Fundamental Mode Rayleigh Waves Research Articles

Eikonal surface wave tomography of central and eastern China

SUMMARY Eikonal tomography has become a key approach to image lithospheric structures with surface waves recorded by dense regional arrays. Its main advantage is that phase velocities can be determined directly from phase measurements without resolving a tomographic inverse problem. Here, we apply a new smoothing spline eikonal tomography approach to a selection of 40 large (Mw larger than 6.5) teleseismic events recorded by the permanent seismic stations of the CEArray. We first apply a time–frequency filter to isolate the fundamental mode of Rayleigh waves and cross-correlate the cleaned and isolated surface wave records to measure precise relative traveltimes. The phase measurements are then unwrapped and corrected for cycle skipping. Finally, we derive phase-velocity maps from the gradient of the reconstructed traveltime fields, using the eikonal equation. We obtain finely resolved phase-velocity maps from 25 to 150 s period that are inverted to obtain a 3-D shear wave velocity model which is in good agreement with previous tomographic studies. The lithospheric architecture emerging from the phase-velocity maps and 3-D S-wave velocities correlates with surface geology and major tectonic provinces. In particular, the architecture of the narrow rift systems, the South China Craton, and the North China Craton (NCC) are revealed with unprecedented details. The South China Craton is characterized by very high velocities beneath the Sichuan Basin. The NCC shows more complex structures with two high-velocity anomalies beneath the Ordos Basin and the southeastern NCC and low-velocity anomalies beneath the Cenozoic rift systems.

Surface wave tomography using dense 3D data around the Scrovegni Chapel in Padua, Italy

A dense single-node 3D seismic survey has been carried out around the Scrovegni Chapel in Padua (Italy), in order to give new insights about the archaeological setting of the area. The survey made use of nearly 1500 vertical nodes deployed over two rectangular grids. 38 shot positions were fired all around the two receiver patches. The fundamental mode Rayleigh wave signal is here analysed: traveltimes are directly inferred from the signal phases, and phase velocity maps are obtained using Eikonal tomography. Also surface wave amplitudes are used, to produce autospectrum gradient maps. The joint analysis of phase velocity and autospectrum gradient allowed the identification of several buried features, among which possible remains of radial walls of the adjacent Roman amphitheater, structures belonging to a medieval convent, and the root area of an eradicated tree. Finally, depth inversion of 1D dispersion curves allowed the reconstruction of a quasi-3D shear-wave velocity model.

Lithospheric Structure of the South India Precambrian Terrains From Surface Wave Tomography

AbstractWe have developed a 3‐D shear velocity (Vsv) model of Precambrian terrains of South India with a lateral resolution of 55 km to a depth of 250 km by inversion of fundamental mode Rayleigh wave phase velocity dispersion data in the period of 30–140s, combined with a well‐constrained crustal velocity model from an earlier study. The dispersion data were computed from 748 earthquakes recorded at 85 homogeneously distributed seismographs. Our velocity model shows 150–200 km thick lithosphere in most of the Archean Dharwar craton where Vsv is >4.7 km/s in the depth of 50–100 km and progressively decreases to 4.6 km/s at 120–150 km followed by a constant velocity of 4.5 km/s beyond 150–200 km depth. It correlates well with the results of the petrological studies of kimberlite xenoliths. An extraordinary high shear velocity (up to 4.8 km/s) and thick lithosphere (150 km) are observed beneath the Proterozoic Carbonatite complex, located at the south‐eastern edge of the Dharwar craton. We infer compositional modification of lower lithosphere at the south‐western margin of the Dharwar craton and lithospheric erosion at the Granulite terrain both possibly due to interaction with the Marion mantle plume at ∼90 Ma. The region is underlain by a 2%–3% lower velocity channel at ∼180–220 km depth in the asthenosphere, uncorrelated with the overlying lithosphere, possibly due to relative motion between them. It may be attributed to deep Earth‐processes such as asthenospheric upwelling.

