Low-velocity Anomalies Research Articles

Ab initio determination of melting and sound velocity of neon up to the deep interior of the Earth.

The thermophysical properties and elemental abundances of the noble gases in terrestrial materials can provide unique insights into the Earth's evolution and mantle dynamics. Here, we perform extensive abinitio molecular dynamics simulations to determine the melting temperature and sound velocity of neon up to 370GPa and 7500K to constrain its physical state and storage capacity, together with to reveal its implications for the deep interior of the Earth. It is found that solid neon can exist stably under the lower mantle and inner core conditions, and the abnormal melting of neon is not observed under the entire temperature (T) and pressure (P) region inside the Earth owing to its peculiar electronic structure, which is substantially distinct from other heavier noble gases. An inspection of the reduction for sound velocity along the Earth's geotherm evidences that neon can be used as a light element to account for the low-velocity anomaly and density deficit in the deep Earth. A comparison of the pair distribution functions and mean square displacements of MgSiO3-Ne and Fe-Ne alloys further reveals that MgSiO3 has a larger neon storage capacity than the liquid iron under the deep Earth condition, indicating that the lower mantle may be a natural deep noble gas storage reservoir. Our results provide valuable information for studying the fundamental behavior and phase transition of neon in a higher T-P regime, and further enhance our understanding for the interior structure and evolution processes inside the Earth.

Intrusive mechanism of the 2018 Sierra Negra Galápagos eruption, constrained by 4D tomographic images

Understanding the mechanisms of magma intrusion underpins our ability to interpret geophysical monitoring signals at volcanoes and thus issue reliable forecasts of future activity. The basaltic caldera volcanoes of the western Galápagos Islands, Ecuador, exhibit exceptionally high rates of deformation driven by shallow magma accumulation and migration. However, the nature and evolution of magma storage at these volcanoes is poorly constrained by earthquake hypocentre locations or geodetic inversions. Here we show that transient variations in seismic velocity before, during, and after the 2018 eruption of the Sierra Negra volcano track the accumulation of magma in a shallow sill complex and the emplacement of a lateral flank intrusion. A four-dimensional tomographic technique applied to the P-wave arrivals of local earthquakes provides high spatiotemporal resolution of changes in the physical properties of the shallow volcanic system. In the month before the eruption, the expansion of a low-velocity zone above the sub-caldera sill complex coincides with caldera uplift and near-surface fracturing, driven by persistent shallow magma accumulation. A new low-velocity anomaly appeared progressively in the western flank of the volcano in the days after the onset of the eruption, coinciding with the opening of a curved sill that supplied magma to secondary eruptive fissures. The anomaly disappeared as the curved sill deflated after the initial opening, despite it remaining the conduit for magma from the caldera complex to the flank fissures. Low velocities across the shallow caldera after the end of the eruption likely result from rapid inflation due to recharge with fresh magma from depth. These results indicate a previously unknown complexity to the magmatic plumbing system at Sierra Negra and suggest velocity changes resulting from an interplay of thermal and stress perturbations.

Fine Shear-Wave Velocity Structures of Subsurface beneath the Guangdong–Hong Kong–Macao Greater Bay Area with Dense Seismic Array and SPAC Method

Abstract We apply the spatial autocorrelation (SPAC) method to construct the 3D subsurface shear-wave velocity structure model using the short-period dense seismic array (containing 725 nodal geophones) located at the Guangdong–Hong Kong–Macao Greater Bay area (GBA). We first divided the dense array into numerous subarrays, with each subarray consisting of nine nodal geophones, and obtained 562 subarrays that can provide 1D VS profiles of the same quantity. Then, the SPAC method and genetic algorithm are utilized to extract the dispersion curve of the Rayleigh wave from the raw microtremor data and invert VS structure, respectively. Finally, a 3D VS structure model from the surface to 3.3 km depth is derived by combining all 1D VS structures. Relatively low-velocity anomalies above 700 m are considered unconsolidated shallow sediments as well as relatively high-velocity anomalies beneath 1100 m are attributed to consolidated granite bedrock. Meanwhile, low-velocity anomalies that are identified through the vertical VS profile at a depth of about 900–3000 m can be contributed to the fractured zone, and striped low-velocity anomalies in the horizontal VS maps reveal the location of the deeply buried faults in the study area. The results also mean that the SPAC method combined with the records of short-period dense seismic array can be effectively applied to image subsurface structures in high-populated urban area. The development of this noise-resistance and environment-friendly geophysical technique provides a reliable and effective way to explore the complicated subsurface geological structures, which is of great significance to urban engineering construction and earthquake disaster reduction work in densely populated urban agglomerations.

