Low-velocity Zone Research Articles

Structural constraints of induced earthquakes in the Weiyuan Shale Gas Field revealed by high-resolution body-wave tomography and earthquake relocation

Increasing concerns have been raised about hydraulic fracturing (HF) due to its capability of triggering damaging earthquakes. Although there is a common consensus that local geological settings play significant roles in these earthquakes, to what extent they influence earthquake locations and magnitudes within a shale gas field remains unclear. Here we conduct body-wave travel time tomography, earthquake relocation, and structural analysis in the Weiyuan Shale Gas Field (WSGF), Sichuan Basin, China. We find that structural deformation of the Weiyuan anticline and the newly revealed basement rift constrained the locations and magnitudes of ML ≥ 3.0 earthquakes in the WSGF. We also observe that ML ≥ 3.0 earthquakes are more prone to occur in low-velocity or velocity transition zones that are geologically susceptible. Based on high-resolution earthquake locations, we use aftershocks to delineate the fault plane of the largest Mw 5.0 in the WSGF. Our analysis shows the Mw 5.0 event nucleated in the basement rift and ruptured a fault plane strikes along northeast. Our results shed light on mitigating destructive earthquakes in shale gas fields by identifying geologically susceptible zones using structural deformation simulation and high-resolution tomography methods.

Geothermal and structural features of La Palma island (Canary Islands) imaged by ambient noise tomography

La Palma island is located in the NW of the Canary Islands and is one of the most volcanically active of the archipelago, therefore the existence of geothermal resources on the island is highly probable. The main objective of this work is to detect velocity anomalies potentially related to active geothermal reservoirs on La Palma island, by achieving a high-resolution seismic velocity model of the first few kilometres of the crust using Ambient Noise Tomography (ANT). The obtained ANT model is merged with a recent local earthquake tomography model. Our findings reveal two high-velocity zones in the island’s northern and southern parts, that could be related to a plutonic intrusion and old oceanic crust materials. Conversely, four low-velocity zones are imaged in the southern part of the island. Two of them can be related to hydrothermal alteration zones located beneath the Cumbre Vieja volcanic complex. This hypothesis is reinforced by comparing the S-wave velocity model with the seismicity recorded during the pre-eruptive phase of the 2021 Tajogaite eruption, which revealed an aseismic volume coinciding with these low-velocity zones. Another low-velocity zone is observed in the southern part of the island, which we interpret as highly fractured rocks which could favour the ascent of hot fluids. A last low-velocity zone is observed in the central part of the island and associated with loose deposits generated by the Aridane valley mega landslide.

Evidence for partial melting and alkali-rich fluids in the crust from a 3-D electrical resistivity model in the vicinity of the Coqen region, western Lhasa terrane, Tibetan Plateau

Both low resistivity zones and low velocity zones are distributed in the middle-lower crust of the western Lhasa terrane, Tibetan Plateau, China. Some estimates from electrical resistivity data suggest large volume fractions of silicate melts that are difficult to reconcile with seismic velocity data that prefer lower volumes. A second conductive phase, such as saline fluids, that drastically reduces the conductivity but does not significantly affect the seismic velocity because of its low volume may be able to explain these differences. In this study, a 3-D model of the electrical resistivity structure is generated on a profile along longitude 85°E from a latitude of 29°N to 32.5°N. Based on experimental measurement of melts and alkali-rich fluids (e.g., H2O-NaCl), we estimate the volume fraction of each phase that is required to explain the conductive anomalies observed in the geophysical model. The model reveals that the maximum bulk conductivity of the mid-lower crust in the south (1.52 S/m) is much higher than the conductivity of the mid-lower crust in the north (0.18 S/m) when taking 31°N as a rough boundary, near Coqen region. We hypothesize that the conductive zones in the south of the Coqen region may result from a silicate melt and alkali-rich fluid (multicomponent) system. In contrast, partial melting alone can explain the conductive zones in the north. The hypothesis can reconcile the predictions from electrical resistivity data and seismic data, and it corresponds well with zircon Hf isotope data. For example, a combination such as the presence of <1% NaCl-bearing aqueous fluids in addition to 5-10% partial melt can reconcile electrical conductivity data and seismic data. We propose that the contributions from partial melt or saline fluids are controlled by the distinct tectonic dynamics in each region. Furthermore, the model compatible with the idea that the Indian lower crust subducted northwards beneath the Lhasa terrane and may not extend far beyond the Indus-Yarlung Zangbo suture (approximately 30-31°N). The widespread distribution and interconnection of crustal conductors at different depths is consistent with the lateral migration of materials. However, both geophysical data sets agree that some anomalies are discontinuous along the profile. Furthermore, the low-angle subducted Indian Plate with no obvious tearing feature and a low volume of melts may have contributed to the absence of long, continuous, N-S-trending normal faults in this region.

