Group Velocity Dispersion Curves Research Articles

Supercontinuum generation in heavy-metal oxide glass based suspended-core photonic crystal fibers

We investigate supercontinuum generation in several suspended-core soft-glass photonic crystal fibers pumped by an optical parametric oscillator tunable around 1550 nm. The fibers were drawn from lead-bismuth-gallium-cadmium-oxide glass (PBG-81) with a wide transmission window from 0.5-2.7 micron and a high nonlinear refractive index up to 4.3.10^(-19) m^2/W. They have been specifically designed with a microscale suspended hexagonal core for efficient supercontinuum generation around 1550 nm. We experimentally demonstrate two supercontinuum spectra spanning from 1.07-2.31 micron and 0.89-2.46 micron by pumping two PCFs in both normal and anomalous dispersion regimes, respectively. We also numerically model the group velocity dispersion curves for these fibers from their scanning electron microscope images. Results are in good agreement with numerical simulations based on the generalized nonlinear Schrodinger equation including the pump frequency chirp.

Dispersion features of transmitted channel waves and inversion of coal seam thickness

In-seam seismic survey currently is a hot geophysical exploration technology used for the prediction of coal seam thickness in China. Many studies have investigated the relationship between the group velocity of channel wave at certain frequency and the actual thickness of exposed coal beds. But these results are based on statistics and not universally applicable to predict the thickness of coal seams. In this study, we first theoretically analyzed the relationship between the depth and energy distribution of multi-order Love-type channel waves and found that when the channel wave wavelength is smaller than the thickness of the coal seam, the energy is more concentrated, while when the wavelength is greater than the thickness, the energy reduces linearly. We then utilized the numerical simulation technology to obtain the signal of the simulated Love-type channel wave, analyzed its frequency dispersion, and calculated the theoretical dispersion curves. The results showed that the dispersion characteristics of the channel wave are closely related to the thickness of coal seam, and the shear wave velocity of the coal seam and its surrounding rocks. In addition, we for the first time realized the joint inversion of multi-order Love-type channel waves based on the genetic algorithm and inversely calculated the velocities of shear wave in both coal seam and its surrounding rocks and the thickness of the coal seam. In addition, we found the group velocity dispersion curve of the single-channel transmitted channel wave using the time–frequency analysis and obtained the phase velocity dispersion curve based on the mathematical relationship between the group and phase velocities. Moreover, we employed the phase velocity dispersion curve to complete the inversion of the above method and obtain the predicted coal seam thickness. By comparing the geological sketch of the coal mining face, we found that the predicted coal seam thickness is in good agreement with the actual thickness. Overall, adopting the channel wave inversion method that creatively uses the complete dispersion curve can obtain the shear wave velocities of the coal and its surrounding rocks, and analyzing the depth of the abruptly changed shear wave velocity can accurately obtain the thickness of the coal seam. Therefore, our study proved that this inversion method is feasible to be used in both simulation experiments and actual detection.

Flyover Crustal Structures Beneath the Qinling Orogenic Belt and Its Tectonic Implications

AbstractDetermining high‐resolution three‐dimensional (3‐D) crustal structures of the Qinling Orogenic Belt (QOB) can reveal significant evidences to enhance our understanding of its tectonic evolution. By jointly inverting group and phase velocity dispersion curves, we obtain a high‐resolution 3‐D shear wave velocity model for the QOB. According to the obviously layered features, which include an E‐W trending high‐velocity structure in the upper crust (0–10 km), a transitional zone in the middle crust (10–30 km), and an approximately N‐S trending low‐velocity zone in the middle‐lower crust (20–40 km), we elucidate the crustal structure as a flyover model to explain how the crust beneath the QOB was deformed under the perpendicular force systems from the NE‐SW and E‐W directions during the Meso‐Cenozoic. First, the apparent NE‐SW trending low‐velocity zone in the middle‐lower crust indicates that the low‐viscosity crustal materials beneath the northeastern Tibetan Plateau did not flow eastward into the eastern Qinling terrane during the Cenozoic. Second, the extension in the northern Qinling Mountains and Weihe Basin was not largely affected by the lateral crustal growth of the NE Tibetan Plateau, which might be the result of deep mantle upwelling and/or residual effects from the subduction of the western Pacific plate. Third, during the Mesozoic, the high‐velocity structures beneath the Hannan‐Micang and Shennongjia‐Huangling domes served as anchors resisting the northward subduction, rotation, and continuous collision of the Yangtze Block. The eastward extruding Hannan‐Micang dome served as an indenter that eventually shaped the present‐day asymmetric style of the Dabashan Orocline during the Cenozoic.

