Sulawesi and Borneo are tectonically complex islands with multistage subduction histories stretching back through the Cenozoic. Seismic studies have played an important role in helping to unravel this history, with spatial distributions of earthquakes tracking actively subducting slabs. In contrast, old or relict aseismic slabs are illuminated not by earthquakes within the slab, but rather by seismic waves that transmit through the slab and reveal them as regions of anomalously high velocity in the mantle via tomographic imaging. We use the relative arrival times of teleseismic body-waves from 1359 unique teleseismic earthquakes between 2019 and 2023, recorded by a network of 134 land and ocean bottom seismometers in the study region (corresponding to 72760 relative arrival times), to tomographically image the P wave velocity structure beneath Sulawesi and the Makassar Strait, which lies between eastern Borneo and Sulawesi. The resulting velocity model robustly uncovers a tabular high velocity anomaly at depths between ~300 and 700 km dipping NW beneath the Makassar Strait. By comparing tomographic models to plate tectonic reconstructions, we connect this anomaly to the hypothesised NW-Celebes slab, which actively subducted beneath northern Borneo between ~21 and 9 Ma. Geochemical evidence from contemporaneous volcanics suggests that slab break-off and the cessation of subduction may have been caused by the subduction of the Celebes mid ocean ridge. Comparison of our findings to pre-existing geodynamic modelling suggests that sinking of the broken-off NW Celebes slab may have helped to initiate S Celebes subduction via subduction polarity reversal.

The North Sulawesi Subduction Zone is one of the youngest active subduction systems in the western Pacific. In western Sulawesi, the Palu-Koro strike-slip fault connects with the westward-extending North Sulawesi Trench, forming a distinctive subduction-transform fault system. Understanding the crustal structure beneath the Celebes Sea and the geometry of the Palu-Koro fault is crucial for assessing regional deformation, rupture dynamics, and seismic hazards. In this study, we analyse data from nine ocean bottom seismometers (OBSs) deployed across the Palu-Koro fault using the receiver function H-kappa stacking method to estimate crustal thickness. Our results reveal a shallow Moho (similar to 8 km depth) beneath the Celebes Sea, in contrast to significantly greater depths (similar to 25 km) beneath eastern Kalimantan and northern Sulawesi. Sharp variations in Moho depth near the Palu-Koro fault suggest the juxtaposition of two distinct crustal blocks. Combining S-wave velocity structures and local seismicity catalogue, we infer that the Palu-Koro fault is a left-lateral, through-going strike-slip fault extending into the Celebes Sea. These findings provide new geophysical constraints on the interplay between strike-slip faulting and subduction retreat, with implications for the generation of tsunamis by submarine earthquakes in this tectonically complex region.

The seabed S-wave velocity structure is a critical parameter in offshore geotechnical engineering and marine geophysical research. However, its direct acquisition remains challenging due to limitations such as high cost and difficulty of exciting S-wave on the seabed. While surface wave dispersion imaging has proven to be an effective tool for obtaining high-resolution S-wave velocity models of shallow marine sediments, conventionalland-based acquisition methods are difficult to apply directly in offshore environments. To address this, weemployed a large-volume air-gun source to generate interface waves, namely Scholte waves, which propagatealong the seabed interface and can be recorded by ocean bottom seismometers (OBS). This study presents acomplete workflow and methodology for inverting S-wave velocity structure using multi-mode Scholte wavedispersion curves derived from such active-source OBS data. By selecting common receiver gathers withdifferent offset ranges for dispersion energy spectrum imaging, the strong lateral averaging effect of Scholtewave dispersion energy spectrum is effectively mitigated, particularly in areas with significant lateral velocitystructure variations. Furthermore, stacking the multi-component dispersion energy spectra from OBS enabled theextraction of the higher mode dispersion curve of Scholte waves. Applying this method, we obtained the firsthigh-resolution S-wave velocity profile of the shallow marine sediment layer within 0.4 km in the Jinjiangoffshore area of the Taiwan Strait region. The inverted S-wave velocity structure shows a high degree ofconsistency with a collocated multi-channel seismic (MCS) profile. Additionally, by integrating with the MCSprofile, an important fault zone in the Taiwan Strait was identified, and its structural characteristics within theshallow marine sediment layer were investigated.

