Measured Rayleigh Wave Research Articles

Asymmetric seafloor depth across the Juan de Fuca Ridge caused by lithospheric heating

Previous studies attribute asymmetries across the East Pacific Rise to horizontal temperature or pressure gradients in the deep asthenosphere caused by the Pacific Superswell, which, however, cannot explain asymmetries observed across the Juan de Fuca Ridge. Here, we provide seismic evidence that the asymmetric seafloor depth across the Juan de Fuca Ridge is primarily caused by thermal buoyancy due to lithospheric heating and thinning. Based on a seismic model generated from Rayleigh wave measurements, we demonstrate that the seafloor depth on the western flank of the ridge, which is shallower ( > 150 m) than the prediction from the plate age, agrees with the relatively younger apparent thermal age inferred from the seismic data, whereas the buoyancy of the deeper asthenosphere alone can only account for <25% of the rise. On the eastern flank, both plate age and apparent thermal age are consistent with observed seafloor depth.

Nonlinear ultrasonic techniques to quantify oxidation damage in carbon/carbon composite material

This research applies linear and nonlinear ultrasonic (NLU) techniques to quantify oxidation damage in a carbon/carbon (C/C) composite material cut from an automotive friction disc ring. The oxidation as a result of gasification of carbon at high temperatures (T >∼ 500 °C) is a major degradation mechanism in C/C composites. This is the first research to successfully use a dry-contact mediator wedge setup for nonlinear ultrasonic applications, enabling repeatable in-situ Rayleigh wave measurements on slow wave velocity materials where submersion is impractical. The samples are subjected to thermal damage by exposure to a high temperature atmosphere over multiple holding times, and the oxidation damage is visualized using scanning electron microscopy (SEM). The sample mass is recorded and both linear (velocity and attenuation) and nonlinear (β′) ultrasonic measurements are performed after each holding time. Superior sensitivity to early stage oxidation affirms the suitability of nonlinear Rayleigh wave measurements for the quantification of thermally induced oxidation damage in C/C composites, while the double mediator wedge setup is well suited for high attenuation, slow wave velocity materials, demonstrating potential applicability for in-situ condition monitoring. The results show that a systematic oxidation starts after 3 min of exposure to the temperature in this C/C composite. The oxidation induced fiber–matrix debonding, matrix defects (appearing to be microcracking), and void growth are consistently detected by the Rayleigh wave velocity and acoustic nonlinearity. In addition to the higher sensitivity, the nonlinearity parameter has the capability of selectively measuring the strength related damage such as the observed fiber–matrix debonding, while the wave velocity can detect the oxidation voids. A combined use of these two parameters can provide useful information on the microstructural evolution in C/C composites subjected to high temperatures.

Asymptotic formula of Rayleigh wave speed in an orthotropic half-space coated by a thin piezoelectric film

ABSTRACT Piezoelectric thin films have been widely used in electro-acoustic devices including ones measuring Rayleigh wave speed. The main objective of this work is to find an explicit formula of Rayleigh speed taking into account the effect of the thin piezoelectric film. First the problem of Rayleigh surface waves propagating along an orthotropic substrate coated by a thin piezoelectric film is considered using Stroh’s formalism approach. Then, the exact secular equation of Rayleigh wave is obtained in implicit form. In case when the thickness of the thin layer is much smaller than the considering wavelength of Rayleigh waves, an asymptotic formula of Rayleigh speed is derived from the asymptotic form of the exact secular equation. The obtained asymptotic formula is simple enough and could be used in practice when Rayleigh speed is measured by using piezoelectric device. Numerical illustrations are carried out to show the validity of the obtained approximate formulas and its potential application in inverse problem.