Preliminary Results on Receiver Function Study in Mt Merapi, Central Java, Indonesia

Mount Merapi, a stratovolcano, is the world’s most active volcano, with a relatively short eruption period. Mount Merapi formed in the Java region as a result of regional tectonics dominated by the Sunda Arc, resulting in a large earthquake. Many earth scientists are interested in studying the volcano’s subsurface conditions due to its relatively short eruption period and interesting geological features. The Receiver Function method was used in this study to determine the crust’s depth and assess the presence of a LVZ (low velocity zone) by reprocessing receiver function data. The Receiver function is used to identify the Moho discontinuity area by converting P to S waves. A total of 100 earthquake data from 8 teleseismic stations were successfully downloaded from the IRIS website, that was distributed into sections A-A′ (west side of Mount Merapi) and B-B’ (east side of Mount Merapi). The processing of the receiver function data, as shown by the stacking align results, shows that the closest teleseismic station at west side of Mount Merapi has a very strong negative amplitude response, which is represented as a LVZ or magma reservoir after the arrival of P wave. To estimate the zone for LVZ, a forward modeling receiver function technique was used to find the best correlation between the Synthetic Receiver Function curve and the Receiver Function observation curve. A forward modeling receiver function technique was used to find the best correlation between the Synthetic RFcurve and the RF observation curve to estimate the zone for the LVZ. The correlation between the synthetic RF curve from Ramdhan et al’s (2019) tomographic velocity model and the observed RF curve is poor. To improve the correlation, include the main signal source that affects the receiver function curve in the form of seismic wave velocity particularly Vs, LVZ Zone, thin sedimen layer or shallow reservoir, and depth of discontinuity by Suhardja et al (2019). The estimated depth of the LVZ at 10 - 17 km is thinning towards the south or towards Mount Merapi, according to the results of the synthetic receiver function curve modelling at the closest station to Mount Merapi.

On the Control of Soil Heterogeneity, Péclet Number and Spatially Variable Diffusion Over Unsaturated Transport

AbstractPhysical properties of soils are ubiquitously heterogeneous. This spatial variability has a profound, yet still partially understood, impact on conservative transport. Moreover, molecular diffusion is often a disregarded process that can have an important counter‐intuitive effect on transport: diffusion can prevent non‐Fickian tailing by mobilizing mass otherwise trapped in low velocity zones. Here, we focus on macroscopically homogeneous soils presenting small scale heterogeneity, as described by the Miller‐Miller theory. We then analyze the dynamic control of soil heterogeneity, advection and diffusion on conservative transport. We focus especially on the importance of diffusion and of its tortuosity‐dependent spatial variability on the overall transport. Our results confirm previous finding that relatively high Péclet number systems are highly sensitive to the degree of heterogeneity, which promotes non‐Fickian transport. Also, diffusion appears to have a profound impact on transport, depending on both the degree of heterogeneity and the Péclet number. For a high Péclet number and a very heterogeneous system, diffusion leads to the counter‐intuitive decrease of non‐Fickian spreading described previously. This is not observed for a low Péclet number due to the impact of the spatial variability in the diffusion coefficient, which appears to be a significant controlling factor of transport by promoting or preventing the accumulation of mass in low velocity zones. Globally, this work (a) highlights the complex, synergistic effect of soil heterogeneity, advective fluxes and diffusion on transport and (b), alerts on potential upscaling challenges when the spatial variability of such key processes cannot be properly described.

