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This study presents the velocity structures of the seafloor sediment across the Pacific using teleseismic body waves recorded by arrays of ocean bottom seismometers (OBSs). We apply the method inverting the multicomponent stacked autocorrelation functions and radial-component waveform of teleseismic body waves. We analyze OBSs distributed across the Pacific Ocean, through a wide range of water depths and equipped with various sensor types. The inverted models agreeing with measurements of seafloor drilling samples demonstrate the broad applicability of employing the inversion method to resolve sediment thicknesses ranging from <100 m to a few kilometers. In addition to various broad-band OBSs, we confirm our method works also for short-period sensors, since our retrieved the model using data from a short-period sensor in Northeast Japan agrees with active-source seismic survey analyses. The constrained models show various numbers of intrasediment discontinuities across different locations. Furthermore, the newly characterized sediment models have higher values of P -wave to S -wave velocity ratio (⁠|${{V}_P}$| / |${{V}_S}$|⁠) than the previously known scaling relationships, which suggests a larger effect of sediment-layer reverberation on the body-wave analysis than we have anticipated. The sediment velocity models obtained in this study produce Rayleigh-wave admittances that are consistent with those in previous studies but with much finer structural details within the sediment layer. The reverberation waveform inversion method will enable improved predictions of the impact of the water and sediment layers in body-wave records from existing and future OBS observations. [ABSTRACT FROM AUTHOR]

The interior of the Ordos block, located northeast of the Qinghai–Xizang Plateau, is not entirely stable or homogeneous and may have been subjected to tectonic deformation. However, there has been no definite conclusion regarding the extent and mechanism of this deformation. Here, we obtained a high-resolution P -wave velocity (Vp) model for the northeastern Qinghai–Xizang (Tibetan) Plateau, Ordos block and surrounding areas using traveltime data involving over 1200 stations from a newly deployed dense temporary array and permanent seismic network. Our results showed that the northern part of Ordos has a relatively high Vp with minimal lateral change. In contrast, the Vp structure in the southern Ordos block varied in the lateral direction, with low-velocity anomalies at a depth of 15 km. The lower crust with Vp from 6.8 to 7.3 km s−1 is thicker in the west and gradually thins towards the east. The northern part of the Ordos block is relatively stable, whereas the southern part has undergone crustal deformation. This deformation may be related to the eastward compressive forces from the Longxi block, which could be associated with the change in the Haiyuan fault from thrust to strike-slip. In the Bayan Har block, a thick low-velocity anomaly exists in the middle and upper crusts, whereas the lower crust is relatively thin. The crust in this area likely experienced crustal shortening and delamination. Beneath the Longxi and Alxa blocks, a low-velocity layer appears in the middle crust, which may be related to ductile shear in the crustal brittle–ductile transition zone caused by plateau expansion. The range of this low-velocity layer indicates that the influence of the plateau expansion exceeded that of the Haiyuan–Tianjingshan fault zone. [ABSTRACT FROM AUTHOR]

Earthquakes with very similar waveforms have been recognized as repeating earthquakes, or repeaters, which are interpreted as repeated ruptures of an isolated, frictionally locked asperity patch that are caused by stress loading due to aseismic slip in the surrounding stable regime of the fault. Ideally, the repeated ruptures of the same asperity result in the same source dimension and stress drop. However, significant variations in the stress drop have been observed during fluid-injection experiments even for a single repeater sequence. This study focuses repeater sequences in three different tectonic regimes beneath the Japanese Islands and investigates variations in the source dimension and stress drop in the same repeater sequences. The obtained results show that the variation in the source radius is mostly within 20–30 per cent in individual repeater sequences but the stress drop is largely variable by one order of magnitude or more. We propose a fluid-invasion hypothesis to explain our observations. When pore-fluid pressures gradually invade into an asperity, the edge of the asperity tends to behave as aseismic because of the reduction of the effective normal stress to values below the critical value of effective normal stress, which makes the source radius somewhat smaller. Simultaneously, the amount of the fluid invasion into the asperity fluctuates the shear strength of the asperity, with the stress drop being smaller for higher pore-fluid pressures. The reduced source radius and stress drop expected from the fluid-invasion hypothesis can explain positive correlations between the source radius and seismic moment. We propose a conceptual model for earthquake generation by involving the triggering of aseismic slip by the enhancement of pore-fluid pressures and subsequent rupture of asperity by the stress loading due to the aseismic slip, which can be potentially applied to the genesis of repeaters and non-repeaters. [ABSTRACT FROM AUTHOR]

The Three Gorges (TG) in central China is an earthquake prone region that lacks active-source seismic surveys, hence requiring high-resolution passive seismic imaging to reveal crustal structures in detail. While teleseismic virtual-source reflection (TVR) profiling is effective for imaging gently dipping upper crustal structures, it is unsuitable for mapping steep structures beneath rugged topography in the TG region. Here we develop a teleseismic virtual-source reverse time migration (TV-RTM) to improve the imaging of steep structures. Synthetic tests are conducted to demonstrate the validity and resolution of the TV-RTM method. To mitigate the imaging inaccuracy due to sparse station spacing, we interpolate direct-arrival waveforms to achieve a doubling of the minimally required station spacing from 2 to 4 km. The TV-RTM images of the upper crustal structure beneath a 2-D array in the TG region reveal significant fold and thrust structures that correlate well with the surface geology and tectonic framework. The resolution of the images is assessed using 2-D and 3-D synthetic models with the station and source geometry of field data. The TV-RTM method provides a new passive seismic solution for studying upper crustal structures in regions that lack active-source seismic data. [ABSTRACT FROM AUTHOR]

We investigate temporal and spatial variations of the spectral decay parameter kappa (κ) before and after the 6 April 2009, L'Aquila earthquake (M w 6.1), in Central Italy. We analysed foreshocks 10 days before and aftershocks occurring 10 days and 6 months after this main event. We select earthquakes with magnitudes M w ≥ 3.2 registered by the seismic network of Central Italy within a radius of 20 km from the epicentre of the L'Aquila main shock and having hypocentre distances of less than 170 km. We separate near-source, along-path and near-site contributions of κ for each group of events and we detected temporal variations of this S -wave attenuation parameter. We find that 10 days before the main shock κ along the path has the lowest values, probably due to high tectonic stress accumulated, in agreement with previous investigations performed with other techniques, then κ increases during the main event and remains constant during the first 10 days of aftershocks. The aftershocks that occurred 6 months after show an increase in the regional attenuation probably due to the tectonic stress released during the main shock and the earlier aftershocks. From the spatial point of view, 10 days before the principal event the foreshocks located to the south show an increase in the near-source attenuation towards the northeast, in the direction of the main shock. These spatial variations of κ may be related to the presence of crustal fluids near the rupture area, as evidenced by other previous studies. The first 10 days of aftershocks that concentrate around the main earthquake have high near-source κ , and those located north of the main rupture have lower values. These observations are consistent with previous investigations that show variations of elastic and anisotropic crustal properties during the L'Aquila earthquake sequence due to dilatancy and fluid diffusion processes within the nucleation zone. We conclude that temporal variations of the spectral decay parameter κ provide important clues for the earthquake cycle in Central Italy. [ABSTRACT FROM AUTHOR]

A g*-Hamiltonian method for tracing real rays was developed that can handle cusps and triplication of quasi-shear wave in a general viscoelastic anisotropic medium. We demonstrate that the g*-Hamiltonian method can produce homogeneous ray-velocity vectors (with parallel real and imaginary parts) and the slowness vectors of reflected and transmitted waves at the interface based on the real Snell's law (RSL), which constrains only the continuity of the real parts of the slowness vectors, or the real slowness direction (RSD) method, which ignores the inhomogeneous component of the slowness vector. These methods are based on the characteristic lines with different Hamiltonians. Our research indicates that these methods are limited to pre-critical incidence ranges. Moreover, we derived a complex energy velocity vector (energy flux velocity) expression, which is always homogenous. We found that directions of corresponding energy velocity calculated with complex Snell's law (CSL) at a contact of two viscoelastic anisotropic materials well match the solutions of the RSL and RSD methods for all R / T waves except post-critical incidence in which the RSL and RSD methods fail to obtain homogenous ray velocities. The RSL and RSD methods result in discrepancies in the ray quality factor, R / T coefficients, and energy ratios, especially for post-critical incidence. However, the exact critical angle determined by the RSD method approximates the 'critical' angle for anelastic/inhomogeneous waves, which was a previous challenge. Our calculations suggest that the energy velocity and energy quality factor obtained with the CSL method can be used for real ray tracing at the interface of viscoelastic anisotropic media, and the complex energy flux velocity vector is always exactly homogeneous. For the post-critical incidence, the RSL and RSD methods fail because the ray quality factor drastically changes from the infinite down to near 2, which contradicts homogeneous ray velocity even in elastic anisotropic materials for RSD method. However, the energy flux quality factor for the elastic-anisotropic material is all infinite, even for post-critical incidence, which is correct. We also show that the CSL method has the same efficiency as the RSD method. [ABSTRACT FROM AUTHOR]

Over the last decade there has been an international effort to find methods to recover and digitise recordings from historical earthquakes and explosions that occurred during the 1950s through to the 1980s. Making these recordings accessible in digital format offers opportunities to study what signatures are encoded in the data, and to apply state-of-the-art techniques and methods to historical data. In this study, we employ unsupervised machine learning to cluster historical teleseismic waveforms from nuclear explosions conducted at the former USSR Degelen test site, in Kazakhstan, recorded at seismic arrays in the UK (Eskdalemuir array), Canada (Yellowknife array), Australia (Warramunga array), and India (Gauribidanur array). In particular, we use two unsupervised algorithms to cluster waveforms using shape-based clustering: kernel k-means and k-Shape. The algorithms clearly split waveforms into distinct clusters that are spatially related, even when waveform differences are subtle, and we show with local and teleseismic numerical simulations that the clusters are related to the topography. The topography at the Degelen test site has characteristic wavelengths of 2–4 km and local simulations highlight that the seismic wavefield is trapped in reverberating mountain peaks. The location of the explosion is crucial in determining which section of the mountain range reverberates, influencing the outgoing wavefield. Teleseismic waveform simulations confirm that it is this superposition of energy leaving the reverberating peaks that results in the observed teleseismic waveform differences we observe. [ABSTRACT FROM AUTHOR]

