Inversion Of Phase Velocities Research Articles

Novel techniques for in situ estimation of shear-wave velocity and damping ratio through MASW testing part II: a Monte Carlo algorithm for the joint inversion of phase velocity and phase attenuation

SUMMARY This paper deals with in situ characterization of the small-strain shear-wave velocity VS and damping ratio DS from an advanced interpretation of Multi-channel Analysis of Surface Waves (MASW) surveys. A new approach based on extracting Rayleigh wave data using the CFDBFa method has been discussed in the companion paper. This paper focuses on mapping the experimental Rayleigh wave phase velocity and attenuation into profiles of VS and DS versus depth, which is achieved through a joint inversion procedure. The joint inversion of phase velocity and attenuation data utilizes a newly developed Monte Carlo global search algorithm, which implements a smart sampling procedure. This scheme exploits the scaling properties of the solution of the Rayleigh eigenvalue problem to modify the trial earth models and improve the matching with the experimental data. Thus, a reliable result can be achieved with a limited number of trial ground models. The proposed algorithm is applied to the inversion of synthetic data and of experimental data collected at the Garner Valley Downhole Array site, as described in the companion paper. In general, inverted soil models exhibit well-defined VS profiles, whereas DS profiles are affected by larger uncertainties. Greater uncertainty in the inverted DS profiles is a direct result of higher variability in the experimental attenuation data, the limited wavelength range at which reliable values of attenuation parameters can be retrieved, and the sensitivity of attenuation data to both DS and VS. Nonetheless, the resulting inverted earth models agree well with alternative in situ estimates and geological data. The results stress the feasibility of retrieving both stiffness and attenuation parameters from active-source MASW testing and the effectiveness of extracting in situ damping ratio estimates from surface wave data.

Novel techniques for in situ estimation of shear-wave velocity and damping ratio through MASW testing – I: a beamforming procedure for extracting Rayleigh-wave phase velocity and phase attenuation

SUMMARY A robust, in situ estimate of shear-wave velocity VS and the small-strain damping ratio DS (or equivalently, the quality factor QS) is crucial for the design of buildings and geotechnical systems subjected to vibrations or earthquake ground shaking. A promising technique for simultaneously obtaining both VS and DS relies on the Multichannel Analysis of Surface Waves (MASW) method. MASW can be used to extract the Rayleigh wave phase velocity and phase attenuation data from active-source seismic traces recorded along linear arrays. Then, these data can be inverted to obtain VS and DS profiles. This paper introduces two novel methodologies for extracting the phase velocity and attenuation data. These new approaches are based on an extension of the beamforming technique which can be combined with a modal filter to isolate different Rayleigh propagation modes. Thus, the techniques return reliable phase velocity and attenuation estimates even in the presence of a multimode wavefield, which is typical of complex stratigraphic conditions. The reliability and effectiveness of the proposed approaches are assessed on a suite of synthetic wavefields and on experimental data collected at the Garner Valley Downhole Array and Mirandola sites. The results reveal that, under proper modelling of wavefield conditions, accurate estimates of Rayleigh wave phase velocity and attenuation can be extracted from active-source MASW wavefields over a broad frequency range. Eventually, the estimation of soil mechanical parameters also requires a robust inversion procedure to map the experimental Rayleigh wave parameters into soil models describing VS and DS with depth. The simultaneous inversion of phase velocity and attenuation data is discussed in detail in the companion paper.

