes eventually brought active landform construction in the plateau’s interior and partly contributing to the contemporaneous intensification of monsoon in SE Asia.

In order to determine the thickness of the crust, this study offers the coherently stacked P-wave receiver functions and Bouguer anomaly mapping of the Western Himalayas (longitude 71–74E and latitude 31–34N). Gravity data is taken from Topex, which is in the public domain and is open for research, whereas the information utilized for the P-wave receiver function is taken from the local seismic network of Pakistan. By coherently stacking the Ps, PpPs, PpSs + PsPs phases of 15 seismic stations, it is possible to determine the crust thickness and average crust Vp/Vs ratio at each station in the network. The events selected for this study were those with magnitudes ≥ 6 and epicentral distances of 300 to 950. Data were gathered from 2012 to 2019. The Moho depth beneath the broadband seismic stations employed for the study varies significantly. Moho depths range from 36 km in the south to an average of 46 km in the middle and 52 km in the study area's northernmost seismic station. In the research area, the crust generally dips from south to north. Residual calculation of Bouguer anomaly data was also performed in order to validate this inferred argument of moho-depth variation from the P-Wave receiver function. These gravity residuals from the investigated area's anomalous data ranging from − 89 to 193 mGal have shown a promising association and supported the theory of crustal dipping put forth by the P-wave function from the seismic network. The tendency supports the theory that the collision of the Indian and Asian plates is what is causing the thickening and shortening of the crust. [ABSTRACT FROM AUTHOR]

The Mesozoic tectonic framework of the eastern South China is mainly controlled by subduction, turning toward, and rollback of the Pacific Plate. Recent studies of receiver function imaging and ambient noise tomography have revealed the "Y-shaped" thinnest crustal belt in the eastern South China under the overall extension of the lithosphere. However, the deep dynamic environment and formation mechanisms of the thin crustal belt remain debatable. Here we obtained high-resolution images of the crustal thickness and Poisson's ratio in the eastern South China Block applying the recently proposed H-κ-c receiver function method, using data recorded by 305 dense portable broadband stations and 219 permanent stations surrounding. Additionally, we discussed the deep dynamic formation mechanism of the "Y-shaped" thinnest crustal belt coupled with two common conversion point stacked images at key locations. Results show that the average crustal thickness of the study area is 33 km (thin crust) and the average Poisson's ratio is 0.24 (low ratio). The overall crustal thinning toward the continental margin is likely because eastern South China was in a back-arc extension environment, which was induced by the rollback of the subducted plate in the Early Cretaceous. The crustal thickness of the "Y-shaped" thinnest crustal belt is <30 km, which is 3–5 km thinner than that outside the zone. The eastern branch is distributed along the trajectory of Nanchang-Ji'an-Ganzhou-Shaoguan-Guangzhou, and the western branch is around the Jianghan-Xiangzhong Basin, both of which intersect in Nanling. The eastern branch of the thin crustal zone indicates the potential location of the Pacific subduction slab breakoff, and the formation mechanism may be related to the interaction of deep-shallow processes, including the upwelling of mantle heat flow through the slab window and transtensional pre-existing faults. We developed a dynamic model that combines subduction-breakoff-rollback processes of the Paleo-Pacific Plate and accompanying deep fluid upwelling to explain the regional extension of the South China lithosphere, the formation mechanism of the thinnest crustal belt, and the distribution of granitic plutons. [ABSTRACT FROM AUTHOR]

The Himalayan region witnesses several natural hazards like earthquakes and landslides due to the continental collisions between the Indian and Eurasian plates. This has given rise to extreme topographic variations throughout the Himalayan belt. The Kumaun–Garhwal region is a classic example of such geological consequences and is prone to several earthquakes. High-quality three-component teleseismic waveform data recorded at seven seismological stations operated by the Wadia Institute of Himalayan Geology (WIHG) are used to investigate the detailed subsurface structure of the crust, the intra-crustal low-velocity layer (LVL), and the upper mantle discontinuities beneath the Kumaun–Garhwal, north-west Himalaya. The results, derived from the inversion of individual station's stacked P-receiver functions (PRFs) using the neighbourhood algorithm approach, show that the crustal thickness varies from 44 to 54 km beneath the study region. The depth of LVL observed beneath six stations from individual and stacked PRFs, varies from 9 to 24 km. The LVL zone with a high Vp/Vs ratio may be due to fluid or partial melt, leading to shallow seismic activity within the study region. The presence of fluid or partial melts in the LVL may be due to the shear heating, cooling, and decompression. The 2D PRF migration image depicts a thick mantle transition zone due to the elevated 410 km discontinuity with respect to the global average values predicted by the IASP91 velocity model. The present research suggests that this might be due to the colder transition zone in this region, indicating the cool underthrust Indian plate with respect to the ambient mantle has reached down to the upper mantle transition zone. [ABSTRACT FROM AUTHOR]