We present a frequency-domain finite-element (FDFE) method for simulating PSV -mode seismic waves (including two slow compressional waves) in 2-D double-porosity media. A functional, whose extremum corresponds to the governing equations with boundary conditions for 2-D PSV waves, is constructed. Solving the boundary value problem of the governing equations is thus equivalent to finding the extremum of the associated functional. We use structured rectangular elements to discretize the computational domain and employ perfectly matched layers (PMLs) to absorb the seismic waves. The FDFE method is validated by comparing the results with those from a semi-analytic method. Numerical simulations are conducted to investigate the propagation of seismic waves in double-porosity media. Assuming a non-viscous pore fluid, we simulate the propagation of slow P waves (P 2 and P 3 waves) in a two-layer model consisting of double-porosity media. We observe the reflection and transmission of the P 2 and P 3 waves at the interface. Additionally, we simulate the propagation of seismic waves in a model with irregular interfaces. Seismic signals collected from horizontal and vertical receiver arrays have demonstrated that the P 1 wave experiences higher attenuation in a double-porosity medium. [ABSTRACT FROM AUTHOR]

Changes in pore pressure within geological reservoirs, due to, for example hydrocarbon production, CO2 and energy storage or wastewater disposal, may cause substantial stress changes in the overburden, altering propagation velocities of elastic waves. The corresponding time-shifts are detected and quantified using time-lapse (4-D) seismic analysis. To invert seismic time-shifts for changes in stress and strains, stress sensitivity of rocks is studied in laboratory experiments on core plugs. Such measurements are typically conducted at ultrasonic frequencies. However, previous studies indicate that the stress sensitivity of velocities at seismic frequencies could be higher than that at ultrasonic frequencies. Therefore, calibration based on laboratory ultrasonic data may lead to inaccurate prediction of stresses and strains when applied to 4-D seismic data. To study the influence of frequency on stress sensitivity of acoustic wave velocities, a series of laboratory experiments was performed on two overburden shales with different petrophysical properties. In a low-frequency apparatus—a triaxial pressure cell that combines measurements at low (seismic) and high (ultrasonic) frequencies—the shale samples underwent stress changes with different ratios of horizontal to vertical stress amplitudes to mimic stress variations across the overburden. High-frequency velocity changes were directly recorded, while low-frequency velocity changes were obtained indirectly from the elastic parameters measured at seismic frequency by applying a rock physics inversion using third-order elasticity model. The experiments were conducted at undrained conditions, a representative state for reservoir overburden composed of shales. The results suggest that the stress sensitivities and strain sensitivities (R-factor) of P -wave velocities could be two to four times greater at seismic frequencies than at ultrasonic frequencies. Furthermore, it was found that the previously reported linear relation between the stress sensitivity and stress-path parameter (horizontal/vertical stress change) at ultrasonic frequencies also holds for seismic frequencies. We discuss the theoretical background for frequency-dependent stress sensitivity of wave velocities that supports the experimental findings. The effect of frequency when using laboratory-based calibrations should be taken into account when inverting time-lapse seismic data for changes in stresses and strains. [ABSTRACT FROM AUTHOR]

Body-wave speeds and polarization directions in a homogeneous and isotropic medium can be influenced by the presence of an initial deviatoric stress, which is conventionally ignored in seismic-wave propagation theories. To the first order, the P -wave speed is sensitive to the initial deviatoric stress, and the P -wave polarization has a perturbation perpendicular to the propagation direction. In addition, the S waves travel at two distinct speeds, and the S -wave polarizations are in two orthogonal directions, which are constrained by the initial deviatoric stress and involve a perturbation parallel to the propagation direction. Using a subduction zone setting with an initial deviatoric stress on the order of tens of MPa, the P -wave speed perturbation, S -wave splitting time and polarization perturbation are estimated to be on the order of 0.01 per cent, 0.01 s and 0.01 per cent, respectively. While these effects are too small to be reliably resolved using seismic observations, the theory presented in this paper could be useful for ultrasonic stress measurements in engineering applications. [ABSTRACT FROM AUTHOR]

We present an array method for constraining near-surface shear wave speed values from P -wave polarization data. As the direction of P -wave particle motion is determined both by the incident ray parameter and the local surficial shear wave speed, the latter can be inferred from three-component seismic records if the ray parameter is estimated first. Previous studies have analysed data recorded at a single station from various earthquakes in order to estimate near-surface shear wave speed. In contrast, this paper analyses data recorded at an array of stations. As such, this method can estimate both ray parameter data for the recorded earthquakes and near-surface shear wave speed values at the studied sites. And as an advantage, the method can analyse records of local and regional earthquakes where ray parameter information cannot be reliably estimated using 1D reference velocity models. Additionally, by processing polarization data spanning multiple events and stations, our method provides robust estimates for near-surface shear wave speed values. We apply this array-based method to a seismic array covering the Valley of Mexico. Analysed records correspond to regional earthquakes with epicentral distances of roughly 300 km. The corresponding ray parameter values for all earthquakes were estimated from the delay of the seismic waves arrival throughout the network as all stations have a reliable time reference. Near-surface shear wave speed values estimated from the array-based polarization analysis are correlated with major geological features of the area such as the Trans-Mexican Volcanic Belt and the local basin. We also apply the presented method to a minimal data scenario. The flexibility of this method allows it to employ a single ray parameter value as input, producing robust estimates for near-surface shear wave speed values within our case study. This feature becomes useful in a variety of scenarios, such as utilizing seismic data lacking timing information. The presented method constitutes an accessible and inexpensive alternative to study shallow seismic structure, opening opportunities to expand and complement current tools and techniques to assess seismic hazards. [ABSTRACT FROM AUTHOR]

The Central African Plateau comprises a mosaic of numerous Archean terranes—the Congo, Bangweulu and Kalahari Cratons—sutured in a series of Proterozoic to early Cambrian orogenic events. Major upper-crustal deformation and complex craton margin fault zones reflect the region's diverse tectonic history: rifting during the Neoproterozoic, collision during the Pan-African orogeny, and more recently, Permo-Triassic Karoo rifting and the Pliocene development of the Southwestern branch of the East African Rift. The tectonic evolution and extent to which the lithospheric mantle has been re-worked by each tectonic event is poorly understood. New seismograph networks across the Plateau provide fresh opportunity to place constraints on the plate-scale Precambrian-to-Phanerozoic processes that have acted across the region. Utilizing data from seismograph deployments across the Central African Plateau, including the new Copper Basin Exploration Science network—a NW–SE-trending, 750-km-long profile of 35 broad-band stations—we explore lithospheric deformation fabrics associated with past and present tectonic events via a shear-wave splitting study of mantle seismic anisotropy. Results reveal short length-scale variations in splitting parameters (fast direction: |$\phi$|⁠ , delay time: |$\delta$| t), suggestive of a fossil lithospheric fabric cause for the observed anisotropy. A lack of fault-parallel |$\phi$| across the Mwembeshi shear zone, suggests it may be too narrow at mantle depths, a thin-skinned, crustal-scale feature, and/or did not experience sufficient fault parallel shear-strain during its last active phase to form a lithospheric deformation fabric discernible via teleseismic shear-wave splitting. In the heart of the Lufilian Arc, we observe abrupt changes in splitting parameters with NE–SW, N–S and NW–SE |$\phi$| and 0.5 s |$< $| |$\delta$| t |$< $| 1.2 s evident at short length-scales: no single, uniform, anisotropic lattice preferred orientation (LPO) fabric defines the entire region. This is consistent with the view that multiple episodes of deformation shaped the Lufilian Arc, or perhaps that pre-existing fabrics, relating to Neoproterozoic Katangan Basin development, have failed to be completely overprinted by the Pan-African orogeny. Near the Domes, where most intense crustal re-working is thought to have taken place during the Pan-African orogeny, there is a cluster of null and low |$\delta$| t splits which likely reflects the lack of organized LPO fabrics, perhaps due to the presence of depth-dependent anisotropy. The neighbouring Congo Craton margin is marked by consistently weak anisotropy (⁠|$\delta$| t |$\lt $| 0.7 s) indicating a weak horizontal alignment of olivine at mantle lithospheric depths, typical of several Archean terranes worldwide. [ABSTRACT FROM AUTHOR]

The grid search method is a common approach to estimate the three spatial coordinates of event hypocentres. However, locating events in large search spaces with small grid spacings is computationally prohibitive. This study accelerates the grid searches over large search spaces using a quadratic interpolation technique. We start with the coarse-grid-estimated location, where we have the minimum value of the difference in the traveltimes between S and P waves summed over all receivers. Then, we select the neighbouring grid points and build a 3D quadratic function. The unknown coefficients of the 3D quadratic function are computed by solving a system of linear equations. After that, we interpolate the location by solving partial derivatives of the quadratic function. The quadratic interpolation technique performs well on both synthetic and real microseismic data examples, typically leading to similar event locations as those obtained using 10 times smaller grid spacings in all three directions, at a minor additional computational expense, and without the need to generate traveltimes at new spatial positions. [ABSTRACT FROM AUTHOR]