In situ nonlinear Rayleigh wave technique to characterize the tensile plastic deformation of stainless steel 316L

The acoustic nonlinearity parameter β is sensitive to dislocation parameters, which continuously change during plastic deformation. Dislocation-based damage in structures/components is the source of the failure; thus, β has been studied as a metric for non-destructive evaluation. This work consists of two parts: the development of an in situ experimental setup for nonlinear Rayleigh wave measurements, and characterization of the dependence of β on applied stress at different levels of initial plastic strain. First, we introduce an experimental setup and methods for repeatable in situ nonlinear ultrasonic measurements. Details on design considerations and measurement schemes are provided. In the second part, β was measured in situ during an incremental monotonic tensile test. The measured β monotonically decreases with plastic strain, but it is relatively insensitive to the applied stress during elastic deformation. This result highlights three aspects of the evolution of β, which have not been sufficiently emphasized in prior work: the apparent insensitivity of β to the applied stress during elastic deformation, decreasing β with plastic deformation, and the saturation of β. We attribute the trend of decreasing β to a scaling of β with monopole loop length during plastic deformation, which depends on initial microstructure. The saturation of β at 1.8% coincides with a planar-to-wavy transition of dislocation structures. The in situ nonlinear ultrasonic experimental method presented in this work is significant as the in situ results can provide broader insights on β and dislocation-based damage evolution than ex situ measurements alone.

3D-ambient noise surface wave tomography of Fogo volcano, Cape Verde

Fogo volcano belongs to the Cape Verde Archipelago, and it is one of the most active volcanoes in the Atlantic Ocean, which most recent eruption occurred from November 2014 to February 2015. We analyzed ambient seismic noise recordings of three different networks deployed in the island, totalizing 14 seismic stations, to derive a crustal 3D shear-wave crustal velocity model of the volcano. Through the phase cross-correlation technique followed by a time-domain phase weighted stack, we were able to measure Rayleigh wave group-velocity dispersion measurements in the period range from 1.0 to 10 s. These dispersion measurements were used to invert for 2D group velocity maps at selected periods, and then inverted to produce a 3D shear-wave velocity model of the island. The tomographic model shows three velocity domains. First, an asymmetric upper layer, above 5–6 km of depth, with lower velocities concentrated in the northeastern sector of the island and a clear higher-velocity horizontal body at 3–4 km of depth in the southwestern sector of the island; the spatial correlation between these two velocity zones and the Galinheiros normal fault suggests a genetic link between the high velocities and long-term surface deformation, which we related to sill intrusions between 3 and 4.5 km depth, beneath the southwestern sector of the island. Second, a marked higher-velocity horizontal layer in between 5 and 6 km and 8–9 km, interpreted as the seismic expression of pervasive sill and laccolith intrusions, now cooled, beneath the volcanic edifice and within the underlying oceanic crust. Third, a lower velocity layer below 8–9 km of depth, more pronounced beneath the northeastern sector, which could be explained by a hotter and possibly melt-rich zone beneath the volcano or a significantly altered/serpentinized crust. Finally, our study also confirms that Fogo lacks any sizable magma chambers (ancient or recent) within the volcanic edifice, in agreement with other geophysical and petrological studies. These observations demonstrate that 3D-ambient noise Rayleigh wave tomography is a powerful tool to image the crustal and upper mantle structure beneath volcanic islands, as shown here for Fogo volcano.

Ultrasonic Rayleigh Wave Interrogation of Directed Energy Deposition Ti–6Al–4V Having a Rough Surface

Abstract In-situ monitoring techniques for additive manufacturing are in high demand to help produce reliable parts. The structural integrity of these parts depends on both the presence of flaws and their microstructure. Ultrasonic Rayleigh waves have the potential to identify flaws and assess the local microstructure during directed energy deposition (DED) additive manufacturing processes, but the scattering associated with the surface roughness degrades the ultrasonic signal and must be understood to extract useful information. Herein, the microstructures and surface profiles of DED and wrought Ti–6Al–4V are compared to provide context for measured Rayleigh wave speeds and second harmonic generation. The Rayleigh wave speed and second harmonic generation for DED and wrought Ti–6Al–4V materials having comparable surface roughness are significantly different. The wave speed measured in DED material is 3% slower than in wrought material, and the relative nonlinearity parameter, commonly used to characterize second harmonic generation, is 3.5–6.0 times higher for polished surfaces. Wave speed and second harmonic generation measurements are also made along the hatch and across the hatch for both as-built and glazed DED surfaces. Based on our results, we conclude that in-situ Rayleigh wave linear and nonlinear measurements are possible; although we acknowledge that in-situ angle-beam transducer generation would be challenging, and thus we will investigate pulsed laser generation in future work.