Uppermost mantle structure and dynamics of the Tanlu fault zone: New insights from Pn anisotropic tomography

Whether the segmentation of the Tanlu fault zone and mechanism of strong crustal earthquakes are related to the uppermost-mantle structure is still unclear. Here a new high-resolution uppermost-mantle velocity and anisotropy model of the Tanlu fault zone and surrounding areas is determined by using 13,045 Pn arrival-time data. These data are handpicked from seismograms recorded at 120 newly deployed TanluArray portable seismic stations and 337 Chinese provincial seismic stations. The pattern of Pn velocity and anisotropy agrees well with the surface geological and tectonic features. Obvious low-velocity (low-V) anomalies are observed in tectonically active areas, such as the Taihang mountain orogenic belt, the eastern Dabie mountain orogenic belt, and the Sulu fold belt, whereas areas with stable tectonics, such as the North China Basin and the South Yellow Sea Basin, exhibit high-velocity (high-V) anomalies. The fast propagation directions (FPDs) of Pn waves in the study area exhibit a complex distribution, but the FPDs show a rotating feature around the margin of the South Yellow Sea Basin. The block feature of the high-V anomalies beneath the North China Basin and segmentation of the low-V anomalies along the Tanlu fault zone may be related to lithospheric delamination and thermal erosion caused by hot and wet upwelling flows in the big mantle wedge. The 1668 Tancheng earthquake (M8.5) occurred near a low-V anomaly, and recent moderate and strong earthquakes (M > 4.0) took place along the boundaries of the velocity anomalies. These results offer new perspectives on the mantle dynamics and mechanism of strong earthquakes in and around the Tanlu fault zone.

The crust-mantle velocity structure beneath the Wudalianchi-Erkeshan-Keluo volcanic belt by joint inversion of receiver functions and ambient noise

The Wudalianchi-Erkeshan-Keluo (WEK) volcanic belt is a significant component of intraplate volcanism in Northeast China and is composed of the Wudalianchi, Erkeshan, and Keluo volcanic clusters. Using joint inversion of receiver functions and ambient noise, we construct a high-resolution 3-D S-wave velocity model of the WEK volcanic belt and its adjacent region, taking advantage of a deployed dense seismic array around this volcanic belt. There is a prominent low-velocity anomaly at 8–15 km depth beneath the Wudalianchi volcanic cluster, suggesting the presence of a crustal magma chamber. Low-velocity anomalies are also observed at 30–35 km depth beneath the Erkeshan volcanic cluster and 30–40 km depth beneath the Keluo volcanic cluster, resulting in discontinuous velocity structures at the Moho discontinuity. We further find a distinct low-velocity anomaly in the uppermost mantle beneath the WEK volcanic belt. Combined with previous geophysical and geochemistry studies, we propose a magma system scenario for the WEK volcanic belt. The upwelling molten material from the asthenosphere accumulated in the uppermost mantle and the magma chamber was formed, which provided the same uppermost mantle magma sources for the WEK volcanic belt.