ANISOTROPIC PROPERTIES OF THE UPPER MANTLE IN CENTRAL ASIA ACCORDING TO THE GROUP VELOCITY DISPERSION CURVES FOR RAYLEIGH AND LOVE WAVES

The article presents the results of the study focused on the anisotropic properties of the upper mantle in Central Asia. The study is based on a representative set of the group velocity dispersion curves for Rayleigh and Love waves. The dispersion curves were calculated in the range of 10–250 s. The maps of group velocity distribution pat‐ terns and the horizontal resolution estimates were calculated by the surface‐wave tomography method developed for a spherical surface. Based on the maps, the local group velocity dispersion curves were reconstructed for the given points within the study region, which were then converted into the one‐dimensional velocity sections of SV‐ and SH‐waves, and a vertical anisotropy coefficient was estimated. A three‐dimensional anisotropic model shows the ve‐ locity distribution pattern of S‐waves in the crust and the mantle to the depth of 500 km. According to this model, ver‐ tical anisotropy in the upper mantle is observed to the depth of about 250 km and has maximum values in the depth interval from the crustal bottom to 150 km. The anisotropic properties are unevenly distributed and reflect the geo‐ logical structure of the study area. Therefore, tectonically active regions are characterized by the high values of the anisotropy coefficient and the reduced values of the S‐wave velocities. The presented results can contribute to the further development of more detailed and strictly proved geodynamic models of the study area.

The Capability Assessment of the Spectrum Decomposition Technique for Measurements of the Group Velocity of Lamb Waves

The ultrasonic guided waves are dispersive waves characterized by the phase and group velocities dispersion curves. In order to use guided waves in various industrial applications, their parameters must be known. Since these guided wave velocities depend on the frequency and thickness of the material, they propagate differently comparing to bulk ultrasonic waves. Therefore, to analyze the parameters of such waves, new measurement techniques should be proposed and possibilities of their application have to be investigated. In this paper possibilities to measure the group velocity based on the spectrum decomposition approach are presented. The investigations are carried out using the simulated and experimental signals of Lamb wave propagating in a 2 mm thickness aluminium plate. The two fundamental modes $$\hbox {A}_{0}$$ and $$\hbox {S}_{0}$$ are selected. Using the proposed technique, segments of the group velocity dispersion curves have been reconstructed and compared with the dispersion curves calculated by the SAFE method to estimate errors. Accordingly to the obtained lower absolute and relative errors, an optimal set of narrowband filters with bandwidth from 20 kHz up to 100 kHz for the $$\hbox {A}_{0}$$ mode and from 80 kHz up to 160 kHz for the $$\hbox {S}_{0}$$ mode are proposed. Applying the proposed optimal frequency sets of narrowband filters for the experimental signals, segments of the group velocity dispersion curves for both modes are reconstructed. The average relative error for the $$\hbox {A}_{0}$$ mode is 1.7–2.2% (expanded relative uncertainty ± (2.2–2.8)%) and 0.78–1.2% (expanded relative uncertainty ± (0.5–0.8)%) for the $$\hbox {S}_{0}$$ mode.

Dispersion of group velocities of Rayleigh and Love waves and anisotropic properties of the Asian continent

We have studied the structures of the Earth’s crust and upper mantle of the Asian continent using a representative sample of dispersion curves of group velocities of fundamental-mode Rayleigh and Love waves for more than 3200 seismic paths. Maps of distributions of variations in group velocities with periods of 10 to 250 s over a spherical surface were calculated by the 2D tomography method. The maps reflect the deep structure of the Earth’s crust and upper mantle of the study area and give a tentative idea of the horizontal distribution of the anisotropic properties of the mantle matter. The obtained data are confirmed by the calculations of the velocity profiles of SV- and SH-waves for the entire Asian continent and for its regions. Vertically, anisotropy is observed to the depths of ~250 km, with its maximum in the depth range from the bottom of the crust to 150 km.