Crustal Structure across the West Antarctic Rift System from Multicomponent Ambient Noise Surface Wave Tomography

Abstract Approximately 2 yr (2010–2011) of continuous seismic records from a subset of the Antarctic component of the Polar Earth Observing Network (POLENET-ANET) seismic network deployed in West Antarctica are used to compute the nine components of the correlation tensor between each pair of stations in the network. Rayleigh wave velocity information from the vertical and radial components was extracted in the form of group and phase velocity dispersion curves, whereas the transverse component provided complimentary Love wave velocity information. The multicomponent Rayleigh wave measurements (ZZ, RR, ZR, and RZ) were averaged and used to infer the measurement uncertainties. The Rayleigh and Love wave group and phase velocities were then regionalized in space using a 2D deterministic tomography. A transect that spans the West Antarctic rift system was extracted from the tomography at individual periods between 7 and 60 s for the four types of surface wave velocities (i.e., Rayleigh and Love phase and group velocities). A transdimensional Bayesian joint inversion algorithm was used to invert these four datasets for a 1D model of isotropic shear-wave velocity versus depth at each point along the transect. In this way, surface wave dispersion curves from multicomponent noise correlations were used to build a 2D isotropic shear-wave velocity model down to ∼55 km depth. In this model, the top of the large low-velocity zone beneath Marie Byrd Land was imaged (up to a 5% decrease in velocity at ∼50 km depth), which provides further evidence for a mantle hot spot beneath the crust that supports the high topography in this region. We also observed a large velocity contrast in the lower crust beneath Marie Byrd Land at a depth where previous long-period seismicity has been observed. This strong contrast occurs more shallow than in previous crustal models, which compared to our model identify a deeper Moho (∼5–10 km deeper) beneath Marie Byrd Land. This new model has implications for interpreting earthquake locations in this region and perhaps necessitates that we revisit past hypocenter estimation studies using updated velocity models for the region.

Design and experiment of array Rayleigh wave-EMAT for plane stress measurement

The Rayleigh wave excited by the electromagnetic acoustic transducer (EMAT) is an effective selection for surface plane stress measurement. However, the propagation velocity of Rayleigh wave on the metal surface is easily affected by the original rolling process. Besides, the direction of the plane stress state is usually unknown, which means that the propagation velocity cannot be expressed linearly by the stress. As a result, the traditional measurement model of one transmitter and one receiver can only realize the decouple of plane stress components by rotating method, which not only brings position error but also low measurement efficiency. Therefore, this paper focuses on a novel Rayleigh wave-EMAT for plane stress ultrasonic measurement. Firstly, the Rayleigh wave measurement model is established based on the acoustoelastic equation and displacement expression. Furthermore, an array Rayleigh wave-EMATintegrating three transmitters and three receivers is designed. Finally, the typical plane stress state of 5052 aluminum alloy plate after friction stir welding (FSW) is measured. The experimental results show good agreement compared with the hole-drilling method, which verifies the effectiveness of proposed method and designed EMAT.

Azimuthal anisotropy from eikonal tomography: example from ambient-noise measurements in the AlpArray network

SUMMARY Ambient-noise records from the AlpArray network are used to measure Rayleigh wave phase velocities between more than 150 000 station pairs. From these, azimuthally anisotropic phase-velocity maps are obtained by applying the eikonal tomography method. Several synthetic tests are shown to study the bias in the Ψ2 anisotropy. There are two main groups of bias, the first one caused by interference between refracted/reflected waves and the appearance of secondary wave fronts that affect the phase traveltime measurements. This bias can be reduced if the amplitude field can be estimated correctly. Another source of error is related to the incomplete reconstruction of the traveltime field that is only sparsely sampled due to the receiver locations. Both types of bias scale with the magnitude of the velocity heterogeneities. Most affected by the spurious Ψ2 anisotropy are areas inside and at the border of low-velocity zones. In the isotropic velocity distribution, most of the bias cancels out if the azimuthal coverage is good. Despite the lack of resolution in many parts of the surveyed area, we identify a number of anisotropic structures that are robust: in the central Alps, we find a layered anisotropic structure, arc-parallel at mid-crustal depths and arc-perpendicular in the lower crust. In contrast, in the eastern Alps, the pattern is more consistently E–W oriented which we relate to the eastward extrusion. The northern Alpine forleand exhibits a preferential anisotropic orientation that is similar to SKS observations in the lowermost crust and uppermost mantle.