We propose a frequency-domain finite-element (FDFE) method to model the 2-D P–SV waves propagating in porous media. This specific finite-element method (FEM) is based on the framework of variational principles, which differs from previously widely used FEMs that rely on the weak formulations of the governing equations. By applying the calculus of variations, we establish the equivalence between solving the stress–strain relations, equations of motion and boundary conditions that govern the propagation of P–SV waves, and determining the extremum or stationarity of a properly defined functional. The structured rectangular element is utilized to partition the entire computational region. We validate the FDFE method by conducting a comparison with an analytically-based method for models of a horizontal planar contact of two poroelastic half-spaces, and a poroelastic half-space with a free surface. The excellent agreements between the analytically-based solutions and the FDFE solutions indicate the effectiveness of the FDFE method in modelling the poroelastic waves. Modelling results manifest that both propagative and diffusive natures of the Biot slow P wave can be effectively modelled. The proposed FDFE method simulates wavefields in the frequency domain, allowing for easy incorporation of frequency-dependent parameters and enabling parallel computational capabilities at each frequency point (sample). We further employ the developed FDFE method to model two simplified poroelastic reservoirs, one with gas-saturated sandstone and the other with oil-saturated sandstone. The results suggest that changing the fluid phase of the sandstone reservoir from gas to oil can substantially impact the recorded solid and relative fluid–solid displacements. The modelling suggests that the proposed FDFE algorithm is a useful tool for studying the propagation of poroelastic waves. [ABSTRACT FROM AUTHOR]

Within the last decade, thin ultra-low velocity zone (ULVZ) layering, sitting directly on top of the core–mantle boundary (CMB), has begun to be investigated using the flip-reverse-stack (FRS) method. In this method, pre- and post-cursor arrivals that are symmetrical in time about the ScS arrival, but with opposite polarities, are stacked. This same methodology has also been applied to high velocity layering, with indications that ultra-high velocity zones (UHVZs) may also exist. Thus far, studies using the FRS technique have relied on 1-D synthetic predictions to infer material properties of ULVZs. 1-D ULVZ models predominantly show a SdS precursor that reflects off the top of the ULVZ and an ScscS reverberation within the ULVZ that arrives as a post-cursor. 1-D UHVZ models are more complex and have a different number of arrivals depending on a variety of factors including UHVZ thickness, velocity contrast, and lateral extent. 1-D modelling approaches assume that lower mantle heterogeneity is constant and continuous everywhere across the lower mantle. However, lower mantle features display lateral heterogeneity and are either finite in extent or display local thickness variations. We examine the interaction of the ScS wavefield with ULVZs and UHVZs in 2.5-D geometries of finite extent. We show that multiple additional arrivals exist that are not present in 1-D predictions. In particular, multipath ScS arrivals as well as additional post-cursor arrivals are generated. Subsequent processing by the FRS method generates complicated FRS traces with multiple peaks. Furthermore, post-cursor arrivals can be generated even when the ScS ray path does not directly strike the heterogeneity from above. Analysing these predictions for 2.5-D models using 1-D modelling techniques demonstrates that a cautious approach must be adopted in utilization and interpretation of FRS traces to determine if the ScS wavefield is interacting with a ULVZ or UHVZ through a direct strike on the top of the feature. In particular, traveltime delays or advances of the ScS arrival should be documented and symmetrical opposite polarity arrivals should be demonstrated to exist around ScS. The latter can be quantified by calculation of a time domain multiplication trace. Because multiple post-cursor arrivals are generated by finite length heterogeneities, interpretation should be confined to single layer models rather than to interpret the additional peaks as internal layering. Furthermore, strong trade-offs exist between S -wave velocity perturbation and thickness making estimations of ULVZ or UHVZ elastic parameters highly uncertain. We test our analysis methods using data from an event occurring in the Fiji-Tonga region recorded in North America. The ScS bounce points for this event sample the CMB region to the southeast of Hawaii, in a region where ULVZs have been identified in several recent studies. We see additional evidence for a ULVZ in this region centred at 14°N and 153°W with a lateral scale of at least 250 km × 360 km. Assuming a constant S -wave velocity decrease of −10 or −20 per cent with respect to the PREM model implies a ULVZ thickness of up to 16 or 9 km, respectively. [ABSTRACT FROM AUTHOR]

A new automated algorithm for picking the arrival times of the global P -, SH - and SV -wave phases from multi-component seismic waveform data is presented. This picker is based on a sequential approach using autoregressive prediction of the filtered waveform in a sliding time window, the Akaike Information Criterion and the Hilbert transform of the original waveform. The quality of the individual picks is computed by combining signal-to-noise ratios and higher order statistics into a single measure. Synthetic tests are used to find values for high and low quality thresholds. The algorithm is applied to a global data set of waveforms from teleseismic events with magnitude 6 or higher that occurred between 1990 and 2019. This resulted in approximately 4 million P-phase arrival times as well as approximately 3 million SH- and SV-phase arrival times each. These automatic picks are compared to approximately |$830\, 000$| manual P-picks as well as approximately |$70\, 000$| manual S-picks from the ISC-EHB catalogue. An upper bound for the picking errors of the automatic picks is estimated by using high quality picks of neighbouring stations. This upper bound is found to be 0.55 s for the P-picks and 4.3 s for the S-picks. If only high quality picks are considered, this represents 50 per cent of the P-picks and 25 per cent of the S-picks, then these errors decrease to 0.35 s for the P-picks and 1.5 s for the S-picks, respectively. As a by-product of the picking, the dominant periods of the arriving signals are determined as well. [ABSTRACT FROM AUTHOR]

Ultra-low velocity zones (ULVZs) have been identified as regions of extremely low velocity anomalies in the Earth's lowermost mantle using seismic observations from reflected, refracted and diffracted arrivals along the mantle side of the core–mantle boundary. Estimation of ULVZ geometrical (i.e. shape and size) and elastic (i.e. velocity and density) parameters with uncertainties is crucial in understanding the role of ULVZs in the ongoing dynamic processes within the Earth's mantle; however, these parameters are still poorly known due to uncertainties and tradeoffs of the seismically resolved ULVZ geometries and elastic parameters. Computation of synthetic waveforms for 2-D and 3-D ULVZs shapes is currently computationally feasible, but past studies utilize higher dimensional waveform modelling of mostly only low-frequency diffracted waves. Most studies focusing on high-frequency core-reflected waveforms (e.g. ScP) still use 1-D modelling approaches to determine ULVZ properties. This approach might lead to wrong results if the imaged structures have inherently 3-D geometries. This study investigates high-frequency synthetic ScP waveforms for various 2.5-D ULVZ geometries showing that additional seismic arrivals are generated even when the ScP geometrical ray path does not directly strike the location of the ULVZ. The largest amplitude additional phases in the 2.5-D models are post-cursor arrivals that are generated at the edges of the finite-length ULVZs. These newly identified ScP post-cursors can arrive within the ScsP post-cursor time window traditionally analysed in 1-D ULVZ studies. These post-cursors might then be misidentified or constructively/destructively interfere with the ScsP post-cursor, leading to incorrect estimation of ULVZ parameters. In this study we investigate the bias introduced by the 2.5-D morphologies on the 1-D estimated ULVZ elastic properties in a Bayesian waveform inversion scheme. We further expand the Bayesian method by including the data noise covariance matrix in the inversion and compare it to an autoregressive noise model that was utilized in previous studies. From the application to the observed ScP data, we find that the new approach converges faster, particularly for the inversion of data from multiple events, and the new algorithm retrieves ULVZ parameters with more realistic uncertainties. The inversion of 2.5-D synthetic ScP waveforms suggests that the retrieved ULVZ parameters can be misleading with unrealistically high confidence if we do not consider the data noise covariance matrix in the inversion. Our new approach can also retrieve the shape of a multidimensional Gaussian ULVZ if its length is 12° or longer in the great circle arc direction. However, 2.5-D synthetic waveforms show additional waveform complexity which can constructively interfere with the ScP wavefield. Hence, in many cases the estimation of ULVZ properties using 1-D forward modelling can provide incorrect ULVZ parameters. Hence previous ULVZ modelling efforts using 1-D parameter estimation methods may be incorrect. [ABSTRACT FROM AUTHOR]

Globally, there is now a growing evidence for a low velocity layer in the deeper parts of the upper mantle, above the 410 km discontinuity (hereafter called LVL-410). The origin of this layer is primarily attributed to interaction of slabs or plumes with a hydrous mantle transition zone (MTZ) that results in dehydration melting induced by water transport upward out of the MTZ. However, the ubiquitous nature of this layer and its causative remain contentious. In this study, we use high quality receiver functions (RFs) sampling diverse tectonic units of the Indian subcontinent to identify Ps conversions from the LVL-410. Bootstrap and differential slowness stacking of RFs migrated to depth using a 3-D velocity model reveal unequivocal presence of a deep low velocity layer at depths varying from 290 to 400 km. This layer appears more pervasive and deeper beneath the Himalaya, where detached subducted slabs in the MTZ have been previously reported. Interestingly, the layer is shallower in plume affected regions like the Deccan Volcanic Province and Southern Granulite Terrane. Even though a common explanation does not appear currently feasible, our observations reaffirm deep low velocity layers in the bottom part of the upper mantle and add to the list of regions that show strong presence of such layers above the 410 km discontinuity. [ABSTRACT FROM AUTHOR]