Crustal and uppermost mantle structure near the Gloria Fault, North Atlantic, from ocean bottom seismometer surface wave observations

SUMMARY In this work, we present both 1-D and 3-D shear wave velocity (Vs) models of the oceanic crust and uppermost mantle below the Deep OCean Test ARray area, located ∼ 70 km north of the central section of the Gloria Fault, in the eastern North-Atlantic Ocean. The velocity models are inferred from the dispersion of surface waves recorded on ocean bottom seismometers. Dispersion measurements are obtained from the analysis of ambient seismic noise at short periods (< 14 s) and teleseismic surface waves at long periods (> 14 s) using the two-station method. The 1-D Vs model is inferred from the joint inversion of Rayleigh wave phase velocities and Love wave group and phase velocities. The 3-D tomographic model is obtained by inversion of 2-D Love wave group velocity maps as a function of depth, further constrained by the average of Love wave phase velocities obtained from ambient noise (4–9 s) and the average Rayleigh and Love wave phase velocities calculated from teleseismic data (14–44 s). The 1-D Vs model shows a sediment layer with a low velocity of 1.05 km s−1, similar to previous studies in the region. Below the sediments, we find an oceanic crust with velocities ranging from 3.3 to 4.5 km s−1. The model reaches an unusually high velocity of 4.9 km s−1 in a 20 km thick layer at depths between 16 and 36 km. We interpret this fast velocity layer as indicative of the presence of harzburgite, a residue of enhanced melting that might have been formed by the proximity between the Mid-Atlantic Ridge and the Azores mantle plume. At greater depths the velocity decreases, forming a low-velocity zone that reaches a minimum at ∼ 70 km depth, which we interpret as the maximum depth for the lithosphere–asthenosphere boundary. The 3-D model shows a structure that is mostly horizontally layered, with Vs isocontours at 3.5–4.5 km s−1 highlighting oscillations of the crustal structure with wavelengths of ∼25–30 km. These oscillations may be due to changes in the rate of mantle upwelling and magma supply rate.

A New Method for Selecting the Phase and Group Velocity Dispersion Curves of Rayleigh and Love Surface Waves: Real Data Case of Central Anatolia, Turkey (Türkiye)

We propose a new method to select the dispersion curve of Rayleigh and Love surface waves obtained from local and regional earthquakes. This method is efficient for the crustal tomography studies and the Central Anatolia is chosen to test it. The single-station method utilizing both seismograms and accelerograms is used for the group velocities from local earthquakes while the two-station approach utilizing the cross correlograms is employed for the group and phase velocities using both local and regional earthquakes. The proposed method is made of two stages. The first stage based on the multiple filter technique – MFT is designed for an automatic selection of the desired dispersion curve. In case of the single-station approach, the MFT diagrams are computed for both Rayleigh and Love seismograms or accelerograms. An algorithm based on several selection criteria is employed to extract the corresponding group velocity dispersion curve from the MFT diagram. In case of the two-station method, the MFT diagrams are computed for both Rayleigh and Love cross correlograms. The latter algorithm developed for the MFT diagram is again applied to the cross correlogram to select the respective two-station group velocity dispersion curve. To select the corresponding two-station phase velocity dispersion curve, another algorithm is developed where the joint inversion of phase and group velocities is employed. The RMS value quantifying the misfit between the selected and inverted dispersion curves is found minimum when the selected phase velocity curve is compatible with the group velocity curve extracted via the MFT. The selection procedures defined up until this point constitute the first stage. In the second stage, phase and group velocity gathering around base pathways is performed. This way it is possible to compute the averages and standard deviations for the observed pathways traversing the similar geology. The outliers determined by using the averages and standard deviations are eliminated from the data set and this elimination is performed for phase and group velocities, separately.

Joint inversion of Rayleigh group and phase velocities for S-wave velocity structure of the 2021 MS6.0 Luxian earthquake source area, China