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.

Crust and Upper Mantle Structure of the South China Sea and Adjacent Areas From the Joint Inversion of Ambient Noise and Earthquake Surface Wave Dispersions

AbstractIn this study, we built a high‐resolution 3D S‐wave velocity model of the South China Sea (SCS) and adjacent areas by the joint inversion of surface wave dispersions retrieved from ambient noise and earthquake data. We first measured Rayleigh wave phase velocity dispersions from the waveforms recorded by 94 broadband seismic stations. Subsequently, we used a generalized least squares inversion scheme with spatially varying resolution to perform 2D tomographic inversion at different periods. Lastly, we performed 1D inversion at different grid points and obtained the 3D S‐wave velocity model in the depth range of 15–250 km. Our model reveals a prominent high velocity (high‐V) anomaly beneath the SCS basin and shows that the lithosphere beneath the SCS basin is about 70–80 km thick. We observe a prominent low velocity (low‐V) anomaly in the asthenosphere beneath the SCS basin, and we interpret this as evidence for the ongoing partial melting of the asthenosphere. An obvious high‐V anomaly is also revealed beneath the Khorat Plateau (KP) in the depth range of 80–150 km, suggesting that the KP has a cold and thick lithosphere and has suffered limited destruction in the Cenozoic era. In the northern part of Kalimantan Island (KI), a prominent low‐V anomaly is shown in the upper 60 km of the lithosphere revealing the upwelling of the asthenosphere. Our findings shed lights on the tectonic evolution of the SCS and adjacent areas.

Horizontally orthogonal distributed acoustic sensing array for earthquake- and ambient-noise-based multichannel analysis of surface waves

SUMMARY A 2-D orthogonal distributed acoustic sensing (DAS) array designed for seismic experiments was buried horizontally beneath the Kafadar Commons Geophysical Laboratory on the Colorado School of Mines campus at Golden, Colorado. The DAS system using straight fibre-optic cables is a cost-efficient technology that enables dense seismic array deployment for long-term seismic monitoring, favouring both earthquake-based and ambient-noise-based surface wave analysis for subsurface characterization. In our study, the horizontally orthogonal DAS array records ambient noise data for a period of about two months from November 2018 to January 2019. During this time, the array also detected seismic signals from an ML3.6 earthquake at Glenwood Springs, Colorado, which exhibit opposite signal polarities in the orthogonal DAS section recordings. We derive the transformation matrix for DAS strain measurements in horizontally orthogonal cables to retrieve both Rayleigh and Love wave dispersion information from the single-component DAS signals using the 2-D multichannel analysis of surface waves method. In addition, ambient noise interferometry is applied to long-term DAS noise recordings. Our theoretical derivation demonstrates that Rayleigh and Love wave Green's functions are coupled in the noise cross-correlation functions (NCFs) of DAS receiver pairs. Stacking NCFs over the horizontally orthogonal DAS array can constructively recover the radial Rayleigh wave component but destructively suppress the Love wave component. The multimodal Monte Carlo inversion of the earthquake-based Rayleigh wave and Love wave dispersion measurements and the noise-based Rayleigh wave measurement reveals a 1-D layered structure that agrees qualitatively with geological surveys of the site. Our study demonstrates that although straight fibre-optic cables lack broadside sensitivity, using appropriate DAS array configuration and seismic array methods can extend the seismic acquisition ability of DAS and enable its application to a broad range of scenarios.

