High-velocity Layer Research Articles

Deccan Volcanism and Related Seismic Unrest in the Koyna–Warna Region, India

Abstract Koyna–Warna is a region of low-tectonic deformation and normal surface heat flow (∼40 mW/m2) in the Deccan volcanic province, India, where low-to-moderate magnitude earthquakes have continued to occur in the last 60 yr. These earthquakes are uniquely restricted to an 11×16 km2 area and confined to the upper crust between 3 and 9 km depth. Located at the last phase of the interaction of India with the Reunion mantle plume ∼65 Ma ago responsible for extensive volcanism, the cause of sustained seismicity in Koyna region is debated. Using the shear-wave velocity model derived through the joint inversion of the receiver function and surface-wave data from the seismic zone, we propose that earthquakes in the Koyna region occur due to stress concentration arising because of the high-density magma intrusions in the shallow crust at 3–9 km. The high-density mafic-ultramafic body exerts gravitationally induced stress of about ∼12 to 15 MPa. The continuation of earthquakes in the deeper part is inhibited by the possible fluid-filled mush zone imaged as a low-velocity layer at a 9–17 km depth. The magma intrusion as dike can induce a cycle of normal faulting in the overlying rock mass, as observed in the Warna region. We present the first evidence of an extremely high velocity (>4.7 km/s) layer at 40–50 km below Moho, interpreted as the presence of eclogite–peridotite responsible for producing Deccan magma in large volume.

A transcranial multiple waves suppression method for plane wave imaging based on Radon transform

Transcranial ultrasound imaging presents a significant challenge due to the intricate interplay between ultrasound waves and the heterogeneous human skull. The skull’s presence induces distortion, refraction, multiple scattering, and reflection of ultrasound signals, thereby complicating the acquisition of high-quality images. Extracting reflections from the entire waveform is crucial yet exceedingly challenging, as intracranial reflections are often obscured by strong amplitude direct waves and multiple scattering. In this paper, a multiple wave suppression method for ultrasound plane wave imaging is proposed to mitigate the impact of skull interference. Drawing upon prior research, we developed an enhanced high-resolution linear Radon transform using the maximum entropy principle and Bayesian method, facilitating wavefield separation. We detailed the process of wave field separation in the Radon domain through simulation of a model with a high velocity layer. When plane waves emitted at any steering angles, both multiple waves and first arrival waves manifested as distinct energy points. In the brain simulation, we contrasted the characteristic differences between skull reflection and brain-internal signal in Radon domain, and demonstrated that multiples suppression method reduces side and grating lobe levels by approximately 30 dB. Finally, we executed in vitro experiments using a monkey skull to separate weak intracranial reflection signals from strong skull reflections, enhancing the contrast-to-noise ratio by 85 % compared to conventional method using full waveform. This study deeply explores the effect of multiples on effective signal separation, addresses the complexity of wavefield separation, and verifies its efficacy through imaging, thereby significantly advancing ultrasound transcranial imaging techniques.

Deep-learning viscoelastic seismic inversion for mapping subsea permafrost

Marine seismic surveys can be used to map ice-bearing subsea permafrost on a large scale. However, current seismic processing technologies have limited capacity to image permafrost distribution at depth, mainly due to the low sensitivity of primary reflections and refractions to the velocity inversion found at the base of permafrost. Guided waves and multiples are more sensitive to the velocity variations below the top of permafrost, but they remain challenging to use in physics-based inversion approaches. A deep-learning-based seismic inversion has the potential to improve seismic imaging below the top of permafrost by automatically extracting information from all wave modes. We develop a multi-input neural network (NN) to estimate seismic velocities from marine seismic data. The network is trained on synthetic data generated from the representative distributions of the seismic properties of subsea permafrost. We find that our network can image large velocity contrasts and reversals in depth, typical of subsea permafrost. We use our network to estimate the P- and S-wave velocity and Q-factor models from a seismic line in the Beaufort Sea. The NN indicates highly discontinuous subsea permafrost with variable thickness in the area. Our work shows that deep-learning-based seismic inversion could become a cost-effective technology to map the distribution of subsea permafrost on a large scale and, more generally, high-velocity geologic layers located in shallow waters.