On September 16, 2021, a MS6.0 earthquake struck Luxian County, one of the shale gas blocks in the Southeastern Sichuan Basin, China. To understand the seismogenic environment and its mechanism, we inverted a fine three-dimensional S-wave velocity model from ambient noise tomography using data from a newly deployed dense seismic array around the epicenter, by extracting and jointly inverting the Rayleigh phase and group velocities in the period of 1.6–7.2 s. The results showed that the velocity model varied significantly beneath different geological units. The Yujiasi syncline is characterized by low velocity at depths of ∼ 3.0–4.0 km, corresponding to the stable sedimentary layer in the Sichuan Basin. The eastern and western branches of the Huayingshan fault belt generally exhibit high velocities in the NE-SW direction, with a few local low-velocity zones. The Luxian MS6.0 earthquake epicenter is located at the boundary between the high- and low-velocity zones, and the earthquake sequences expand eastward from the epicenter at depths of 3.0–5.0 km. Integrated with the velocity variations around the epicenter, distribution of aftershock sequences, and focal mechanism solution, it is speculated that the seismogenic mechanism of the main shock might be interpreted as the reactivation of pre-existing faults by hydraulic fracturing.

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 (<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.

Sedimentary and crustal structure of eastern North China from joint inversion of Rayleigh wave phase velocity and ellipticity and its implication for magmatism

Understanding the tectonic evolution of the North China Craton and its destruction mechanism requires the knowledge of eastern North China's fine crustal velocity structure. Using the two-year record of 342 ChinArray stations deployed in eastern North China, we obtain the Rayleigh wave phase velocity via ambient noise interferometry and Rayleigh wave ellipticity from ∼1000 teleseismic events. High-resolution sedimentary thickness and crustal 3D S-wave velocity structure are obtained by jointly inverting these observations. The results show that the Bohai Bay Basin is infilled with thick sediments and the Central Bohai Sea is the current depocenter with sedimentary thickness up to about 6 km. The crust and uppermost mantle beneath the Bohai Bay Basin are characterized by low velocity, indicating a high internal temperature of the crust. An upwelling of mantle thermal material under the crust of the Bohai Bay Basin continuously modifies the crust, suggesting that bottom-up thermochemical erosion is likely the primary control factor of crustal modification and thinning in this area. The uplift areas around the basin generally exhibit high velocity with weaker crustal modification, showing the heterogeneity of craton modification. Moreover, localized low-velocity zones observed beneath the Chifeng Volcanos, Dalainor Volcanos, and Kuandian Volcanos indicate the residual magmatism beneath these Quaternary volcanos. Furthermore, the magmatism beneath the Chifeng and Dalainor Volcanos shares a common magmatic source.

The Radial Anisotropy of the Continental Lithosphere From Analysis of Love and Rayleigh Wave Phase Velocities in Fennoscandia

AbstractRadial anisotropy (RA) in the upper mantle of the Fennoscandian Shield is analyzed by joint inversion of Love and Rayleigh wave phase velocities measured from recordings of teleseismic events at the ScanArray network. The phase velocities are measured by beamforming using three geographical subsets of the network as well as the full network. We analyze how different procedures for determining the phase velocities influence the final result and uncertainty. Joint inversion of the phase velocities in the period range 22–100 s reveals the presence of similar RA in the three subregions, with an average ξ value of about 1.05 in the subcrustal lithosphere down to at least 200 km depth. This corresponds to SH waves faster than SV by 2%–3%, a value very similar to those found in other continental regions. Considering this anisotropy together with other observables pertaining to seismic anisotropy in the area, we cannot propose a unique model satisfying all data. We can show, however, in which conditions different types of olivine crystallographic preferred orientations (CPOs) commonly observed in natural samples are compatible with the observations. CPO types associated with the preferred orientation of the a‐axis, in particular the common A‐type CPO, require a‐axes dipping not more than 25° from the horizontal plane to explain our observations. AG‐type CPO, characterized by preferred orientation of the b‐axis and occurring in particular in compressional settings, can be considered as an interesting alternative interpretation of continental lithospheric anisotropy, provided the olivine b‐axis is dipping by at least 60°.

