Wave Velocity Model Research Articles

Azimuthal seismic anisotropy of the Iran plateau: Insights from ambient noise analysis

The continental lithosphere of the Iran plateau is complicated by many tectonic processes that affected both the Arabian and Eurasian plates before and after their convergence. To investigate the deformation mechanisms of the crust and mantle lithosphere, we directly invert Rayleigh wave phase velocity dispersion data (5–60 s) for a 3-D shear wave velocity and depth-dependent azimuthal anisotropy model using ambient noise tomography from the surface down to 100 km with data recorded in 84 seismic stations. The shear wave velocity maps reveal a reasonable match with geological domains and agree with those previously published. The projections of the fast axes of Rayleigh wave azimuthal anisotropy in the subcrustal lithosphere allowed us to divide the Iran plateau into two main regions: the Zagros Mountains and the rest of the country. Furthermore, the anisotropy pattern illustrates a prominent contrast between the NW and SE Zagros Mountains. In both the crust and subcrustal lithosphere, the NW Zagros shows relatively weak but coherent azimuthal anisotropy in the NE-SW direction (i.e., orogen-perpendicular orientation). We ascribe the orogen-perpendicular fast axis in NW Zagros to stress-induced anisotropy. However, in the SE Zagros, the north-northwest orientations of the fast axes are attributed to the N-S trending basement structures, which are inherited from the Pan-African construction phase. The azimuthal anisotropy pattern displays an overall NW-SE trend dominant over the rest of the country. This NW-SE direction can be explained by an NW-SE extension due to transpressional deformation beneath Central Iran resulting from the oblique indentation of the Arabian plate into Eurasia. Nevertheless, strike-parallel anisotropy directions along the western and central Alborz Mountains throughout the entire lithosphere may be related to pure shear deformation. The persistence of azimuthal anisotropy patterns in the crust and subcrustal lithosphere implies that the whole lithosphere deforms coherently in the NW Zagros, west Iran, the Alborz Mountains, and across the Lut block. However, strong contrasts between the crustal and subcrustal pattern of anisotropy observed in the SE Zagros as well as in north Central Iran suggest that in these regions, the crust and the underlying mantle lithosphere do not deform coherently. A strong correlation between the Rayleigh wave anisotropy directions at subcrustal depths and the anisotropy patterns estimated from the shear-wave core phases suggests that in many places over the plateau, the SKS directions may have been dominated by the deformation of the lithosphere.

Ambient Noise Tomography of Northern Borneo Reveals Evidence of Subduction and Post‐Subduction Processes

AbstractThe region of northern Borneo in South East Asia sits within a post‐subduction setting formed by the recent termination of two sequential but opposed subduction systems. In this study we use seismic data from a recent temporary array deployment to image the crustal velocity structure beneath northern Borneo using a two‐stage Bayesian trans‐dimensional tomography scheme, in which period dependent phase velocity maps are first generated, and then used to build a 3‐D shear wave model through a series of 1‐D inversions. In the second stage, we also apply an Artificial Neural Network to solve the 1D inverse problem, which results in a smoother 3‐D model compared to the TransD approach without sacrificing data fit. Our shear wave velocity model reveals a complex crustal structure. Under the Crocker Range, a heterogeneous velocity structure likely represents remnants of early Miocene subduction, including underthrust continental crust from subsequent continent‐continent collision. In the east we observe high velocities that are interpreted to be igneous rocks in the crust generated by melting due to mid Miocene Celebes Sea subduction and later decompression melting as well as a low velocity zone that could represent underthrust sediment or duplexes from Celebes Sea subduction. A low velocity zone in the lower crust is present in a region of apparent crustal thinning. Our preferred explanation for this anomaly is remnant thermal upwelling within a failed rift that represents the on‐shore continuation of the extension of the Sulu Sea, most likely caused by rollback of the Celebes Sea slab.