Dispersion curves for Lamb wave propagation in prestressed plates using a semi-analytical finite element analysis.

Acoustoelastic techniques have been recently used to characterize the state of prestress in structures such as plates. The velocity of guided wave modes propagating through plates is sensitive to the magnitude and orientation of the initial state of stress. Dispersion curves for phase velocities of plate guided waves can be computed using the superposition of partial bulk waves (SPBW) method. Here, a semi-analytical finite element (SAFE) method is formulated for the acoustoelastic problem of guided waves in weakly nonlinear elastic plates. The SAFE formulation is shown to provide phase velocity dispersion curve results identical with those provided by the SPBW method for the problem of a plate under a uniaxial and uniform tensile stress. Analytical phase and group velocity dispersion curves are also obtained for a plate with an initial prestress gradient through its thickness using the SAFE method. The magnitude of the prestress gradient is shown to have a significant effect on phase and group velocities of the fundamental and first order Lamb modes, only in certain frequency-thickness regimes.

Effective time-frequency characterization of Lamb wave dispersion in plate-like structures with non-reflecting boundaries

Research on Lamb wave-based damage identification in plate-like structures depends on precise knowledge of dispersive wave velocity. However, boundary reflections with the same frequency of interest and greater amplitude contaminate direct waves and thus compromise measurement of Lamb wave dispersion in different materials. In this study, non-reflecting boundaries were proposed in both numerical and experimental cases to facilitate time-frequency characterization of Lamb wave dispersion. First, the Lamb wave equations in isotropic and laminated materials were analytically solved. Second, the non-reflecting boundaries were used as a series of frames with gradually increased damping coefficients in finite element models to absorb waves at boundaries while avoiding wave reflections due to abrupt property changes of each frame. Third, damping clay was sealed at plate edges to reduce the boundary reflection in experimental test. Finally, the direct waves were subjected to the slant-stack and short-time Fourier transformations to calculate the dispersion curves of phase and group velocities, respectively. Both the numerical and experimental results suggest that the boundary reflections are effectively alleviated, and the dispersion curves generated by the time-frequency analysis are consistent with the analytical solutions, demonstrating that the combination of non-reflecting boundary and time-frequency analysis is a feasible and reliable scheme for characterizing Lamb wave dispersion in plate-like structures.

The application of seismic interferometry for estimating a 1D S-wave velocity model with the use of mining induced seismicity

Abstract The main objective of this paper is to present the usefulness of the seismic interferometry method to determine the S-wave velocity model of the rock mass affected by exploitation in the KGHM Rudna copper ore mine. The research aim was achieved on the basis of seismic data, acquired from seismograms, of 10 strong seismic events of magnitude greater than 2.6. They were recorded by a pair of seismometers deployed on mining terrain. In the first stage, the Rayleigh wave between seismometers was estimated. Then, the group velocity dispersion curves of fundamental and first higher modes were identified. Finally, inversion of the dispersion curves to a 1D S-wave velocity model up to 500 m in depth was obtained. The velocity model was determined for the part of the rock mass partially affected by mining. The results confirm similar rock mass structure and velocities of the subsurface layers as those obtained by the archival 3D model. In both models, a high degree of correlation in the boundary location between the overburden of the Cenozoic formations and the bedrock of the Triassic formations was observed. The applied methodology can be used to estimate the S-wave velocity model in other mining regions characterized by strong seismicity.