Crustal structure beneath Central Kamchatka inferred from ambient noise tomography

In this study, we investigated the crustal structure beneath the area of Central Kamchatka (Kamchatka Peninsula, Russian Far East) limited by latitudes 52.8 and 54.1 degrees. This area comprises parts of the active Eastern Volcanic Front and two mountain ridges Sredinny and Ganalsky, as well as several backarc volcanoes. We used the continuous data from the permanent stations and the temporary network installed in 2019–2020 to perform ambient noise tomography. We implemented a two-step tomographic inversion algorithm that initially generated the two-dimensional maps of group velocities for periods ranging from 1 s to 30 s, then found the optimal 1D model of shear wave velocities and derived the three-dimensional S-wave velocity distribution. In the resulting model, we observe a prominent low-velocity anomaly below the active volcanoes of Avacha and Koryaksky, which is traced to depths of ∼40 km and may be associated with a zone of heated rocks suitable for petrothermal energy extraction. We propose that this anomaly represents a common magma pathway through the crust feeding both volcanoes. Beneath another active Zhupanovsky volcano, we observe three low-velocity layers at depths of 2 km, 8 km and 20 km indicative of a multilevel magma feeding system. We believe our findings will further the understanding of the complex processes occurring within these active volcanoes, which pose a tangible hazard to the relatively populated Petropavlovsk-Kamchatsky urban district. Beneath the Sredinny Ridge we reveal a high-velocity pattern at shallow depths, likely associated with consolidated magmatic structures. The low-velocity in the lower crust is interpreted as a high-temperature zone conserved here since the time when this area hosted an active volcanic arc. Upon further exploration, this high-temperature zone could prove to be a prospective source of geothermal energy. Below the Ganalsky Ridge, a prominent high-velocity anomaly extending to a depth of ∼15 km can be attributed to the Precambrian to Carboniferous metamorphic rocks of predominantly mafic composition.

Seismic structure of the 2015 Mw7.8 Gorkha earthquake revealed by ambient seismic noise and teleseismic surface wave tomography

The destructive 2015 Mw7.8 Gorkha earthquake occurred in the Main Himalayan Thrust due to the collision of the Indian and Asian plates, which provides a unique opportunity to understand the deep dynamic processes and seismogenic mechanisms of strong earthquakes. We construct a regional-scale shear-wave velocity model of the crust and uppermost mantle using ambient seismic noise and teleseismic surface wave at periods of 5–100 s around the Gorkha earthquake region. The new shear-wave velocity model exhibits prominently lateral heterogeneities in the Gorkha earthquake areas. We observe a high-velocity (high-V) zone around the Gorkha main shock in the Main Himalayan Thrust, indicating the existence of a high-strength asperity that sustains the stress accumulating. The aftershocks are primarily located in the low-velocity (low-V) anomalies and enclosed by two high-V anomalies, which appear to act as structural barriers that influence the spread of the aftershocks. Prominent low-Vanomalies from the lower crust to the mantle lithosphere are observed along the north–south trending rifts, suggesting the hot materials upwelling due to the tearing of the northward subducting Indian lithosphere. These observations may indicate that seismic velocity heterogeneity could play an essential role in earthquake initiation and the rupture process.

Computation and Modelling of Seismic Refraction Tomography for Anisotropic Medium Based on Pseudo Bending Method

Seismic refraction tomography is one of the imaging techniques in geophysical methods used to remodel the near-surface velocity layer structure of the Earth. In this study, we carried out a new computational approach and modelling of seismic refraction tomography using the pseudo-bending method. The true model of the near-surface is designed to be anisotropic medium which is having a low velocity anomaly distribution. This anomaly is constructed in such a way as to be similar to a model of liquid waste away spreading, which exhibits seismic velocities ranging from 1600 m/s to 1800 m/s. Based on our computations and numerical modelling results, it was found that the ray tracing path using pseudo-bending method displays an asymmetrical trajectory when the positions of the source and geophone are exchanged. Altering the shooting configuration from direct shoot (DS) to reversed shoot (RS) also reveals a significant difference in travel time values. The results of delay time tomography inversion, which represents the difference between travel times in the true model and the initial model using the SIRT method, indicate the presence of a low velocity anomaly that can be interpreted as the distribution of liquid waste.

