Geophysical Observations Research Articles

Bayesian Inversion of Lithology and Liquid Phase Parameters From Seismic Velocity and Electrical Conductivity in the Crust and Uppermost Mantle

AbstractTo deeply understand various geodynamic processes, including volcanic activities and earthquakes, it is essential to extract detailed information about Earth materials, such as lithology and geofluid, from geophysical, petrological, and geochemical observations of Earth's interior. We developed a Bayesian probabilistic framework that can estimate the lithology and geofluid type (aqueous fluid or melt), geofluid amount (porosity), and parameters related to the fluid geometry (aspect ratio and critical fluid fraction related to connectivity) from P‐wave and S‐wave seismic velocities and electrical conductivity data obtained from geophysical tomography. By conducting synthetic inversion tests, we showed that methods based on a joint probability distribution, which simultaneously determines all parameters, sometimes fail to narrow the number of possible answers from 78 lithologies (e.g., basalt, granite, and eclogite) × two geofluid type (melt or aqueous fluid) candidate sets. This failure is derived directly from the difficult nature of inversion problems with relatively large data uncertainties. The proposed method uses a marginalization technique that first estimates lithology and geofluid type and then quantifies geofluid parameter values, which can narrow the probable lithology, geofluid type, and geofluid parameter sets in many cases. In addition, the computational cost of the proposed marginalization method is comparative to a former joint‐estimation method, which is theoretically identical to a previous heuristic method based on the least squares method. Therefore, the marginalization method is useful for geophysical data analyses that involve large amounts of observational data with relatively large uncertainties.

Assimilating Seismic Data And Soil Basic Information to Predict Near-surface Permeability And Water Content

Soil permeability plays a key role in the study of earth sciences for agriculture and forestry. Because the traditional laboratory measurement is time-consuming and laborious, and only discrete profile data can be obtained. In this paper, a simple and efficient method using soil transfer function (PTFs), is proposed to estimate permeability and other parameters indirectly by seismic wave observation. Based on Biot’s theory establishing the relationship between shear-wave velocity and permeability, and the basic soil information in the study area, including land survey, logging and drilling data, using the PTFs constraint to estimate saturated hydraulic conductivity, soil components, saturated water content and residual water content may favor the physical corrections of the PTFs. The results show that there are three distinct layers of permeability in the study area, consistent with the velocity structure. The measured basic information further verifies the accuracy of the saturated permeability coefficient estimation, which shows the reliability of the improved PTFs. The established near surface permeability estimation method by integrating seismic wave observation and soil basic information may provide further assimilation for parameter estimation combining geophysical observation and hydrogeographic statistics.

Construction and Significance of Comprehensive Geophysical Observatory Array in Dabie Monitoring and Forecasting Experimental Site

This paper reviews the development process of the seismic scientific experimental site at home and abroad and introduces the background, significance and construction of the the Dabie Monitoring and Prediction Experimental Site, which led by Anhui Earthquake Agency. During the 12th Five-Year Plan, the office of this experimental site was built, which is located in Lu’an Seismic Station. During the 13th Five-Year Plan, a comprehensive geophysical observation array is built, including a seismic science array, a flux-gate array, a optical-fiber array, a infra-sound array and a baseline of the gravity field at this experimental site. Through these constructions, it can not only play a important role in the study of earthquake monitoring and prediction, but also contribute to the study of the crustal structure detection, crustal strain analysis, seismic acoustic characteristics and gravity equipment calibration.

GIA imaging of 3-D mantle viscosity based on palaeo sea level observations - Part I: Sensitivity kernels for an Earth with laterally varying viscosity.