A triangular acoustic emission source location method considering its anisotropic propagation behavior on the surface of finger-joint-laminated board

ABSTRACT To understand the anisotropic propagation characteristics of acoustic emission (AE) signals in finger-joint-laminated boards, an improved planar localization method for AE sources is proposed. Initially, the AE source was generated by the pencil-lead break (PLB) at a fixed position on the surface of the glued wood specimen, and two AE sensors were arranged in different directions at a fixed distance of 60 mm, and the AE propagation velocity was calculated according to the time difference of arrival (TDOA). For clarity, the grain parallel direction was defined as 0 degrees, with velocities measured every 10 degrees through a counterclockwise rotation. Using these measurements, two angular anisotropic wave velocity models were developed. Subsequently, three AE sensors arranged linearly on the specimen surface pinpointed the AE source’s location. This source location problem was formulated as a multi-parameter optimization model, constrained by the geometric relationships between the AE source and the sensors. A particle swarm optimization (PSO) algorithm was utilized to estimate the AE source’s angles relative to each sensor. The findings indicate that the located AE sources deviated by 6.0–19.0 mm from the predetermined PLB positions, with inaccuracies largely attributed to the anisotropic AE wave velocity models’ precision.

Seismic site conditions of RESNOM network

The Northwest Seismic Network of Mexico (RESNOM) is operated by personnel from the Center for Scientific Research and Higher Education of Ensenada, Baja California (CICESE), which supervises station installation, improvement, and maintenance. We employed seismic noise and the Horizontal to Vertical Spectral Ratio (HVSR) method to determine, for each station, the following site condition parameters: the depth of the rock layer (Heng_bed), and the geotechnical parameter VS30, obtained from 1D shear wave velocity models. Other parameters as the fundamental frequency (f0) and the average amplitude at the fundamental frequency (A0) were also estimated. Our results show clear differences between the values obtained for the Mexicali Valley and the Peninsular ranges regions. The VS30 obtained for stations of the Mexicali Valley region falls in the range from 173 m/s to 535 m/s, while for the Peninsular Ranges region is between 213 m/s and 958 m/s. Regarding the Heng_bed parameter, the values are similar between both regions, from 23 m to 850 m for the Peninsular and from 42 m to 926 m for the Mexicali Valley. Additionally, from the VS30 values, we propose the site classification according to the U.S. National Earthquake Hazards Reduction Program (NEHRP).

Correction to: AI based 1-D P- and S-wave velocity models for the greater alpine region from local earthquake data

Correction to: AI based 1-D <i>P</i>- and <i>S</i>-wave velocity models for the greater alpine region from local earthquake data

Three-dimensional shear-wave velocity structure of the Adana–Iskenderun basins by ambient noise tomography

SUMMARY We construct a 3-D shear-wave velocity model for the crustal structure and the geometry of the Adana and Iskenderun basins by using ambient noise tomography of Rayleigh waves. For this purpose, we compute interstation Green's functions and measure the group velocity dispersion in the period range of 8–25 s. Then Rayleigh wave group velocity maps obtained by tomographic inversion are used to derive a shear wave velocity model by linearized inversion. Both Rayleigh wave group velocity maps and the 3-D shear-wave velocity structure are correlated with the geology and the major tectonic features of the region. Cross sections taken from the velocity model suggest a sediment thickness of up to 11 km in the wedge-shaped Adana Basin with the velocity ranging between 2.4 and 2.9 km s−1. The horseshoe-like high velocities surrounding the basin correspond to the Taurus Mountains in the west and north, and the Amanos Mountains in the east. In the region, down to a depth of 35 km the crustal velocity varies between 2.9 and 3.7 km s−1. Our investigations reveal the detailed 3-D basin geometry and crustal structure that can be beneficial for hazard assessment, geodynamic modelling as well as hydrocarbon exploration studies.

Construction and experimental verification of wave velocity model for source location in goaf overlying rock strata

The geological structure of the goaf overlying rock is complex, a consequence of coal mining that has modified the original stratified structure of the sedimentary strata. To enhance the accuracy of microseismic source location in such intricate geological formations, a wave velocity model for the “three zones” goaf was constructed based on natural divisions within the strata using Snell's law and assuming a homogeneous medium. The model took into account the effects of rock deformation and fracture development, enabling the derivation of formulas for microseismic wave propagation path and travel time calculation. Additionally, the concept of equivalent wave velocity was defined. An indoor simulation test using similar materials was conducted to establish a geological model of the goaf. By comparing the errors between the theoretical and measured values of equivalent wave velocity, assessing the locating effects before and after implementing the wave velocity model of the goaf, and verifying the feasibility of the model, it was demonstrated that establishing a wave velocity model based on the characteristics of the strata structure was crucial for improving the accuracy of the microseismic source location. Notably, as the propagation path of microseismic waves in the goaf increased, the equivalent wave velocity decreased. The wave velocity structure in the goaf exhibited nonuniformity, with the relative error between the theoretical and measured values of equivalent wave velocity being limited to 10 %. The incorporation of this established wave velocity model into the location method resulted in a substantial 58.57 % increase in locating accuracy.