Uppermost Mantle Pn Velocity and Anisotropy Structures Beneath the Sakhalin–Kuril–Kamchatka Region

AbstractIn this study, we used the Pn tomography method to obtain detailed velocity and anisotropy structures of the uppermost mantle beneath Sakhalin–Kuril–Kamchatka region for improving the understanding of plate subduction, arc–arc collision, and volcanism. We found low Pn velocities beneath volcanoes and areas characterized by pronounced tectonic activity and high Pn velocities with strong anisotropy in the subducting plate. Low Pn velocity anomalies beneath southern Sakhalin connected the low‐velocity anomalies in the mantle and crust, indicating the ascent of fluid or melt, and may provide a magmatic source for the rear‐arc Rishiri volcano. In the absence of plate subduction, low‐velocity anomalies and north–south Pn anisotropy manifested beneath northern Kamchatka, revealing the lateral propagation of mantle flow beneath this northern region. We suggest that the eastern boundary of the slab window at the Kamchatka–Aleutian junction is likely to be located near the Komandorsky Islands.

High P‐T Sound Velocities of Amphiboles: Implications for Low‐Velocity Anomalies in Metasomatized Upper Mantle

AbstractMetasomatized mantle xenoliths containing hydrous minerals, such as amphiboles, serpentine, and phlogopite, likely represent the potential mineralogical compositions of the metasomatized upper mantle, where low seismic velocities are commonly observed. This study presents the first experimentally determined single‐crystal elasticity model of an Fe‐free near Ca, Mg‐endmember amphibole tremolite at high pressure and/or temperature conditions (maximum pressure 7.3(1) GPa, maximum temperature 700 K) using Brillouin spectroscopy. We found that sound velocities of amphiboles strongly depend on the Fe content. We then calculated the sound velocities of 441 hydrous‐mineral‐bearing mantle xenoliths collected around the globe, and quantitatively evaluated the roles that amphiboles, phlogopite and serpentine played in producing the low velocity anomalies in the metasomatized upper mantle.

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.

Geophysical constraints on continental rejuvenation in central China: Implications for outward growth of the Tibetan Plateau

Continental rejuvenation results from the tectonic reactivation of crustal structures and lithospheric reworking by mantle flow. Geochemical observations and field mapping have traditionally provided the primary evidence for the secular evolution of crustal composition and tectonic processes during continental rejuvenation. Nonetheless, the impact of continental rejuvenation on the observed present-day strain rate and orogenic-scale lithospheric structure has not been well constrained. The pre-existing E-W−trending Central China Orogenic Belt has been overprinted by the N-S−trending Central Longitudinal Seismic Belt and constitutes the intracontinental West Qinling Syntaxis in central China, where the tectonic setting changes eastward from contraction to extension. Combining updated global positioning system data and high-resolution crustal seismic tomography, we reveal a modern continental rejuvenation process within the West Qinling Syntaxis in central China. The northward extrusion of the Tibetan Plateau’s weak lithospheric layer (middle-lower crust and lithospheric mantle) of southwestern China relative to the rigid Sichuan Basin/Ordos Block of the eastern West Qinling Syntaxis results in regional dextral shearing that shapes the Central Longitudinal Seismic Belt and defines the eastern Tibetan Plateau margin. The pre-existing E-W−trending Central China Orogenic Belt has been preserved above the brittle-ductile transition zone, and the northward movement of the deep lithospheric layer drives the deformation of the upper crust in the West Qinling Syntaxis. Our results, along with previous studies, suggest the presence of an intracontinental lithospheric interchange structure in central China. The continental rejuvenation of the West Qinling Syntaxis results from a combination of fault reactivation in the upper crust (Stage I, Eocene−Oligocene) and reworking of the deep lithosphere (Stage II, middle−late Miocene) related to the plateau-wide shift in stress accommodation ultimately driven by the redistribution of mass outward from the central Tibetan Plateau. At present, the transition zone between the high- and low-velocity anomalies along the Central Longitudinal Seismic Belt not only shapes the landscape boundary but controls the size and recurrence interval of earthquakes within the West Qinling Syntaxis in central China.

