Rayleigh Wave Group Velocity Research Articles

Scattering enhancement of partially buried sphere at the water-sand interfaces due to reflected subsonic Rayleigh waves

In view of the development needs of maritime security, the detecting of buried targets over long distances at low frequencies needs to be solved urgently. Through well designed experiments at low frequency (ka = 5–10), this paper presents a study on the scattering echoes from polymethylmethacrylate (PMMA) spheres in partially buried cases (50%, 75% and 100% buried), and the propagation characteristics of subsonic Rayleigh surface waves on the surface of PMMA spheres at the water-sand interface can be clearly observed. It is shown that the fine sand medium with larger impedance brings about smaller circumferential phase and group velocities of subsonic Rayleigh waves on the surface of the PMMA sphere; the resonance peak caused by subsonic Rayleigh waves is shifted toward the low frequency with the deepening of the burial depth, and the offset can be predicted by the circumferential phase velocity of the subsonic Rayleigh wave in different media and burial depth of PMMA sphere, and the experimental and finite element simulation results validate the shift of the Rayleigh resonance peak. The research results of this paper can provide certain reference value for the detection of low frequency buried targets.

Shear wave crustal velocity structure in the Garhwal-Kumaon Himalaya based on noise cross-correlation of Rayleigh wave

The tectonics of the Garwal-Kumaon Himalaya is characterized by thrusts, tectonic windows, and klippen. We investigate crustal shear wave velocity variations using ambient noise cross-correlation tomography. The studied region encompasses the Kali River valley in the east to Satluj valley in the west, with the adjoining Indo-Gangetic Plain in the south covering the northern part of the Delhi-Haridwar ridge. The fundamental mode group velocities of Rayleigh waves are extracted from cross-correlation data of 33 broadband seismological stations from a regional seismic network. A total of 374 dispersion curves with a period range of 4–29 s show a group velocity variation between ∼2.3 and ∼ 3.4 km/s. The shear wave velocity structure of the uppermost lithosphere down to ∼50 km obtained by non-linear inversion of the Rayleigh wave dispersion data provides new insight into the geometry of the crust. A large variation in Vs in the range of ∼2.8 to ∼4.7 km/s corresponds to a variety of changes in the tectonic deformation, structure, and crustal thickness of the Himalayan wedge. Thick low-velocity sedimentary formations are identified beneath the Indo-Gangetic Plain and the frontal Himalaya. Anomalous low Vs zones are also observed in the mid-crust beneath the higher Himalaya and southern Tibet, indicating partial melting or the presence of aqueous fluid zones. The high-velocity anomalies may be correlated with duplex structures beneath the Lesser Himalaya and with lithospheric flexure.

High-resolution shallow crustal shear wave velocity structure of Anyuan mining area and its adjacent region in Jiangxi Province, China

High-resolution seismic image is critically important for mining minerals. In this study, we collected seismic data from a local dense seismic array consisting of 154 stations around the Anyuan mining area and its adjacent region of Pingxiang City, Jiangxi Province in South China, and applied the ambient noise tomography (ANT) method to image the shear wave velocity structure in the study area. Shallow crustal velocities at depths less than 3.3 km were determined by direct inversion of Rayleigh wave group velocity dispersion curves at the period range of 0.5–5.0 s. Overall, the S-wave velocity structure has a tight correlation with surface geological and tectonic features in the study area. The shear wave velocity structure in the shallow crust of the Anyuan Mine and its adjacent areas displayed distinct low-velocity anomalies, which can be attributed to the depression of sedimentary structures and coal mining activities in the Pingxiang-Leping region. The zones surrounding the Anyuan fault (AYF) and Wangkeng fault (WKF) zones exhibited low-velocity anomalies from the ground surface to ~ 3.3 km underground. And the low-velocity anomalies at depths less than 1.2 km could be related to the sedimentary environment of coal mine and the coal mining activities, while the low-velocity anomalies at depths below 1.2 km are caused by the presence of fracture medium, oil and gas in the fault zone. The shear wave velocity changes sharply across the AYF, and the characteristics of the velocity change interface indicate that the AYF is inclined toward the northwest, with its extension reaching depths of approximately 3 km underground.Graphical