Antarctic Bottom Water in the Vema Fracture Zone

AbstractA section of 46 CTD/Lowered Acoustic Doppler Current Profiler stations along the Vema Fracture Zone (VFZ) was made for the first time with a repeat of the transverse section on the main sill (at 41° W) for the sixth time. Potential temperature of Antarctic Bottom Water when it flows into the fracture increases from 1.3°С to 1.6°C. This temperature increase is much smaller than in the Romanche Fracture Zone because in the VFZ there are not many transverse sills that force the flow to become turbulent. A zone with minimal velocities of the bottom flows exists in the western part of the VFZ up to the main sill. A current with velocities of 0.25 m/s flows above the low velocity zone at a depth of about 4,000 m. Underwater spillways with hydraulically controlled flow of bottom water down the slope from 4,500 m to 5,000 m at speeds up to 0.40 m/s were found east of the sill (in the central and southern channels). The bottom current accelerates as it flows down, but then its kinetic energy decreases. The bottom current slows down and mixes with surrounding waters. A thin (∼30 m) bottom flow descends further to the deep depression. In the southern channel of the VFZ, bottom water flows into a deep (5,400 m) basin. The coldest and densest water reaches this depression during rare inflows.

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

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

Study on Hydrodynamics of Fish-Friendly Open-Channel Flow

To elucidate the hydrodynamic characteristics of a fish-friendly open channel, a three-dimensional numerical hydrodynamic model is constructed in this study employing the Large-eddy Simulation (LES) technique. The reliability of this numerical model is thoroughly validated by incorporating periodic boundaries in the flow direction, allowing for the recurrent development of water flow within the computational domain and utilizing experimental data from a flume. The results demonstrate that the fish-friendly channel is partitioned into a high-velocity mainstream zone and a low-velocity habitat zone. The time-averaged flow velocity in the mainstream zone is approximately 2.0 times the cross-sectional average flow velocity. In the low-velocity habitat zone, the average hourly flow velocity and turbulence are at a relatively low level, rendering the zone suitable for fish habitation and the incubation of adhesive fish eggs. A significant vortex structure is present in the mixing layer at the junction of the main flow area and the habitat area, which not only markedly increases the turbulence intensity but also serves as the primary driver of momentum exchange within the open channel.

Variations of intrinsic and scattering seismic attenuation with depth in western Tibet

In this study, the depth-dependent seismic wave attenuation beneath western Tibet have been investigated using 628 local earthquakes recorded at Y2 network operated by US-China collaboration group from July 2007 to May 2011. Initially, we have calculated the attenuation of shear wave (Qs−1) using coda normalization method and coda waves (Qc−1) using single isotropic scattering model. Thereafter, we have estimated the quantitative values of scattering (Qsc−1) and intrinsic (Qi−1) attenuation using Wennerberg approach. The observations are made at five different central frequencies (1.5, 3, 6, 12, and 18 Hz) and five lapse time windows (30, 40, 50, 60, and 70 s). The results show frequency and lapse time dependence of Qc−1, Qsc−1 and Qi−1 values for western Tibet. Qc−1 and Qi−1 shows similar trend suggesting that the key mechanism is intrinsic dissipation for the decay of coda wave in the medium. Qi−1 is found to be as a dominant mechanism over Qsc−1 at all the investigated depth beneath western Tibet. Interestingly, the values of Qc−1 and Qi−1 decrease and get closer to Qsc−1 with increasing lapse time window/depth. It signifies the impact of ongoing underthrusting Indian lithosphere at deeper depth. A stronger intrinsic attenuation at shallow depth reflects the effect of the mid crustal low-velocity zone (LVZ) below Tethyan Himalaya and the southern Lhasa terrane. The obtained results in this study are also in good agreement with other reported results from seismically active regions in the world.