A non-destructive material characterization framework for retrieving a stiffness matrix using bulk waves

The accuracy of the stiffness matrix used as input in dispersion curve algorithm determine the accuracy of the predicted wave speeds. Common practice is to use standard mechanical testing procedures to determining the E11,E22,G12 and ν12. The other engineering constants are then based on assumptions such as: E33=E22. The engineering constants are converted to the stiffness matrix and used as input. Due to this approach the dispersion curves can vary significantly from those obtained experimentally.In this research the stiffness matrix components are determined non-destructively using a newly introduced ultrasonic immersion technique, the LAMSS approach. The LAMSS approach utilizes the symmetry planes within an orthotropic transversely isotropic material and the critical angle approach to divide the stiffness matrix retrieval process into several steps to reduce the complexity of the process and increase the accuracy of the solution.As last, the predicted group velocity dispersion curves obtained using a stiffness matrix based on mechanical testing and the ultrasonic immersion technique are compared to experimentally obtained velocities.

VS of the uppermost crust structure of the Campi Flegrei caldera (southern Italy) from ambient noise Rayleigh wave analysis

Shear wave velocities (VS) are defined in the uppermost 1–2km of the Campi Flegrei caldera through the non-linear inversion of the group velocity dispersion curves of fundamental-mode Rayleigh waves extracted from ambient noise cross-correlations between two receivers. Noise recordings, three months long, at 12 seismic stations are cross-correlated between all couples of stations. The experiment provided successful results along 54 paths (inter-stations distance), of which 27 sampled a depth > 1km.VS contour lines are drawn from 0.06km b.s.l. to 1km depth b.s.l. and show difference between the offshore (gulf of Pozzuoli and coastline) and the onshore areas. At 0.06km b.s.l., the gulf of Pozzuoli and the coastline are characterized by VS of 0.3–0.5km/s and of 0.5–0.7km/s, respectively. Such velocities are typical of Neapolitan pyroclastic soils and fractured or altered tuffs. The inland shows VS in the range 0.7–0.9km/s, typical of Neapolitan compact tuffs. Velocities increase with depth and, at 1km depth b.s.l., velocities lower than 1.5km/s are still present in the gulf and along the coastline while velocities higher than 1.9km/s characterize the eastern sector (grossly coincident with the Neapolitan Yellow Tuff caldera rim), the S. Vito plain and the area between Solfatara and SW of Astroni.Such features are much more evident along two cross-sections drawn in the offshore and onshore sectors by integrating our VS models with literature data. Our models join previous noise cross-correlation studies at greater scale at depths of 0.7–0.8km, hence the picture of the Campi Flegrei caldera is shown up to a depth of 15km. VS of about 1.7km/s, corresponding to compression velocities (VP) of about 3km/s (computed by using the VP/VS ratio resulted in the inversion), are found at depths of 1.1km, in the centre of the gulf of Pozzuoli, and at a depth of about 0.7km b.s.l. onshore. An increment of VS velocity (~1.9–2.0km/s) is locally observed onshore which might be attributed to a layer of tuffs and tuffites interbedded with thin lava beds, according to the correlation of VS with stratigraphies in the deep drillings of S. Vito.

Identifying the Transition Zone Between East and West Dharwar Craton by Seismic Imaging

The data from 12 temporary broadband seismic stations operated across east–west corridor in Dharwar region of Indian Peninsula along with ten other seismic stations operated by CSIR National Geophysical Research Institute (NGRI) in the region have been analysed that provide high-resolution image of southern Dharwar crust. Crust along the corridor is imaged by receiver function $$H-k$$ stacking, common conversion point stacking using data from 22 sites in combination with joint inversion modeling of receiver functions and Rayleigh wave group velocity dispersion curves. The velocity image reveals thinner crust (36–38 km) except one site (coinciding with Cuddapah basin on the surface) in East Dharwar Craton (EDC), while crust beneath the West Dharwar Craton (WDC) is thicker (46–50 km). This study also observed a transition zone between EDC and WDC starting west of Closepet granite to the east of Chitradurga Schist Belt (CSB), which shows diffused Moho with a thickness of 40–44 km. Chitradurga Schist Belt is identified as the contact between Mesoarchean (WDC) and Neoarchean (EDC) crustal blocks. The lowermost part of the crust ( $$V_\mathrm{s} > 4.0$$ ) is thin (2–6 km) beneath EDC, intermediate (6–8 km) beneath transition zone and thicker (14–30 km) beneath WDC across the profile.