The amalgamation and breakup of the West Gondwana shaped the South American platform. The dynamics during the processes can be reflected by crust anisotropy of the platform, but there are no specialized crustal anisotropic measurements yet. Splitting analysis of Moho-converted shear waves in P -wave receiver functions (Pms) can reveal crustal-scale anisotropy, which is important for understanding the dynamic evolution of the crust and for the interpretation of mantle anisotropy from splitting analysis of core–mantle refracted shear waves (XKS phases). This study measured crustal anisotropy for the old and stable South American platform by Pms splitting analysis. The splitting times vary mainly in the range of 0–0.5 s, with a regional mean of 0.2 s, slightly lower than that observed in tectonically active regions. The detected crustal anisotropy shows distinct characteristics and spatial zoning, providing insights into tectonic processes. (1) Fast polarization directions at stations close to the Transbrasiliano Lineament (TBL) are oriented NNE–SSW, generally consistent with the strike of the TBL but inconsistent with the maximum horizontal compressive stress, implying that they might be formed by dynamic metamorphism during the formation of the TBL. (2) Crustal anisotropy along the passive continental margin in the east and northeast is weak. Still, the fast polarization directions tend to be oriented along the margin, implying the existence of fossil extensional crustal fabrics formed during the continental rifting of West Gondwana. (3) The Paraná Basin, one of the world's largest Large Igneous Provinces (LIP) covered by continental flood basalts, shows distinctively strong anisotropy, with fast polarization directions highly aligned with mantle anisotropy, implying that synchronous crust–mantle deformation occurred in these regions as a result of magmatism during the breakup of West Gondwana. [ABSTRACT FROM AUTHOR]

Backazimuthal variations in the shear wave splitting of core-refracted shear waves (SKS, SKKS and PKS phases, jointly referred to as X KS) at the Black Forest Observatory (BFO, Southwest Germany) indicate small-scale lateral and partly vertical variations of the seismic anisotropy. However, existing anisotropy studies and models for the nearby Upper Rhine Graben (URG) area in the northern Alpine foreland are mostly based on short-term recordings and by this suffer from a limited backazimuthal coverage and averaging over a wide or the whole backazimuth range. To identify and delimit laterally confined anisotropy regimes in this region, we carry out X KS splitting measurements at six neighbouring (semi-)permanent broad-band seismological recording stations (interstation distance 10–80 km). We manually analyse long-term (partly > 20 yr) recordings to achieve a sufficient backazimuthal coverage to resolve complex anisotropy. The splitting parameters (fast polarization direction |$\phi $|⁠ , delay time |$\delta t$|⁠) are determined in a single- and multi-event analysis. We test structural anisotropy models with one layer with horizontal or tilted symmetry axis and with two layers with horizontal symmetry axes (transverse isotropy). To account for lateral variations around a single recording site, modelling is compared for the whole and for limited backazimuth ranges. Based on this, we provide a 3-D block model with spatial variation of anisotropic properties. Based on delay times > 0.3 s and missing discrepancies between SKS and SKKS phases, which do not support lower mantle anisotropy, the found anisotropy is placed in the lithosphere and asthenosphere. The spatial distribution as well as the lateral and backazimuthal variations of the splitting parameters confirm lateral and partly vertical variations in anisotropy. On the east side of the URG, we suggest two anisotropic layers in the Moldanubian Zone (south) and one anisotropic layer in the Saxothuringian Zone (north). In the Moldanubian Zone, a change of the fast polarization directions is observed between the east and the west side of the URG, indicating different textures. At the boundary between the two terranes, an inclined anisotropy is modelled which may be related with deformation during Variscan subduction. Regarding the observation of numerous null measurements and inconsistent splitting parameters, especially (southwest of BFO) in the southern URG, different hypothesis are tested: scattering of the seismic wavefield due to small-scale lateral heterogeneities, a vertical a -axis due to a vertical mantle flow related to the Kaiserstuhl Volcanic Complex, as well as a different preferred orientation of the olivine crystals (not A-type, but C-type) due to specific ambient conditions (high temperature, water content). [ABSTRACT FROM AUTHOR]

Thirty-seven portable accelerometers were deployed in the eastern San Francisco Bay communities of Walnut Creek and Concord to study site response in a fault-bounded, urban, sedimentary basin. Local earthquakes were recorded for a period of two years from 2017 to 2019 resulting in 101 well-recorded events. Site response is estimated by two methods: the reference site spectral ratio method and a source-site spectral inversion method. The reference site spectral ratio method allows investigation of the variability of site amplification with source azimuth and frequency. The source-site spectral inversion method yields the best least-squares fit to site response for a database of ground-motion records. Both methods show substantial amplification in the Walnut Creek-Concord basin below 2 Hz indicating strong surface-wave development. Greater amplification is seen for sources aligned along the long axis of the basin. Inversion using close-in sources at short distances yields lower amplification at longer periods than the entire data set due to reduced surface-wave generation for steeper angles of incidence. Inversion of site response spectra for shallow shear-wave velocity using a global search algorithm yields VS30 values consistent with generalized mapping results based on geology and topography but with greater variability due to local site variations. 3D finite-element modeling shows greater amplification in the Walnut Creek-Concord basin with a basin-edge effect likely contributing to higher ground motions. Topography is also seen to lead to increased scattering and shadowing effects. [ABSTRACT FROM AUTHOR]

On-site earthquake early warning (EEW) requires the best estimates of earthquake magnitude and distance parameters within a few seconds after the P-wave arrival for estimating the subsequent S-wave parameters. The errors in the estimates of the earthquake P-wave parameters will propagate into the estimates of the S-wave parameters. To solve this problem, we used the methodology by Zhao and Zhao (2019), which uses the spectral ratio between the response spectral values of the first 3 s of the S wave and that of the first 3 s of the P wave, referred to as the R3P model. The modeling presented here was based on strong-motion records from the Next Generation Attenuation (NGA)-West2 dataset. We also used the spectral ratio between the response spectral values from the full records from the S-wave arrival time to the end of the record and that of the first 3 s P wave to develop a second EEW model (the RFP model). An advantage of these two models is that the magnitude and hypocentral distance are not required in considerable magnitude and distance ranges. This means that the errors in the estimated source and path parameters from the first 3 s of the P wave will not affect the model predictions. A theoretical justification for these results is that the magnitude and the distance scaling rates for the first 3 s of the P wave are similar to those of the first 3 s of the S wave. This may also apply to the full S-wave window within useful EEW magnitude and distance ranges. We also found that when the estimated magnitude and distance for a record are necessary, the effect of the corresponding errors would be smaller than using a ground-motion prediction equation (GMPE), because the magnitude and distance scaling rates from this study are smaller than those of many GMPEs. [ABSTRACT FROM AUTHOR]

The time-averaged shear-wave velocity in the top 30 m, VS30, is used to represent the site condition in many ground-motion models (GMMs). Regionalized GMMs account for regional differences in the in(VS30) scaling by including region-specific coefficients for the site term. For example, recent GMMs developed for subduction zone earthquakes as part of the Next Generation Attenuation-Subduction (NGA-Sub) project include region-specific VS30 scaling for seven regions: South America, Central America, Japan, New Zealand, Taiwan, Cascadia (Pacific Northwest America), and Alaska. Of these seven regions, South America has the largest within-event standard deviation. One cause for the larger within-event standard deviation is a weaker relation between VS30 and the site term in South America compared to other regions. A cause for this weak correlation is that most of the VS30 values for the South American region in the NGA-Sub data set are based on proxies. The relation between VS30 and the site term improves significantly for periods less than 1 s if only stations with measured VS30 are considered, but the correlation is weak for periods greater than 1 s even for stations with measured VS30, indicating that, for the South America region, VS30 is not well correlated with the deeper shear-wave velocity profile that controls the long-period amplification. To improve the site terms for South America, we develop a model based on the horizontal-to-vertical spectral ratio (HVSR) from microtremors, similar to the approach used by Pinilla-Ramos et al. (2022) for California. The data set includes 660 recordings from 51 earthquakes recorded at 274 sites. The site-term model includes the period-dependent HVSR amplitude and the geometric mean of the average HVSR amplitude over the frequency band of 0.25 to 15 Hz. Including these two parameters reduces the standard deviation of site-specific site terms over the period range 0.3-3 s, with the largest reduction in the period range 1-2 s. This site-term model can be incorporated into subduction GMMs and implemented in probabilistic seismic hazard analyses for South America. [ABSTRACT FROM AUTHOR]

To estimate predicted ground motion from a teleseismic slip model, we use a low- and high-frequency hybrid method to simulate the regional, strong ground motions observed following the 18 April 2014 moment magnitude (MW) 7.3 Papanoa, Mexico, earthquake. To generate the regional ground motion at low frequencies (<1 Hz), a teleseismically derived, finite-fault, kinematic model is used to define the earthquake source, taking into account slip-model variations identified with a parameter sampling approach that considers possible errors in the fault geometry, the hypocenter depth, and the rupture velocity. A 3D crustal model is used to calculate the low-frequency ground motions using a finite-element calculation that includes topography and considers variations in the source model to estimate the uncertainty in the calculations. High frequencies (>1 Hz) are added using a 1D full-wave propagation code that estimates uncertainties by considering multiple random distributions of slip with different spatial correlation lengths. The synthetic, broadband (0.05-10.0 Hz) ground motions are obtained by combining the low- and high-frequency portions match filtered at 1 Hz. These synthetic ground motions are compared with the regional observations using velocity records, peak ground acceleration, and medians of the orientation-independent response spectra of the horizontal components (RotD50) calculated at periods of 0.2, 0.3, 0.5, 1.0, 2.0, 3.0, 5.0, 7.5, and 10.0 s. The results indicate that ground motions estimated at these periods using our hybrid approach based primarily on a teleseismically derived source model are comparable to the values observed for the 2014 Papanoa earthquake at regional distances. The approach could be used to estimate strong-motion spectral levels expected for regions with limited local and regional recordings and could also fill in magnitude or distance gaps in ground-motion prediction relations utilized in the assessment of seismic hazard. [ABSTRACT FROM AUTHOR]