Simultaneous Inversion for Surface Wave Phase Velocity and Earthquake Centroid Parameters: Methodology and Application

AbstractBoth the regional earthquake surface wave and seismic ambient noise provide important constraints on the Earth's structure, and yet no study satisfactorily combined them for the best imaging of subsurface seismic structure. In this study, we address this issue by developing a new method to simultaneously determine surface wave phase velocity and earthquake centroid parameters in three steps: (a) preliminary phase velocity inversion based on seismic ambient noise, (b) preliminary earthquake relocation based on earthquake surface wave data, and (c) simultaneous inversion for phase velocity and earthquake centroid parameters with constraints of interstation phase velocity measurements based on seismic ambient noise and event‐station phase velocity measurements based on earthquake surface wave data. Application of the method in the North South Seismic Belt region in China results in high‐resolution Rayleigh wave phase velocity maps and accurate earthquake centroid parameters. The additional earthquake data notably improve resolution of the inverted phase velocity model in west Yunnan and central Tibetan blocks, the regions with sparse seismic station coverage. The inverted phase velocity model exhibits high‐velocity anomalies in cratonic regions and the Emeishan Large Igneous Province, and low‐velocity anomalies in the interior and surrounding regions of the Tibetan Plateau. Relocation places earthquakes in shallow depths with geotherm above the crustal rock's brittle‐ductile transition temperature of , revealing thermal control on thickness of the seismogenic zone. With earthquake centroid parameters constrained, earthquake data are expected to provide further constraints on the deep seismic structure that is beyond the sampling limit of seismic ambient noise.

Lithospheric Structure in Central-East China from Joint Inversion of Surface-Wave Dispersion and CCP-Derived Receiver Function: Implications for Regional Tectonics

Abstract To investigate the lithospheric deformation beneath central-east China, we constructed a 3D crustal and uppermost-mantle velocity model of this region by joint inversion of Rayleigh-wave phase velocities and common conversion point-derived receiver functions. The velocity images at the east and west sides of Xuefeng Mountain presented quite different features and a sharp change in lithospheric thickness beneath this mountain, suggesting that this mountain may be the southwestern segment of the boundary between the Yangtze craton (YZC) and the Cathaysia block (CAB). We observed extensive lithospheric thinning in the eastern South China block (SCB), including the CAB and lower YZC. Considering that the eastern SCB is within the influence of the circum-Pacific system and features intensive Mesozoic magmatic activity, we deduced that subduction of the Pacific plate was the main controlling factor of lithospheric thinning. The westward extension of lithospheric thinning in the SCB terminates in the area east of the north–south gravity lineament. We observed a large-scale, low-velocity zone in the uppermost mantle of the southern Trans-North China orogen that penetrates northwestward into the southeastern corner of the Ordos block. The low-velocity zone intrudes into the bottom of the crust and results in crustal thinning in the region. We proposed that both the southern Trans-North China craton and the southeastern Ordos block are suffering ongoing lithospheric mantle modification.

S-Wave Velocity Structure of the Crust and Upper Mantle beneath the North China Craton Determined by Joint Inversion of Rayleigh-Wave Phase Velocity and Z/H Ratio

Abstract The North China craton (NCC) is one of the oldest craton in the world. Since the Cenozoic, the NCC has undergone severe lithospheric thinning, accompanied by extensive crustal deformations and volcanic activities. To better understand the mechanisms of the crustal and lithospheric deformations and intraplate volcanisms, we construct a high-resolution 3D S-wave velocity model for the NCC by jointly inverting Rayleigh-wave phase velocity dispersion and Z/H ratio measurements. Across the NCC, our model reveals significant lateral variations. In the shallow crust, prominent low-velocity anomalies associated with thick sediments are resolved in the Bohai Bay basin, and the Ordos basin and its surrounding grabens. Meanwhile, our model also shows that sediments are thin or even missing in the southeast of the Ordos basin and other orogens in the study area. The sedimentary structures of the Bohai Bay basin and the surrounding grabens of the Ordos basin may be the superficial response to the subduction of the Pacific plate and the northeastward push of the Tibetan plateau, respectively. Under the Datong volcano, our model reveals an integrated low-velocity anomalies from the mid-to-lower crust to the mantle, which verifies that the Datong volcano has a deep origin. Besides, the low-velocity zone beneath the eastern NCC (ENCC) connects to the low-velocity anomalies under the Datong volcano in the upper mantle. Combining the previous studies, we further propose that the asthenospheric upwelling of the Datong volcano and the subduction of the Pacific plate may jointly contribute to the lithospheric thinning of the ENCC.