Broadband ground-motion simulation of the 24 May 2014 Gokceada (North Aegean Sea) earthquake (Mw 6.9) in NW Turkey considering local soil effects

On 24 May 2014, a Mw 6.9 earthquake occurred in the west of Gokceada Island, northern Aegean Sea. The earthquake was close to Canakkale, Enez, Tekirdag cities, and damaged 300 buildings in the Marmara Region, NW Turkey. We simulated its broadband (0.1–10 Hz) ground motions including 1D deep and shallow structures soil amplification effects at the 12 strong ground motion stations in the western Marmara Region. The 1D deep velocity structures from the focal layer to the engineering bedrock with an S-wave velocity of 0.78 km/s in different azimuthal directions were tuned by comparing the observed group-velocity dispersion curves of Rayleigh and Love waves from the mainshock with theoretical ones. We also added the shallow parts from previous surveys into the 1D models. Synthetic seismograms on the engineering bedrock were generated using the discrete wave number method with a source model and the 1D deep velocity structures. Then the surface motion was generated considering shallow soil amplification. The synthetic seismograms are generally in good agreement with the observed low and high-frequency parts at most of the stations indicating an appropriateness of the source model and the 1D structural model.

Ambient noise tomography of the western Corinth Rift, Greece

Three years of continuous waveform data recorded at 22 stations from the Corinth Rift Laboratory and the Hellenic Unified Seismological Network are used to perform an ambient noise surface-wave tomography of the western Corinth Rift. All available vertical component time-series were cross-correlated to extract empirical Rayleigh-wave Green's functions. Group velocity dispersion curves were measured for each station-pair by applying frequency-time analysis and then inverted to build 2D group velocity maps between 1 and 6 s period. Finally, we locally inverted these velocity maps using a neighborhood algorithm to assess the 3D shear-velocity model of the shallow crustal structure of the western Corinth Rift. Across all studied periods the southern coast of the Corinth Gulf is generally imaged as a region of lower velocities compared to the northern coast. At periods up to 3 s, the spatial variation of the group velocities is correlated with the surface geology of the area. Lower velocities are observed in areas where mostly Plio-Quaternary syn-rift sediments are present, such as off-shore regions of the rift, the Mornos delta and the largest part of the southern coast. Higher velocities are observed in pre-rift basement structures which are dominated mostly by carbonates. At periods above 3 s, where Rayleigh-waves begin to sense deeper structures below the sediments within the underlying basement, our study highlights the presence of a distinct zone of lower velocities across the southern part of the rift with an elongation in the WNW-ESE direction. The interpretation of this low velocity includes two arguments, the present-day active tectonic regime and the possible involvement of fluids circulation processes at depth within a highly fractured upper crust in the vicinity of the major faults zones. In general, the results demonstrate good agreement with the major geological and tectonic features of the area, as well as with previous local earthquake tomography studies and support the assumption of fluid circulations at depth. This work intends to be the base for further investigations towards the study of the Corinth Rift structure using long-time series of ambient noise data.

Finite element modelling of lamb waves propagation in 2D plates and thin sheets for damage detection

AbstractStructural health monitoring is an evolving technology applied to identify, locate and quantify severity of damages in structures before failure. Lamb waves have become a keen interest for inspection since they can be used to monitor a large area from one single location. The objective of this research is to simulate lamb wave response using finite element method and its application to crack detection and identification in thin metallic engineering structures. Two types of specimen i. e. two‐dimensional aluminium plate and thin aluminium sheets are simulated using commercially available finite element package ABAQUS. Initially phase velocity and group velocity dispersion curve are plotted for aluminium material. Thereafter simulation of individual specimens with cracks and without cracks is performed. Simulation results were compared and validated with actual results and were found to be in reasonably good agreement. This is certainly done by employing group velocity and time of flight for the distance travelled between the monitoring point and crack position. Assessment of lamb waves sensitivity to various sizes and shapes of cracks like rectangular and circular are also investigated and its effect on the structure is discussed in detail.