Ocean Bottom Seismometers (OBSs) are new high-tech instruments which are urgently demanded in marine geophysical survey. And the sea tests, applications in deep seismic survey of these independent research and development instruments rely highly upon a suitable research vessel and the specialty cruise. In the first decade of the 21st century, the Chinese OBS became successful; in the same time, the Shiptime Sharing Project was initiated and implemented by National Natural Science Foundation of China (NSFC). The two developing processes met at the right time, which were originally independent although. These two events together promoted the rapid progress of OBS survey in South China Sea, and pushed the "shiptime-instrument-data-achievement" sharing cooperative schema. In the geophysical sharing cruises of South China Sea during continuous 15 years (2010-2024), totally near 760 set-times of Chinese OBS were deployed and the average recovery rate reached as high as 95%. The shooting lines of large volume airguns were summed up to about 18 thousand kilometers, and mass data were acquired and accumulated. Based on these data, onboard scientists have published a few dozens of high-level scientific papers, which greatly pushed forward the deep crustal structure studies of continental margins and oceanic basins in South China Sea. Although the instrument performance fluctuated little before 2018, but tended to stable and mature overall. During 2010-2018, active OBS survey of the sharing cruises was only distributed in the northern and central parts of South China Sea, north of latitude 14 degrees N. The survey areas covered tectonic units such as northeast passive margin and subduction zone, northwest marginal rifting and separating blocks, as well as Extinct Spreading Ridge (ESR) and ZhongNan Fault (ZNF) in the central basin. The related researches have achieved important scientific understandings. This paper systematically reviews development process of the historic segment, summarizes cruise experiences and paper highlights, and looks forward to the future.

The Challenger Deep, located within the southern Mariana Trench, represents the deepest point in the global ocean and is characterized by an ancient subducting slab with a steep dip angle and intense basal deformation. However, the seismic anisotropy and its underlying formation mechanisms remain poorly constrained. To address this concern, we conducted SK(K)S wave splitting analyses using ocean bottom seismometer data to investigate anisotropic characteristics and tectonic implications in the Southern Mariana Subduction Zone for the first time. Results show that near the trench, the fast-wave polarization directions are subparallel to the trench axis and exhibit a gradual transition in splitting patterns. With increasing distance from the trench, the fast directions systematically rotate and become subperpendicular to the trench orientations. Observed delay times range from 0.8 to 2.8 s, with an average value of 1.6 s, suggesting the presence of multiple sources of seismic anisotropic in the region. These observations suggest strong mantle anisotropy beneath the Challenger Deep, indicative of trench-parallel mantle flow and significant deformation. Oblique anisotropy likely reflects a transition in fast-polarization orientations from shallow low-angle to deep high-angle subduction regimes. In contrast, trench-perpendicular anisotropy is likely dominated by serpentinized mantle layers. The relatively large delay times provide direct evidence for enhanced plate flexural fracturing and hydration. Overall, this study improves our understanding of subduction dynamics and water-material cycling in the southern Mariana Trench, highlighting the complex interplay of mantle flow, slab deformation, and fluid-mediated processes.

Seamount subduction plays a pivotal role in shaping subduction zone dynamics, significantly influencing deformation processes and seismicity. This study examines the crustal and upper mantle deformation associated with seamount subduction beneath northern Luzon, where the South China Sea Plate underthrusts the region. We employed local S-wave splitting techniques to characterize the deformation and present seismological evidence of seamount subduction’s role in modulating subduction dynamics.Our findings reveal a dominant trench-normal fast-axis orientation, aligned with the P-axis from crustal earthquake focal mechanisms, across most forearc stations. This pattern differs from the trench-parallel fast-axis commonly observed in other forearc settings such as northeastern Japan, Cascadia, and Sumatra. The frequency-dependent delay times and trench-normal fast-axis orientation suggest seismic anisotropy associated with fluid-filled cracks aligned with the prevailing stress field, influenced by the seamount subduction.Notably, delay times increase with focal depth, highlighting that the effects of seamount subduction extend from the overriding crust into the subducting slab. These results offer direct seismological evidence of seamount subduction shaping subduction zone dynamics, promoting aseismic creep and small earthquakes through fracture network formation. This study enhances to the understanding of the complex interactions within subduction zones and underscores the importance of seamount subduction in these processes.