Origin of a high-velocity layer: Insights from seismic reflection imaging (South China Sea)

Magmatism and crustal structural characteristics are well-known phenomena that support understanding the marginal sea's tectonic evolution. The South China Sea (SCS) has become a natural laboratory for researchers to investigate the evolution of the marginal sea because of its complex tectonic background and multiple phases of magmatic activities. However, due to limitations in resolution and depth of observations, the evolutionary process of the SCS is still controversial. We use the latest processed multi-channel seismic reflection profile Line AB to evaluate the sedimentary sequences, crustal tectonics, and the Cenozoic magmatism in the southern SCS. The gravity modeling results indicate a reduction in crustal thickness from 18 km in the proximal domain to 7 km in the Continent-Ocean Transition (COT). The COT is roughly 35 km wide, representing a rapid break up process of the SCS. A high-velocity layer (HVL) of about 3 km thickness was formed in the continental slope crust, interpreted as post-spreading magmatic underplating. In addition, four sedimentary sequences, post-spreading magmatic intrusion, and the possible magma conduits were identified in the profile. Combining with previous ocean bottom seismometer (OBS) data, multi-channel seismic data, fault system data, and petrologic observations, we indicate the deep mantle magmatic material upwelled to the crust and further intruded into sediments along the hyper-extended crust or detachment faults during the post-spreading stage. We contend that cooling and heat shrinking within the SCS oceanic lithosphere have the potential to initiate decompression melt deformation in the continental slope. In summary, we construct a model of the origin and migration mechanism of extensive post-spreading magmatism in the SCS. Our research reveals the Cenozoic evolution and the HVL of the southern SCS, and provides a possible genesis for the anomalously young magmatism observed at the continental margin.

Microtremor Full-Wavefield Modeling of Effective Phase Velocity and Horizontal-to-Vertical Spectral Ratio at Kyoto Reference Borehole Site: Comparison with Surface-Wavefield Modeling Based on a Velocity Structure with a Cap Layer

ABSTRACT Two types of data commonly used for microtremor exploration are phase-velocity dispersion curves obtained through an array measurement and horizontal-to-vertical spectral ratios (HVSRs) obtainable by a single-station measurement. Phase-velocity dispersion curves obtained by applying the spatial autocorrelation method to the array waveforms have a characteristic peaked shape in some cases. This dispersion curve shape has traditionally been explained as a consequence of the predominance of higher modes over fundamental mode in the Rayleigh waves. In this study, the effects of body waves on phase velocities and HVSRs were investigated based on both field measurements and theoretical calculations of microtremors. We used vertical-component array waveforms and single-station three-component waveforms of microtremors, obtained at and around a site where combined P-wave–S-wave (PS) and density loggings were conducted in the Kyoto basin, Japan (site KD-1), to identify phase velocities and HVSRs at frequencies in the range 0.2–2 Hz. The corresponding theoretical phase velocities and HVSRs were identified using full-wavefield synthetic data, which were generated assuming excitation points randomly distributed over the surface of a horizontally stratified velocity structure model created based on the logging data. The following key results were obtained. The measured phase-velocity dispersion curve exhibits a peaked shape with the value exceeding the S-wave velocity of the Tamba Group (Tb-Group), which is the bedrock (half-space) of the velocity structure model. Theoretical calculations based on the surface-wavefield theory were unable to reproduce this peaked shape; however, theoretical calculations based on the full-wavefield theory reproduced it with extraordinary accuracy. To reproduce the peaked shape based on the surface-wavefield theory, it was necessary to construct a model containing a cap (i.e., high-velocity layer) connected under the Tb-Group. The theoretical calculation based on the full wavefield also accurately reproduced the peak value and peak frequency of the measured HVSRs.