Shear‐Wave Velocity Structure Beneath Northeast China From Joint Inversion of Receiver Functions and Rayleigh Wave Phase Velocities: Implications for Intraplate Volcanism

AbstractA high‐resolution 3D crustal and upper‐mantle shear‐wave velocity model of Northeast China is established by joint inversion of receiver functions and fundamental‐mode Rayleigh wave phase velocities. The teleseismic data used to calculate receiver functions are collected from 107 CEA permanent sites and 118 NECESSArray portable stations. Rayleigh wave dispersion measurements are extracted from an independent tomographic study. Our model exhibits significant detail in S wave velocity structure. Particularly, we observe a nearly constant S wave velocity of 3.4–3.6 km/s from shallow to deep crystalline crust under the study area, which we attribute to a high thermal gradient. Some modestly positive S wave velocity anomalies in the crust beneath the Songliao basin are interpreted as solidified late‐Mesozoic mafic intrusions. In the upper mantle, we confirm the local presence of low‐velocity zones below the Changbai mountains and Lesser Xing'an mountain range, consistent with asthenospheric mantle upwelling models. Furthermore, moderately low shear velocities imaged beneath the Halaha and Abaga volcanoes indicate possible pathways of magma ascent through the lithospheric mantle from the asthenosphere. At a regional scale, the average lithosphere‐asthenosphere boundary depth increases from ∼70 km under the greater Changbai mountains to ∼100 km below the Songliao basin, and reaches ∼110–120 km beneath the Greater Xing'an mountain range in the west. The conjectured dense mantle lid under the Songliao basin, characterized by fast S velocities, may have prevented sublithospheric melts from migrating to the surface.

Joint inversion of Rayleigh wave phase velocities and P-receiver functions along the East Anatolian fault zone

Joint inversion of Rayleigh wave phase velocities and P-receiver functions along the East Anatolian fault zone

Joint Inversion of Rayleigh Group and Phase Velocities for S-Wave Velocity Structure of the 2021 Ms 6.0 Luxian Earthquake Source Area, China

Joint Inversion of Rayleigh Group and Phase Velocities for S-Wave Velocity Structure of the 2021 Ms 6.0 Luxian Earthquake Source Area, China

Sedimentary and crustal structure of the US Gulf Coast revealed by Rayleigh wave and teleseismic P coda data with implications for continent rifting

We have developed an S-wave model of the south-central US focusing on the Gulf Coast sedimentary basin and its crust to understand continental rifting and regional tectonics. The model was derived by a joint inversion of Rayleigh wave phase velocities, Z/H ratios and P-coda data. The surface- and body-wave measurements were made, respectively, from ambient noise and teleseismic earthquakes recorded by 215 USArray stations in a rectangular area of 100°–87° west and 28°–37° north. We employed a cross-convolution function and H-κ analysis to better constrain sedimentary and Moho structure. We find that the southern edge of the Ouachita fold-and-thrust belt (OFTB) appears as a boundary in measured phase velocities, Z/H ratios, sediment basement depths, Moho depths, average crustal Vs and Vp/Vs ratios. The model shows southeastward thickening of the sedimentary basin, accompanied by thinning of the crystalline crust. The Moho gradient suggests that early rifting between North America and the Yucatan block commenced in a SE direction and involved most of the Pangea crust south of the OFTB boundary (i.e., Gondwana crust). We believe that a high velocity feature in the lowermost crust and upper mantle parallel to the southeast Texas coast was emplaced as a mafic body and is the source of the Houston magnetic anomaly. The seismic structures of the crust and uppermost mantle observed beneath the Mississippi Embayment and the Mississippi Valley Graben are consistent with plume induced Cretaceous uplift of the Mississippi Embayment as North America passed over the Bermuda hotspot.