Measuring the phase of ambient noise cross correlations: anisotropic Rayleigh and Love wave tomography across the Oman Mountains

SUMMARY Ambient seismic noise tomography has, over the last two decades, developed into a well-established tool for imaging seismic properties of the Earth’s crust. Fundamental mode Rayleigh and Love wave phase velocity dispersion curves can be measured from ambient noise cross-correlation functions (CCF) either using a high-frequency approximation theory, or by fitting the spectrum of the CCF to a Bessel function. Here, we advance the latter approach and present an automated algorithm that fits the phase of the Hankel function to the phase of the causal symmetric part of the CCF in order to determine phase velocity curves as continuous functions of frequency. Synthetic tests verify the reliability of the proposed method in the presence of low signal-to-noise ratio (SNR). Moreover, usage of the phase allows for robust phase velocity measurements at longer periods than when using the zero crossings of the Bessel function only and is, therefore, particularly useful at short inter-station distances. In the frequency domain, acceptable bandwidths of smooth phase velocity curves are obtained in an automated procedure using a set of fine-tuned quality criteria. We apply the method to 2.5 yr of continuous waveform data recorded by 58 temporary and permanent broad-band seismic stations in northern Oman. We obtain 1072 and 670 phase velocity curves for Rayleigh and Love waves, respectively, in the period range of 2–40 s. The data are inverted for isotropic and azimuthally anisotropic period-dependent phase velocity maps. Synthetic reconstruction tests show that the phase velocity maps have a lateral resolution of ∼30 km. The results suggest distinctly different middle to lower crustal architecture between the northern and eastern Oman Mountains. Azimuthal anisotropy shows contrasting fast propagation orientations in the shallow and deep crust, which we attribute to stress-induced and structural anisotropy in the upper crust and to lattice-preferred orientation in the lower crust.

Seismic site characterization in Fraser River Delta in Metropolitan Vancouver

Seismic site characteristics, specifically depth to stiff glaciated Pleistocene sediments (zgl) and shear wave velocity (Vs) profiles, of the Fraser River Delta (FRD), located in southern Metropolitan (Metro) Vancouver, British Columbia, Canada are determined using the most comprehensive geodatabase to date. The relationships between zgl and theoretical fundamental frequency determined from known thickness of post-glacial sediments and experimental peak frequency from microtremor horizontal-to-vertical ratios (f1,HVSR) are investigated. A zgl predictive model based on f1,HVSR is developed for shallow FRD sites (zgl ≤ 56 m) and is compared to existing microtremor horizontal-to-vertical spectral ratio (MHVSR) peak frequency-sediment thickness models developed in other regions in the world. At 16 FRD sites, a combination of active- and passive-source surface wave array methods are performed to obtain the fundamental-mode Rayleigh wave dispersion curve which is jointly inverted with the MHVSR fundamental peak frequency (f0,HVSR) using three parametrization models with varied layering. The jointly inverted Vs profiles with their uncertainties are presented for the 16 sites and compared with existing proximal Vs profiles measured by other in situ Vs profiling methods. The comparison shows a notable match (9.1% depth-averaged absolute relative difference) to a significant depth of 220 m from the two datasets. In addition, 8 of the developed Vs profiles are co-located with strong-motion accelerograph stations, which allows for correlation between recorded earthquake ground motions and seismic site parameters. The proposed predictive zgl model and validation of evolving non-invasive in situ Vs profiling methods are important for 1D and 2D seismic site effects quantification and mapping in the FRD.

Integrated analysis of surface wave velocity and gravity data for the development of new density-velocity models of the crust and upper mantle in SE Iran