Ambient vibration analysis on seismic arrays to investigate the properties of the upper crust: an example from Herdern in Switzerland

SUMMARYThe difficulty and the high cost to assess the subsurface properties led to the development of several geophysical techniques. Generally, the focus of a site study is the reconstruction of the S-wave velocity profile down to few tens to hundreds of metres (e.g. 30–300 m), but not the investigation of deeper structures, such as the transition to the crystalline basement. However, such deeper structures are of interest when seismic hazard products have to relate to a reference rock-velocity profile, for example in regional seismic hazard assessment and microzonation studies. To estimate the S-wave velocity profiles down to several kilometres, we study the potential of Rayleigh and Love waves at low (down to 0.1 Hz) and high (up to 20 Hz) frequencies using two seismic arrays of increasing size. The small array, with a maximum inter-station distance of 900 m and a recording time of 3 hr, was aimed at constraining the shallow subsurface down to about 350–400 m, while the big one, with a maximum inter-station distance of more than 29 km and 23 hr of recording had the goal to constrain the deeper structure. The arrays were deployed in northern Switzerland (east of the village of Herdern) within the Swiss Molasse basin, a sedimentary basin north of the Alps stretching from Lake Constance to Lake Geneva; its thickness increases from 800 to 900 m in the northeast to more than 5 km in the southwest. The seismic data recorded by the two arrays were analysed using the techniques developed for the analysis of small-aperture arrays. The results were inverted for the S-wave velocity profile in two steps: first, the Rayleigh and Love wave phase dispersion curves were inverted together. Secondly, the previous dispersion curves were jointly inverted with the measured Rayleigh wave ellipticity angle. The resulting S-wave velocity profiles are similar and show agreement with the available geological and geophysical data, confirming the potential of surface waves to investigate deep structures. Moreover, our analysis proves the feasibility of site characterization techniques to large arrays and the possibility to estimate the P- and S-wave velocity profiles down to 5 km, deeper than the contrast between Molasse basin and crystalline rock at around 2.1 km.

Shear attenuation and anelastic mechanisms in the central Pacific upper mantle

We determine the mantle attenuation (1/Qμ) structure beneath 70 Myr seafloors in the central Pacific. We use long-period (33-100 sec) Rayleigh waves recorded by the NoMelt array of broadband ocean-bottom seismometers. After the removal of tilt and compliance noise, we are able to measure Rayleigh wave phase and amplitude for 125 earthquakes. The compliance correction for ocean wave pressure on the seafloor is particularly important for improving signal-to-noise at periods longer than 55 sec. Attenuation and azimuthally anisotropic phase velocity in the study area are determined by approximating the wavefield as the interference of two plane waves. We find that the amplitude decay of Rayleigh waves across the NoMelt array can be adequately explained using a two-layer model: Qμ=1400 in the shallow layer, Qμ=110 in the deeper layer, and a transition depth at 70 km, although the sharpness of the transition is not well resolved by the Rayleigh wave data. Notably, Qμ observed in the NoMelt lithosphere is significantly higher than values in this area from global attenuation models. When compared with lithospheric Qμ measured at higher frequency (∼3 Hz), the frequency dependence of attenuation is very slight, revising previous interpretations. The effect of anelasticity on shear velocity (VS) is estimated from the ratio of observed velocity to the predicted anharmonic value. We use laboratory-based parameters to predict attenuation and velocity-dispersion spectra that result from the superposition of a weakly frequency dependent high-temperature background and an absorption peak. We test a large range of frequencies for the position of the absorption peak (fe) and determine, at each depth, which values of fe predict Qμ and VS that can fit the NoMelt Qμ and VS values simultaneously. We show that between depths of 60 and 80 km the seismic models require an increase in fe by at least 3-4 orders of magnitude. Under the assumption that the absorption peak is caused by elastically accommodated grain-boundary sliding, this increase in fe reflects a decrease in grain-boundary viscosity of 3-4 orders of magnitude. A likely explanation is an increase in the water content of the mantle, with the base of the dehydrated lid located at ∼70-km depth.