CRUSTAL STRUCTURE OF THE MENDELEEV RISE IN THE ARCTIC OCEAN: A SYNTHESIS OF SEISMIC PROFILES AND ROCK SAMPLING DATA

The Mendeleev Rise is located in the Amerasia Basin of the Arctic Ocean. The work is based on a synthesis of interpretation of regional seismic profiles of the OGT 2D DOM and data from rock sampling using special underwater vehicles on the slopes of seamounts and scarps. The uplift is represented by alternation of highs (horsts) and half-grabens. At the base of the horst sections, bright reflectors are distinguished, which are interpreted as volcanics. Half-graben sections are wedge-shaped in section and are similar in geometry to seaward-dipping reflectors (SDRs) of continental passive volcanic margins. Rock sampling has shown that the horsts are composed of sedimentary rocks of Palaeozoic age, penetrated by intrusions. Aptian-Albian sections with volcanics (basalts, trachybasalts, trachyandesites) were identified on the horsts. U/Pb dating of igneous rocks showed that typical age of rocks is 110-114 Ma. Magmatic Cretaceous rocks contain zircons with ages ranging from pre-Barremian Mesozoic to Palaeozoic and Precambrian. These zircons were captured by basaltic magma during its upward movement. The presence of these ancient zircons indicates that the Mendeleev Rise is composed of continental crust. A model of the crustal structure of the Mendeleev Rise is proposed. The base of the section visible on seismic profiles is dominated by volcanics (on horsts from basalts to trachyandesites, in half-grabens mainly basalts). The upper and lower crust is approximately 20-30% saturated with intrusions of basic composition. At the base of the crust, a high-velocity layer up to 5 km thick is distinguished. It is assumed that its lower part is entirely represented by gabbro-type intrusions, and the upper part is the lowest part of the lower crust, maximally saturated with intrusions.

Joint wave-equation inversion of Rayleigh and Love dispersion curves

Wave-equation dispersion (WD) inversion techniques for surface waves have proven to be a robust way to invert the S-wave velocity model. Unlike 1D dispersion curve inversion, WD method obviates the need for a layered model assumption and reduces the susceptibility to cycle-skipping issues in surface wave full-waveform inversion. Previous WD inversion experiments conducted on Rayleigh and Love waves have highlighted that inverting Love waves yields better stability due to their independence from the P-wave velocity model. Nevertheless, Rayleigh waves possess the advantage of greater penetration depth compared with Love waves with similar wavelengths. Therefore, combining the two types of surface waves is a feasible way to improve the accuracy of S velocity tomograms. In light of this, we develop a novel approach: a joint WD inversion encompassing Rayleigh and Love waves. This innovative technique adjusts the weighting of individual WD gradients using the sensitivity factor of an equivalent layered model, offering a significant advancement in subsurface characterization. Synthetic model tests demonstrate that the joint WD inversion method can generate a more accurate S-wave velocity model, particularly in the presence of complex low-velocity layers or high-velocity layers, when compared with individual wave WD inversion techniques. Simultaneously, the results of field tests validate the effectiveness of the proposed joint WD inversion strategy in producing a more dependable S-wave velocity distribution that aligns closely with the actual geologic structure.

Gravity-Seismic Joint Inversion of Lithospheric Density Structure in the Qiongdongnan Basin, Northwest South China Sea