Inversion of Rayleigh wave dispersion curves is challenging due to its nonlinearity and multimodality. In this paper, a Simulated Annealing (SA) algorithm is applied to the nonlinear inversion of fundamental-mode Rayleigh wave group dispersion curves for shear and compressional wave velocities. In our approach, we invert thickness, velocities and densities and their vertical gradients of four layers, sediments, upper-crust, lower-crust and lithospheric mantle.At first, to determine the efficiency and stability of the method, noise-free and noisy synthetic data sets are inverted. Results from the synthetic data demonstrate that the SA applied to the nonlinear inversion of surface wave data is interesting not only in terms of accuracy but also in terms of the convergence speed. In fact, the SA method is suitable for large-scale optimization problems, especially for those in which a desired global minimum is hidden among many local minima. In a second step, real data in SE Iran are inverted to examine the usage and robustness of the proposed approach on real surface wave data. Then, we applied 3D gravity modeling based on surface wave analysis results to obtain the density structure of each layer. The reason for using both types of data sets is that the gravity anomaly does not have a good vertical resolution and surface wave group velocities are more appropriate for placing layer limits at depth, but they are not very sensitive to density variations. Therefore, the use of gravity data increases the overall resolution of density distribution. Compared with the Shuffle Complex Evolution (SCE) method that was implemented in a previous study, we found out that the SA method is more stable and has less variability of model parameter values in successive tests. In the final step, we reapplied the SA method to invert the fundamental-mode Rayleigh wave group velocities based on the results of gravity modeling. Gravity results, such as thicknesses and densities were used to limit the search space in the SA method.Our results show that the Moho depth across the Makran subduction zone is increasing from the Oman seafloor (24–30 km) and Makran forearc setting (34–42 km) to the Taftan-Bazman volcanic arc (47–49 km). Also, the results show high shear and compressional velocities under the Gulf of Oman, decreasing to the north of the Makran region. The density image shows an average crustal density with maximum values under the Gulf of Oman, decreasing northward to the Makran region.

Deep learning inversion of Rayleigh-wave dispersion curves with geological constraints for near-surface investigations

SUMMARY With the emergence of massive seismic data sets, surface wave methods using deep learning (DL) can effectively obtain shear wave velocity (Vs) structure for non-invasive near-surface investigations. Previous studies on DL inversion for deep geophysical investigation have a reference model to generate the training data set, while near-surface investigations have no model. Therefore, we systematically give a set of training data set generation processes. In the process, we use both prior information and the observed data to constrain the data set so that the DL inversion model can learn the local geological characteristics of the survey area. Because the space of inverted Vs models is constrained and thus narrowed, the inversion non-uniqueness can be reduced. Furthermore, the mean squared error, which is commonly used as loss function, may cause a poor fitting accuracy of phase velocities at high frequencies in near-surface applications. To make the fitting accuracy evenly in all frequency bands, we modify the loss function into a weighted mean squared relative error. We designed a convolutional neural network (CNN) to directly invert fundamental-mode Rayleigh-wave phase velocity for 1-D Vs models. To verify the feasibility and reliability of the proposed algorithm, we tested and compared it with the Levenberg–Marquardt (L-M) inversion and neighbourhood algorithm (NA) using field data from the Lawrence experiment (USA) and the Wuwei experiment (China). In both experiments, the inverted Vs models by CNN are consistent with the borehole information and are similar to that from existing methods after fine tuning of model parameters. The average root mean squares errors (RMSEs) of the CNN, NA and L-M methods are also similar, except in the Lawrence experiment, the RMSE of CNN is 17.33 m s−1 lower than previous studies using the L-M method. Moreover, the comparison of different loss functions for the Wuwei experiment indicates that the modified loss function can achieve higher accuracy than the traditional one. The proposed CNN is therefore ideally suited for rapid, repeated near-surface subsurface imaging and monitoring under similar geological settings.