Finite Element Modelling of Lamb Waves Propagation in 3D Plates and Brass Tubes for Damage Detection

Structural health monitoring (SHM) is becoming very significant for preventing catastrophic failures and uninterrupted operation. Due to continuous developments in SHM Lamb wave technique, finite element simulation is emerging as an initial step to be performed to visualize the potential solution to problem. The present study focus on the simulation of Lamb wave response using finite element method and its application to crack detection and identification in 3D aluminium plates and brass tubes using commercially available finite element package ABAQUS. Phase velocity and group velocity dispersion curve are initially plotted for aluminium and brass material. Subsequently simulation of distinct specimens with and without presence cracks is performed. Simulation results were validated and compared with actual results and were found to be in reasonably good agreement. A damage index parameter i.e. amplitude ratio is defined to notice the effect of crack dimensions variation. Interaction of Lamb waves with rectangular, semi-elliptical and semi-circular cracks (shallow to deep) is studied in detail. Also the application of signal processing techniques such as Spectrogram analysis, Fourier transform and Hilbert transform is also reported in this paper.

Laser Ultrasound Measurement of Diaphragm Thickness, Young's Modulus and Poisson's Ratio in an MEMS Device

Laser-generated Lamb waves, coupled with a large bandwidth Michelson interferometer, have been demonstrated to accurately measure the thickness of a microelectromechanical systems pressure sensor diaphragm in the [110] direction of a silicon wafer. Using the reassigned Gabor time–frequency method to produce group velocity dispersion curves, the technique facilitates the measurement of thickness, Young's modulus, and Poisson's ratio from just one noncontact measurement. In this paper, thickness was determined to be 35.01 μm ± 0.18 μm. For comparison, the thickness was measured using an independent optical technique, obtaining a value of 34.60 μm ± 0.27 μm. Values for Young's modulus and Poisson's ratio were also determined to be 163 GPa ± 11.7 GPa and 0.351, respectively, and these are in good agreement with values found in the literature.

레일리파 토모그래피를 사용한 동아시아의 3차원 SV파 속도구조

동아시아 지역에 대한 지각과 상부맨틀의 속도구조를 파악하기 위해서, 레일리파 분산곡선에서 측정한 군속도 값을 사용하여 SV파 토모그래피 역산을 수행하였다. 한국, 일본, 그리고 중국에 설치된 253개의 광대역 관측소에 기록된 321개의 지진자료를 사용하였다. 다중필터기법(multiple filtering technique)을 사용하여, 3 ~ 150초 사이의 주기 범위에 대하여, 5,167개의 레일리파 분산 곡선을 획득한 후, 역산을 수행하여 깊이 10 ~ 100 km에 대한 SV파 토모그래피 모델을 얻었다. 다른 지역과는 다르게 동해지역 하부의 깊이 10 km에서 고속도 이상이 관찰되며 깊이 15 ~ 20 km에서는 강한 고속도 이상이 관측되는데, 이는 맨틀물질의 존재를 나타내는 것으로 생각된다. 따라서 모호면의 깊이가 주변지역에 비해 얕다는 것으로 추정되며, 이를 바탕으로 동해지역의 지각 두께는 10 ~ 20 km 사이로 판단된다. 고속도 이상의 존재 여부로 판단할 때, 한반도 지역의 모호면은 35 km 깊이 부근에 존재하는 것으로 생각된다. 일본열도 북동쪽 지역에서 좁은 범위지만 태평양판의 섭입이 관찰된다. 강원도 연안 하부의 100 km 깊이에서 강한 저속도 이상이 보이며, 이는 울릉도와 울릉분지 형성에 영향을 끼친 용융체의 존재로 생각된다. 또한, 야마토 분지 하부의 100 km 깊이에서 강한 저속도 이상체가 관찰되며, 이는 유체탈수(dehydration) 작용으로 인해 태평양판으로부터 유체가 빠져 나온 후 상승하면서 생성된 저속도 이상체로 해석된다. We construct 3D SV-wave velocity structure of the crust and the upper mantle beneath East Asia from Rayleighwave group-velocity measurements. For the construction of the SV-wave velocity model at 10 ~ 100 km depth, we used seismic data recorded at 321 broadband stations in Korea, Japan, and China. Rayleigh-wave group-velocity dispersion curves were obtained by using the multiple filtering technique in the period range from 3 to 150 s. High SV-velocity anomalies are imaged beneath the East Sea from 10 km depth to deeper depth, implying that the Moho beneath the East Sea is between at 10 ~ 20 km depth. We estimated the Moho beneath the Korean peninsula to be around 35 km based on the depth where a high-velocity anomaly is observed. The SV-wave velocity model shows prominent fast S-velocity anomalies near northeastern Japan, associated with the subducting Pacific plate. Low-velocity anomalies are found beneath the east coast of the Korean peninsula at 100 km depth, which may play a role in the formation of the Ulleungdo and the Ulleung basin. We observed low-velocity anomalies beneath the Yamato basin at 100 km depth as well, which may indicate the upwelling of fluid from the Pacific plate via dehydration at deeper depth.