A key initial step in geophysical imaging is to devise an effective means of mapping the sensitivity of an observation to the model parameters, that is to compute its Fréchet derivatives or sensitivity kernel. In the absence of any simplifying assumptions and when faced with a large number of free parameters, the adjoint method can be an effective and efficient approach to calculating Fréchet derivatives and requires just two numerical simulations. In the Glacial Isostatic Adjustment problem, these consist of a forward simulation driven by changes in ice mass and an adjoint simulation driven by fictitious loads that are applied at the observation sites. The theoretical basis for this approach has seen considerable development over the last decade. Here, we present the final elements needed to image 3-D mantle viscosity using a dataset of palaeo sea-level observations. Developments include the calculation of viscosity Fréchet derivatives (i.e. sensitivity kernels) for relative sea-level observations, a modification to the numerical implementation of the forward and adjoint problem that permits application to 3-D viscosity structure, and a recalibration of initial sea level that ensures the forward simulation honours present-day topography. In the process of addressing these items, we build intuition concerning how absolute sea-level and relative sea-level observations sense Earth's viscosity structure and the physical processes involved. We discuss examples for potential observations located in the near field (Andenes, Norway), far field (Seychelles), and edge of the forebulge of the Laurentide ice sheet (Barbados). Examination of these kernels: (1) reveals why 1-D estimates of mantle viscosity from far-field relative sea-level observations can be biased; (2) hints at why an appropriate differential relative sea-level observation can provide a better constraint on local mantle viscosity and (3) demonstrates that sea-level observations have non-negligible 3-D sensitivity to deep mantle viscosity structure, which is counter to the intuition gained from 1-D radial viscosity Fréchet derivatives. Finally, we explore the influence of lateral variations in viscosity on relative sea-level observations in the Amundsen Sea Embayment and at Barbados. These predictions are based on a new global 3-D viscosity inference derived from the shear-wave speeds of GLAD-M25 and an inverse calibration scheme that ensures compatibility with certain fundamental geophysical observations. Use of the 3-D viscosity inference leads to: (1) generally greater complexity within the kernel; (2) an increase in sensitivity and presence of shorter length-scale features within lower viscosity regions; (3) a zeroing out of the sensitivity kernel within high-viscosity regions where elastic deformation dominates and (4) shifting of sensitivity at a given depth towards distal regions of weaker viscosity. The tools and intuition built here provide the necessary framework to explore inversions for 3-D mantle viscosity based on palaeo sea-level data.

Intra−Neo-Tethyan subduction initiation inferred from the Indawgyi mafic rocks in the Central Ophiolite Belt, Myanmar

Geological evidence has demonstrated the presence of an intra−Neo-Tethyan subduction system during the Cretaceous. However, when and how this intra-oceanic subduction was initiated, especially for the eastern Neo-Tethys, are still not well constrained. Here we present geochemical and geochronological analyses of the Indawgyi mafic rocks from the Central Ophiolite Belt in the West Burma Block (Myanmar), which record early forearc spreading during the intra−Neo-Tethyan subduction initiation. Zircon U-Pb ages of gabbros indicate the ophiolitic crust formation at ca. 120 Ma. Gabbros show mid-oceanic-ridge basalt−like rare earth element patterns and depleted Sr-Nd-Pb isotopic compositions with negative anomalies of high field strength elements (e.g., Nb, Ta, Zr, and Hf), similar to forearc basalt characteristics. Basalts show more slab-derived component signatures than the gabbros and represent mantle wedge magmas most likely formed between forearc spreading and arc maturation. These data, together with regional geological records and geophysical observations, suggest that the Indawgyi gabbros were derived from an intra−Neo-Tethyan forearc setting during the early stage of subduction initiation. Considering the timing of supra-subduction zone ophiolites and metamorphic sole in the Indo-Burma Range, we propose that spontaneous subduction initiation and sinking of the eastern Neo-Tethyan lithosphere during the Early Cretaceous (ca. 120 Ma) led to formation of the Indawgyi forearc crust, whereas subsequent mature subduction resulted in the Middle Cretaceous (ca. 108‒90 Ma) arc magmatism in the West Burma Block. These findings confirm the double-subduction model of the Neo-Tethys Ocean and shed new light on the intra−Neo-Tethyan subduction initiation.

Extracting crucial microstructures to characterize the elastic wave velocity and resistivity of Berea sandstone using convolutional neural networks