Rayleigh wave scattering at shallow cavities in layered half spaces

For layered half spaces in which the shear wave (S-wave) velocity of layer increases with the layer depth, the subsurface seismic wave field generated by surface sources is dominated by the fundamental mode Rayleigh waves (R-waves). When a cavity full of air is present in the underlying half space, the forward (or incident) R-waves are scattered. The scattered wave field in front of the cavity is dominated by the back-scattered R-waves and the interference fringes are formed. Under the conditions that the length (i.e. horizontal extent) of cavity is enough large relative to the wavelength and the burial depth, the majority of the diffracted waves over the cavity approach the fundamental mode plate waves. The displacement structures (variations of displacement with depth) of the diffracted waves are different from those of R-waves within the burial depth. The differences could result in the decrease of the spectral amplitude over the cavity relative to that in front of the cavity. In the surface wave-field behind the cavity, the diffracted waves under the cavity could cause the increase of the spectral amplitude relative to that over the cavity at some low frequencies. By transforming the spectrum into the wavelength domain, a method is proposed to estimate the horizontal extent and burial depth of a cavity from the spectral changes in the offset-wavelength domain. The feasibility is verified by a preliminary experiment. ∙ The normalized depth of energy distribution in a layered half space is smaller than that in a half space with the Poisson’s ratio equal to that of the first layer. ∙The majority of the diffracted waves over a cavity behave as the fundamental mode plate waves under conditions that the length of cavity is large compared to the wavelength and the burial depth. ∙The spectral changes over a cavity are explained using displacement structure differences between R-waves and the diffracted waves. ∙The spectral density over a cavity is weaker than that in front of the cavity. ∙In the wave field behind a cavity, the frequency content relatively increases for the waves with the wavelengths larger than the burial depth. ∙The location and burial depth of cavity can be estimated from the spectral changes in front of and behind a cavity in the offset-wavelength domain.

Determination of near-surface shear-velocity structure based on the joint inversion of Rayleigh-wave dispersion and ellipticity from multistation active-seismic records

The dispersive characteristics of high-frequency (≥2 Hz) Rayleigh waves have been widely applied to determine subsurface shear velocity ([Formula: see text]). However, the [Formula: see text] of the high-velocity surface layers (e.g., rigid roads in urban areas) and underlying layers is poorly constrained in the Rayleigh-wave dispersion inversion. Compared to the phase velocity, the Rayleigh-wave horizontal-to-vertical (H/V) ratios of particle motion (i.e., ellipticity) are particularly sensitive to shallow [Formula: see text] structures. Therefore, we have proposed a robust method for obtaining near-surface [Formula: see text] structures by inverting Rayleigh-wave dispersion and H/V data. Initially, we separate the fundamental-mode Rayleigh waves from the active-seismic records using a high-resolution linear Radon transformation method. Then, we extract the high-frequency H/V and interstation phase traveltimes from the separated fundamental mode using deconvolution interferometry and the frequency-time analysis method. Based on the phase traveltimes, we calculate the phase velocity dispersion using ray-based tomography. Finally, we perform the joint inversion of dispersion and H/V data to obtain the [Formula: see text] structure. According to the inversion results of synthetic and field data, the accuracy of the [Formula: see text] in the rigid surface layer and underlying layers is considerably improved. We find that the joint inversion of Rayleigh-wave phase velocity dispersion and H/V data from surface-wave surveys is a useful tool for accurately determining the complex shallow [Formula: see text] structures.

Semi‐Supervised Surface Wave Tomography With Wasserstein Cycle‐Consistent GAN: Method and Application to Southern California Plate Boundary Region

AbstractMachine learning algorithm has been applied to shear wave velocity (Vs) inversion in surface wave tomography, where a set of starting 1‐D Vs profiles and their corresponding synthetic dispersion curves are used in network training. Previous studies showed that the performance of such trained network is dependent on the diversity of the training data set, which limits its application to previously poorly understood regions. Here, we present an improved semi‐supervised algorithm‐based network that takes both model‐generated and observed surface wave dispersion data in the training process. The algorithm is termed Wasserstein cycle‐consistent generative adversarial networks (Wasserstein Cycle‐GAN [Wcycle‐GAN]). Different from conventional supervised approaches, the GAN architecture enables the inclusion of unlabeled data (the observed surface wave dispersion) in the training process that can complement the model‐generated data set. The cycle‐consistency and Wasserstein metric significantly improve the training stability of the proposed algorithm. We benchmark the Wcycle‐GAN method using 4,076 pairs of fundamental mode Rayleigh wave phase and group velocity dispersion curves derived in periods from 3 to 16 s in Southern California. The final 3‐D Vs model given by the best trained network shows large‐scale features consistent with the surface geology. The resulting Vs model has reasonable data misfits and provides sharper images of structures near faults in the top 15 km compared with those from conventional machine learning methods.