Early Results of Modelling Subsurface Study in Lake Toba, North Sumatra Using Receiver Function

The island of Sumatra was dominated by tectonic interaction between the Indo-Australian plate subducting beneath the Eurasian plate. Lake Toba is one of the results of the collision and had a large eruption several tens of thousands of years ago. The goal of this study was to use the receiver function approach to calculate the depth variation of the Moho layer and identify the presence of a low-velocity zone (LVZ) under Lake Toba, and then compare the results of the deconvolution method. To process data on receiver functions around Lake Toba, 31 data stations from the Lake Toba (LT) project have been chosen, with data sources coming from Geofon (Germany). Water level (frequency domain deconvolution) and iterative time-domain deconvolution are the receiver function methods employed. According to the findings, the depth of the Moho layer varies between 27.5 and 36.95 kilometres, and it thickens to the northeast of Lake Toba. The average depth of Moho is roughly 31.05 km, and the Vp/Vs ratio obtained is around 1.86 on average. The presence of a low-velocity zone was then detected at a depth of 15 km to 25 km in the A-A′ section leading from northwest to southeast on the east side of Lake Toba, and at a depth of 8 km to 22 km in the B-B′ section leading from southwest to northeast across Lake Toba, as indicated by low amplitude. The Investigator Fracture Zone (IFZ) beneath Lake Toba was identified as a magma reservoir and source of volcanic activity. The iterative time-domain deconvolution approach was then used to illustrate the result of the best receiver function in this investigation, which revealed that the arrival time of the Ps wave was more spiky than the result of the water level.

The Main Himalayan Thrust Beneath Nepal and Southern Tibet Illuminated by Seismic Ambient Noise and Teleseismic P Wave Coda Autocorrelation

AbstractNepal is an actively deforming region due to its tectonic setting that hosts destructive earthquakes including the recent 2015 Mw 7.8 Gorkha earthquake. To better understand the physics of earthquakes and better assess the seismic hazard in the region, a highly resolved 3‐D structure of the crust is essential. This study presents a new 3‐D S‐wave velocity structure of the crust using ambient noise tomography (ANT). In relation to the 2015 earthquake, we show that the lateral variation of S‐wave velocity likely controls the rupture propagation and arrest. This study further constrains crustal discontinuities beneath Nepal including the Main Himalayan Thrust (MHT) using teleseismic P‐wave coda autocorrelation. The MHT geometry correlates well with sizeable low S‐wave velocity zones obtained from the ANT and separated by two ramps. The southernmost first ramp is where the 2015 Gorkha earthquake nucleated that we map up to the source model of the 1934 historical earthquake in Eastern Nepal. The northernmost second ramp is adjacent to the mid‐crustal low velocity zone beneath south Tibet. In between the two ramps, we interpret the horizontal low velocity zone as the interseismic creeping patch of the MHT with down‐dip extent of 50–70 km length and 90–100 km in correspondence with the Mw 7.8 2015 and 8.4 1934 source models, respectively. A systematic mapping of the low‐velocity zones in relation to the MHT integrated with GPS geodesy along with the respective slip from individual past earthquake ruptures can help decipher the paradox of maximum Himalayan earthquake magnitudes.

Numerical modeling and application of the effects of fish movement on flow field in recirculating aquaculture system

Hydrodynamics plays a crucial role in the recirculating aquaculture system (RAS). An efficient flow field is essential for removing residual bait and feces, creating a healthy environment for fish growth, development, and overall welfare. Hydrodynamics not only influences fish growth and development, but the presence and movement of fish also impact the flow dynamics within the culture tank. This article proposes a fluid-solid coupled model based on computational fluid dynamics (CFD) to investigate the interaction between fish movement and the flow field. Firstly, we established a numerical model of a circular tank using tetrahedral meshing. Several bionic fish were introduced into the tank simultaneously, and their movements were defined using UDF (User-Defined Function) files. Secondly, we evaluated the effects of different numbers of fish movements on the velocity and uniformity of the flow field within the tank. To quantitatively analyze the impact of different numbers of bionic fish on the flow field in the culture tank, we calculated the average velocity, velocity uniformity coefficients, and energy-effective utilization coefficients in three different planes (Y = 0.05, Y = 0.1, and Y = 0.15). This study demonstrated that the presence of fish and their tail-swinging motion enhanced the stability of the flow field and reduced low flow velocity zones in the culture tank. As the number of fish increased, the velocity significantly decreased, and the velocity uniformity decreased as well. The results highlight the effectiveness of the numerical modeling method for the fluid-solid coupling of fish interactions with the system's flow field. Furthermore, the model aligns with experimental results and can be utilized to study fish movement and flow fields effectively.