Three-dimensional joint inversion of surface wave dispersion and gravity data using a petrophysical approach: an application to Los Humeros Geothermal Field

SUMMARY We present a method to jointly invert surface wave dispersion data and gravity measurements for 3-D shear wave velocity and density models. We implemented a petrophysical approach to combine the kernels of both methodologies in a single process. The synthetic experiments show that jointly inverted models recover shear wave velocity and density better than separate inversions. In particular, density models benefit from the good vertical resolution of surface wave dispersion data, while shear velocity models benefit from the good lateral resolution of gravity data. We also proposed two methods to stabilize the solution when using high-grade polynomials. We applied the methodology to the Los Humeros Geothermal area to demonstrate its applicability in a complex geological scenario. Compared with separate inversion, the joint inversion contributes to enhancing key aspects of the geothermal system by (i) delimitating better the geometry of the caldera deposits in the first 0–2.8 km deep by increasing the vertical resolution in density, (ii) delimitating better the lateral borders of low-Vs bodies at different depths interpreted as a part of a complex magmatic chamber system and (iii) estimating the local shear wave velocity–density relationship that conforms to other known relationships for sedimentary and igneous rocks but with some differences that bring us additional information.

Crustal and upper mantle 3-D Vs structure of the Pannonian region from joint earthquake and ambient noise Rayleigh wave tomography

SUMMARY The Pannonian Basin, situated in Central Europe, is surrounded by the Alpine, Carpathian and Dinaric orogens. To understand its tectonic characteristics and evolution, we determine a shear wave velocity model of its crust, mantle lithosphere and asthenosphere consistently by jointly inverting Rayleigh wave phase velocities measured consistently from earthquake (EQ) and ambient noise (AN) data. For the AN data, continuous waveform data were collected from 1254 stations, covering an area within 9° from the centre of the Pannonian Basin during the time period from 2006 to 2018. This data set enabled the extraction of over 164 464 interstation Rayleigh phase-velocity curves, after applying a strict quality control workflow. For the EQ data set more than 2000 seismic events and about 1350 seismic stations were used in the broader Central and Eastern European region between the time-span of 1990 to 2015, allowing us to extract 139 987 quality controlled Rayleigh wave phase-velocity curve. Using the combined data set, a small period- and distance-dependent bias between ambient noise and earthquake measurements, mostly below 1 per cent but becoming larger towards longer periods has been found. After applying a period and distance dependent correction, we generated phase-velocity maps, spanning periods from 5 to 250 s. 33 981 local dispersion curves were extracted and a new approach is introduced to link their period-dependent roughness to the standard deviation. Using a non-linear stochastic particle swarm optimization, a consistent 3-D shear wave velocity model (PanREA2023) encompassing the crust and upper mantle down to 300 km depth was obtained with a lateral resolution reaching about 50 km at the centre of the study area for shorter periods. The crust beneath the Carpathian orogen exhibits a distinct low-velocity anomaly extending down to the Moho. It is referred to as Peri-Carpathian anomaly. Similar anomalies were observed in the Northern Apennines, while the Eastern Alps and Dinarides, as collisional orogens, generally demonstrate higher velocities in the upper crust. High crustal shear wave velocities are also evident in the Bohemian Massif and the East European Craton. The brittle upper crust of the Pannonian Basin is characterized by alternating NE–SW trending high- and low-velocity anomalies: the western and central Pannonian low-velocity anomalies and the Transdanubian and Apuseni high-velocity anomalies related to Miocene sedimentary basins and intervening intervening interbasinal highs exposing Pre-Cenozoic rocks including crystalline basement rocks. Beneath the Southeastern Carpathians, a NE-dipping slab was identified, extending to depths of at least 200 km, while a slab gap is evident beneath the Western Carpathians. A short south-dipping Eurasian slab was imaged beneath the Eastern Alps down to only 150–200 km depth. The Adriatic lithosphere is subducting near-vertically dipping beneath the Northern Apennines, and a slab gap was observed beneath the Central Apennines. In the Northern Dinarides, a short slab was evident, reaching depths of around 150 km. The Southern Dinarides featured a thinned but possibly incompletely detached slab.