Oceanic plate subduction and continental extrusion in Sumatra: Insight from S-wave anisotropic tomography

We determine the first high-resolution 3-D model of S-wave azimuthal anisotropy down to ∼200 km depth of the Sumatra subduction zone using high-quality amplitude and phase data of teleseismic fundamental mode Rayleigh waves at periods of 20-70 s. The subducting Indo-Australian slab is imaged as a high-velocity (high-V) zone beneath the Sumatra arc, whereas obvious low-V anomalies exist in the mantle wedge beneath the arc and back-arc areas. Our results also reveal significant depth-varying azimuthal anisotropy beneath Sumatra. The subducting slab mainly exhibits a trench-parallel (NW-SE) fast velocity direction (FVD) normal to the fossil seafloor isochrons on the slab, except for an area near the Toba volcano where trench-normal (NE-SW) FVDs exist in the slab. The mantle wedge is dominated by trench-normal FVDs parallel or sub-parallel to the subduction direction of the oceanic slab, which may indicate trench-normal corner flow in the mantle wedge. The overriding plate beneath the Sumatra Island exhibits FVDs sub-parallel or oblique to the Sumatra fault, which may be related to the continental extrusion in Southeast Asia due to the Cenozoic collision between the Indo-Australian plate and the Eurasian plate. These results suggest that structural fabrics related to the oceanic plate subduction and continental extrusion are decoupled from each other beneath Sumatra.

Analysis of earthquakes recordings of tidal gravimeters in the period range of 10-1000 s

The physical properties of tidal gravimetric instruments allow recording, not only tidal effects, but also waves generated by earthquakes. Three gravimetric stations with determined transfer functions and co-located seismic stations from the observatories in Western and Central Europe were selected for analysis. The observatories are equipped with almost all types of sensors available on the market, which allow for thorough analysis of earthquake recordings in the period range of 10–1000 s. In total, over 10,000 traces of worldwide earthquakes were investigated. The saturation levels of gravimeters as well as a correlation between the gravimetric and seismometric signals of an earthquake were carefully analysed. A simple processing scheme of gravimetric signal of earthquakes was adopted thanks to the probabilistic power spectral density analysis of continuous recordings. The detail analysis of transfer function of gravimeters allowed to define a period range when a sensitivity coefficient (calibration factor) and a time lag value only can be used to properly describe the properties of instruments. What’s more, it has been shown based on the calculated group-velocity dispersion curves of fundamental mode of Rayleigh waves, that the Earth’s mantle structure can be determined for greater depths from the recording of tidal gravimeters than from typical broad-band seismometers.

Seismic Attenuation at the Equatorial Mid‐Atlantic Ridge Constrained by Local Rayleigh Wave Analysis From the PI‐LAB Experiment

AbstractThe ocean lithosphere represents a simple realisation of the tectonic plate, offering a unique opportunity to better understand its physical and chemical properties in relationship to those of the underlying asthenosphere. While seismic velocity is frequently used to image the plate, seismic attenuation (Qμ−1) offers an important complimentary observation. We use fundamental mode Rayleigh waves from 17 local, M > 4.2 earthquakes recorded at stations located on 0–80 My old lithosphere near the equatorial Mid‐Atlantic Ridge. We determine the attenuation coefficient () for periods between 15 and 40 s and invert for 1‐D average shear wave quality factor values () and shear wave velocity (Vs). We find values of 175 ± 16 at 50 km depth, decreasing to 90 ± 15 at greater than 60 km. Comparison of our and Vs measurements to previous observations from oceanic settings shows agreement in terms of higher and Vs in the lithosphere in comparison to the asthenosphere. The observations from oceanic settings are in general agreement with the laboratory predictions for ‐Vs relationships for thermal models. However, a small amount of partial melt (1%) is required to explain several previous observations. Our result also compares favorably to previous observations of lithospheric and asthenospheric attenuation with respect to frequency. Melt is not required for the 1‐D average of our study area, consistent with previous electromagnetic and seismic imaging that suggested melt in punctuated and/or thin channel anomalies rather than over broad regions of the mantle.