Ambient Seismic Noise Cross – Correlation of the Tangkuban Parahu Volcano, West Java Indonesia: Preliminary Result

Tangkuban Parahu is an active volcano in Indonesia that is part of the Quaternary Sunda volcanic arc. We used the vertical component of waveform recorded by 55 temporary seismic stations deployed around Tangkuban Parahu volcano from October 2021 to February 2022 to gain a thorough understanding of its seismic structure. To extract empirical Green’s functions, we computed cross-correlations of the vertical component of continuous record data. The empirical Green’s function was extracted from the daily data series, and the cross-correlation data from each day was stacked into a single inter-station cross-correlation data set. The empirical Green’s Function is seen at the band period 1-10 s. In this preliminary result, we obtained the Rayleigh wave group velocity map in period 1 s. The result shows a low-velocity value around the crater surrounded by high velocities. The low velocity is related to the weak zone or fluid presence.

Ambient Noise Data Processing to Obtain Group Velocity for Subsurface Structure Identification: Preliminary Research in Hululais Geothermal Field, Sumatra, Indonesia

Hululais area lies in the pull-apart basins of the Ketaun Segment and Musi Segment fault as a part of the Sumatra Fault Zone (SFZ). The boundary normal faults of pull-apart basins play an important role as major discharge zones for geothermal fluid because the extensional stress is concentrated in the boundary normal faults. In order to identify the geothermal reservoir structure in Hululais Geothermal Field (HGF), we introduce the local-scale study of the Rayleigh wave group velocity structure using ambient noise tomography (ANT). The ANT studies were collected using 18 seismometers inside 12 km2 area with a spacing of 125 – 500 meters, deployed across the fault structure for 1 month. More than two thousand Rayleigh Green’s Functions are extracted by cross-correlation at available station pairs. Using the estimated green function in this preliminary research, the group velocity as a function of the period can measure the dispersion curve by using multiple filter technique (MFT) and fast marching surface tomography (FMST) scheme to obtain group velocity images. The tomography result as group velocity image shows the subsurface Rayleigh wave structure variation. The NW-SE main structure is reflected by the contrast velocity structure between the central part and the north eastern-south western sides. The central part shows the low periods which are associated with low wave velocity However the margin of the central part shows the high velocity in all periods. The ANT studies have been efficient in time and cost, however useful in subsurface structure interpretation in Hululais Geothermal Field.

Lithospheric architecture beneath the Amsterdam-St. Paul plateau, Southern Indian Ocean using the integrated gravity, magnetic and seismological study

Evolution of the Amsterdam-St. Paul (ASP) plateau is debated as a consequence of the Kerguelen mantle plume interaction with the South-East Indian Ridge (SEIR) or related with a weaker ASP plume origin. We performed an integrated geophysical approach using gravity and magnetic modeling along with the joint inversion of Ps receiver function and Rayleigh wave group velocity dispersion data to determine the crustal architecture, and upper mantle structure below the plateau. The gravity-magnetic model revealed that the plateau is associated with three crustal layers as basaltic layer, gabbroic layer and underplated material underlain by sedimentary strata. Our models further suggests that beneath the ASP plateau the Moho depth varies from ∼6.6 to 18.0 km. Joint inversion of Ps receiver function and Rayleigh wave group velocity dispersion curve suggested the Lithosphere-Asthenosphere Boundary at depth of 50 km below the Amsterdam Island. Further seismological result suggests evidence of a Low Velocity Zone below the Moho at depth of 20–36 km which may represent a magma chamber. Our model suggests that the evolution of the ASP plateau is guided by the Amsterdam and St. Paul fracture zones, which acted as a thermal boundary confining the horizontal movement of the ASP plume. We propose that the ASP plateau is not linked with the Kerguelen plume and emphasize origin of the ASP plateau by the interaction of SEIR with the ASP plume.