Crustal structure of the Bushveld complex, South Africa from 1D shear wave velocity models: Evidence for complex-wide crustal modification

Thirty-nine 1D shear wave velocity profiles, obtained by jointly inverting receiver functions and Rayleigh wave group velocities, are used to investigate the crustal structure of the Bushveld Complex in northern South Africa. Data from teleseismic earthquakes recorded on broadband seismic stations between 1997 and 1999 and 2015–2020 were used to compute P-wave receiver functions. Rayleigh wave group velocities between 5 and 30 s period were obtained from an ambient noise tomography and combined with group velocities between 30 and 60 s period from a published continental-scale surface wave tomography model. Moho depths of 45–47.5 km are found under the center of the complex compared to 40 km thick crust, on average, surrounding the complex, indicating ∼5–7 of crustal thickening. The bottom ∼10 km or more of the lower crust across much of the Bushveld Complex has a Vs ≥ 4.0 km/s, consistent with a mafic composition, whereas in most areas around the margins of the complex the thickness of the mafic lower crust is much less than 10 km. In the upper crust higher velocity structure (Vs > 3.6 km/s) above 15 km depth underlain by lower velocity structure is seen in many locations, suggesting the presence of mafic/ultramafic layering. These results favor the continuous-sheet model for the structure of the Bushveld Complex because the ensemble of 1D models is characterized by three diagnostic features consistent with that model: (1) thicker crust under the center of the complex than away from the complex; (2) a greater thickness of high-velocity (i.e., mafic) layering in the lower crust under the complex compared to away from the complex; (3) high-velocity (i.e., mafic/ultramafic) layering in the upper crust beneath much of the complex. The lack of upper crustal mafic/ultramafic layering beneath some parts of the complex is consistent with the post-emplacement tectonic and magmatic history of the complex.

3D shear wave velocity and azimuthal anisotropy structure in the shallow crust of Binchuan Basin in Yunnan, Southwest China, from ambient noise tomography

The Binchuan Basin in northwest Yunnan, southwest China, is a rift basin developed at the intersection of the Red River Fault and Chenghai Fault, where historical earthquakes have occurred. Understanding the fine velocity structure of the shallow crust in this region can help improve earthquake location accuracy and our understanding of the relationship between fault zone structures and fault slip behaviors. Using the continuous waveform data recorded by 381 dense array stations in 2017, we obtained 7 915 Rayleigh-wave phase velocity dispersion curves in the period band of 0.2–6 ​s from ambient noise cross-correlation functions after rigorous data processing and quality control. We determined 3D isotropic and azimuthally anisotropic shear wave velocity models at depths above 6 ​km in the shallow crust based on the direct surface wave azimuthal anisotropic tomography method. The isotropic model reveals a strong correspondence between the S-wave velocity structure at depths of 0–1 ​km and the regional topography and lithology. The Binchuan depocenter, Zhoucheng depocenter, Xiangyun Basin, and Xihai Rift Basin are primarily composed of Quaternary deposits, which show low-velocity anomalies, while the regions with the Paleozoic shale, limestone, and basalt exhibit high-velocity anomalies. The nearly N–S orientation of fast directions from azimuthal anisotropy models are mainly controlled by the active Binchuan Fault with N–S strike as well as the NNW-oriented primary compressive stress.