To reveal the internal structure of the earth, rock physical models are insightful for interpreting geophysical observation data and are often used for estimating subsurface structures. However, as the estimated value of each model significantly depends on the assumed microstructure, the obtained subsurface structure may be biased. Despite the innovations of high-resolution imaging technologies and their application to digital rock physics (DRP) and machine learning (ML), the characteristic microstructure controlling rock physical properties remains unclear. We apply ML and extraction of regression activation mapping (RAM) and learned filters to visualize the characteristic microstructure underlying the elastic and electrical properties of rock (P-wave velocity [Formula: see text], S-wave velocity [Formula: see text], [Formula: see text]/[Formula: see text] ratio, and electrical resistivity). This regression task is implemented using convolutional neural networks (CNNs) that are trained on these properties and calculated with the DRP approach using Berea sandstone images as samples. Although the calculated geophysical properties show perturbations even among digital rocks that have the same porosity, the results predicted using the best-performing CNN model exhibit a relative error of 2%–7% with respect to elastic properties. The visualized RAM revealed that the machine focuses on larger grains for predicting P-wave and S-wave velocities, whereas their ratio ([Formula: see text]/[Formula: see text]) takes into account the pore space. In contrast, electrical properties consider grain edges and surrounding pores, which may be related to the tortuosity of the pore space. Moreover, filter visualizations demonstrated that the geometric characteristics of Berea sandstone could be representative of circular or elliptical shapes for determining elastic properties, whereas tube-like shapes were extracted for electrical resistivity. These findings shed new light on the crucial internal structures used for defining each rock property.

СОДЕРЖАНИЕ ОЗОНА НАД ТЕРРИТОРИЕЙ РОССИЙСКОЙ ФЕДЕРАЦИИ В ТРЕТЬЕМ КВАРТАЛЕ 2023 Г

The review was compiled based on the results of the operation of the total ozone (TO) monitoring system over Russia and adjacent territories, operating online at the Central Aerological Observatory (CAO). The monitoring system uses data from the domestic network of filter ozonometers M-124, operating under the methodological guidance of the Main Geophysical Observatory; The quality of operation of the entire system is quickly monitored using observations from OMI satellite equipment (NASA, USA). The main TO observation data for each month of the third quarter of 2023 and for the quarter as a whole are summarized. The results of regular measurements of ground-level ozone carried out in the Moscow region are also presented.

Lithospheric Structure Controls Sequentially Active Detachment Faulting at the Longqi Segment on the Southwest Indian Ridge

AbstractOceanic detachment faulting, a major mode of seafloor accretion at slow and ultraslow spreading ridges, is thought to occur during magma‐poor phases and be abandoned when magmatism increases. In this framework, detachment faulting is the result of temporal variations in magma flux, which is inconsistent with recent geophysical observations at the Longqi segment on the Southwest Indian Ridge (49°42′E). In this paper, we focus on this sequentially active detachment faulting system that includes an old, inactive detachment fault and a younger, active detachment fault. We investigate the mechanisms controlling the temporal evolution of this tectonomagmatic system by using 2D mid‐ocean ridge spreading models that simulate faulting and magma intrusion into a visco‐elasto‐plastic continuum. Our models show that temporal variations in magma flux alone are insufficient to match the inferred temporal evolution of the sequentially active detachment system. Rather we find that sequentially active detachment faulting spontaneously occurs at the Longqi segment as a function of lithospheric thickness. This finding is in agreement with an analytical model, which shows that a thicker axial lithosphere results in a smaller fault heave and that a flatter angle in lithosphere thickening away from the accretion axis stabilizes the active fault. A thicker axial lithosphere and its flatter off‐axis angle combined have the potential to modulate sequentially active detachment faulting at the Longqi segment. Our results thus suggest that temporal changes of magmatism are not necessary for the development and abandonment of detachment faults at ultraslow spreading ridges.

Lower Crustal Rheology Controls Strain Partitioning and Mode of Intracontinental Deformation

AbstractThe factors that control strain partitioning along plate boundaries and within continental interiors remains poorly resolved. Plate convergence may be accommodated via distributed crustal shortening or discrete crustal‐scale strike‐slip faulting, but what controls these differing modes of deformation is debated. Here we address this question by examining the actively deforming regions that surround the Tarim Basin in central Asia, where deformation is uniquely partitioned into predominately strike‐slip faults in the east and distributed fold‐thrust belts in the west to accommodate Cenozoic India‐Asia plate convergence. We present integrated geological and geophysical observations to elucidate patterns in crustal deformation and compositional structure in and around the Tarim Basin. The thrust‐dominated western Tarim Basin correlates with a strongly‐magnetic lower crust, whereas strike‐slip faulting along the eastern margins of the Tarim Basin lack such magnetic signals. We suggest that the lower crust of the western Tarim is more mafic and stronger than in the east, which impacts intra‐plate strain partitioning. A stronger lower crust results in vertical decoupling to drive mid‐crust horizontal detachments and facilitate thrust faulting, whereas a more homogenized crust favored vertical transcrustal strike‐slip faulting. These rheological differences likely originated from the impingement of the Permian Tarim plume focused in the west. A comparison with the Longmen Shan of eastern Tibetan Plateau reveals remarkably similar strain partitioning that correlates with variations in foreland rheology. Our results highlight how variations in lower‐crust viscosity impact strain partitioning in an intra‐plate setting and how plume processes exert a strong control on later continental tectonic processes.