Influence of a low viscosity zone on the evolution of post-eruption deformation: A case study of the crustal deformation of Aira Caldera after the 1914 eruption of Sakurajima Volcano

Spatial viscosity variations in the crust can be inferred from geophysical imaging and may be essential for interpreting volcano deformations that show more complex behaviours than what simple uni-viscous models predict. In southern Kyushu, Japan, recent seismic imaging implies the presence of a low viscosity zone (LVZ) beneath Aira Caldera, and geodetic data in and around the caldera require uni-viscous model to have lower and higher viscosities earliest and later, respectively, in the period following the 1914 eruption of Sakurajima Volcano. Here, we use a 3D finite element model comprising an elastic layer underlain by a linear Maxwell viscoelastic layer to examine the influence of a LVZ on ground surface displacement in response to two different deformation source modes, i.e., instantaneous source deflation during a major eruption and subsequent continuous source inflation due to magma recharge. A LVZ is introduced into the viscoelastic layer by gradually reducing its viscosity towards the centre. The behaviour of the LVZ model quantified by comparison with the behaviour of a uniform viscosity (UNV) model reveals that, for a given LVZ structure, an apparent UNV model that best represents the LVZ model displacement in response to the instantaneous deflation has lower viscosity than that in response to gradual inflation, i.e., the rate-controlling viscosities of the LVZ model are those in the inner and outer parts of the LVZ for the instantaneous source deflation and subsequent continuous inflation, respectively. Such LVZ model behaviour, for a LVZ spatial extent comparable with the imaged low velocity anomaly, explains well the geodetic data in and around Aira Caldera at any stage after the 1914 eruption, as the predominant viscoelastic response changes with time from that to the instantaneous deflation to that to the subsequent continuous inflation. This study highlights the need for interdisciplinary investigations that integrate geodetic and geophysical datasets to better understand volcanic unrest.

Lithospheric architecture beneath the Amsterdam-St. Paul plateau, Southern Indian Ocean using the integrated gravity, magnetic and seismological study

Evolution of the Amsterdam-St. Paul (ASP) plateau is debated as a consequence of the Kerguelen mantle plume interaction with the South-East Indian Ridge (SEIR) or related with a weaker ASP plume origin. We performed an integrated geophysical approach using gravity and magnetic modeling along with the joint inversion of Ps receiver function and Rayleigh wave group velocity dispersion data to determine the crustal architecture, and upper mantle structure below the plateau. The gravity-magnetic model revealed that the plateau is associated with three crustal layers as basaltic layer, gabbroic layer and underplated material underlain by sedimentary strata. Our models further suggests that beneath the ASP plateau the Moho depth varies from ∼6.6 to 18.0 km. Joint inversion of Ps receiver function and Rayleigh wave group velocity dispersion curve suggested the Lithosphere-Asthenosphere Boundary at depth of 50 km below the Amsterdam Island. Further seismological result suggests evidence of a Low Velocity Zone below the Moho at depth of 20–36 km which may represent a magma chamber. Our model suggests that the evolution of the ASP plateau is guided by the Amsterdam and St. Paul fracture zones, which acted as a thermal boundary confining the horizontal movement of the ASP plume. We propose that the ASP plateau is not linked with the Kerguelen plume and emphasize origin of the ASP plateau by the interaction of SEIR with the ASP plume.