The Scholte wave dispersion curves can be used to invert the shear wave velocity structure of shallow marine sedimentary layer.The competitive swarm optimizer algorithm is a kind of improved algorithm of the particle swarm optimization algorithm,which has advantages in convergence speed,memory space saving,etc.To solve the three major problems of convergence difficulty,unstable results,and difficulty in inverting layer thickness in the multimodal Scholte wave dispersion curves inversion,this paper introduce the competitive swarm optimizer algorithm to achieve the joint inversion of seabed multimodal Scholte wave dispersion curves.Besides,the introduction of Latin hypercube sampling makes the particle initialization more uniform;The model selection is optimized and forward computation workload is reduced through the particle two-by-two competition mechanism.Three typical theoretical models of marine sedimentary layers are designed,and the algorithm is tested using the theoretical five-mode Scholte wave dispersion curves,and the results show that the inversion method can be converged effectively and robustly.The inversion method is validated using multimodal dispersion curves extracted from Scholte wave data excited by an air gun seismic source and recorded by the ocean bottom seismometers.The inversion results are consistent with the expected level,further verifying the practicality of the proposed method.Both theoretical models and the measured data tests show that the competitive swarm optimizer algorithm can be applied to Scholte wave multimodal dispersion curves inversion,which can invert the layer thickness and shear wave velocity simultaneously.It has good application potential and practical value in the seabed Scholte wave survey method.

Tectonic tremors (TTs), composed of a swarm of low-frequency earthquakes (LFEs), constitute a type of slow earthquakes characterized by a lack of high-frequency energy. Previous studies have suggested that slow earthquakes, which usually occur near the megathrust earthquake rupture zones, are crucial for deepening our understanding of seismic activity. The Northern Sulawesi subduction zone (NSSZ) is situated at the convergence of the Eurasian, Australian, and Philippine Sea plates, experiencing frequent earthquakes that may trigger local tsunamis due to the complex tectonic setting. Until now, the lack of shallow observations has limited the understanding of the shallow tectonic structure beneath the NSSZ. We observe episodic shallow TTs using 8 Ocean Bottom Seismometers (OBS) deployed near the NSSZ, indicating the presence of stable sliding near the subduction boundary. Our research results reveal that the locations of shallow TTs align with the boundary of the weakly coupled plate interface where the relative Coulomb stress is weaker. Additionally, we discover that the sedimentary environment in the shallow subduction zone and the dehydration of serpentinite within the plates provide favorable conditions for high pore fluid pressure, thereby promoting the occurrence of shallow TTs. Furthermore, we try to establish a connection between deep earthquakes and shallow TTs, exploring the possibility of deep-seated stress propagating from the deep crust to the shallow seismic zone through faults or plate boundaries.

Seamount subduction influences subduction zone dynamics by altering stress fields, fracture patterns, and seismic anisotropy. This study utilizes local S ‐wave splitting analysis to investigate crustal and upper mantle deformation associated with seamount subduction beneath northern Luzon. Our observations reveal predominantly trench‐normal fast‐axis orientations and frequency‐dependent delay times. These patterns suggest that anisotropy arises primarily from fluid‐filled cracks and possible serpentinization, with effects extending from the overriding crust into the subducting slab, spanning multiple structural depths. In contrast, trench‐parallel directions in southern non‐seamount subduction regions indicate either ductile overriding lithosphere deformation or toroidal mantle flow around the slab edge. Event depth and raypath geometry further indicate that seamount subduction promotes stress heterogeneity and vertical anisotropic layering. These findings demonstrate that subducting features such as seamounts produce distinct anisotropy signatures, offering new insights into subduction dynamics and lithospheric deformation.