The seismic structure of Villarrica Volcano revealed by ambient noise tomography

We compute the first ambient noise tomography of Villarrica Volcano, obtaining a 3D Vs model. Using ambient noise recorded from May to July 2014 from 18 seismic stations, we measured the Rayleigh wave group velocity by stacking daily cross-correlations between station pairs. The resulting group velocity curves were inverted into 2D tomographic maps with periods ranging between 1.0 and 4.9 s. These tomographic maps were locally inverted through the neighborhood algorithm to obtain an accurate Vs model in depth. Our findings reveal velocity variations within the Villarrica Volcano region, reaching a depth of around 7.5 km. The resulting 3D shear-wave velocity model has provided novel insights into the internal structure of the area. Our study has identified slow velocity anomalies in the area of the Liquiñe-Ofqui Fault Zone, and horizontal variations in shear-wave velocity that run parallel to the volcanic lineament from the Mocha-Villarrica Fault Zone. We also note the presence of a negative velocity anomaly located southeast of the Villarrica summit, in a position consistent with the magmatic plumbing system proposed by earlier geophysical studies conducted in the region.

Seismic Phenomena Associated with the Eruption of a Volcano in the Area of the Tonga Archipelago on January 15, 2022

The study of changes in the seismic process associated with the eruption and the assessment of the energy parameters and structure of the wave field from seismic data is the subject of this work. Three types of disturbances are distinguished in the structure of the wave field. First of all, these are Rayleigh surface waves with an average oscillation period of 23 s, well traceable at distances up to 100 degrees. The group velocity of Rayleigh waves is 3.6–3.8 km/s. The magnitude calculated from them at stations mainly with oceanic propagation paths is Ms = 5.52 ± 0.18, and the corresponding seismic energy was on the order of Ec = (1–7) × 1013 J, and the scalar seismic moment M0 =2.4 × 1017 J. Two other types of oscillations were detected on seismic channels with a frequency band from 0.0003 to 0.1 Hz of the stations. For the first, the apparent velocity of propagation of the disturbance lies in the range of 0.28–0.37 km/s with a characteristic period of 268 s. This type of disturbance is associated with the gravitational response of the seismometer to an acoustic disturbance in the atmosphere. For the second type of seismic disturbances, the propagation velocity is 0.21–0.28 km/s with characteristic periods of 450 s on the horizontal components of seismic receivers. This type of seismic disturbance is probably caused by the interaction of the gravitational wave in the hydrosphere with the coast of the islands on which the seismic stations are located.

3-D geometry of the Lonar impact crater, India, imaged from cultural seismic noise

SUMMARY The Lonar impact crater in the Deccan Volcanic Province of India is an excellent analogue for impact-induced structures on the Moon and other terrestrial planets. We present a detailed architecture of the crater using a high-resolution 3-D seismic velocity image to a depth of 1.5 km through the inversion of high-frequency ambient noise data recorded over 20 broad-band seismographs operating around the crater. The ambient noise waveform is dominated by cultural noise in the 1–10 Hz band. The shear wave velocity (Vs) model is created from Rayleigh wave group velocity data with a horizontal resolution of 0.5–1 km in the period range of 0.1–1.2 s. A key feature of the model is a velocity reduction of 10–15 per cent below the crater compared to outside the ejecta zone. The low-velocity zone below the crater is nearly circular and extends to a depth of ∼500 m. This estimated crater's depth is consistent with global depth–diameter scaling relations for simple craters. The basement, with a Vs of more than 2.5 km s−1, lies beneath the Deccan basalt, which has a Vs of ∼2.4 km s−1. These results are consistent with laboratory-measured data from the Lonar crater and borehole data in the western Deccan trap. This study opens a new window for exploring impact craters and sub-basalt structures using high-frequency ambient noise tomography.