Upper Mantle Structure Beneath Mariana: Insights From Rayleigh‐Wave Anisotropic Tomography

AbstractWe determine Rayleigh‐wave phase‐velocity azimuthal anisotropy tomography beneath Mariana using a large number of high‐quality amplitude and phase data of teleseismic fundamental mode Rayleigh waves at periods of 25–130 s. The obtained phase‐velocity model is then inverted for a 3D azimuthal anisotropic shear‐wave velocity (Vs) model down to ∼300 km depth. The old (∼150 Ma) Pacific oceanic lithosphere near the Mariana trench is imaged as a clear high‐Vs zone with a thickness of ∼100 km, whereas obvious low‐Vs anomalies exist in the mantle wedge and in the asthenosphere beneath the incoming plate. The Pacific oceanic lithosphere near the Mariana trench mainly exhibits a fast‐velocity direction (FVD) of NW‐SE normal to seafloor isochrons, which may reflect frozen‐in lattice‐preferred orientation formed at the mid‐ocean ridge, whereas the dominant FVD in the subducting Pacific slab at depths >∼100 km is parallel to the Mariana trench, probably reflecting subduction‐related structural deformation in the slab. Significant trench‐normal FVDs exist in the mantle wedge at depths <∼100 km, whereas the deeper portion of the mantle wedge mainly exhibits trench‐parallel FVDs, which may indicate a flow pattern in the mantle wedge due to the slab deep subduction and dehydration.

Global reference seismological data sets: multimode surface wave dispersion

SUMMARYGlobal variations in the propagation of fundamental-mode and overtone surface waves provide unique constraints on the low-frequency source properties and structure of the Earth’s upper mantle, transition zone and mid mantle. We construct a reference data set of multimode dispersion measurements by reconciling large and diverse catalogues of Love-wave (49.65 million) and Rayleigh-wave dispersion (177.66 million) from eight groups worldwide. The reference data set summarizes measurements of dispersion of fundamental-mode surface waves and up to six overtone branches from 44 871 earthquakes recorded on 12 222 globally distributed seismographic stations. Dispersion curves are specified at a set of reference periods between 25 and 250 s to determine propagation-phase anomalies with respect to a reference Earth model. Our procedures for reconciling data sets include: (1) controlling quality and salvaging missing metadata; (2) identifying discrepant measurements and reasons for discrepancies; (3) equalizing geographic coverage by constructing summary rays for travel-time observations and (4) constructing phase velocity maps at various wavelengths with combination of data types to evaluate inter-dataset consistency. We retrieved missing station and earthquake metadata in several legacy compilations and codified scalable formats to facilitate reproducibility, easy storage and fast input/output on high-performance-computing systems. Outliers can be attributed to cycle skipping, station polarity issues or overtone interference at specific epicentral distances. By assessing inter-dataset consistency across similar paths, we empirically quantified uncertainties in traveltime measurements. More than 95 per cent measurements of fundamental-mode dispersion are internally consistent, but agreement deteriorates for overtones especially branches 5 and 6. Systematic discrepancies between raw phase anomalies from various techniques can be attributed to discrepant theoretical approximations, reference Earth models and processing schemes. Phase-velocity variations yielded by the inversion of the summary data set are highly correlated (R ≥ 0.8) with those from the quality-controlled contributing data sets. Long-wavelength variations in fundamental-mode dispersion (50–100 s) are largely independent of the measurement technique with high correlations extending up to degree ∼25. Agreement degrades with increasing branch number and period; highly correlated structure is found only up to degree ∼10 at longer periods (T > 150 s) and up to degree ∼8 for overtones. Only 2ζ azimuthal variations in phase velocity of fundamental-mode Rayleigh waves were required by the reference data set; maps of 2ζ azimuthal variations are highly consistent between catalogues ( R = 0.6–0.8). Reference data with uncertainties are useful for improving existing measurement techniques, validating models of interior structure, calculating teleseismic data corrections in local or multiscale investigations and developing a 3-D reference Earth model.