The northern region of the Indochina Peninsula is adjacent to the southeastern margin of the Tibetan Plateau and has historically undergone complex tectonic deformation processes such as the closure of the Old and New Tethys Oceans,especially the southeastward material extrusion from the uplift of the Tibetan Plateau since the Cenozoic,which has further influenced the tectonic deformation in the region.We obtained 13 stable transform wave double-layer anisotropy results by calculating the receiver functions of 140 collected mobile seismic stations.The results show that the upper and lower crust may be decoupled near the Simao block in southern Yunnan and the Dien Bien Phu Fault Zone in Vietnam,and the anisotropy of the upper crust is mainly influenced by the strike-slip faults and the directional alignment of minerals in the region,while the anisotropy of the lower crust may be related to the crustal material flow;the Dien Bien Phu Fault Zone in the Vietnamese section only cuts through the upper crust,and the lower crustal material flow may be extruded through the Dien Bien Phu Fault Zone into the northwestern region of Vietnam,but not through to the South China Sea.

The North Sulawesi Subduction is formed by the Celebes Sea Basin subducting beneath the northern branch of Sulawesi Island.Since the 20th century,it has experienced several large earthquakes,accompanied by frequent small earthquakes,making it a critical area for studying seismicity.In this study,we utilized continuous waveform data recorded by 6 ocean-bottom seismometers deployed around the North Sulawesi Trench and applied a Bayesian inversion method based on envelope cross-correlation to investigate slow earthquakes.We identified 39 shallow tectonic tremors in the central part of the North Sulawesi Subduction.The distribution of these tremors is closely related to the sedimentary structure of the Celebes Sea Basin.The tremors concentrate on the surface of the subducting slab at depths shallower than 20 km,with a clear boundary separating them from deeper regular earthquakes.The central part of the North Sulawesi Subduction where tremors frequently occurred not only continuously releases stress,but also hinders the propagation of surrounding earthquake ruptures,which account for the observed low seismicity in this region.

The landing point of the Ocean Bottom Seismometer(OBS)may drift away from its deploying position due to ocean currents or others during its descent.This will seriously affect the accuracy of subsequent data processing(such as seismic phase fitting),so the OBS landing point needs to be relocated.Currently,active-source OBS relocations primarily use the inversion method based on the fitting of arrival times of direct waves from air gun sources.However,these methods involve substantial manual effort in picking direct wave arrival times and are susceptible to subjective errors.Additionally,the inversion accuracy is affected by the selection of the mean seawater wave velocity.Therefore,this paper presents an innovative OBS relocation method based on the arrival time differences of direct waves,which employs an algorithm of modified maximum likelihood with coherence to quickly calculate the arrival time differences between any two direct waves and combines the Monte Carlo with least squares algorithm to iteratively optimize the arrival time difference objective function,with the final convergence point representing the relocation position of the OBS.Using this method,the landing positions of seven OBSs with a depth over ten kilometers deployed on landers in the Mariana Trench were relocated and the relocation results indicate that the objective function converges stably and the relocation results are reliable.The advantages of this method are mainly reflected in:First,it utilizes cross-correlation to compute arrival time differences,thereby eliminating subjective errors associated with manual picking of arrival times and enhancing efficiency in batch data processing.Second,this method exhibits low dependence on the sea water velocity,resulting in high robustness of the inversion process.The method will play a broader role in deep structure detection by OBS.

The South Yellow Sea and its environs are pivotal for unraveling the complexities of crustal dynamics and continental collision processes. A holistic assessment of deep structural variations from northern China to the Korean Peninsula is essential for a comprehensive and accurate determination of the tectonic affinity of the Korean Peninsula.Thus, we deployed a pioneering active-source seismic profile (Line2016) spanning the South Yellow Sea and the eastern onshore region of the Korean Peninsula, provides crucial insights into the collision dynamics between the Sino-Korean Block and the Yangtze Block. Our innovative approach, incorporating forward modeling, tomography, and finite-difference wavefield modeling, yielded a high-resolution crustal P-wave velocity model, addressing a significant knowledge gap in understanding the geological intricacies between northern China and the Korean Peninsula. The results confirmed and precisely located the West Marginal Fault of the Korean Peninsula, a significant crustalscale tectonic structure, likely representing the eastern boundary between the Yangtze Block and the Sino-Korean Block. The study advocates for classifying the Korean Peninsula as part of the Sino-Korean Block, presenting evidence for the one-part affinity hypothesis. This collision resulted in the creation of two distinct suture zones-an orogenic belt in the northern part and a significant strike-slip fault zone in the eastern part of the South Yellow Sea. The study emphasizes the pivotal role of block morphology in regulating plate convergence, providing valuable insights for understanding similar phenomena in other collision zones. (c) 2024 Published by Elsevier B.V. on behalf of International Association for Gondwana Research.