Analysis of fish migration in correspondence with wood and rock-made instream structures

Anthropic exploitation of soil, forests, and water bodies contributes to the modification of river flow regimes and sediment transport dynamics, thus affecting fish habitats and water quality. In the last decades, the increasing awareness of the consequences of such modifications on natural contexts has stimulated researchers and practitioners to provide more innovative solutions aiming at enhancing river biodiversity. Recently developed eco-friendly approaches for river restoration evidenced the necessity of more careful planning and management of natural resources, including the maintenance and enhancement of fish species' habitats. The present study aims at assessing the interaction of different types of wood- and rock-made instream structures with river flow regimes and habitat requirements of many freshwater fish species, accounting for the influence of downstream scour potholes to facilitate fish species migration and spawning. Experimental tests showed that instream structures cause deep scour holes characterized by low flow velocity zones, thus decreasing the required fish swimming power and potentially providing suitable conditions for the migration of freshwater fish species. Using dimensional analysis, empirical relationships were derived to optimize the longitudinal distance between subsequent instream structures, accounting for the effects of various structure geometries and fish biological characteristics. Nevertheless, further investigation is needed to validate the proposed relationships at prototype scale. This study ultimately contributes at providing tools to design river instream structures favorable to fish migration.

Turbulent flow simulations in a simplified channel bend model of Dragon style rivers with constant curvature and different single-short-branch characteristics

This study investigates the turbulent flows in simplified Dragon style river channels with a constant critical curvature and different single-short-branch characteristics, using an effective numerical method based on real geometry statistics. First, when the branch outlet is closed, in the main channel, strong bed shear stresses are concentrated at the inner bend apex and downstream; as the branch widens, more flow enters the branch, reducing the downstream velocities, increasing the helical flow strength (I) and turbulent kinetic energy (TKE); however, when the branch radius is about 3.5 times the main channel width, the opposite occurs. Second, in this non-free-flowing branch, upstream of a critical section, the cross-sectional I and TKE increase with increasing width, but both decrease with increasing radius or decreasing orientation angle (relative to the direction tangent to the main channel). Third, when the branch outlet is open, in the main channel, the bifurcation location primarily affects the main channel flow, moving it upstream creates a more regular low-velocity zone and recirculation zone, and lower shear stresses downstream, and meanwhile, the cross-sectional I, TKE, and shear stresses decrease upstream but increase downstream of the bifurcation location, respectively. Finally, in this free-flowing branch, shear stresses occupy the inner branch; the recirculation zone grows with increasing branch width, reaches a maximum around a relative branch radius of 3.0 (scaled with main-channel width), and disappears at a branch orientation angle of about 67°; both the cross-sectional I and TKE become negative, and the graphical cross-sectional I (along the flow direction) curves downward with increasing width, upward with increasing radius, and straightens with decreasing orientation angle. Findings provide fundamental conditions for harmonious vegetative integration and gradual development of Dragon style river regulation patterns, especially to mitigate crises like the ‘suspended river’ issues in the lower Yellow River.

Slab Tearing Underneath the Bransfield Strait, Antarctica

AbstractWe conduct a P‐wave receiver function analysis of the Bransfield Strait (West Antarctica) to determine the lithospheric structure of this back‐arc basin, thanks to 31 temporary and permanent stations. Our main finding is a 15 km tear of the Phoenix slab, coinciding with the location of the 2020–2021 Orca earthquake swarm's epicenters. Teleseismic wave modeling reveals that the two major earthquakes occurred at the base of the crust, suggesting that the swarm could have been triggered by active underplating driven by mantle flow through the slab tear. There is evidence for such an underplating layer at least under Deception Island and for a widespread low velocity zone in the mantle wedge probably undergoing partial melting. We found average crustal thickness (30.5 ± 1.0 km) and Vp/Vs (1.81 ± 0.04) values close to average extended continental crust, although results in the South Shetland Islands are significantly more heterogeneous than in the Antarctic Peninsula.