Group Velocity Measurements of Earthquake Rayleigh Wave by S Transform and Comparison with MFT

Based upon the synthetic Rayleigh wave at different epicentral distances and real earthquake Rayleigh wave, S transform is used to measure their group velocities, compared with the Multiple Filter Technique (MFT) which is the most commonly used method for group-velocity measurements. When the period is greater than 15 s, especially than 40 s, S transform has higher accuracy than MFT at all epicenter distances. When the period is less than or equal to 15 s, the accuracy of S transform is lower than that of MFT at epicentral distances of 1000 km and 8000 km (especially 8000 km), and the accuracy of such two methods is similar at the other epicentral distances. On the whole, S transform is more accurate than MFT. Furthermore, MFT is dominantly dependent on the value of the Gaussian filter parameter α, but S transform is self-adaptive. Therefore, S transform is a more stable and accurate method than MFT for group velocity measurement of earthquake Rayleigh waves.

Optical coherence elastography for evaluating Rayleigh wave propagation in soft matter-micellar fluids

Wormlike micellar (WM) fluids exhibit a self-assembly property and viscoelastic behavior which make them a promising material for representing biological fluids, ultrasoft tissue phantoms and drug delivery carriers. Although WM biochemistry has been investigated, the mechanical properties have been rarely discussed because of the complexity of soft matter systems. Here, we demonstrate that optical coherence elastography (OCE) is capable of measuring Rayleigh wave of micellar fluids. Three solutions with concentrations of 100, 200, 300 and 400 mM, were considered in the study. Ultrasound excitation to produce Rayleigh waves was accomplished using 7.5 MHz burst excitation signals of length 4.0 ms repeated at a rate of 19 Hz. The optical coherence tomography (OCT) acquisition was set to 50 lateral positions and 4000 A-scans for each lateral position at a 76 kHz scan rate for total acquisition time of 52.6 ms. Ultrasound-based shear wave elastography, implemented on a Verasonics system, was used to obtain group and phase velocity of the shear wave in all micellar fluids as reference. Our research results demonstrated that Rayleigh wave in micellar fluids can be evaluated by OCE and mechanical properties like Young’s modulus can be assessed. A wide of concentration ranges of WM fluids would provide a more realistic model to mimic biological fluids and tissues.

Upper mantle shear velocity structure beneath the equatorial East Pacific Rise from array-based teleseismic surface-wave dispersion analysis

Summary The Discovery/Gofar transform faults system is associated with a fast-spreading center on the equatorial East Pacific Rise. Most previous studies focus on its regular seismic cycle and crustal fault zone structure, but the characteristics of the upper mantle structure beneath this mid-ocean ridge system are not well known. Here we invert upper mantle shear velocity structure in this region using both teleseismic surface waves and ambient seismic noise from 24 ocean bottom seismometers (OBSs) deployed in this region in 2008. We develop an array analysis method with multi-dimensional stacking and tracing to determine the average fundamental mode Rayleigh wave phase-velocity dispersion curve (period band 20–100 s) for 94 teleseismic events distributed along the E-W array direction. Then, we combine with the previously measured Rayleigh wave phase-velocity dispersion (period band 2–25 s) from ambient seismic noise to obtain the average fundamental mode (period band 2–100 s) and the first-higher mode (period band 3–7 s) Rayleigh wave phase-velocity dispersion. The average dispersion data are inverted for the 1-D average shear wave velocity (Vs) structure from crust to 200-km depth in the upper mantle beneath our study region. The average Vs between the Moho and 200-km depth of the final model is about 4.18 km/s. There exists an ∼5-km thickness high-velocity lid (LID) beneath the Moho with the maximum Vs of 4.37 km/s. Below the LID, the Vs of a pronounced low-velocity zone (LVZ) in the uppermost mantle (15–60 km depth) is 4.03–4.23 km/s (∼10 per cent lower than the global average). This pronounced LVZ is thinner and shallower than the LVZs beneath other oceanic areas with older lithospheric ages. We infer that partial melting (0.5–5 per cent) mainly occurs in the shallow upper mantle zone beneath this young (0–2 Myr) oceanic region. In the deeper portion (60–200 km depth), the Vs of a weak LVZ is 4.15–4.27 km/s (∼5 per cent lower than the global average). Furthermore, the inferred lithosphere-asthenosphere boundary (LAB) with ∼15-km thickness can fit well with the conductive cooling model. These results are useful for understanding the depth distribution and melting characteristics of the upper mantle lithosphere and asthenosphere in this active ridge-transform fault region.