A deep-learning-based approach for seismic surface-wave dispersion inversion (SfNet) with application to the Chinese mainland

Surface-wave tomography is an important and widely used method for imaging the crust and upper mantle velocity structure of the Earth. In this study, we proposed a deep learning (DL) method based on convolutional neural network (CNN), named SfNet, to derive the vS model from the Rayleigh wave phase and group velocity dispersion curves. Training a network model usually requires large amount of training datasets, which is labor-intensive and expensive to acquire. Here we relied on synthetics generated automatically from various spline-based vS models instead of directly using the existing vS models of an area to build the training dataset, which enhances the generalization of the DL method. In addition, we used a random sampling strategy of the dispersion periods in the training dataset, which alleviates the problem that the real data used must be sampled strictly according to the periods of training dataset. Tests using synthetic data demonstrate that the proposed method is much faster, and the results for the vS model are more accurate and robust than those of conventional methods. We applied our method to a dataset for the Chinese mainland and obtained a new reference velocity model of the Chinese continent (ChinaVs-DL1.0), which has smaller dispersion misfits than those from the traditional method. The high accuracy and efficiency of our DL approach makes it an important method for vS model inversions from large amounts of surface-wave dispersion data.

Crustal Structure beneath SLY1 Station from Joint Inversion of Receiver Function and Surface Wave Dispersion Analyses, Sulaymaniyah Area, NE Iraq

The crustal structure beneath the Sulaymaniyah area in northeast Iraq which is located in a seismically active zone, was investigated and the results were presented in this study. Teleseismic and regional earthquakes recorded by the Sulaymaniyah broadband seismic station (SLY1) were used to conduct the P-wave receiver functions and Rayleigh wave group velocities using Computer Programs in Seismology (CPS). Joint inversion of receiver functions and surface waves dispersion curves were used to infer the crustal structure below the station. The waveforms were analyzed for 72 earthquakes with epicentral distances ranging from 30º to 90º that occurred during the period from October 2015 to Oct. 2021 with magnitudes ≥ 7Mw. The generated crustal model shows that the sedimentary cover in the area is around 6km with a P-wave velocity of 5.4817km/sec and Moho discontinuity around 48km with a P-wave velocity of 8.0560km/sec. These outcomes indicate that the crystalline crustal thickness at Sulaymaniyah station is 42 km with a P-wave velocity ranging from 5.4817km/sec to 8.0560km/sec. There are two sharp discontinuities around 20km and 38km with P-wave velocities of 5.9182km/sec and 7.1485km/sec respectively. It is believed that these are the discontinuities between the upper, middle, and lower crust.

3-D Inversion of Gravity Data of the Central and Eastern Gonghe Basin for Geothermal Exploration

The Gonghe Basin is one of the most important regions for the exploration and development of hot dry rock geothermal resources in China. However, there is still some controversy about the main heat source of hot dry rock geothermal resources in the Gonghe Basin. Combined with previous research results including three-dimensional magnetotelluric imaging and linear inversion of Rayleigh wave group and phase velocity result, we obtained a high-resolution underground spatial density distribution model of the Gonghe Basin based on satellite gravity data by using 3-D gravity focusing inversion method. According to the results, there are widely distributed low density anomalies relative to surrounding rock in the middle crust of the study area. The low-density layer is speculated to be a low-velocity, high-conductivity partial melting layer in the crust of the Gonghe Basin. The inversion result confirms for the first time the existence of a partial melt layer from the gravity point of view, and this high temperature melt layer may be the main heat source of the hot dry rock geothermal resources in the Gonghe Basin. It can provide a new basis for further research on the genesis of the hot dry rock geothermal system in the Gonghe Basin.