The fine shallow velocity model of the Tangshan Fault Zone (TSFZ) is of paramount importance in understanding the intricate geological structure and seismogenesis in this area. In this study, a dense seismic array consisting of 110 short-period seismometers was deployed in the TSFZ for similar to 2 months. With this dataset, a 3-D shallow structure model deep to 4 km with the highest resolution up to now are obtained by using an ambient noise tomography, which can provide clear seismotectonic characteristics of the 1976 M-S 7.8 Tangshan earthquake sequence in the upper crust. The results show that the V-S model is influenced by sedimentary strata and characterized by a conspicuous high-velocity anomaly in the northern region and a contrasting low-velocity anomaly in the southern region of the TSFZ. The velocity structure correlates well with the geological structure, and a significant velocity contrast can be observed at depths greater than 1 km on both sides of the TSFZ. By combining previous research findings with our novel discoveries, we suggest that the distribution of low-velocity sediments in the TSFZ is not only consistent with the shallow fault system but also related to local transtensional effects. Earthquakes in the middle and upper crust are primarily concentrated near the high-resistivity and high-velocity body in the northeastern part of the Tangshan Fault. Under the influence of right-lateral strike-slip motion, this high-resistivity and high-velocity body will rupture when the accumulated stress reaches a critical threshold.

Geometry and frictional states of the megathrust in subduction zones play critical roles on the rupture extent, hence the magnitude, of great earthquakes. However, their details are often difficult to obtain, primarily due to limited offshore geophysical observations. Here we overcome this challenge by precisely determining source parameters (i.e., location and focal mechanism) of medium-sized earthquakes using global broadband seismic waveform data. The dip angles of 108 Mw 5.0+ plate interface events from 1990 to 2017 in the central Sumatran subduction zone are tightly constrained by waveform inversion, and their locations are refined with surface-wave relocation and depth phases modeling. Our results reveal compact trench parallel earthquake belts at the up-dip and down-dip boundaries of the ruptures of the Mentawai 2007 Mw 8.4 and Mw 7.9 earthquakes. The down-dip seismicity belt marks ∼8º bending of the plate interface, suggesting that the change of fault geometry may have played a role in halting the megathrust rupture along with thermal effects. The up-dip seismicity belt shows a clear spatial complementarity to the largest events’ slip distribution. This belt is quite narrow (∼20 km) for the Mw 8.4 rupture but much broader (∼50 km) for the Mw 7.9 event. The combination of the up-dip seismicity belt and slip distributions matches well with the positive residual bathymetry and gravity anomalies, suggesting that these events resulted from long-term coupling on the plate boundary. Our findings suggest the importance of considering both up-dip and down-dip seismicity belts, along with residual bathymetry and gravity data, in understanding megathrust rupture behaviors and assessing the associated hazards.