Sedimentary and crustal velocity structure of Trans-North China Orogen from joint inversion of Rayleigh wave phase velocity and ellipticity and some implication for Syn-rift volcanism

We present a high-resolution 3-D shear wave velocity model of the crust and uppermost mantle beneath the Trans-North China Orogen and its adjacent regions derived from a joint inversion of Rayleigh wave phase velocity and ellipticity computed from ambient noise data. The 3-D velocity model reveals large lateral variations in sedimentary and crustal structure across the study area. In addition to the large velocity contrast between the North China Basin and the Ordos Basin located in the east and west sides of the orogen, we found a northward thinning of sediments within the Weihe-Shanxi Rift System that sits in the middle of the orogen, suggesting that the rifting has likely propagated from the Weihe Rift in the south to the Shanxi Rift in the north during the Oligocene-Pliocene. While the north-south trending rift is underlaid by a low Vs across the entire range in the upper and middle crust, only the north segment, which houses the Datong volcanic complex, exhibits a substantial low velocity structure in the lower crust and the uppermost mantle. The center of the low velocity anomaly extends into the uppermost mantle at a location approximately 150 km west to the volcanic complex, which is speculated for a nearly horizontal channel in the lower crust for the upwelling of the hot materials from the asthenosphere. These observations suggest that Shanxi Rift in the north is a more active segment within the rift system, which is manifested by the Quaternary syn-rift Datong volcanic complex.

Joint inversion of Rayleigh wave ellipticity and phase velocity for crustal structure in Taiwan

The convergence between the Eurasian Plate and the Philippine Sea Plate results in rapid uplifting of the Taiwan Orogen. To study the mountain building in Taiwan, we present three transects of shear wave velocity across the Taiwan mountain belt with joint inversion of teleseismic Rayleigh wave H/V ratios and phase velocities derived from ambient noise tomography. The Particle Swarm Optimization algorithm is introduced into the joint inversion and is proved to be an efficient tool in solving the geophysical inverse problems. In determining the H/V ratios, cross-correlation techniques are used to reduce the non-great circle effects of the Rayleigh wave packets and to correct sensor misorientations. The cross-correlation techniques improve the accuracy of the H/V measurements, especially at the short periods. The observed up-to-middle crustal thickening in the central segment of the Central Range suggests lithospheric collision tectonism, whereas the lower crustal thickening in southern segment is proposed to be related to subduction of the Eurasian continental crust. In the north, however, no crust thickening is observed. The variations of the crust architecture along the axis of the Taiwan Orogen favor a close coupling relationship between mountain building and slab interactions in the mantle. Beneath the Central Range, the twist of the fate of the Eurasian continental crust from consumption in the south to collision in the central segment is proposed to be resulted from the slab breakoff beneath central Taiwan.

High‐Resolution 3‐D Shear Wave Velocity Model of Northern Taiwan via Bayesian Joint Inversion of Rayleigh Wave Ellipticity and Phase Velocity With Formosa Array

AbstractThe Formosa array, with 137 broadband seismometers and ∼5 km station spacing, was deployed recently in Northern Taiwan. Here by using eight months of continuous ambient noise records, we construct the first high‐resolution three‐dimensional (3‐D) shear wave velocity model of the crust in the area. We first calculate multi‐component cross‐correlations to extract robust Rayleigh wave signals. We then determine phase velocity maps between 3 and 10 s periods using Eikonal tomography and measure Rayleigh wave ellipticity at each station location between 2 and 13 s periods. For each location, we jointly invert the two types of Rayleigh wave measurements with a Bayesian‐based inversion method for a one‐dimensional shear wave velocity model. All piecewise continuous one‐dimensional models are then used to construct the final 3‐D model. Our 3‐D model reveals upper crustal structures that correlate well with surface geological features. Near the surface, the model delineates the low‐velocity Taipei and Ilan Basins from the adjacent fast‐velocity mountainous areas, with basin geometries consistent with the results of previous geophysical exploration and geological studies. At a greater depth, low velocity anomalies are observed associated with the Linkou Tableland, Tatun Volcano Group, and a possible dyke intrusion beneath the Southern Ilan Basin. The model also provides new geometrical constraints on the major active fault systems in the area, which are important to understand the basin formation, orogeny dynamics, and regional seismic hazard. The new 3‐D shear wave velocity model allows a comprehensive investigation of shallow geologic structures in the Northern Taiwan.