Strong effect of liquid Fe–S on elastic wave velocity of olivine aggregate: Implication for the low velocity anomaly at the base of the lunar mantle

The origin of the seismic low velocity zone (LVZ) observed at the base of the lunar mantle is important for understanding the nature of the lunar mantle. Liquid Fe–S is considered as a possible candidate of the cause of the LVZ. However, effect of liquid Fe–S on elastic wave velocity of mantle rock has been considered to be minor, according to theoretical estimation based on the high dihedral angle of liquid Fe–S in olivine aggregate observed in previous microstructural analysis of polished sample cross sections. Here we carry out direct measurements of elastic wave velocities of liquid Fe–S-bearing olivine aggregate and report strong reduction of the elastic wave velocities in the presence of liquid Fe–S. The effects of liquid Fe–S in olivine aggregates are much more pronounced than the theoretical estimation. Three-dimensional X-ray microtomography analysis reveals that the geometry of liquid Fe–S strongly depends on the size, with larger liquid Fe–S blobs having low aspect ratios, which may predominantly affect elastic wave velocity. Since smaller spherical Fe–S blobs are much more abundant than the larger elongated blobs, conventional dihedral angle analyses based on scanning electron microscope images tend to statistically over-sample smaller blobs and under-sample larger blobs. Our results suggest that the LVZ at the base of the lunar mantle can be explained by the presence of 4.3–5.0 vol.% liquid Fe–S. The LVZ may be a hidden reservoir of highly siderophile elements (HSEs), which may explain the low abundance of HSEs in the lunar mantle compared to the Earth.

Crustal and Upper Mantle Velocity Structure of SE Tibet From Joint Inversion of Rayleigh Wave Phase Velocity and Teleseismic Body Wave Data

AbstractSince the continental collision between the Indian and Eurasian plates began about 50 Ma ago, southeastern Tibet (SET) has undergone complex tectonic deformation. In this study, we investigate fine scale structural features of the crustal and upper mantle depths (&lt;250 km) beneath SET, which hold important clues to understanding the dynamic processes related to this collision. A 3D shear velocity model is constructed through jointly inverting Rayleigh wave phase velocity and teleseismic body wave data from more than 650 stations. Our 3D model identifies three independent low‐velocity zones (LVZs) in the mid‐lower crust with unprecedented details. More specifically, we observe a prominent LVZ beneath the North Chuan‐Dian Block, which is well separated from another LVZ beneath the Tengchong volcano in the south. This LVZ beneath the volcano represents a focused magma reservoir in the crust whose origin is potentially linked to the mantle upwelling associated with the eastward subduction of the Indian plate. The third LVZ, observed around the Xiaojiang Fault, likely represents a separated and mechanically weak layer in the mid‐lower crust due to the combined effects of regional crustal thickening under the southeastward plateau expansion, mantle upwelling, and shear heating of strike‐slip faults. In the upper mantle, we observe strong velocity reductions both in localized areas beneath the Tibetan Plateau and the broad region south of 26°N. These low velocity anomalies are sitting above high velocity anomalies at deeper depths, suggesting their association with lithospheric thickening and delamination processes.

Rayleigh phase velocity and azimuthal anisotropy from ambient noise data in the Sanjiang lateral collision zone in the SE margin of the Tibetan plateau

The Sanjiang lateral collision zone in the SE margin of the Tibetan Plateau is located at the east edge of the junction of the Eurasian and Indian plates. Using the continuous seismic waveforms recorded by 146 temporary and 21 permanent seismic stations in the study area, we obtain Rayleigh wave phase velocity and azimuthal anisotropy for periods 2 s to 40 s from the surface wave direct tomography method. This direct tomography method can obtain finer high-resolution results than the traditional surface wave tomography. Our results show that the low-velocity anomalies are found beneath the Lijiang-Xiaojinhe fault (LXF), Red River fault (RRF), Chuxiong fault and Tengchong volcanoes, the high-velocity anomalies are in the region of Weixi and Panzhihua at periods 5 ~ 8 s. The fast velocity directions mainly align N-S. At periods 10 ~ 15 s, the distributions of low-velocity anomalies are consistent with the strikes of LXF and RRF. At periods 20 ~ 35 s, the high- and low-velocity anomalies are bounded by the RRF, which may imply the fault is divided by the thick crust (indicated by low-velocity anomalies) and the thin crust with shallow mantle (indicated by high-velocity anomalies). The fast velocity directions at the periods 10 ~ 35 s rotate clockwise from north to south of the study area. The intensity of anisotropy in the low-velocity zone is stronger than that in the high-velocity zone, and the intensity in the north of the study area is stronger than that in the south. Results indicate the source of anisotropy may be different in each subzone.