The North Sulawesi subduction zone is characterized by southward subduction of the Celebes Sea slab to a depth of similar to 250 km, mainly overlying the Sangihe slab that subducts west from the Molucca Sea and penetrates the mantle transition zone. The palaeo-subducted Sula slab dips northward and partially underlies both the Sangihe and Celebes Sea slabs. Adjacent subduction zones with horizontal overlapping subducting slabs in the upper mantle have unclear dynamic interactions. An extensive strike-slip fault forms the western boundary of the active North Sulawesi subduction zone, providing an ideal setting to study mantle flow between overlapping slabs. We use local S-wave and teleseismic S and SK(K)S waveform splitting analysis to measure seismic anisotropy in the northern Sulawesi region. Our observations reveal typical mantle wedge corner flow within the Sangihe subduction system. In the Gulf of Tomini, the observed trench-oblique fast-axis orientations above the Celebes Sea slab are likely a consequence of the interaction between two subducting slabs. The southernmost measurement with an E-W-trending fast direction in the mantle wedge might be related to the subduction of the Sula slab. Furthermore, fault-parallel fast-axis orientations of anisotropy near the southern segment of the Palu-Koro fault are attributed to large-scale shearing across this lithospheric-scale strike-slip fault system. Overall, our observations suggest that the strain caused by lithospheric and asthenospheric deformation is mainly confined within the microplate, displaying a restricted flow pattern and localized effects due to the size of the plate boundaries, such as the Palu-Koro fault. North Sulawesi is bordered by the Makassar Strait, Celebes Sea, Molucca Sea, and Gulf of Tomini and hosts the Palu-Koro fault. The region hosts two active subduction zones: the smaller, southward dipping North Sulawesi subduction zone and the larger westward dipping Sangihe subducting slab. Another fossilized submerged slab -Sula slab lies beneath both the Celebes Sea and Sangihe slabs to the south. Thus, it is an ideal area to investigate how the interaction between neighboring subduction zones can affect the upper-mantle deformation and whether a strike-slip fault could contribute to the mantle dynamics. We use shear-wave splitting analysis to measure seismic anisotropy to reveal the lithospheric deformation and upper mantle flow patterns of this interactive area. Our results show that the Celebes Sea slab controls mantle wedge dynamics in the North Sulawesi subduction zone, and the interaction between the Celebes Sea and Sangihe slabs occurs where two slabs entirely overlap. The strike-slip fault at the western boundary only appears to have a local influence on the regional strain field in western Sulawesi. We employ shear-wave splitting analysis to characterize the seismic anisotropy to infer upper-mantle deformation in north Sulawesi We find evidence for slab-slab dynamic interactions and constrain their influencing spatial range The Palu-Koro fault marks the west limit of lithospheric deformation imposed by the complex subduction system

The Molucca Sea area, situated on the southwestern side of the Philippine Sea in eastern Indonesia, is critical research area for the study of the ocean-continent coupling effect and subduction dynamic system of the western Pacific. Despite its significance as a research hotspot, several research gaps exist in this region. We aim to address two outstanding scientific issues: the cause of the Sangihe Forearc Thrust (SFT) in Molucca Sea, and the mechanism of volcanic discontinuity and migration of Halmahera arc. Numerical simulation is utilized to analyze these issues. For the SFT in Molucca Sea, our results show that plate boundary stress and volcanic loading are two critical factors affecting forearc thrusting during asymmetric divergent double subduction (DDS). In the northern part of the DDS in Molucca Sea, the SFT is primarily caused by plate boundary stress. This stress is mainly generated by the southwestward subduction of the Philippine Sea Plate. In contrast, the SFT in the southern part of the DDS is mainly caused by the effects of differential volcanic loading. The effect of volcanic loading on the Halmahera forearc is considerably stronger than that on the Sangihe forearc, resulting in more severe vertical deformation and subsidence of the former. Consequently, the Sangihe forearc, which exhibits less vertical deformation, is thrust over the Halmahera forearc. For the mechanism of volcanic arc migration in Halmahera, our results show that the dehydration depth of subduction slab and the temperature structure of mantle wedge are closely linked to the rates of subducting or overriding plates. A lower rate of subducting plate or a higher rate of overriding plate is favorable for arc magmatism. Changes in the rates of subducting and overriding plates are identified as the cause of the magmatic activity interruption and volcanic migration of Halmahera arc after the Middle Miocene. During the Miocene-Pliocene period, the rate of subducting plate was lower than the westward convergence rate of the eastern microplate, which created a high-temperature zone favorable for arc magmatism. However, starting in the Middle Pliocene, the rate of subducting plate became close to the westward convergence rate of the eastern microplate fragments due to the tilted subduction of the Philippine Plate, which was unfavorable for arc magmatism. This led to the interruption of volcanic activity and the westward migration of the volcanic arc. In the Holocene, the westward migration of Halmahera arc was blocked, leading to an accelerated convergence of the eastern microplate. As a result, the volcanic activity of Halmahera arc resumed in a new location with a high-temperature zone favorable for arc magmatism.