Rayleigh Wave Propagation in the Bighorn Mountains Region, Wyoming

ABSTRACT Short-period Rayleigh waves, Rg, provide strong constraints on the depth of shallow seismic events and are of interest for monitoring small explosions. Characterizing the seismic sources that generate Rg requires an understanding of how shallow crustal structure affects Rayleigh wave propagation. In support of these efforts, this study utilizes observed waveforms from small shallow explosions recorded on temporary seismic network deployments in the Bighorn region, Wyoming. We study regional near-surface geology by measuring changes in surface-wave amplitude and polarization during propagation through basins, foothills, and mountains. We develop additional insight by carrying out surface-wave eigenfunction analyses and numerical-wave simulations, which together reproduce many characteristics seen in the observed waveforms. Our results show how sedimentary basins in the Bighorn region allow for amplified prograde-polarized higher-mode and retrograde-polarized fundamental-mode Rayleigh waves, whereas adjacent mountains only support retrograde motion. These different modes provide distinct constraints on the Earth structure and source characteristics, potentially enabling targeted inversions in future studies. Our findings provide insight into Rg propagation through complex near-surface geology, improving our understanding of shallow propagation and source effects that are relevant to explosion monitoring efforts.

Rayleigh wave group velocity maps at periods of 10–150 s beneath South America

SUMMARY Based on new data from permanent and temporary networks, we present fundamental mode Rayleigh wave group velocity maps at periods of 10–150 s related to the lithosphere beneath South America. We analyse waveform data from 1043 earthquakes, from 2002 to 2019, which were recorded by 282 stations. To isolate fundamental mode Rayleigh waves, a phase-matched filter is applied, and the measurements of group velocity are obtained from multiple filter analysis techniques. Thus, we obtain 17 838 paths, covering most of the South American continent, which reach their maximum at the period of 30 s and decrease for both shorter and longer periods. We calculate average dispersion curves and probability density distribution of all measured curves to check the consistency of our data set. Then, regionalized group velocity maps are obtained by iteratively combining the fast marching method and the subspace inversion method. The resolution of our models is assessed by checkerboard tests, which show that the synthetic group velocities are well recovered, despite some amplitude and smearing effects in some portions of the model, probably owing to regularization and uneven ray path coverage. Compared to previous group velocity studies for South America, our models present better resolution, mainly for shorter periods. Our maps of 10 and 20 s, for example, show an excellent correlation with the sedimentary thickness (CRUST1.0) and topography density (UNB$\_$TopoDens). Regions of exposed basement and high-density are related to fast group velocities, while sedimentary basins and low-densities are observed as areas of slow group velocities. We identify small-scale fast group velocity heterogeneities that may be linked to the Rio Apa and Rio Tebicuary cratons as well as to the geochronological provinces of the Amazonian Craton. The most striking feature of our map at 40 s is a fast group velocity structure with the same NE trend of the Transbrasiliano lineament, a Neoproterozoic megashear fault that crosses a large part of the South American continent. Our long-period maps sample lithospheric depths, revealing that cratonic areas of South America, such as the Amazonian and São Francisco cratons, correlate well with fast group velocities. Another interesting feature is the presence of a strong group velocity gradient between the Paraná and Chaco-Paraná basins, which nearly coincides with the location of the Western Paraná Suture, a continental-scale gravity discontinuity. From our group velocity maps, we estimate 1-D S-wave velocity depth profiles at 10 locations in South America: Chaco-Tarija Basin, Borborema Province (BP), Amazonian Craton, Paraná Basin, Tocantins Province, Acre Basin (AcB), Altiplano-Puna Volcanic Complex, Mantiqueira Province (MP), Parnaíba Basin and São Francisco Craton. Most of our inverted S-wave velocity profiles show good agreement with the SL2013sv model at lithospheric depths, except the BP, AcB and MP profiles. Particularly for the BP, a low shear wave velocity, from about 75 to 150 km depth, is a feature that is not present in the SL2013sv model and was probably resolved in our model because of our denser ray path coverage. This decreased S-wave velocity may be due to a lithospheric thinning beneath the BP, as already pointed out by previous studies.