The P-wave peak distribution in azimuth and takeoff angles, corrected for the distance through empirical attenuation laws, can reveal the radiation pattern amplitude of the source (Tarantino et al., 2019). This piece of information, jointly combined with the available polarities and S-wave peak amplitudes, can provide the full focal mechanism, constraining the solution also when only a few seismic station records are available, that is, in the case of microseismicity. We proposed a new technique, named P-, S-wave amplitude, and polarities (P-SAP), designed to compute the focal mechanism by jointly inverting the P-, S-wave amplitude ratios and P-wave polarities in a Bayesian framework and employing the octree strategy (Fang et al., 1996) to explore the space of possible solutions. The outputs are strike, dip, and rake angles of the most likely triplet (principal and auxiliary planes) with the related uncertainties, as well as other multiple solutions, if present. We tested the methodology to synthetic data, and we applied it to a microseismic sequence that occurred in Irpinia region, southern Italy. A background microseismicity occurs in a volume delimited by the faults activated during the 1980 M 6.9 Irpinia earthquake. This faults system is complex and composed of northwest-southeast-striking normal faults along the Apennines chain and an approximately east-west-oriented strike-slip fault, deep-seated in the Potenza area. A network of three-component accelerometers and velocimeters are currently deployed to monitor the area. The major topic of this work is validating the proposed P-SAP methodology for microseismicity studies. We also inferred the optimum stress tensor of the sequence, confirming that the microseismicity is controlled by the regional stress field and can reveal characteristics useful to highlight behaviors of larger-scale seismicity. [ABSTRACT FROM AUTHOR]

The objective functions adopted by earthquake early warning (EEW) location algorithms are inadequate for out-of-network earthquakes. As a result, the real-time locations of these earthquakes are often erroneous. The consequences of mislocating out-of-network earthquakes are that their magnitudes are miscalculated, and the loci of their shaking predictions map are shifted. Given that the largest earthquakes occur in subduction settings, improving real-time out-of-network earthquake location is of great importance. In this study, the Virtual Dynamically Assembled Array Algorithm (ViDA³) is introduced, which addresses the location issue of offshore and off-network earthquakes. The guiding principle underlying the new EEW location algorithm is that standard seismic networks may be viewed as a collection of medium-sized seismic arrays, with each array consisting of three or more network stations. The potential of array seismology for EEW against out-of-network earthquakes stems primarily from the slowness vector, which points at the direction of the epicentral region. Thus, this region may be constrained merely by intersecting two or more such vectors. In addition, the length of the slowness vector depends on the hypocentral distance and depth and is thus vital for addressing an acute problem in a subduction setting--discriminating between upper crust and deep slab earthquakes. Furthermore, when the slowness of the P phase is known, the slowness of the S phase is deduced, and the S-phase arrival is searched for using the shift-and-sum practice. What makes ViDA³ so attractive is that, in locations where a real-time network is already in place, these added values may be achieved without extra hardware or substantial budget requirements. We present the result of ViDA³ real-time operation on a shallow earthquake offshore Vancouver Island and the result of its replay on a deep slab earthquake in northern Chile. performance is further assessed using a dataset of seismograms from the Mendocino Triple Junction area. It is concluded that ViDA³ location scheme outperforms currently available EEW location algorithms for out-of-network earthquakes. [ABSTRACT FROM AUTHOR]

Determining the depths of small crustal earthquakes is challenging in many regions of the world, because most seismic networks are too sparse to resolve trade-offs between depth and origin time with conventional arrival-time methods. Precise and accurate depth estimation is important, because it can help seismologists discriminate between earthquakes and explosions, which is relevant to monitoring nuclear test ban treaties and producing earthquake catalogs that are uncontaminated by mining blasts. Here, we examine the depth sensitivity of several physics-based waveform features for ~8000 earthquakes in southern California that have well-resolved depths from arrival-time inversion. We focus on small earthquakes (2 < ML < 4) recorded at local distances (<150 km), for which depth estimation is especially challenging. We find that differential magnitudes (MW/ML-MC) are positively correlated with focal depth, implying that coda wave excitation decreases with focal depth. We analyze a simple proxy for relative frequency content, ɸ ≊ log10(M0) + 3log10(fc)ɸ, and find that source spectra are preferentially enriched in high frequencies, or "blue-shifted," as focal depth increases. We also find that two spectral amplitude ratios Rg 0.5-2 Hz/Sg 0.5-8 Hz and Pg/Sg at 3-8 Hz decrease as focal depth increases. Using multilinear regression with these features as predictor variables, we develop models that can explain 11%-59% of the variance in depths within 10 subregions and 25% of the depth variance across southern California as a whole. We suggest that incorporating these features into a machine learning workflow could help resolve focal depths in regions that are poorly instrumented and lack large databases of well-located events. Some of the waveform features we evaluate in this study have previously been used as source discriminants, and our results imply that their effectiveness in discrimination is partially because explosions generally occur at shallower depths than earthquakes. [ABSTRACT FROM AUTHOR]

The detection of microseismic events with low signal-to-noise ratios (SNRs) can expand the seismic catalog and provide opportunities for a deeper understanding of subsurface reservoir features. We propose a novel polarization analysis method for comprehensively detecting S-wave arrival and P-S travel time of low-SNR events from the particle motion of P and S waves in the time and frequency domain. In most circumstances, the direct S-wave particle motion shows a flat plane, and that is perpendicular to the direct P-wave motion direction. We combine these two properties to detect the S-wave arrival of low-SNR events. Our previous study applied spectral matrix (SPM) analysis to characterize the 3D particle motion of P waves. However, SPM analysis had limitations in detecting S-wave arrivals. We then introduce the time-delay components of the SPM (complex spectral matrix [cSPM]) to characterize the S-wave particle motion, separate the S-wave from the noise, and detect S-wave arrivals. Using the cSPM analysis method, we assess the planarity and perpendicularity of the S-wave polarization in the time and frequency domains. We then define a characteristic function that detects S-wave arrivals by combining two properties, planarity and perpendicularity, to detect more low-SNR events. The P-S travel time is obtained by setting the threshold values for the P- and S-wave characteristic functions. We apply our method to 4 hr and 2 months of field data recorded at the Groningen field in the Netherlands. Our method successfully detects the P-S travel time of all catalog events and several additional undetected events. We locate the hypocenter of all events using the detected P-S travel times with a grid-based search method. [ABSTRACT FROM AUTHOR]

Estimating the corner frequency (fc) of an earthquake is of fundamental importance to improving our knowledge of the physics of the rupture that gives rise to a seismic event, while also having important implications by providing information on the high-frequency radiation for seismic hazard studies. However, the estimation of fc through spectral-fitting methods suffers from trade-offs with the estimation of seismic wave attenuation, making the obtained values precise but not necessarily accurate. For this reason, after a review of the source model proposed by Brune (1970), a new method of estimating fc, based on the phase fitting of Fourier spectra of the Brune's seismic pulse related to S-waves, called Fourier phase spectra fitting (FPS), is proposed and evaluated in this study. The method can be applied in cases in which Brune's model may be appropriate, while also considering the effects of propagation on impulse deformation, for which the synthetic tests have been conducted. The results, obtained first using synthetic seismograms generated under controlled conditions and then on a data set of recordings of real seismic events collected at the Groningen gas field in the Netherlands, showed the method to be promising (being accurate and precise) and at the same time pointed out its limitations, with its applicability being restricted to short hypocentral distances of <20 km. [ABSTRACT FROM AUTHOR]

SUMMARY: Seismic traveltime tomography is a geophysical imaging method to infer the 3-D interior structure of the solid Earth. Most commonly formulated as a linearized inverse problem, it maps differences between observed and expected wave traveltimes to interior regions where waves propagate faster or slower than the expected average. The Earth's interior is typically parametrized by a single kind of localized basis function. Here we present an alternative approach that uses matching pursuits on large dictionaries of basis functions.Within the past decade the (Learning) Inverse Problem Matching Pursuits [(L)IPMPs] have been developed. They combine global and local trial functions. An approximation is built in a so-called best basis, chosen iteratively from an intentionally overcomplete set or dictionary. In each iteration, the choice for the next best basis element reduces the Tikhonov–Phillips functional. This is in contrast to classical methods that use either global or local basis functions. The LIPMPs have proven their applicability in inverse problems like the downward continuation of the gravitational potential as well as the MEG-/EEG-problem from medical imaging. Here, we remodel the Learning Regularized Functional Matching Pursuit (LRFMP), which is one of the LIPMPs, for traveltime tomography in a ray theoretical setting. In particular, we introduce the operator, some possible trial functions and the regularization. We show a numerical proof of concept for artificial traveltime delays obtained from a contrived model for velocity differences. The corresponding code is available online. [ABSTRACT FROM AUTHOR]

SUMMARY: Mode conversion of P waves at the boundary between Earth's crust and upper mantle, when analysed using receiver functions (RFs), allows characterization of Earth structure where seismic station density is high and earthquake sources are favourably distributed. We applied two ensemble decision tree algorithms—Random Forest (RanFor) and eXtreme Gradient Boost (XGBoost)—to synthetic and real RF data to assess these machine learning techniques' potential for crustal imaging when available data are sparse. The synthetic RFs, entailing both sharp increases in seismic velocity across the Moho and gradational Moho structures, calculated with and without added random noise, correspond to idealized crustal structures: a dipping Moho, Moho offset by crustal-scale faults, anti- and synform Moho structures and combinations of these. The RanFor/XGBoost algorithm recovers input structures well regardless of event-station distributions. Useful crustal and upper mantle seismic velocities can also be determined using RanFor and XGBoost, making it possible to image crustal thickness and P- and S-wave velocities simultaneously from RFs alone. We applied the trained RanFor/XGBoost to RFs determined from real seismic data recorded in the contiguous United States, producing a map of the Moho and P- and S-wave velocities of the lowermost crust and uppermost mantle. Use of XGBoost, which evaluates residuals between input RFs and ground-truth to update the decision tree using the gradient of a penalty function, improves the crustal thickness estimates. [ABSTRACT FROM AUTHOR]