Rayleigh wave group velocity tomography of Gujarat region, Western India and its implications to mantle dynamics

In the present study, fundamental Rayleigh waves with varying period from 10 to 80 s are used to obtain group velocity maps in the northwest Deccan Volcanic Province of India. About 350 paths are obtained using 53 earthquakes (4.8 ≤ M ≥ 7.9) recorded by the SeisNetG (Seismic Network of Gujarat). Individual dispersion curves of group velocity of Rayleigh wave for each source-station path are estimated using multiple filter technique. These curves are used to determine lateral distribution of Rayleigh wave group velocity by tomographic inversion method. Our estimated Rayleigh group velocity at varying depths showed conspicuous corroboration with three tectonic blocks [Kachchh Rift Basin (KRB), Saurashtra Horst (SH), and Mainland Gujarat (MG)] in the region. The seismically active KRB with a thicker crust is characterized as a low velocity zone at a period varying from 10 to 30 s as indicative of mantle downwarping or sagging of the mantle beneath the KRB, while the SH and MG are found to be associated with higher group velocities, indicating the existence of the reduced crustal thickness. The trend of higher group velocity was found prevailed adjacent to the Narmada and Cambay rift basins that also correspond to the reduced crust, suggesting the processes of mantle upwarping or uplifting due to mantle upwelling. The low velocities at periods longer than 40 s beneath the KRB indicate thicker lithosphere. The known Moho depth correlates well with the observed velocities at a period of about 30 s in the Gujarat region. Our estimates of relatively lower group velocities at periods varying from 70 to 80 s may correspond to the asthenospheric flow beneath the region. It is interesting to image higher group velocity for the thinner crust beneath the Arabian Sea adjacent to the west coast of Gujarat at the period of 40 s that may correspond to the upwarped or upwelled mantle beneath the Arabian Sea. Our results have better resolution estimated by a radius of equivalent circular averaging area for each period.

Inferences on the lithospheric structure of Campi Flegrei District (southern Italy) from seismic noise cross-correlation

Lithospheric VS models are defined in the Campi Flegrei District (southern Italy) through the non-linear inversion of the group velocity dispersion curves of fundamental-mode Rayleigh waves extracted from ambient noise cross-correlations between two receivers, and the regional group and phase velocities of the Italian cellular lithospheric model (1°×1°cells). Four paths are investigated, of which one (ISCHIA-MIS) across two adjoining cells.The distribution of VS shows a pyroclastic covering with VS increasing from 0.3–0.7km/s to 2.1km/s. It rests on a lava or carbonate basement, about 5–6km thick, with VS increasing from 2.1km/s to 3.1km/s at about 2km of depth and rising to ∼0.6km towards the island of Procida. A metamorphic layer is detected at an average depth of 7.7km with VS of 3.8–3.9km/s, about 5km thick, overlying a low velocity layer (VS of 3.5km/s) at about 11–12km of depth. The VS model along the ISCHIA-MIS path, as average of the models obtained by combining local and regional dispersion data of the two adjoining cells, is well consistent with the other paths. The Moho discontinuity is retrieved at about 23km of depth with VS of 4.2km/s.