Principal component analysis on twenty years (2000–2020) of geochemical and geophysical observations at Campi Flegrei active caldera

Campi Flegrei (CF) is an active and densely populated caldera in Southern Italy, which has manifested signs of significant unrest in the last 50 years. Due to the high volcanic risk, monitoring networks of the most sensitive unrest indicators have been implemented and improved over time. Precious database constituted by geophysical and geochemical data allowed the study of the caldera unrest phases. In this paper we retrace the caldera history in the time span 2000–2020 by analyzing displacement, seismicity and geochemical time series in a unified framework. To this end, Principal Component Analysis (PCA) was firstly applied only on geochemical data because of their compositional nature. The retrieved first three components were successively analyzed via PCA together with the geophysical and thermodynamical variables. Our results suggest that three independent processes relay on geochemical observations: a heating/pressurizing of the hydrothermal system, a process related to magmatic fluids injection at the hydrothermal system roots, and third process probably connected with a deeper magmatic dynamic. The actual volcano alert state seems mainly linked to the variation of the hydrothermal system activity. Our approach made it possible to explore the interrelation among observations of different nature highlighting the importance of the relative driving processes over time.

Towards linking slab window geodynamics with the geophysical and geochemical signature of the upper mantle

Slab windows have clear consequences for surface observations that are the manifestations of asthenospheric upwelling and uppermost mantle temperature anomalies. In this contribution we link geophysical and geochemical observations from modern slab windows to the asthenospheric flow, temperature field and extent of melting using different geometries. We use an analytical solution for the slab window geometry and implement this solution in 3D steady-state thermomechanical models to calculate the temperature and velocity field. Our results show that upwelling (>1 cm/yr) and temperature anomalies (>1400 °C) are controlled by the ratio between the half-spreading rate and velocity of the overriding plate. The extent of melting depends on both the ratio of these velocities and ridge obliquity while flow patterns are controlled only by ridge obliquity. Finally, we construct ternary diagrams to estimate the efficiency of matrix/melt upwelling and maximum mean mantle temperatures based on plate kinematics and ridge obliquity. This novel approach was used to investigate the slab windows in Antarctica, South America and western North America.

Relationship Between Permeability and Resistivity of Sheared Rock Fractures: The Role of Tortuosity and Flow Path Percolation

AbstractThe fluid‐flow properties of fractures have received increasing attention regarding the role of geofluids in the genesis of slow and fast earthquakes and recent advances in geoengineering developments. Geophysical observations are promising tools to remotely estimate crustal permeability changes; however, quantitative interpretations are limited by the rock‐physical models' paucity for fractures. This study investigated changes in permeability, resistivity, and their respective relationships at elevated stress by performing numerical simulations of different fracture models with varying fracture size, roughness, and shear displacement. Numerical results and microscopic flow analysis demonstrate that permeability–resistivity relationships are controlled by percolation and are less dependent on fracture geometric characteristics. Our finding suggests that the permeability evolution of fractures can be formulated with resistivity changes independent of both fracture size and microstructure, the trends of which can be predicted using Archie's exponent. The extension to the electro‐mechanical relationship further derives the potential applications of estimating stress changes.

New Seismic Imaging of the Crustal Structure beneath the Eastern Sichuan and Wuling Mountains, South China: Insights into the Formation of Fold-and-Thrust Belts

Abstract Intracontinental deformation is out of the theory of conventional plate tectonics. It is widely recognized with deformation within the continental interior instead of the plate margin, yet its formation mechanism has long been controversial. The eastern Sichuan–Wuling mountains (ESWM) area is located ∼1300 km away from the subduction plate boundary and had developed intracontinental deformations, including crustal shortening and fold-and-thrust (FAT) tectonics, making it an ideal place to understand the mechanism of intracontinental deformation. In this study, we obtain a new seismic image of the 3D crustal structure of the ESWM area using the continuous ambient noise data of 67 broadband seismic stations. We invert the Rayleigh-wave dispersions of 5–30 s derived from cross-correlating the Z-component of all station pairs and obtain the fine crustal VS model. Our new seismic image reveals distinct velocity characteristics between the thin-skinned chevron anticline FAT tectonics in the eastern Sichuan basin and the thick-skinned chevron syncline FAT tectonics in the Wuling mountains area. Specifically, a low-VS layer observed beneath the Wuling mountains area, together with the crystalline basement beneath the eastern Sichuan basin, marks the ductile décollements confining the folding and thrusting deformation. Based on our new VS model and some previous studies, we propose a geodynamic model, which is associated with the far-field effect of the westward paleo-Pacific subduction during the late Mesozoic. Our model meets all the structural investigations at surface and geophysical observations at depth, and is reliable and valuable for further studies on similar intracontinental deformation in other regions.