Abstract Qiongdongnan Basin (QDNB), located at the northwestern corner of the South China Sea (SCS), is a key juncture between the extensional tectonic regime in the northern continental margin and the shear tectonic regime in the western continental margin. Analyzing the crustal density structure and tracking the thermodynamic controlling factors are effective approaches to reveal the nonuniform breakup process of the northwestern SCS. Herein, focusing on the obvious tectonic deformation with distinct eastern and western parts in the QDNB, we present the crustal density structures of five profiles and identify the high-density anomaly related to the synrifting mantle underplating and postrifting magmatic intrusions. The crustal density model was constructed from the Bouguer gravity anomaly, ocean bottom seismic profiles, and multichannel seismic reflection profiles. The northern part of QDNB, with normal crustal density, lower surface heat flow of <55 mW/m2, and limited extension factor of 1.25–1.70, is recognized as the initial nonuniform extension continental crust. The mantle underplating beneath the QDNB is identified as a high mantle density of 3.30–3.40 g/cm3 and a high lower crustal density of 2.92–2.96 g/cm3, which is usually recognized by the high-velocity layers in the northeastern margin of SCS. The magmatic intrusions are identified as the high-density bodies ranging from 3.26 g/cm3 at the base to 2.64 g/cm3 at the top, which become stronger from the west to east. The central part of Xisha Trough is featured by the cooling of the heavily thinned lower crust in the final continental rifting stage, which is close to the cold and rigid oceanic crust. Lateral variations in the deep magmatic anomaly should be the crucial factor for the nonuniform breakup process in the northwestern margin of SCS.

Numerical Simulation of Wave Equation Using the Finite Difference Method with Variable Refined Grid in Complex Near-surface Condition

The horizontal layered refined grid is adopted in the conventional finite difference method (FDM) of variable refined grid for the numerical simulation of wave equation. But, the method cannot well adapt to the characteristics of undulated topography and the variation of velocity structure in near-surface region. To solve this problem, a numerical simulation of wave equation using FDM with undulated variable refined grid is proposed in this paper. The grids are generated according to the undulated topography and the spatial distribution of velocity in the near-surface region from low-velocity to the high-velocity layer. For ensuring the accuracy while taking into account the computational efficiency, the FDM with variable coefficient in above grids is used to discretize the acoustic wave equation. The numerical evaluation and example show that the same accuracy as the method using fine grid can be obtained by the new method. However, its computational time is only 43.6% of the one spent by the fine grid. Furthermore, the new method can also stably adapt to the real complex loess plateau near-surface model in a working area of Ordos Basins.

S-wave velocity structure and Vp/Vs ratios beneath Liyuexi Trough, Nansha Block, the South China Sea: Implications for crustal lithology and tectonic nature of the southern continental margin

By processing the vertical component data of OBS2019–2, a wide angle seismic line situated adjacent to the Liyuexi Trough in the Nansha Block of the South China Sea (SCS), we obtained a crustal model of P-wave velocity structure beneath the line and acquired information about the deep crustal structure in the Liyuexi Trough region. However, the lithology remains to be fully constrained. This paper presents the results of processing the horizontal component data of OBS2019–2 to obtain the S-wave velocity structure model and Vp/Vs ratios. The results show that the Vp/Vs ratios of sedimentary layer range from 1.90 to 4.60, while the corresponding S-wave velocities range from 0.4 to 1.8 km/s. As for the crust, the Vp/Vs ratios fall between 1.70 and 1.89, while the S-wave velocities range between 2.9 and 4.2 km/s. The lithological compositions of oceanic crust from top to bottom may consist of basalt, diabase, and gabbro. In contrast, the lithologies of continental crust transition from felsic to mafic downwards, including andesite, granodiorite, granite, greenschist, and anorthosite. The rock compositions of the Continent-Ocean Transition (COT) region are intermediate between the continental and oceanic crusts, exhibiting transitional features. The rock composition of the high-velocity layer (HVL) in the lower crust may consist of amphibolite, and the presence of magma intrusions and underplating in this region could be associated with the Zhongnan-Liyue Fault. Compared to the Dongsha Block, the lithologies of the Liyue Block and Zhongsha Block are more similar, indicating a higher possibility of conjugation between the Liyue Block and the Zhongsha Block.