Ambient noise tomography of El Hierro island (Canary Islands)

El Hierro island is one of the most active islands in the Canary Islands from a volcanological point of view. This is the reason why the imaging of the internal crustal structure is of huge importance. The geophysical exploration methods employed on El Hierro Island, such as gravimetry and seismic tomography, allowed obtaining the high-resolution characterization of the crust’s deep part. However, these methods did not yield significant information about the surface and the shallower part of the crust. To gain a deeper insight into the shallow geological structure of El Hierro island, we employed Ambient Noise Tomography to construct a 3D S-wave velocity model. Our investigation revealed the presence of seven significant seismic velocity anomalies, partly identified by previous studies. We identified two high-velocity anomalies located in the eastern and western parts of the island at a depth between 0 and 3 km below sea level (b.s.l.). We interpreted these anomalies as dense intrusive complexes of dikes, possibly linked to the Tanganasoga volcano and the formation of the Tiñor edifice. Additionally, we observed two high-velocity anomalies in the northern and southern parts of the island at a depth between 3 and 4 km b.s.l., which we related to the accumulation of solidified igneous rocks. On the other hand, a low-velocity anomaly was observed in the Golfo valley, between 0 and 0.5 km b.s.l., and we interpreted it as megalandslide deposits. This anomaly was evidenced for the first time in the present study. Finally, two low-velocity anomalies were observed in the southern part of the island at different depths, between 0–0.5 km b.s.l. and 0–2 km b.s.l. These were interpreted as fractures generated by Quaternary volcanism along the SSE Rift. Also, one of them was evidenced for the first time in this study, corresponding to the zone of the fractures produced during the Quaternary volcanism. This study has allowed us to gain a more detailed understanding of the shallow geological structure of the island. Even if most of the anomalies had been evidenced previously, we could observe the existence of two low-velocity zones in the shallow crust that have not been observed before.

Crustal Velocity Structure of the Sichuan–Yunnan Block Revealed by High-Quality Crustal Phase Travel Time

Abstract The Sichuan–Yunnan block is located at the southeastern margin of the Tibetan Plateau, which is the key area as a transition belt from the active plate extrusion zone to the stable Yangtze Craton. Using a semiautomatic measuring method based on a graphical interface, we pick 81,585 precise travel times from 449 local earthquake records and finally obtain a crustal 3D P-wave velocity model of the Sichuan–Yunnan block. The model reveals an unexpected velocity contrast between the shallower and deeper crusts. It is summarized as weakly perturbed low-velocity belts encircling a high-velocity zone in the upper crust and strongly perturbed low-velocity anomalies in the mid-lower crust, respectively. The weak low-velocity anomalies are revealed along the major strike-slip faults, and their small perturbations may imply a slip-driven mechanism. The strong low-velocity anomalies are distributed extensively in the Sichuan–Yunnan block, and their great perturbations may be related to the partial melting of weak material extruded from Tibet. Besides, our result shows noticeable high-velocity anomalies in the core zone of the Emeishan Large Igneous Province (ELIP), which may be an indication of magma solidification from the ancient mantle plume. The result further exhibits an interesting pattern that the strong low-velocity anomalies are partially separated by the high-velocity anomalies in the ELIP. Such a specific pattern probably reflects that the stable zone in the ELIP leads to the bifurcation of weak Tibetan material.