Numerical simulation of ultrasonic P-wave propagation in water-bearing coal based on gas-liquid homogeneous wave velocity model

This paper establishes the equivalent density and equivalent P-wave velocity models of coal bodies with different water saturation, and investigates the P-wave propagation evolution law of coal bodies with different water saturation from the perspective of numerical simulation. When the experimental coal samples continue to use high-pressure water after reaching the natural saturation state, the water saturation can be further increased, and the increase in water saturation is larger, and the corresponding P-wave velocity also appears to be increased more substantially, and this phenomenon also occurs in numerical simulation results, which indicates that the P-wave velocity will have an obvious increasing trend when the coal body is from nearly saturated to fully saturated. The experimental and numerical simulation results show that the P-wave velocity of coal samples increases with the water saturation degree in a similar exponential function, which further verifies the reasonableness and feasibility of the modelling based on the assumption of gas-liquid homogeneous medium. In addition, the propagation of P-wave in coal samples is affected by the pore and fracture structure, and the pore and fracture with small pore size has less influence on the P-wave, and vice versa.

Seismic and Potential Field Constraints on Upper Crustal Architecture of Inner Bering Shelf, Offshore Southwestern Alaska

Southwestern Alaska encompasses a group of fault-bounded tectonostratigraphic terranes that were accreted to North America during the Mesozoic and Paleogene. To characterize the offshore extension of these terranes and several significant faults identified onshore, we reprocessed three intersecting multichannel deep seismic reflection profiles totaling ∼750 line-km that were shot by the R/V Ewing across part of the inner Bering continental shelf in 1994. Since the uppermost seismic section is often contaminated by high amplitude water layer multiples from the hard and shallow seafloor, the migrated reflection images are supplemented with high-resolution P wave velocity models derived by traveltime tomography of the recorded first-arrivals to depths of up to 2000 m. Additionally, other geophysical datasets such as seismicity, well logs, high resolution satellite-altimetry gravity, air-borne magnetics, ship-board gravity and magnetics, are also incorporated into an integrated regional interpretation. We delineate the offshore extension of the major mapped geological elements, including the Togiak fault (TGF), East Kulukak fault (EKF), Chilchitna fault (CF), Lake Clark fault (LCF), Togiak terrane (TT), Goodnews terrane (GT), Peninsular terrane (PT), Northern Kahiltna flysch (NKF) and Southern Kahiltna flysch (SKF) deposits, and the regional suture zone. The geophysical evidence from this study suggest that the major faults and terrane boundaries of southwestern Alaska, excluding the LCF, not only extend beneath the Bering shelf offshore but also appear to rotate, forming a trend parallel to the inactive Beringian margin. The LCF extends offshore but likely terminates in the northeastern part of the Bristol Bay basin. Additionally, the constraints from seismicity data indicates that while the major faults in southwestern Alaska exihibit some activity onshore, they remain dormant in the offshore region. These new findings will contribute to a better understanding of terrane accretion processes and existing fault models of southwestern Alaska.

Lithospheric Evolution of the South‐Central United States Constrained by Joint Inversion of Receiver Functions and Surface Wave Dispersion

AbstractIn the present study, we use broadband seismic data recorded by 190 stations of the EarthScope program's Transportable Array to construct a 3‐D shear wave velocity model for the upper 180 km using a non‐linear Bayesian Monte‐Carlo joint inversion of receiver functions (RFs) and Rayleigh wave dispersion curves. Ambient noise and teleseismic data are used for obtaining Rayleigh wave phase velocity dispersion curves. A resonance removal filtering technique is applied to the RFs contaminated by reverberations from the thick sedimentary layers that cover most of the region. Our observations of the higher crustal shear velocities (∼3.40 km/s) beneath the Sabine Block (SB), along with the estimated relatively thicker crust (∼34.0 km) and lower crustal Vp/Vs estimates (∼1.80) in comparison with the rest of the Gulf Coastal Plain (GCP) (∼3.10 km/s for crustal shear velocities, ∼29.0 km for crustal thickness, and ∼1.90 for crustal Vp/Vs estimates), indicating that this crustal block has different crustal properties from the surrounding coastal plain regions. The southern Ouachita Mountains have a thin crust (∼30.0 km) and low mean crustal Vp/Vs value (∼1.73), suggesting that lower crustal delamination has occurred in this region. Low velocities in the upper mantle beneath most of the GCP are interpreted as a combined result of thin lithosphere, higher‐than‐normal temperatures, and possibly compositional variations.