Spatial and temporal control of the volcanic activities over the distribution of sediments in the Magadi basin, Kenya Rift

High-velocity layers in basins with significant sub-basalt coverage, such as the Magadi basin, can impede geophysical imaging of potential subsurface hydrocarbon deposits. Using magnetic and gravity methods, this study examines the influence of volcano-tectonic processes and the basement on the evolution of the Magadi basin. Regional-residual separation analyses identified key structures and signatures. Lineaments and depths of the causative sources were determined using Euler deconvolution. Butterworth low-pass filters on aeromagnetic data isolated depth-varying magnetic layer responses. The study demonstrates a westward displacement of volcanic flows in the Magadi basin, originating from nearly all volcanic plugs. Depocenters near the western boundary fault are more affected by westerly movements of the volcano-tectonic flows than depocenters near the eastern boundary fault. The Rift underwent internal compartmentalization, resulting in depocenters with unique and intermittent tectonic and sedimentary occurrences. Rift segmentation led to modifications in catchment areas, which caused sudden variations in sediment flux, ultimately leading to the isolation of the depocenters. This study identifies the initial compartmentalized depocenters of Nguruman, Koora, and Forty-six. The initial Nguruman depocenter is oriented SW-NE and spans approximately 2512 sq. km with a sediment thickness range of 2000–4000 m. The presence of horsts in the southern and northern parts precludes volcaniclastics from the depocenter. Active tectonics and inversion caused the initial Nguruman depocenter to split into two halves: the northern Musenke depocenter and the southern Pakase. Euler depth analysis indicates sedimentary formations are thicker in compartmentalized depocenters in Magadi's western and southern regions than near the eastern boundary fault. The sediment thickness range in the Koora basin is approximately 1000–3000 m, while Pakase on the western flank has a sediment thickness range of 2000–3800 m.

Seismic structure of the Eastern European crust and upper mantle from probabilistic ambient noise tomography

The Eastern European lithosphere is a natural laboratory to study continental formation and evolution through time, comprising Archean continental remnants, Proterozoic rifts and belts, and younger accreted terranes. We investigate the seismic structure of the East European Craton (EEC) crust and uppermost mantle, and the transition from Precambrian to Phanerozoic Europe across the Trans European Suture Zone (TESZ) using probabilistic transdimensional ambient noise tomography. We cross-correlate noise recorded at broadband seismic stations from Eastern, Northern, and Central Europe, remove earthquake signals using continuous wavelet transform, and extract Rayleigh wave phase velocity dispersion curves. We invert these for the highest resolution shear wave velocity model of the Eastern European lithosphere to date, using Markov chain Monte Carlo Bayesian inversion. Our shear wave velocity model exhibits spatial correlation with major tectonic units and bears similarities with active seismic survey profiles in terms of seismic velocity patterns and main discontinuities. The crust thickens across the TESZ boundary and the mantle is seismically faster than beneath younger terranes, consistent with a less dense Precambrian lithosphere in the EEC. The crust and lithosphere beneath the Pannonian region is hyper-extended but the adjacent Transylvanian basin crust shows significant heterogeneity. The Precambrian building blocks of the EEC exhibit contrasting seismic fabrics. The Baltic orogens of Fennoscandia are underlain by uniform crust with a flat Moho, while Sarmatia shows alternating high and low velocity layers and a regional south-dipping crustal boundary from beneath the Ukrainian Shield towards the Crimean Peninsula. The last observation supports a geodynamic style driven by horizontal rather than vertical tectonics, with fundamental implications for the formation and evolution of early continents.

The Crustal and Uppermost Mantle Vs Structure of the Middle and Lower Reaches of the Yangtze River Metallogenic Belt: Implications for Metallogenic Process