Linear polarization signatures of atmospheric dust with the SolPol direct-sun polarimeter

Abstract. Dust particles in lofted atmospheric layers may present a preferential orientation, which could be detected from the resulting dichroic extinction of the transmitted sunlight. The first indications were provided relatively recently on atmospheric dust layers using passive polarimetry, when astronomical starlight observations of known polarization were found to exhibit an excess in linear polarization, during desert dust events that reached the observational site. We revisit the previous observational methodology by targeting dichroic extinction of transmitted sunlight through extensive atmospheric dust layers utilizing a direct-sun polarimeter, which is capable to continuously monitor the polarization of elevated aerosol layers. In this study, we present the unique observations from the Solar Polarimeter (SolPol) for different periods within 2 years, when the instrument was installed in the remote monitoring station of PANGEA – the PANhellenic GEophysical observatory of Antikythera – in Greece. SolPol records polarization, providing all four Stokes parameters, at a default wavelength band centred at 550 nm with a detection limit of 10−7. We, overall, report on detected increasing trends of linear polarization, reaching up to 700 parts per million, when the instrument is targeting away from its zenith and direct sunlight propagates through dust concentrations over the observatory. This distinct behaviour is absent on measurements we acquire on days with lack of dust particle concentrations and in general of low aerosol content. Moreover, we investigate the dependence of the degree of linear polarization on the layers' optical depth under various dust loads and solar zenith angles and attempt to interpret these observations as an indication of dust particles being preferentially aligned in the Earth's atmosphere.

The continuation of the Mull Dyke Swarm into the Southern North Sea

We present new geophysical observations of the Mull Dyke Swarm in the Southern North Sea. Two- and three-dimensional seismic reflection and aeromagnetic data were used to map the dykes. The three main dyke groups recognized onshore (Cleveland, Blyth and Hawick–Acklington) are found to extend to varying distances into the North Sea, crossing a number of major crustal-scale fault zones and domain boundaries, with almost no reorientation. The Blyth Dyke Group extends furthest, for a distance of 672 km from the source on Mull. The seismic data show extensive development of pit chain craters above the upper tips of these dykes, which can be approximately dated to the late Paleocene from the ages of crater fills. Volumetric estimates are made of the intrusive volumes associated with each group, ranging from 90 to 202 km 3 . These three main axes of intrusion probably formed in different intrusive events within a c. 1 myr period, from 59 to 58 Ma, during magnetic chron C26R.

Contaminant source and aquifer characterization: An application of ES-MDA demonstrating the assimilation of geophysical data

Contaminant source and aquifer characterization (CSAC) is critical in groundwater pollution evaluation and remediation. The ensemble smoother with multiple data assimilation (ES-MDA) is utilized to jointly identify contaminant source information and hydraulic conductivities by assimilating time-lapse electrical resistivity tomography (ERT) data. In a synthetic profile with a time-varying release history in a heterogeneous aquifer, we verify the performance of the proposed data assimilation framework. The results show that the CSAC problem could be handled by the proposed approach. The time-varying release history and the high permeability area can be identified with adequate time-lapse ERT measurements. Further reproduction of the evolution of the plume after CSAC also shows consistency with the reference plume. The poor conditioning inversion caused by the filter inbreeding is analyzed by comparing four scenarios with different apparent resistivity measurements. Furthermore, we also evaluate the impact of uncertainties in the petrophysical properties and geophysical observations on our data assimilation framework. The results show that the proposed ES-MDA data assimilation framework could provide a convincing inversion of the time-varying release history and hydraulic conductivities.