Shear-wave velocity structure beneath Alaska from a Bayesian joint inversion of Sp receiver functions and Rayleigh wave phase velocities

The present-day lithospheric structure of Alaska is the result of a unique tectonic history of subduction and terrane accretion that controls upper plate thickness and rheology. To provide new constraints on the structure of the crust and upper mantle beneath Alaska, we jointly inverted Sp receiver functions and Rayleigh wave phase velocities to calculate shear-wave velocity profiles. Robust Sp receiver functions were obtained using a broad range of frequencies (2–100 s), time-domain deconvolution, and K-means cluster analysis. Tests of the Bayesian joint inversion with synthetic data illustrate that Sp receiver functions enhance the resolution of the velocity gradients at the Moho and the lithosphere-asthenosphere boundary, while Rayleigh surface waves provide information about absolute velocities. Our results show that in central Alaska, above the shallow slab, the continental lithosphere is thinnest (∼60 km) and the asthenosphere has its lowest velocities. This zone coincides with the Denali Volcanic Gap. The continental lithosphere thickens to the north beneath the Brooks Range and the northern Arctic Alaska terrane, reaching values of 110 to 130 km, with high lithospheric velocities that are comparable to Archean cratons. This pattern is consistent with a northward decrease in upper plate modification by melt and volatiles derived from the slab, in addition to intrinsic mantle velocity and viscosity differences between inherited lithospheric terranes. Lithospheric and asthenospheric velocities are not significantly different inside and outside of the Denali Volcanic Gap, but at the boundaries of this region lithospheric thickness increases rapidly to the north and gradually to the south. In the south, the subducting Yakutat plate is thicker (∼100–120 km) than the subducting Pacific plate (∼80–90 km), likely due to its thicker crust.

A Weighted Average Phase Velocity Inversion Model for Depth-Resolved Elasticity Evaluation in Human Skin In-Vivo.

In current surface acoustic wave (SAW) elastography field, wavelength-depth inversion model is a straightforward and widely used inversion model for depth-resolved elasticity profile reconstruction. However, the elasticity directly evaluated from the wavelength-depth relationship is biased. Thus, a new inversion model, termed weighted average phase velocity (WAPV) inversion model, is proposed to provide depth-resolved Young's modulus estimate with better accuracy. The forward model for SAW phase velocity dispersion curve generation was derived from the numerical simulations of SAWs in layered materials, and inversion was implemented by matching the measured phase velocity dispersion curve to the one generated from the forward model using the least squares fitting. Three two-layer agar phantoms with different top-layer thicknesses and one three-layer agar phantom were tested to validate the proposed inversion model. Then the model was demonstrated on human skin at various sites (palm, forearm and back of hand) in-vivo. In multi-layered agar phantoms, depth-resolved elasticity estimates provided by the model have a maximal total inversion error of 15.2% per sample after inversion error compensation. In in-vivo human skin, the quantified bulk Young's moduli (palm: 212 ± 78 kPa; forearm: 32 ± 11 kPa and back of hand: 29 ± 8 kPa) are comparable to the reference values in the literature. The WAPV inversion model can provide accurate depth-resolved Young's modulus estimates in layered biological soft tissues. The proposed model can predict depth-resolved elasticity in layered biological soft tissues with a reasonable accuracy which traditional wavelength-depth inversion model cannot provide.