AbstractThe Middle and Lower Reaches of the Yangtze River metallogenic belt (MLYMB) is one of the most important Fe‐Cu polymetallic belts in China. However, the mechanism and deep geodynamical process for the formation of this belt are still controversial. Here, we obtain the crustal and the uppermost mantle structures using ambient noise data from a dense seismic profile. A low velocity zone is revealed beneath the Moho of MLYMB, interpreted as the source of the deep mineralization materials. In addition, a low velocity layer (LVL) and a high velocity layer (HVL) are observed in the crust of the southern segment of the profile. The LVL is interpreted as a tectonic detachment layer between the upper and the lower crust, and the HVL is interpreted as the aggregation zone for mineralizing melts or crystallized magma chambers. Based on the observed velocity features, we propose a three‐stage model for the formation of ore deposits in MLYMB. Our model suggests that an upwelling of asthenosphere triggered by the delamination of a previously thickened lithosphere leads to the partial melting of upper mantle rocks, which eventually ponders under the Moho. The magma then infiltrates through the ductile lower crust and reaches a depth of ∼7–13 km, forming a minerals‐enriched magma chamber. Minerals‐rich hot fluids originating from the magma chamber continue to move upward along the pre‐existent faults and the minerals finally precipitate in dense veinlets when reaching shallow depths, forming the ore deposits in and around the MLYMB.

RELEVANCE OF RECEIVER FUNCTION TECHNIQUE IN SUBDUCTION ZONE (AVACHA BAY)

This paper elaborates on specific aspects of P- and S-receiver functions. The functions that are researched in this paper were calculated using waveforms obtained by three adjacent broadband seismic stations within the Avacha bay area in proximity to the subduction zone of the Pacific plate. The subduction zone in seismological context manifests as a layer of high seismic velocities, which are known to introduce a level of distortion to the receiver functions. To specify the level of this effect we parsed through two sets of P and S receiver functions in this research. The first set contains events that pass through and theoretically are affected by the subduction zone of the Pacific plate and the second set contains events that do not. The paper demonstrates that converted waves and their multiples formed at the boundaries of the high-velocity layer significantly affect P-receiver functions starting with 30-th second after the primary phase. However, no notable effects on S-receiver functions were revealed. Thus, we empirically confirm that [at least in the investigated area] local single-dimensional models are valid to be used for the inversion of the receiver functions to the depth of up to 200 km after which point the seismic noise produced by the subducting plate effectively limits the applicability of such models.

Nature and origin of an undetected seismic phase in waveforms from Southern Tyrrhenian (Italy) intermediate-depth and deep earthquakes: first evidence for the phase-A in the subducted uppermost lithospheric mantle?

We have found a previously unreported later seismic phase in seismograms of European seismic stations from intermediate-depth and deep earthquakes of the Southern Tyrrhenian subduction zone. We observe this phase at stations from 6 to 9° from the epicentre, towards north. Only seismograms of earthquakes located in a well-defined region of the slab, in the depth range of 215–320 km, show the later x-phase. In this work, we describe the nature and possible origin of this phase, and we provide a simple 2D model to explain the observed arrival times. Our analyses reveal that the x-phase propagates downward in a high velocity layer, possibly located within the deepest part of the slab. We suggest that this layer reveals the presence of the dense hydrous magnesium silicate phase A, introduced from petrological laboratory experiments, inferred to carry water in the upper mantle and predicted to be found in cold subduction zones.

Imaging exhumed continental and proto-oceanic crusts in the Camamu triple junction, Brazil

During the SALSA experiment, in 2014, twelve combined wide-angle refraction and coincident multi-channel seismic profiles were acquired in the Jequitinhonha-Almada-Camamu, Jacuípe, and Sergipe-Alagoas basins, NE Brazil. Profile SL09 images the Almada-Camamu basin and the São Francisco craton, with 18 four-channel ocean-bottom seismometers and 22 land stations. The datasets were forward modelled and combined with pre-stack depth migration to increase the horizontal resolution of the velocity models.Our results show that sediment thickness varies between 3.8 km in the oceanward part of the profile, 4.3 km in the Almada basin and 6.5 km in the Camamu basin. Crustal thickness at the north-western edge of the profile is of around 40 km, with velocity gradients indicating a continental origin.The Camamu basin, which corresponds to the triple junction between the aborted N–S oriented Tucano rift, the SW-NE oriented Jacuipe-Sergipe-Alagoas branch, and the N–S Jequitinhonha-Almada branch, presents two crustal layers: a very thin upper layer, about 1.5 km thick, which increases seawards to 3 km in the Almada basin, and an higher velocity (HV) layer (6.8–7.2 km/s) about 4 km thick. This lower layer gradually disappears in the Almada basin. At the south-eastern edge of the profile, the resolution is lower but the thickness of the crust seems to increase up to 5 km. Deep wide-angle reflections indicate upper mantle stratification.Crustal organisation and P-wave propagation velocities in the Almada and Camamu basins indicate a transitional crust domain of exhumed continental crust affinity. In the Camamu triple junction and beneath this thin exhumed continental crust, the HV layer may probably reflect intruded materials. No exhumed upper mantle is observed along the entire profile. The easternmost part of the profile may correspond to a proto-oceanic crust. Typical oceanic crust is never imaged along the 260 km-long offshore profile.