DISTURBANCE OF GEOPHYSICAL FIELDS AND IONOSPHERE DURING A STRONG GEOMAGNETIC STORM ON APRIL 23, 2023

We present the data of instrumental observations made at the Mikhnevo geophysical observatory of the IDG RAS of magnetic field variations and the response of microbaric oscillations and the electric field in the Earth’s surface atmosphere to a strong magnetic storm on April 23, 2023 (Kp = 8). When determining the response of the ionosphere to a magnetic disturbance in the form of a change in the critical frequency of the F2 layer, we use the results of ionospheric sounding performed at the IAP (Leibniz-Institut Fur Atmospharen Physik). It is shown that during periods of maximum magnetic field variations, increased microbaric variations are recorded and increased electric field variations are noted. The magnetic storm was also accompanied by a change in the critical frequency f0F2, as well as in the spectral characteristics of variations in the electric field strength.

Lithospheric Flexure Controls on Geomorphology, Hydrology, and River Chemistry in the Andean Foreland Basin

AbstractTectonics exerts a strong control over the morphology of Earth's surface that is apparent in active mountain belts. In lowland areas, subtle processes like lithospheric flexure and isostatic rebound can impact Earth surface dynamics, hydrologic connectivity, and topography, suggesting that geomorphic and hydrologic analyses can shed light on underlying lithospheric properties. Here we examine the effect of lithospheric flexure on the geomorphology, hydrology, and river water chemistry of the Rio Bermejo fluvial system in the east Andean foreland basin of northern Argentina. Results show that proximal to the mountain front, foredeep basin subsidence causes sedimentation along a braided channel belt that is superelevated relative to the surrounding flood basin. During floods, water flows from the superelevated channel into the groundwater reservoir, causing a net loss of discharge with distance downstream. Further downstream, forebulge uplift forces channel narrowing, high lateral migration rates, and incision up to 13 m into older river deposits. This incision locally allows groundwater flow into the river, causing a ∼20% increase in river solute load. Groundwater emerges from the forebulge into the backbulge, predominantly as spring‐fed channels. Here, channel migration rates decrease, suggesting a switch from net uplift to subsidence that reduces the depth to the groundwater table. This analysis shows that subtle lithospheric flexure can have significant effects on river channel morphology that determine hydrologic flow paths, and ultimately influence geochemical and ecological patterns. We suggest that these effects may elucidate lithospheric properties that are otherwise inferred from bulk geophysical observations.

MANGO: An Optical Network to Study the Dynamics of the Earth's Upper Atmosphere

AbstractThe Mid‐latitude All‐sky‐imaging Network for Geophysical Observations (MANGO) employs a combination of two powerful optical techniques used to observe the dynamics of Earth's upper atmosphere: wide‐field imaging and high‐resolution spectral interferometry. Both techniques observe the naturally occurring airglow emissions produced in the upper atmosphere at 630.0‐ and 557.7‐nm wavelengths. Instruments are deployed to sites across the continental United States, providing the capability to make measurements spanning mid to sub‐auroral latitudes. The current instrument suite in MANGO has six all‐sky imagers (ASIs) observing the 630.0‐nm emission (integrated between ∼200 and 400 km altitude), six ASIs observing the 557.7‐nm emission (integrated between ∼90 and 100 km altitude), and four Fabry‐Perot interferometers measuring neutral winds and temperature at these wavelengths. The deployment of additional imagers is planned. The network makes unprecedented observations of the nighttime thermosphere‐ionosphere dynamics with the expanded field‐of‐view provided by the distributed network of instruments. This paper describes the network, the instruments, the data products, and first results from this effort.

Elastic properties of amphibole at high pressure: A new explanation of the low velocities anomaly in the mantle wedges

Amphibole is an important hydrous mineral in mantle wedges, and its chemical composition and crystal structure are complex. The elastic properties of amphibole at high pressure are useful to explain the geophysical observations. In this study, the compressional (VP) and shear (VS) wave velocities of four amphibole aggregates were measured simultaneously at pressure of up to 7 GPa by ultrasonic interferometry. The P- and S-wave velocities increased monotonically with pressure, but the four amphibole samples exhibited different trends. Compared to mantle minerals, amphibole exhibits lower P- and S- wave velocities but a similar VP/VS ratio under the condition of upper mantle pressures. Based on the modeling results, the increase in velocity may result from the decomposition of amphibole in H2O-saturated peridotites. The results of this study provide a new explanation for the origin of low velocity anomalies in the mantle wedges.