3D shear wave velocity imaging of the subsurface structure of granite rocks in the arid climate of Pan de Azúcar, Chile, revealed by Bayesian inversion of HVSR curves

Abstract. Seismic methods are emerging as efficient tools for imaging the subsurface to investigate the weathering zone. The structure of the weathering zone can be identified by differing shear wave velocities as various weathering processes will alter the properties of rocks. Currently, 3D subsurface modelling of the weathering zone is gaining increasing importance as results allow the identification of the weathering imprint in the subsurface not only from top to bottom but also in three dimensions. We investigated the 3D weathering structure of monzogranite bedrock near the Pan de Azúcar National Park (Atacama Desert, northern Chile), where the weathering is weak due to the arid climate conditions. We set up an array measurement that records seismic ambient noise, which we used to extract the horizontal-to-vertical spectral ratio (HVSR) curves. The curves were then used to invert for 1D shear wave velocity (Vs) models, which we then used to compile a pseudo-3D model of the subsurface structure in our study area. To invert the 1D Vs model, we applied a transdimensional hierarchical Bayesian inversion scheme, allowing us to invert the HVSR curve with minimal prior information. The resulting 3D model allowed us to image the granite gradient from the surface down to ca. 50 m depth and confirmed the presence of dikes of mafic composition intruding the granite. We identified three main zones of fractured granite, altered granite, and the granite bedrock in addition to the mafic dikes with relatively higher Vs. The fractured granite layer was identified with Vs of 1.4 km s−1 at 30–40 m depth, while the granite bedrock was delineated with Vs of 2.5 km s−1 and a depth range between 10 and 50 m depth. We compared the resulting subsurface structure to other sites in the Chilean coastal cordillera located in various climatic conditions and found that the weathering depth and structure at a given location depend on a complex interaction between surface processes such as precipitation rate, tectonic uplift and fracturing, and erosion. Moreover, these local geological features such as the intrusion of mafic dikes can create significant spatial variations to the weathering structure and therefore emphasize the importance of 3D imaging of the weathering structure. The imaged structure of the subsurface in Pan de Azúcar provides the unique opportunity to image the heterogeneities of a rock preconditioned for weathering but one that has never experienced extensive weathering given the absence of precipitation.

Structure and dynamics of southern Mariana margin: Constraints from seismicity, tomography and focal mechanisms

The southern Mariana margin is a tectonically distinctive and rapidly deforming region with the world deepest trench and unusual deformation and magmatism. However, its related deep structure and dynamics are still poorly understood. In this study, we determine robust 3-D P and S wave velocity models down to 130 km depth beneath the southern Mariana margin by using arrival-time data of local earthquakes recorded at 12 near-field seismic stations. Our tomographic results together with local seismicity and focal mechanisms reveal that the subducted Pacific slab is rapidly rolling back with a steep dip angle and narrowly but strongly coupled with the thin forearc block, which results in the deepest trench. An inferred eastward mantle flow transports the enhanced hydrous mantle melt beneath the southwest Mariana rift to the southern Mariana Trough, causing unusual deformation, magmatism and volcanism in the new oceanic crust. In addition, seismicity in the west Mariana ridge lithosphere may indicate active foundering of the arc lower crust.

The Marmara Sea basin as a regional depression constrained from ambient noise correlation tomography

SUMMARY We computed a 3-D shear wave velocity model of the Marmara Sea region from ambient noise tomography. The correlations of up to 8 yr of vertical-component seismic recordings from 80 broad-band stations provided Rayleigh wave group velocity measurements in the period band 6–21 s at more than 1400 selected virtual source–receiver pairs. Rayleigh wave group velocity maps were used to derive a shear wave velocity model through simulated annealing inversion. The resulting crustal model provides coverage of the Marmara Sea along with its surrounding regional tectonic features. This allows for an investigation of the spatial extents of the Marmara Sea on a scale larger than that of basins. The low-velocity structures of the Marmara Sea and the Thrace Basins are coeval to a depth of approximately 9 km. The crustal velocities beneath the Marmara Sea basins exhibit a low vertical gradient and smooth horizontal variations. The regional tectonic structures, such as Istranca Massif, Istanbul and Sakarya Zones, display sharp velocity contrasts with the lower velocity crust beneath the Marmara Sea. The observed low crustal velocities, along with depth variations of the velocity isosurfaces (i.e. 3.4 km s−1) indicate that the Marmara region is a structural depression much deeper and larger than the three basins of the North Marmara Trough. The North Anatolian Fault Zone is unlikely to be the primary factor contributing to the origin of this significant depression, as the basin's development appears to have occurred before the fault propagated into the region.