Differences in Thermo-Rheological Structure between Qiongdongnan Basin and Pearl River Mouth Basin: Implications for the Extension Model in the Northwestern Margin of the South China Sea

This study combines surface heat flow, multi-channel seismic reflection profiles, and ocean-bottom seismometer (OBS) profiles to determine the thermo-rheological structure of the Qiongdongnan Basin (QDNB) and Pearl River Mouth Basin (PRMB), with the aim of researching the west–east variation of the passive continental margin rifting. Based on the initial lithospheric rheological model of a jelly sandwich-1 (JS-1) regime, the current architecture of the continental margin is identified to be the result of a non-uniform extension. Due to the decoupled crust–mantle relationship caused by the weak lower crust, the non-uniform extension led to the rupture of the mantle lithosphere before the crust. The central Xisha Trough falls into the JS-2 regime with only one brittle load layer, which is close to the rigid oceanic lithosphere of the Northwest Sub-basin (NSB). The high-velocity layers (HVLs) and detachment faults beneath the Xisha Trough are considered to be the result of the cooling of a thinned lower crust with mantle underplating during the middle stage of continental margin rifting. A seaward-increasing trend of lithospheric rheological strength is exhibited across the PRMB, from the crème brûlée-1 (CB-1) regime at the continental shelf to the JS-2 regime at the NSB. Unlike the HVLs of the Xisha Trough, the lower crustal HVLs beneath the eastern PRMB formed during the late stage of continental margin rifting due to the mantle lateral flow. The absence of HVLs beneath the western PRMB may indicate that the mantle lateral flow demonstrates a limited impact.

Uncertainty quantification of multimodal surface wave inversion using artificial neural networks

An inversion of surface wave dispersion curves (DCs) is a nonunique and ill-conditioned problem. The inversion result has a probabilistic nature, which becomes apparent when simultaneously restoring the S-wave velocity and layer thickness. Therefore, the problem of uncertainty quantification is relevant. Existing methods through deterministic or global optimization approaches of uncertainty quantification via posterior probability density (PPD) of the model parameters are not computationally efficient because they demand multiple solutions to the inverse problem. We have developed an alternative method based on a multilayer fully connected artificial neural network (ANN). We improve the current unimodal approach, which is known from publications, to multimodal inversion. We use Cox’s and Teague’s algorithm to determine optimal parameterization (number of layers) and the ranges of possible model parameters. We uniformly draw training data sets within estimated ranges and train the ANN. Saved ANN weights map the phase velocity DCs to values of the S-wave velocity and layers thickness. To estimate the uncertainties, we adapt the Monte Carlo simulation strategy, according to which the frequency dependent data noise and the inverse operator errors are projected onto the resulting velocity model. The inverse operator errors are evaluated by the prediction of the training data set. The proposed combination of surface wave data processing methods, configured with each other, provides a novel surface wave multimodal dispersion data inversion and uncertainty quantification approach. We first test our approach on synthetic experiments for various velocity models: a positive gradient velocity, a low-velocity layer, and a high-velocity layer. This is done considering unimodal inversion at first and then compared with the multimodal inversion. Afterward, we apply our approach to field data and compare the resulting models with the body S-wave processing by the generalized reciprocal method. The experiments indicate high-potential results — using ANN yields the possibility to accurately estimate the PPD of restored model parameters without a significant computational effort. The PPD-based comparison demonstrates the advantages of a multimodal inversion over a unimodal inversion. The trained ANN provides reasonable model parameters predictions and related uncertainties in real time.