SUMMARY: A major challenge in seismic tomography consists in quantifying and representing model resolution and uncertainty, particularly at global scales. This information is crucial for interpretations of tomographic images and their technical application in geodynamics. However, due to large computational costs, there have been only few attempts so far to coherently analyse the spatially varying resolving power for a complete set of model parameters. Here, we present a concept for an effective evaluation and global representation of the 3-D resolution information contained in a full set of averaging kernels. In our case, these kernels are constructed using the 'Subtractive Optimally Localized Averages' (SOLA) method, a variant of classic Backus-Gilbert inversion suitable for global tomography. Our assessment strategy incorporates the following steps: (1) a 3-D Gaussian function is fitted to each averaging kernel to measure resolution lengths in different directions and (2) we define a classification scheme for the quality of the averaging kernels based on their focus with respect to the estimated 3-D Gaussian, allowing us to reliably identify whether the inferred resolution lengths are robust. This strategy is not restricted to SOLA inversions, but can, for example, be applied in all cases where point-spread functions are computed in other tomographic frameworks. Together with model uncertainty estimates that are derived from error propagation in the SOLA method, our concept reveals at which locations, resolution lengths and interpretations of model values are actually meaningful. We finally illustrate how the complete information from our analysis can be used to calibrate the SOLA inversion parameters—locally tunable target resolution kernels and trade-off parameters—without the need for visual inspection of the individual resulting averaging kernels. Instead, our global representations provide a tool for designing tomographic models with specific resolution-uncertainty properties that are useful in geodynamic applications, especially for linking seismic inversions to models of mantle flow. [ABSTRACT FROM AUTHOR]

SUMMARY: We define double (S1S2) and triple (PS1S2) singularity points and their degeneracy classes in triclinic anisotropic media. We derive equations for the group velocity image for all these cases. The degeneracy classes are defined by factorization of quadratic (double singularity point) and cubic (triple singularity point) forms with three variables. [ABSTRACT FROM AUTHOR]

SUMMARY: Imaging seismic velocity discontinuities within the Earth's interior offers important insight into our understanding of the tectonic plate, associated mantle dynamics, and the evolution of the planet. However, imaging velocity discontinuities in locations where station coverage is sparse, is sometimes challenging. Here we demonstrate the effectiveness of a new imaging approach using deconvolved SS precursor phases. We demonstrate its effectiveness by applying it to synthetic seismograms. We also apply it to ∼1.6 M SS precursor waveforms from the global seismic database (1990–2018) for comparison with CRUST1.0. We migrate to depth and stack the data in circular 6° bins. The synthetic tests demonstrate that we can recover Moho depths as shallow as 20 km. Globally, the Moho is resolved at 21–67 km depth beneath continental regions. The Moho increases in depth from 21 km ± 4 km beneath the continental shelf to 45–50 km beneath the continental interiors and is as deep as 67 ± 4 km beneath Tibet. We resolve the Moho in 77 percent of all continental bins, within 10 km of CRUST1.0, with all outliers located in coastal regions. We also demonstrate the feasibility of using this method to image discontinuities associated with the mantle transition zone with both synthetic and real data. Overall, the approach shows broad promise for imaging mantle discontinuities. [ABSTRACT FROM AUTHOR]

SUMMARY: This study examines the topographic effect on the body-wave polarization and, subsequently, on near-surface wave speed estimation. We first derive 3-D P- and S-wave polarization angles in the presence of ground tilt, where the angles are functions of the ground tilt orientation, the near-surface wave speeds and the incident wave direction. We find that S-wave polarization angle varies considerably (e.g. more than 100 per cent) when the incident angle is close to the critical angle. The counter-intuitive phenomenon for flat surface, that is, P-wave polarization being only sensitive to S but not P wave speeds, breaks down in the presence of ground tilt, and P-wave polarization becomes sensitive to both P and S wave speeds. Examining the differences in the inferred wave speeds with and without the flat-surface assumption reveals that bias in wave speed estimates is, in general, higher for smaller incident angles, for example, about 50 per cent or higher for a 15° ground tilt and near-vertical (<5°) incidence. The effect on P wave speed estimates is also significant when the S-wave incident angle approaches the critical angle. In order to investigate the topographic effect on wave speed estimates inferred using teleseismic polarization data, we revisit the near-surface wave speed estimates at Hi-net stations from a previous study that did not account for the topography. Based on the ground tilt and strike angles measured for each Hi-net site at 300-m scale—which is found to be the most relevant scale—we constrain P and S wave speeds utilizing the P-wave polarization data. We find that P-wave polarization data alone can effectively constrain not only S but also P wave speeds, especially when the ground tilt is sufficiently large (e.g. >5°). Furthermore, our additional test suggests that including S-wave polarization data with the tilt consideration will improve the near-surface wave speed estimates significantly compared to when the tilt effect is ignored. [ABSTRACT FROM AUTHOR]

In a large body of rock-physics research, seismic wave velocity dispersion and attenuation in fluid-saturated porous rock are studied by constructing analytical or numerical models for time- or frequency-dependent dynamic (effective, or viscoelastic) moduli. A key and broadly used model of such kind is the Zener's, or the standard linear solid (SLS). This model is qualitatively successful in explaining many field and laboratory observations and serves as the key element of many generalizations such as the Burgers model for plastic deformations or the generalized SLS explaining band-limited or near-constant seismic attenuation. However, as a physical model of fluid-saturated porous rock, the SLS has several major limitations: disregard of inertial effects, absence of secondary wave modes and lack of key physical parameters such as porosity and Skempton coefficients. Grainy and porous rock is an unconsolidated material in which the effective density is frequency-dependent, and its effects on wave velocities may exceed those of the dynamic modulus. To overcome these limitations of the empirical SLS, we propose a rigorous rheologic model based on classical continuum mechanics and called the extended SLS, or eSLS. This rheology explains the available attenuation and dispersion observations equally well, but it is also close to Biot's model, honours all poroelastic relations, includes inertial effects, and reveals several new physical properties of the material. Detailed comparison of the eSLS and Biot's models gives a physical-mechanism-based classification of wave-induced fluid flow (WIFF) phenomena. In this classification, the so-called 'global-scale' flows occur in Biot's type structures within the material, whereas the 'local-scale' WIFF occurs in eSLS-type structures. Combining Biot's and eSLS models gives a broad class of rheologies for linear anelastic phenomena within rock with a single type of porosity. The model can be readily generalized to multiple porosities and different types of internal variables, such as describing squirt flows, wetting or thermoelastic effects. Modelling is conducted with relatively little effort, using a single matrix equation similar to a mechanical form of the standard SLS. By combining the eSLS and Biot's models, observations of dynamic-modulus dispersion and attenuation can be inverted for macroscopic mechanical properties of porous materials. [ABSTRACT FROM AUTHOR]

Seismic traveltime anomalies of waves that traverse the uppermost 100–200 km of the outer core have been interpreted as evidence of reduced seismic velocities (relative to radial reference models) just below the core–mantle boundary (CMB). These studies typically investigate differential traveltimes of SmKS waves, which propagate as P waves through the shallowest outer core and reflect from the underside of the CMB m times. The use of SmKS and S(m-1)KS differential traveltimes for core imaging are often assumed to suppress contributions from earthquake location errors and unknown and unmodelled seismic velocity heterogeneity in the mantle. The goal of this study is to understand the extent to which differential SmKS traveltimes are, in fact, affected by anomalous mantle structure, potentially including both velocity heterogeneity and anisotropy. Velocity variations affect not only a wave's traveltime, but also the path of a wave, which can be observed in deviations of the wave's incoming direction. Since radial velocity variations in the outer core will only minimally affect the wave path, in contrast to other potential effects, measuring the incoming direction of SmKS waves provides an additional diagnostic as to the origin of traveltime anomalies. Here we use arrays of seismometers to measure traveltime and direction anomalies of SmKS waves that sample the uppermost outer core. We form subarrays of EarthScope's regional Transportable Array stations, thus measuring local variations in traveltime and direction. We observe systematic lateral variations in both traveltime and incoming wave direction, which cannot be explained by changes to the radial seismic velocity profile of the outer core. Moreover, we find a correlation between incoming wave direction and traveltime anomaly, suggesting that observed traveltime anomalies may be caused, at least in part, by changes to the wave path and not solely by perturbations in outer core velocity. Modelling of 1-D ray and 3-D wave propagation in global 3-D tomographic models of mantle velocity anomalies match the trend of the observed traveltime anomalies. Overall, we demonstrate that observed SmKS traveltime anomalies may have a significant contribution from 3-D mantle structure, and not solely from outer core structure. [ABSTRACT FROM AUTHOR]