A multiscale magma system beneath the Tengchong volcano in western Yunnan revealed by ambient noise tomography

SUMMARY The western Yunnan is located in the SE Tibetan Plateau, and is characterized by the active Tengchong volcano (TCV), complex crust–mantle coupling and intense earthquakes. To elucidate tectonism in the western Yunnan, we construct a 3-D S-wave velocity model to 80 km depth via ambient noise tomography using dense seismic stations. Our model shows significant low-velocity anomalies at different depths in the crust and uppermost mantle. Compared with the results of previous regional tomography, we image low-velocity anomalies consistent with a large-scale source of partial melts in the uppermost mantle beneath the Tengchong and Baoshan blocks, rather than just below the Tengchong block. Our results also reveal a magma chamber extending from the shallow subsurface to the lower crust beneath the TCV, which is fed by the mantle source. Based on these findings, we propose that the mantle source and crustal magma chamber form a multiscale magma system. Moreover, the mantle source is potentially resulted from asthenospheric upwelling, which is related to the subduction of the Indian slab. In addition, our model shows that the 1976 M7.4 and M7.3 Longling earthquakes occurred near a magma chamber. Thus, fluids from the magma chamber likely reduced the frictional coefficient on the seismogenic fault and caused the Longling earthquakes.

Lithospheric underthrusting beneath the northern Eastern Cordillera plateau, Colombian Andes: Insights from analyzing tomographic inversion using local earthquake travel time and gravity data

The northern Eastern Cordillera in the Colombian Andes is a wide (~200 km) high-elevation (>2.5 km) subduction-related asymmetric plateau. This plateau is currently subjected to shortening and magmatic addition, and yet the nature of the lowermost crust and its transition to the underlying mantle remains poorly constrained. To improve our knowledge about the structure of the deep crust beneath the EC plateau, we conducted a joint inversion of travel times of local earthquakes and gravity data. Gravity contributes to up to 0.5% in dVp and dVs with no affectation of the shape or location of the discussed anomalies. Along the latitudinal profile with the highest resolution (~5.7°N), two anomalies are identified at depths of 40–60 km beneath the plateau. A western low-velocity anomaly is interpreted as crustal material underthrust eastward beneath the northwestern EC. This process is triggering the abrupt change in topography between the adjacent low-elevation basin and the orogenic plateau. Additionally, a high-velocity anomaly beneath the eastern flank of the EC is likely related to mantle metasomatism and westward underthrusting of the foreland lithosphere. This metasomatism is either related to the interaction between mafic magmas and the uppermost mantle or silica enrichment that occurred during a past episode of flat subduction. Evidence for foreland underthrusting includes relocated earthquakes at 33–42 km depth within the thrust system between the EC and the foreland, along with increased shortening in the eastern part of the plateau, an increase in exhumation rates, the eastward migration of deformation towards the foreland, and dominated thick-skinned deformation in the upper crust of the foreland with an eastward vergence of thrusts and related folds. Furthermore, within the central part of the plateau, our results show low velocities between 30 and 40 km depth, consistent with previous constraints, suggesting magmatic underplating beneath the Paipa-Iza volcanic complex.

Potential geothermal reservoir systems in the Kenyan Great Rift Valley and volcanic region assessed by ambient noise analysis

To identify potential geothermal reservoirs in the Kenyan Great Rift, we analyzed ambient noise data to reveal seismic velocities and map azimuth-based Rayleigh-wave phase velocities. We extracted phase velocity dispersion curves from the ambient noise cross-correlation using the zero-crossing method and constructed S-wave velocity models using 3D surface-wave tomography. S-wave velocities were lower under Silali and Paka volcanoes (i.e., in magmatic regions) and in fractured areas (i.e., geothermal reservoirs) normal to the rift axis. We detected magmas at depths of >3 km beneath the volcanoes. Low-velocity anomalies may be related to fluid migration pathways. Most potential geothermal reservoirs are located above magma bodies and below the rift axis and are associated with intensive fracturing and porous rocks. Our approach used a limited number of seismometers, making it useful for application in areas with restricted access and challenging deployment; therefore, it could be used for the assessment of preliminary results in other magmatically active regions.