Enigmatic doubly scattered tube waves at a crosswell seismic survey

SUMMARY Enigmatically strong tube waves continue to exist long after the direct wave during repeat crosswell-monitoring surveys at the Nagaoka CO2 injection site in Japan. The tube waves, which have linear moveouts with velocities of 1.29 and 1.41 km s−1 at plastic and steel casings, respectively, are generated by double scattering on the shallow side of the wells. We characterize wavefields to confirm that these late coda waves are tube waves that are excited at the source well, propagate upward, then travel to a receiver well as a body wave, and finally propagate downward along the receiver well. Even though these tube waves result from second-order scattering, they have large amplitudes because of the relatively short distance between source and receiver wells. Because these waves propagate along both wells many times, we can extract detailed information of wave propagation by averaging them to increase the signal-to-noise ratio. We first use the tube waves to relocate sources and receivers between multiple monitoring surveys, demonstrating our ability to correct the severe noise caused by location errors that frequently degrades the repeatability of time-lapse surveys. Then, we apply an inversion based on the physics of tube waves to estimate the location and strength of the scatterers and find that they are predominantly located in the shallow segment of the wells, above the sources, and infer that they are related to local fractures and/or wellbore conditions because their locations do not correspond to the well geometry. Lastly, we use the tube waves to accurately estimate subsurface velocities along the wells. The estimation is stable and robust, and the velocities follow the general trend of subsurface structure seen in the well log data. Due to the receiver spacing, tube-wave analysis cannot resolve a thin, high-velocity layer at the CO2 reservoir. By combining tube waves observed during different stages of the monitoring survey, we can estimate the time-lapse changes of the subsurface velocities.

Crustal structure of the northern Manila subduction zone: Is thinned continental crust or oceanic crust subducting beneath the Luzon arc and forearc?

Forward and inverse P-wave velocity models along an E-W 280-km-long wide-angle reflection/refraction profile at 21°N across the northeastern South China Sea margin and Manila subduction zone reveal a 10–12 km thick thinned continental crust (TCC) located west of the Manila transcurrent fault (MTF) and Manila slab. The TCC is divided in two parts separated by the intra-continental velocity difference boundary (VDB). West of the VDB, the TCC is located above a high velocity layer (HVL). East of the VDB, the TCC consists of three layers: an undefined body with a velocity inversion at its base above the upper and lower crusts. The VDB and MTF merge east of the Hengchun peninsula showing that the TCC located east of the VDB decreases in size and disappears to the north. The distribution of earthquakes (EQs) shows that ∼80 km of crust have subducted east of the MTF since the onset of Luzon arc collision with Eurasia at ∼7 Ma. All published tomographies suggest the subduction of oceanic crust (OC). However, due to crustal corrections, up to a maximum width of 40 km of TCC might eventually exist in the upper part of the Manila slab instead of OC. Two end-members with 40 km of TCC or not in the upper part of the Manila slab were restored at ∼7 Ma. Since that time, the north Luzon island moved ∼600 km in the northwest while part of this motion was absorbed near the Hengchun peninsula by shrinking of Eurasia TCC, resulting in a ∼ 20° clockwise rotation of the Manila slab and MTF. Small Luzon arc blocks started to rotate 10°-30° clockwise at ∼7 Ma below northern Taiwan, 3–4 Ma in the Coastal range and 1 Ma in Lutao and Lanyu islands and not south of the Hengchun peninsula. Thus, both the clockwise rotation of the individual Luzon arc blocks and of the Manila slab and MTF might explain the collision process of the Luzon arc with the Eurasia continent.