This study attempts to interrogate the upper mantle deformation pattern beneath the Kumaon-Garhwal region, located in the western Himalaya, using shear wave splitting (SWS) analysis of core-refracted (XK(K)S) phases recorded at 53 broad-band stations. The fast polarization azimuths (FPAs) revealed by 338 well constrained measurements are dominantly clustered around ENE–WSW, with a few along the NE and E–W directions. The delay times vary from 0.2 to 1.4 s, with an average of 0.6 s that is smaller than that for the Indian shield (∼0.8 s), central and eastern Himalayas. The northern part of the lesser Himalaya shows a slightly smaller delay time compared to the southern part, which is attributed to the weakening of azimuthal anisotropy caused by the dipping of the Indian lithosphere. In order to understand the crustal contribution, its anisotropy is measured by analysing the splitting of Ps conversions from the Moho (Pms), akin to that of the XK(K)S phases. However, reliable results for crustal anisotropy could be obtained only at 10 stations. The average delay time due to crustal anisotropy is 0.47 s, with a variation from 0.2 to 0.9 s. Although the dominant period of Pms is smaller than that of SK(K)S, crustal anisotropy contributing to splitting of the latter phases cannot be ruled out. The orientation of FPAs obtained from Pms phases is found to be parallel or sub-parallel to those from XK(K)S phases, suggesting a similar deformation mechanism in the mid- to lower-crust and upper mantle. On the basis of FPAs derived from XK(K)S measurements, the Kumaon-Garhwal Himalaya (KGH) region can be divided into four subregions. In the western and eastern parts, the FPAs are mostly aligned along NE and ENE–WSW, and NE, respectively. In the central and south-eastern parts, their orientation is along ENE–WSW and NW, respectively. The strong ENE–WSW orientation in the central part could result from a slightly variable anisotropy in the crust to the upper part of the lithosphere or basal topography causing deflection of mantle flow. Also, the NW orientation in the south-eastern part of KGH is associated with a shallow source within the lithosphere. Application of the spatial coherency technique to single-layered anisotropic parameters results in a depth of 220–240 km, implying that the dominant source of anisotropy could lie in the upper mantle. [ABSTRACT FROM AUTHOR]

Full-waveform inversion (FWI) of seismic data provides quantitative constraints on subsurface structures. Despite its widespread success, FWI of data around the critical angle is challenging because of the abrupt change in amplitude and phase at the critical angle and the complex waveforms, especially in the presence of a sharp velocity contrast, such as at the Moho transition zone (MTZ). Furthermore, the interference of refracted lower crustal (Pg) and upper mantle (Pn) arrivals with the critically reflected Moho (PmP) arrivals in crustal and mantle studies makes the application of conventional FWI based on linearized model updates difficult. To address such a complex relationship between the model and data, one should use an inversion method based on a Bayesian formulation. Here, we propose to use a Hamiltonian Monte Carlo (HMC) method for FWI of wide-angle seismic data. HMC is a non-linear inversion technique where model updates follow the Hamiltonian mechanics while using the gradient information present in the probability distribution, making it similar to iterative gradient techniques like FWI. It also involves procedures for generating distant models for sampling the posterior distribution, making it a Bayesian method. We test the performance and applicability of HMC based elastic FWI by inverting the non-linear part of the synthetic seismic data from a three-layer and a complex velocity model, followed by the inversion of wide-angle seismic data recorded by two ocean bottom seismometers over a 70 Ma old oceanic crustal segment in the equatorial Atlantic Ocean. The inversion results from both synthetic and real data suggest that HMC based FWI is an appropriate method for inverting the non-linear part of seismic data for crustal studies. [ABSTRACT FROM AUTHOR]

The Lijiang–Xiaojinhe fault (LXF) and its vicinity are located in the transition zone among the Tibetan Plateau (TP), the South China block and the Indochina block. Researchers believe that this area has acted as a key tectonic zone during the evolution of the TP. Owing to the continuous growth and SE-ward expansion of the TP, the LXF and its vicinity have experienced intense deformation. Although different models, such as the rigid block extrusion and mid-lower crustal flow models, have been proposed to explain this intense deformation, a consensus has not yet been achieved. To better understand the deformation of the LXF and its vicinity, a high-resolution image of the subsurface structure must be constructed. In this study, we construct images of P -wave velocity and azimuthal anisotropy structures by using an eikonal equation-based traveltime tomography method. We collect high-quality seismic data from 276 broad-band seismic stations and manually pick a total of 48   037 first arrivals for the tomography study. Our tomographic results reveal a strong low-velocity body below the LXF and its vicinity. In addition, a strong azimuthal anisotropy structure with an N–S-oriented fast velocity direction is distributed along the low-velocity body. These features indicate the occurrence of mid-lower crustal flow, that penetrates across the LXF and extends to the Dianzhong block (DZB). In addition, we find obvious low-velocity perturbations in the mid-lower crust and uppermost mantle beneath the DZB. The low velocities may be attributed to the upwelling of hot materials from the upper mantle. We consider the limited distribution of mid-lower crustal flow on the margin of the SE TP, and mid-lower crustal flow may not play a significant role in the expansion of the TP. [ABSTRACT FROM AUTHOR]

The recent rapid improvement of machine learning techniques had a large impact on the way seismological data can be processed. During the last years several machine learning algorithms determining seismic onset times have been published facilitating the automatic picking of large data sets. Here we apply the deep neural network PhaseNet to a network of over 900 permanent and temporal broad-band stations that were deployed as part of the AlpArray research initiative in the Greater Alpine Region (GAR) during 2016–2020. We selected 384 well distributed earthquakes with M L ≥ 2.5 for our study and developed a purely data-driven pre-inversion pick selection method to consistently remove outliers from the automatic pick catalogue. This allows us to include observations throughout the crustal triplication zone resulting in 39 599 P and 13 188 S observations. Using the established VELEST and the recently developed McMC codes we invert for the 1-D P - and S -wave velocity structure including station correction terms while simultaneously relocating the events. As a result we present two separate models differing in the maximum included observation distance and therefore their suggested usage. The model AlpsLocPS is based on arrivals from ≤130 km and therefore should be used to consistently (re)locate seismicity based on P and S observations. The model GAR1D_PS includes the entire observable distance range of up to 1000 km and for the first time provides consistent P - and S -phase synthetic traveltimes for the entire Alpine orogen. Comparing our relocated seismicity with hypocentral parameters from other studies in the area we quantify the absolute horizontal and vertical accuracy of event locations as ≈2.0 and ≈6.0 km, respectively. [ABSTRACT FROM AUTHOR]

Around the world the Earth's crust is blanketed to various extents by sediment. For continental regions, knowledge of the distribution and thickness of sediments is crucial for a wide range of applications including seismic hazard, resource potential and our ability to constrain the deeper crustal geology. Excellent constraints on the sediment thickness can be obtained from borehole drilling or active seismic surveys. However, these approaches are expensive and impractical in remote continental interiors such as central Australia. Recently, a method for estimating the sediment thickness using passive seismic data, the collection of which is relatively simple and low cost, was developed and applied to seismic stations in South Australia. This method uses receiver functions, specifically the time delay of the P -to- S converted phase generated at the sediment-basement interface, relative to the direct- P arrival, to generate a first-order estimate of the thickness of sediments. In this work, we expand the analysis to the vast array of over 1500 seismic stations across Australia, covering an entire continent and numerous sedimentary basins that span the entire range from Precambrian to present day. We compare with an established yet separate method to estimate the sediment thickness, which utilizes the autocorrelation of the radial receiver functions to ascertain the two-way traveltime of shear waves reverberating in a sedimentary layer. Across the Australian continent the new results match the broad pattern of expected sedimentary features based on the various geological provinces. We are able to delineate the boundaries of many sedimentary basins, such as the Eucla and Murray Basins, which are Cenozoic, and the boundary between the Karumba Basin and the mineral rich Mount Isa Province. Contrasts in seismic delay time across these boundaries are upwards of 0.4  |$\, \mathrm{s}$|⁠. The delay signal is found to diminish to <0.1  |$\, \mathrm{s}$| for older Proterozoic basins, likely due to compaction and metamorphism of the sediments over time. As an application of the method, a comparison with measurements of sediment thickness from local boreholes allows for a straightforward predictive relationship between the delay time and the cover thickness to be defined. This offers future widespread potential, providing a simple and cheap way to characterize the sediment thickness in underexplored areas from passive seismic data. [ABSTRACT FROM AUTHOR]

The investigation of upper mantle structure beneath the US has revealed a growing diversity of discontinuities within, across, and underneath the sub‐continental lithosphere. As the complexity and variability of these detected discontinuities increase—for example, velocity increase/decrease, number of layers and depth—it is hard to judge which constraints are robust and which explanatory models generalize to the largest set of constraints. Much work has been done to image discontinuities of interest using S‐waves that convert to P‐waves (or top‐side reflected SS waves). A higher resolution method using P‐to‐S scattered waves is preferred but often obscured by multiply reflected waves trapped in a shallower layer, limiting the visibility of deeper boundaries. Here, we address the interference problem and re‐evaluate upper mantle stratification using filtered P‐to‐S receiver functions (Ps‐RFs) interpreted using unsupervised machine‐learning. Robust insight into upper mantle layering is facilitated with CRISP‐RF: Clean Receiver‐Function Imaging using Sparse Radon Filters. Subsequent sequencing and clustering organizes the polarity‐filtered Ps‐RFs into distinct depth‐based clusters. We find three types of upper mantle stratification beneath the old and stable continental US: (a) intra‐lithosphere discontinuities (paired or single boundary), (b) transitional discontinuities (single boundary or with a top layer), and (c) sub‐lithosphere discontinuities. Our findings contribute a more nuanced understanding of mantle discontinuities, offering new perspectives on the nature of upper mantle layering beneath continents. Plain Language Summary: Early investigations of the mantle rocks in the US indicate intricate layering. However, uncertainties remain regarding the origins of these structures. Here, we re‐examine mantle rock stratification using a fine‐resolution approach. We use short waves that improve our ability to identify the depth of thin layers and sharp transitions in rock properties. Until now, these methods haven't been used due to interference with waves trapped in the near‐surface layers. We address this problem with machine learning and the CRISP‐RF (Clean Receiver Function Images Using Sparse Radon‐Filters) method. CRISP‐RF filters out the waves trapped in the crust and machine learning reveals spatially coherent patterns. Underneath the stable continents, we find evidence for different types of rock layering: (a) reflectors within cold stiff rocks (b) reflectors at depth ranges where the rocks become warmer and flow more readily, and (c) reflectors at depths farther down in the upper mantle. Our approach enables the test of hypotheses about the origins of upper mantle layering beneath continents. Key Points: Upper mantle stratification is constrained using Clean Receiver‐function Imaging using Sparse Radon Filters and machine learningStratification is classified into intra‐lithospheric, transitional and sub‐lithosphericHigh‐resolution constraints allow the evaluation of different causal models [ABSTRACT FROM AUTHOR]