Shallow and in depth seismic testing in urban environment: A case study in Lisbon Miocene stiff soils using joint inversion of active and passive Rayleigh wave measurements

The use of Surface Wave Methods for the characterization of the shear wave velocity profile of the soil has become increasingly attractive due to its non-invasive nature and its ability to provide information at a larger scale. Their main drawbacks are the non-uniqueness of the solution and high uncertainty of the results at higher depths. Its application in dense urban areas can be challenging due to the proximity to underground structures, high background noise level and space limitations. This paper describes a case study on the use of surface wave based methods on stiff soil in urban environment within a confined area located in Lisbon city, Portugal, surrounded by buildings and between an underground car park and the subway tunnel. The old building, built in the study area, was demolished for the construction of a new building with five basement floors. During this process, passive and active linear array and passive three-component single-station measurements were made at the surface level, before the works started, and at 2 deeper excavation levels. The results obtained using shallow and in depth measurements were compared, including HVSR and Rayleigh wave ellipticity curves and Vs profiles obtained through the conventional MASW method and through the joint inversion of both Rayleigh wave dispersion and ellipticity curves. A deeper Vs profile with low uncertainty was obtained through the joint analysis of Rayleigh wave dispersion and ellipticity curves, compatible with Vs profile obtained in depth.

S Velocity Model of East Asia From a Cluster Analysis of Localized Dispersion

AbstractWe measured Rayleigh wave group velocity dispersion curves from station pair cross correlations of continuous broadband data from 1,082 seismic stations in regional networks across China, Korea, and Japan. The dispersion curves are localized in the period range 6–40 s to produce group velocity maps, which we then combine with group velocity data in the period range 50–133 s from the global dispersion model of Ma et al. (2014, https://doi.org/10.1093/gji/ggu246). Cluster analysis performed on the local dispersion curves reveals distinct tectonic regions and provides a new way of estimating the number of regions needed to describe a surface wave dispersion data set. We inverted the centroid dispersion curves corresponding to each cluster for a new set of reference models. We used a combination of teleseismic receiver functions and Moho depths from the LITHO1.0 model to create a 3‐D Moho depth reference model and then constructed a 3‐D S velocity model by jointly inverting group velocities with localized phase velocities in the period range 28.57–200 s, also from Ma et al. (2014, https://doi.org/10.1093/gji/ggu246). The cluster analysis and comparisons with other regional models provide evidence for a broad region of deformed lithosphere in East Asia. We image high velocities in the Yangtze and Ordos cratons and low velocities underneath the East Sea (Sea of Japan), and we find that a low‐velocity zone in the midcrust is appropriate for Tibet. The model we present is intended as an alternative reference model that fits a broader range of periods to be used for further studies of regional geophysical phenomena.

Large‐Scale Forces Under Surface Gravity Waves at a Wavy Bottom: A Mechanism for the Generation of Primary Microseisms

AbstractPrimary microseisms are background seismic oscillations recorded everywhere on Earth with typical frequencies 0.05 &lt; f &lt; 0.1 Hz. They appear to be generated by ocean waves of the same frequency f, propagating over shallow bottom topography. Previous quantitative models for the generation of primary microseisms considered wave propagation over topographic features with either large scales, equivalent to a vertical point force, or small scales matching ocean wave wavelengths, equivalent to a horizontal force. While the first requires unrealistic bottom slopes to explain measured Rayleigh wave amplitudes, the second produced Love waves and not enough Rayleigh waves. Here we show how the small scales actually produce comparable horizontal and vertical forces. For example, a realistic rough bottom over an area of 100 km2 with depths around 15 m is enough to explain the vertical ground motion observed at a seismic station located 150 km away. Ocean waves propagating over small‐scale topography is thus a plausible explanation for the observed microseisms at frequencies around 0.07 Hz.