Low‐Velocity Structure of Subducted Oceanic Crust in the Upper Mantle: Insights From High Pressure and Temperature Elasticity Measurements of Aragonite

AbstractThis study examines the effects of water and carbon on the velocity profiles of subducted oceanic crust in the upper mantle. High pressure and temperature Brillouin measurements were conducted to determine the single‐crystal elasticity of aragonite (CaCO3) up to 20 GPa and 600 K, respectively. Using the finite‐strain method, we determined the elastic parameters: KS0 = 70.7 GPa, G0 = 36.1(3) GPa with KS′ = 5.0(1), G′ = 1.3(1), ∂KS/∂T = −0.020(2) GPa/K, and ∂G/∂T = −0.015(1) GPa/K. When combined with literature results, our findings reveal that neither hydrous minerals nor carbonate alone can explain the observed 3%–4% and 3%–7% low velocity anomalies in the compressional (VP) and shear‐wave (VS) velocities of subducted oceanic crust in the circum‐Pacific region at 150–250 km depth. Considering the combined effect of water and carbonate, the addition of 5.9–6.9 vol.% aragonite together with 10 vol.% lawsonite in the oceanic crust can produce a 3%–4% and 6.1%–8.1% low velocity anomalies in the VP and VS at 150–250 km depth, respectively, consistent with the seismic observations in the region. Complete dehydration of lawsonite accompanied by the gradual decarbonization of the subducted oceanic crust explains the absence of low‐velocity anomalies below 300‐km depth. Our findings help for a better understanding of the possible influence of carbon and water on seismic velocities of the mantle. As a result, the circulation of carbon and water may be better understood while taking the complex velocity structure of subduction zones and mineral physics findings into account.

Weighted Adjoint-state Crosshole Traveltime Tomography

The adjoint-state method of first arrival traveltime tomography, featured with implicit ray tracing calculation, was used to compute the gradient of the misfit function without introducing Fréchet derivatives. However, the majority of velocity gradients in the adjoint-state method focus on the high velocity area due to high raypath density. Thus, the current adjoint-state method faces challenges of imaging low velocity anomalies in the near-surface by crosshole seismic acquisition. In this paper, we proposed a weighted adjoint-state Eikonal equation-based crosshole traveltime tomography method. A preconditioning operator is provided to fix the ray path density influence. The weighted method was tested on a homogeneous model, a checkerboard model, and a reduced-scale Marmousi model. Both misfit cures and velocity logs are used to make a comparison between the weighted method and the traditional method. The results show the weighted method has better resolution, faster convergence speed, and lower final residual, which demonstrate the method can mitigate the ray path effect and reduce the inverted velocity error of low velocity anomalies.

Ambient noise tomography of a linear seismic array based on an improved Voronoi tessellation

Ambient noise tomography, when applied to a dense linear seismic array, has the capability to provide detailed insights into the fine velocity structures across diverse tectonic settings. The linear station arrangement naturally generates parallel and concentrated ray paths along the array trend. This unique geometry requires specific optimization of the inversion methodology and model parameterization. The Bayesian-based transdimensional inversion method, characterized by its fully non-linear nature and high degree of freedom in parameter settings, offers a powerful tool for ambient noise inversion. To effectively adapt this method to a linear array layout, we propose a modification to the Voronoi cell tessellation built in the transdimensional method. By introducing spatial priority to the Voronoi kernels, we strategically increased the density of Voronoi cells along the direction of the array. We then applied the modified approach to a linear seismic array in the North China Craton and validated its robustness through phase velocity images and resolution tests. Our improved non-uniform sampling technique in the 2-D model space accelerates convergence while simultaneously enhancing model accuracy. Compared with the conventional damped least-squares method, the proposed algorithm revealed a shear-wave velocity map with notable low-velocity anomalies situated in the middle and lower crust beneath the borders of the Ordos block and its surrounding orogenic belt. Aligned with the crustal structures revealed by receiver function and electrical imaging, our findings indicated that the western and eastern margins of the Ordos block had experienced intensive crustal wedge deformation and re-melting, respectively.