Body wave extraction from oceanic secondary microseismic sources with seismic interferometry provides alternative information to better constrain the Earth's structure. However, sources' spatiotemporal variations raise concerns about travel time measurement robustness. Therefore, we study the cross‐correlations' stability during a single oceanic event. This study focuses on 3 days of data and three seismic arrays' combinations between 8 and 11 December 2014 during storm Alexandra, a "weather bomb" event in southern Greenland. We use the WAVEWATCH III hindcast to model P‐wave noise sources and assess the impact of short‐term source variations on cross‐correlations. Model‐based cross‐correlations compared to data show coherent delays to reference 3D Earth models (∼0–3 s) confirming the robustness of the source model which could explain minor travel time variations (≤1 s). Plain Language Summary: Ocean wave interactions are a significant source of constant seismic wave emissions, known as ambient noise. Methods using correlations between seismic recordings recently highlighted surface waves and, more importantly, body waves to extract properties of the Earth's deep interior. These studies either use continuous recordings to infer medium properties, or focus on wave propagation from a specific storm. However, concerns about measurements can come from the broad oceanic source constantly changing in space and time. We model seismic recordings for 3 days during a powerful oceanic storm in southern Greenland, 8–11 December 2014, to assess the source variations' impact on body wave arrival times. We then compare it to data and measure travel time lags. Our findings explain source‐induced delays and also agree with the known structure of the Earth, with some differences. This tool could add body wave travel time measurements and uncertainties from interferometry to image our planet's deep structures. Key Points: Teleseismic P‐wave sources in the secondary microseismic band are inferred from a hindcast oceanographic modelSeismic interferometry methods are applied to a "weather bomb" event between 8 and 11 December 2014 using an adaptive station pair selectionThree‐hour synthetic cross‐correlation functions are compared to data to assess the impact of continuously varying sources on travel times [ABSTRACT FROM AUTHOR]

Ambient noise technology can efficiently extract surface wave signals from seismic background noise and has been extensively utilized in imaging lithospheric structures. However, retrieving crustal body wave signals, such as PmP or SmS phases, still poses a challenge. Only a limited number of reports have successfully extracted these regional-scale body wave signals from ambient noise in only a few limited study areas. It remains unclear why these signals are difficult to retrieve from ambient noise data. To investigate the mechanism of recovering body wave signals in noise cross-correlations, we calculate cross-correlation functions at four regions and observe the similarity of the recovered body waves. Through a series of synthetic simulations, we demonstrate that the appearance of body wave signals in noise cross-correlations is closely related to the distribution of noise sources. Among these signals, the post-critical SmS wave proves to be the most readily recoverable from ambient noise data, primarily stemming from distant sources. In contrast, the recovery of P -wave requires the array to be in proximity to the sources. Our experiments also reveal that the main origin of PL waves is the multiple reflections of S -waves propagating in the crust. [ABSTRACT FROM AUTHOR]

We have derived an analytical approximate expression to estimate the delay in diving seismic waves due to thin layers of CO2. The expression is valid for high frequencies and can be used to estimate the delay in diving waves at seismic frequencies for large separations between the source and receiver (offset). The approximation may be used to assess CO2 detection limits using diving waves and to support survey planning for CO2 monitoring and full-waveform inversion (FWI) cycle skipping analysis. In this study, we analyse the diving-wave response to a thin layer of CO2 for band-limited data using acoustic finite-difference modelling, and compare the results against the analytical calculations. We find that the responses are offset-dependent and related to double- and single-leg interactions between the diving waves and the CO2. To test the methods, we created a synthetic representation of the 2010 subsurface conditions for the top CO2 layer at the Sleipner storage complex in the North Sea, by combining base and monitor post-stack seismic data with field velocity trends. Using the acoustic finite-difference method, we model pre-stack data that captures the complexity of field data and demonstrate the use of the diving-wave delay for CO2 migration monitoring and CO2 thin layer detection. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing (HF) often stimulates the local earthquake productivity which provides a unique opportunity to characterize the crustal heterogeneities, reservoir properties and fluid injection effects. However, the velocity models acquired solely based on the arrival time records are often undermined due to the seismic network coverage and interpolation techniques. Instead, we adopt the waveform-based approach to apprehend; (1) structural heterogeneities, (2) reservoir distribution and (3) signatures of the injected fluid in the Weiyuan shale gas field. We categorize the waveforms into dominant high and low frequencies based on the qualitative inspection and frequency index analysis of the seismic waveforms. We first inspect the waveform to access the potential controlling mechanisms (source, site and path effects) at both single and multiple stations in different azimuthal orientations. As a result, we find the path effect as a dominant factor to influence the waveform characteristics, for example S -wave amplitude, and frequency. Subsequently, to localize the path effect, we conduct an in-depth examination of events within 10 km of each seismic station and classify the waveform records using their frequency indices. Notably, certain stations record a significant proportion of low-frequency waveforms (LFWs, up to 20 per cent), while others have limited occurrences (∼1 per cent) indicating suspected anomalous zones. Afterward, we identify two suspected anomalous zones based on LFWs intensity and ray tracing map. Both zones are in close proximity to fault zones and preserved reservoirs with no HF activities, where fault damage zones or the fluid-rich reservoir may contribute to our observed LFWs. [ABSTRACT FROM AUTHOR]

A large non-double-couple component of a tectonic earthquake indicates that its rupture likely was complex and likely involved multiple faults. Detailed source models of such earthquakes can add to our understanding of earthquake source complexity. The 2007 Martinique earthquake in the Caribbean Sea is one of the largest recent earthquakes with a known large non-double-couple component. It was an intermediate depth intraslab earthquake within the South American plate where it is subducting beneath the Caribbean plate. We applied potency density tensor inversion (PDTI) to teleseismic P waves generated by the 2007 Martinique earthquake to model its source processes and focal mechanism distribution. We identified two focal mechanisms: a strike-slip mechanism with a north–south tension axis (T- axis), and a downdip extension (DDE) mechanism with an east–west T -axis. Rupture by the DDE mechanism was predominant in the northern part of the source region and strike-slip rupture in the southern part. These two focal mechanisms had approximately parallel pressure axes (P- axes) and approximately orthogonal T -axes. The seismic moments released by both types of rupture were almost equal. These results indicate that the 2007 Martinique earthquake had a large non-double-couple component. We identified five subevents with two predominant directions of rupture propagation: two strike-slip subevents propagated to the southeast and three DDE subevents propagated to the east. Although the directions of propagation were consistent for each focal mechanism, each subevent appears to have occurred in isolation. For example, the rupture of one DDE subevent propagated from the edge of the source region back towards the hypocentre. Complex ruptures that include multiple subevents may be influenced by high pore fluid pressure associated with slab dehydration. Our results show that PDTI can produce stable estimates of complex seismic source processes and provide useful information about the sources of complex intermediate depth intraslab earthquakes for which fault geometry assumptions are difficult. [ABSTRACT FROM AUTHOR]

We present high-resolution 3-D images of isotropic P -wave velocity (Vp), azimuthal anisotropy (AAN) and radial anisotropy (RAN) down to 700 km depth beneath the North China Craton (NCC) and adjacent areas, which are obtained by inverting a great number of high-quality arrival time data recorded at 1374 portable seismic stations and 635 permanent stations in the study region. Our results reveal new and detailed features of the upper mantle structure beneath the NCC. Varying structural heterogeneities are revealed beneath different tectonic blocks, and differences also exist between northern and southern parts of each block. The fast velocity directions (FVDs) of azimuthal anisotropy are mainly NW–SE under the Alaxa block, and NE–SW beneath the Tibetan Plateau. The FVDs present an arc transition along the boundary faults separating the Tibetan Plateau, the Alaxa block, the western NCC, and the Sichuan basin. Low- Vp anomalies with positive RANs (i.e. horizontal Vp  > vertical Vp) are revealed at 100–200 km depths under the Tibetan Plateau, reflecting frozen-in anisotropy in the thick lithosphere. Significant low- Vp anomalies with a circular AAN pattern exist at 0–700 km depths beneath the Datong volcano. In addition, negative RAN occurs right below the volcano, whereas positive RANs appear around it, suggesting that the Datong volcano is fed by hot upwelling flow from the lower mantle associated with collapsing of subducted slab materials down to the lower mantle. The eastern NCC shows complex Vp AANs and RANs. Seismic anisotropy exhibits east–west variations in the upper mantle across the Tanlu fault zone. The west of the Tanlu fault shows negative RANs (vertical Vp  > horizontal Vp), whereas its east shows positive RANs at 300–500 km depths. The low- Vp anomaly under the Datong volcano is connected with a large low- Vp anomaly beneath the eastern NCC above ∼250 km depth, suggesting that the hot upwelling flow under Datong may migrate laterally to the asthenosphere under the eastern NCC and contribute to the lithospheric delamination and destruction there. [ABSTRACT FROM AUTHOR]

We define the degenerate orthorhombic anisotropy models which have two symmetric singularity lines with constant phase velocity for S1 and S2 waves. Depending on the singularity line trajectory, we consider two types of degenerate models (VTI- and HTI-type). In addition to this singularity line, seriethere is always one isolated singularity point in one of non-essential symmetry planes. The degenerate orthorhombic model has seven independent parameters and can be parametrized by different combinations of the stiffness coefficients. Exploiting the fact that the second-order derivatives matrix computed from the Christoffel polynomial is degenerate, we also compute the group velocity image of this singularity line. [ABSTRACT FROM AUTHOR]