Velocity and azimuthal anisotropy structure underneath the Reelfoot Rift region from Rayleigh wave phase velocity dispersion curves

SUMMARYPhase velocity and azimuthal anisotropy maps for fundamental mode Rayleigh waves are determined for a portion of the central United States including the seismically active Reelfoot Rift (RFR) and the enigmatic Illinois Basin. Dense seismic array installations of the Northern Embayment Lithosphere Experiment, the EarthScope transportable array and the Ozarks Illinois Indiana Kentucky array allow a detailed investigation of phase velocity and anisotropy in a broad period range (20–100s).We obtain more than 12 000 well-constrained, unique two-station paths from teleseismic events. The two-station method is used to determine dispersion curves and these are inverted for isotropic phase velocity maps and azimuthal anisotropy maps for each period. The presence of fast phase velocities at lower crustal and uppermost mantle depths is found below the RFR, and Ste. Genevieve and Wabash Valley fault zones. At periods of 30s and higher, the RFR is underlain by slow phase velocities and is flanked to the NW and SE by regions of fast velocity. Fast phase velocities are present below the centre of the Illinois Basin in the period range 75–100s. Anisotropy fast axis orientations display complex patterns for each period and do not trend parallel to the direction of absolute plate motion. Anisotropy fast directions are consistently parallel to the trend of the RFR from 50s to higher periods, suggesting the presence of either frozen-in anisotropic fabric or fabric related to material transport from a recently discovered, pronounced low velocity zone below the Mississippi Embayment.

A Seismic Investigation of the Upper Crustal Structure of the Iranian Plateau

We obtained a three-dimensional (3D) shear wave velocity model of the upper crust of the Iranian Plateau, based on the inversion of fundamental mode Rayleigh wave group velocity. The surface wave group velocity measurements for the period range 5-25 s were extracted from two seismic data sets: ambient noise cross-correlations and regional earthquakes. The low shear wave velocity (Vs) anomalies of the upper crust correspond to regions of thick sediments. The surrounding basins of the Plateau, the South Caspian Basin (SCB) in the north and the Zagros Fold-Thrust Belt (ZFTB) and the Makran accretionary wedge in the south form the thickest sedimentary covers of the region exceeding 20 km. The Dasht-e Kavir. The thickest parts of inland basins such as the Jazmurian depression, and the Dasht-e Kavir are ~10 km. The Vs structure of southern Zagros is almost homogeneous at all levels of the crust, but the low velocity anomaly beneath the southern Lorestan Arc separates the central Zagros from the northernmost Zagros. The volcanic belt of the Makran Subduction zone is another large/deep low velocity zone, where the observed low Vs possibly results from magma migration and heat transfer from the mantle wedge of the Makran Subduction. The high-velocity regions in the upper crust are observed in the Sanandaj-Sirjan Zone (SSZ), the Urmia-Dokhtar Magmatic Assemblage (UDMA), and the south of the Lut Block.

Shallow Shear-Wave Velocity Structure beneath the West Lake Area in Hangzhou, China, from Ambient-Noise Tomography

Urban geophysical exploration plays an important role in the sustainable development of and the mitigation of geological hazards in metropolitan areas. However, it is not suitable to implement active seismic methods in densely populated urban areas. The rapidly developing ambient-noise tomography (ANT) method is a promising technique for imaging the near-surface seismic velocity structure. We selected the West Lake area of the city of Hangzhou as a case study to probe the shallow subsurface shear-wave velocity (Vs) structure using ANT. We conducted seismic interferometry on the ambient-noise data recorded by 28 seismograph stations during a time period of 17 days. Fundamental-mode Rayleigh-wave group- and phase-velocity dispersion data were measured from cross-correlation functions and then inverted for a 3D Vs model of the uppermost 1 km that covers an area of about 7 km × 8 km. The tomographic results reveal two prominent anomalies, with high velocities in the southwest and low velocities in the northeast. The fast anomaly corresponds to the presence of limestone and sandstone, whereas the slow anomaly is due to the relatively low-velocity rhyolite and volcanic tuff in the area. The boundary between the two anomalies lies to the NE of an NW–SE trending fault, indicating that the fault dips toward the NE. In addition, the pronounced low-velocity anomalies appear under the Baoshi mountain, likely due to the thick rhyolite and volcanic tuff beneath the extinct volcano. Our results correlate well with regional geological features and suggest that ANT could be a promising technique for facilitating the exploration of urban underground space.