Crustal structure of Borneo, Makassar Strait and Sulawesi from ambient noise tomography

SUMMARY Borneo and Sulawesi are two large islands separated by the Makassar Strait that lie within the complex tectonic setting of central Indonesia. The seismic structure beneath this region is poorly understood due to the limited data availability. In this study, we present Rayleigh wave tomography results that illuminate the underlying crustal structure. Group velocity is retrieved from dispersion analysis of Rayleigh waves extracted from the ambient noise field by cross-correlating long-term recordings from 108 seismic stations over a period of 8 months. We then produce a 3-D shear wave velocity model via a two-stage process in which group velocity maps are computed across a range of periods and then sampled over a dense grid of points to produce pseudo-dispersion curves; these dispersion curves are then separately inverted for 1-D shear wave velocity (Vs), with the resultant models combined and interpolated to form a 3-D model. In this model, we observed up to ± 1.2 km s−1 lateral Vs heterogeneities as a function of depth. Our models illuminate a strong low shear wave velocity (Vs) anomaly at shallow depth (≤ 14 km) and a strong high Vs anomaly at depths of 20–30 km beneath the North Makassar Strait. We inferred the sediment basement and Moho depth from our 3-D Vs model based on iso-velocity constrained by the positive vertical gradient of the Vs models. The broad and deep sedimentary basement at ∼14 ± 2 km depth beneath the North Makassar Strait is floored by a shallow Moho at ∼22 ± 2 km depth, which is the thinnest crust in the study area. To the east of this region, our model reveals a Moho depth of ∼45 ± 2 km beneath Central Sulawesi, the thickest crust in our study area, which suggests crustal thickening since the late Oligocene. Moreover, the presence of high near-surface Vs anomalies with only slight changes of velocity with increasing depth in southwest Borneo close to Schwaner Mountain confirm the existence of a crustal root beneath this region.

AI based 1-D P- and S-wave velocity models for the greater alpine region from local earthquake data

SUMMARY 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 ML ≥ 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.

The Magmatic Patterns Formed by the Interaction of the Hainan Mantle Plume and Lei–Qiong Crust Revealed through Seismic Ambient Noise Imaging

Magmatism on continental lithospheres induced by mantle plumes is more complex compared to oceanic intraplate volcanism owing to the heterogeneous nature of continental crustal and lithospheric structures. Substantial evidence points to the deep-oriented Hainan mantle plume beneath the Lei–Qiong region, the southernmost of the South China block. In this study, we present a detailed shear wave velocity model of the crust and uppermost mantle in the Lei–Qiong volcanic region, derived from 3-year seismic data (2016–2018) from 34 stations and the use of the ambient noise tomography method. An evident columnar low-velocity anomaly was imaged in the crust and uppermost mantle beneath the Wushi Sag (WSS), Beibu Gulf, potentially suggesting that the center of either one branch or the entirety of the Hainan mantle plume impacts the crust here. This low-velocity anomaly is overlaid by a local Moho deepening, indicative of underplating beneath the existing crust. The Maanling–Leihuling Volcanic Field (MLVF) in northern Hainan Island, previously considered the center of the hotspot, does not exhibit such distinct velocity anomalies. Instead, subtle lower crustal anomalies beneath the MLVF are linked with the upper mantle low-velocity zone beneath the WSS. Additionally, the high-conductivity bodies beneath the MLVF indicate lateral magma transport. Earthquake swarms and deep-seated seismic events beneath the WSS further support the presence of magmatic processes. This study indicates that in the Lei–Qiong region, the interaction of the continental crust with the mantle plume centered in the WSS results in magma exhibiting both vertical ascent and lateral migration, leading to a dual low-velocity shear wave pattern in the upper crust, which significantly influences the surface volcanic activity.