The Sumatra subduction zone is located in the southwest of the Suna plate, between the Euro-Asia Plate and Indo-Austrilian Plates. With the obliquely subducting of the Indo-Austrilian Plate toward the the Euro-Asia Plate, complex tectonics, strong earthquakes and volcanoes have been observed in this area which has become a well experimental field used to study the subduction zone. In this work, we employed receiver function method to evaluate the S-wave velocity structure beneath 5 broadband seismic stations along the Sumatra subduction zone. We selected 332 receiver function waveforms with intelligent software and manual picking methods, including 130, 34, 42, 29 and 97 receiver function waveforms corresponding to BKNI, GSI, LHMI, MNAI, and PMBI stations, respectively. These stacked receiver function waveforms were applied to inversion to estimate Swave structure beneath each station based on a Neighborhood Algorithm (NA). Our results indicate that the sediment layers for GSI, LHMI and MNAI stations are more than 3 km thick, two stations of which are thicker than 6 km (e.g. GSI and LHMI). The difference of receiver function waveforms for NE, SW and W orientation at station GSI where is accompanied with strong thrust earthquakes suggests that there is a complicated structure beneath this station. Station BKNI and PMBI are located on the eastern side of the Sumatra fault and the thickness of their sediment layers is only ∼ 1 km. The crustal thickness for back-arc basin is within 30–36 km. However, the crustal thickness of forearc area varies from ∼ 26 km of the forearc ridge to 26–30 km of the forearc basin toward continent and its, which suggests that the down dip limit (slab-Moho intersection) of seismogenic zone is within 29–36 km in forearc and explains why the shallow-focus earthquakes play a dominant role in this area. The stable state for the inner wedge of forearc within a seismogenic circle provides a favorable environment for storing stress. Meanwhile, these faults caused by the subducting of Indo-Austrilian Plate constructed a condition (e.g. cracking of intact rocks and frictional sliding) in which it would trigger shallow-focus seismic activities (releasing stress).

Denoising is a critical step in signal processing. We develop a method for random noise reduction in active source seismic data using spectrum reconstruction. Two methods are developed for modifying the observed data’s amplitude spectrum: one substitutes it with the source wavelet’s amplitude spectrum, whereas the other involves multiplying the source wavelet’s amplitude spectrum with the observed data’s amplitude spectrum. By reconstructing the modified amplitude spectrum while preserving the observed data’s phase spectrum, noise suppression is achieved. Extensive testing with theoretical models, synthetic shot gathers, and field data indicate a notable improvement in the signal-to-noise ratio (S/N) compared with the traditional band-pass filtering method. This method proves particularly effective for enhancing the S/N in the context of active source wide-angle seismic data used in offshore structural studies, eliminating the need for data segmentation based on offset, and thereby improving processing efficiency. Our method relies solely on a single complete cycle of the source wavelet, making it a purely data-driven solution. It has broad applications in processing active source or controlled source data with consistent source wavelets, including but not limited to seismic exploration, acoustic detection, and signal denoising in various ground-penetrating radars used on Mars, the moon, and earth.

Abstract In Southeast Asia, emerging subduction zones often appear to begin at the corners of small oceanic basins, which have a triangular‐indenter continent–ocean boundary geometry. To investigate the influence of a triangular corner on subduction initiation, we performed a series of three‐dimensional numerical simulations with varying corner angles and base lengths. The results show that the apex of the corner constitutes the initial location of subduction, irrespective of the angle or the extent of the corner. Smaller angle corners are more likely to facilitate subduction initiation. At the same time, wide acute angle corners are difficult to form. Our findings suggest that triangular corner structures may facilitate subduction initiation in smaller basins; however, the role such corners in subduction initiation is limited in larger basins. Our results emphasize the importance of accounting for the three‐dimensional geometry of a subduction zone when examining its subduction dynamics and geological features.