Subcontinental lithospheric mantle discontinuities beneath the Eastern Himalayan Plate Boundary System, NE India

SUMMARY We use P-wave receiver function (P-RF) analysis and joint inversion with Rayleigh wave group velocity dispersion data to model the shear wave velocity (Vs) structure of subcontinental lithospheric mantle (SCLM) discontinuities beneath northeast (NE) India. The most prominent SCLM discontinuity is the Hales Discontinuity (H-D) observed beneath the Eastern Himalayan Foreland Basin (Brahmaputra Valley) and Shillong Plateau. The P-to-SV converted phase from the H-D (Phs) is a positive amplitude arrival at ∼10–12 s and has positive moveout with increasing ray-parameter. From joint inversion, the H-D is modelled at a depth range of 90–106 km, with 11–12 per cent Vs increase beneath the Brahmaputra Valley. Beneath the Shillong Plateau the H-D is at a depth range of 86–99 km, with 6–10 per cent Vs increase. An intralithospheric discontinuity (ILD) has been identified in the Shillong Plateau station P-RFs, as a positive amplitude PILDs phase, arriving at 8–8.5 s. This is modelled at a depth range of 66–75 km with Vs increase of 2–9 per cent. We construct 2-D profiles of depth-migrated common-conversion-point stack of P-RFs to distinguish the SCLM discontinuity arrivals from crustal phases. 3-D spline-interpolated surface of the H-D has been constructed to visualize its lateral variations. We use xenolith data from the Dharwar Craton, which has similar geological age, petrology and seismic structure as the Shillong Plateau, to petrologically model the SCLM H-D and ILD Vs structure in NE-India. From the calculated Vs structure we conjecture that the H-D is a petrological boundary between mantle peridotite and kyanite-eclogite, with its origin as metamorphosed paleosubducted oceanic slab, similar to other global observations. We further speculate that the shallower ILD could be formed as a contact between frozen asthenosphere-derived metasomatic melts within the SCLM.

Shallow subsurface imaging across the Himalayan Frontal Thrust (HFT) analysing ambient noise data from a riverine source

Modelling subsurface geology across the Himalayan Frontal Thrust (HFT) is an essential task as it plays a significant role in the transfer of stresses from infrequent major earthquakes, which influence site characterization, groundwater estimates, and environmental engineering. But it is challenging to carry out a controlled-source seismic survey across HFT due to the difficult terrain. Interestingly, the Himalayan rivers flow downhill across the HFT and generate seismic noise that can be useful in extracting information about the subsurface geology. In this study, we record the noise generated by one such river, the Dabka, near Pawalgarh in the Nainital district of Uttarakhand, India, to understand the shallow subsurface geology across this region of the HFT of the Siwalik Himalayas. We use a network of 23 geophones of 5 Hz natural frequency, deployed for 46 days, to record the noise datasets. We cross-correlate these time series for the station pairs and extract the noise correlation functions (NCFs). Dispersion analysis of the NCFs provides the Rayleigh wave group velocity dispersion curves for the station pairs, using which we created 2D Rayleigh wave group velocity and shear wave velocity maps at different depths. These velocity-depth models demarcate the location of the HFT where there is a jump in the Rayleigh and shear wave velocities, with average standard deviations of ∼50 m/s and ∼ 93 m/s, respectively, across the HFT. The obtained shallow subsurface velocity models provide crucial information for geotechnical assessment and site characterization of this earthquake-prone region and thus can be useful for future hazard assessment.

Ambient noise tomography in the upper crust of North Harrat Rahat, Saudi Arabia

Images of Rayleigh-wave group velocity were derived for the 1–15 s period range that is sensitive to shear wave velocities in the crust of north Harat Rahat, Saudi Arabia. We used a continuous dataset of ambient noise that was processed by applying the technique depending on the Rayleigh-wave group velocity tomography. The dataset comprises six months of recordings extracted from 12 broadband seismic stations in north Harrat Rahat, Saudi Arabia. Using a cross-correlation technique, the Rayleigh waves have been generated and stacked along the whole time series. Dispersion curves representing fundamental mode Rayleigh waves were derived by a multiple filter technique. Dispersion curves for each station pair were inverted to obtain the one-dimensional model of corresponding shear wave velocities. The spatial distribution of shear wave velocities was depicted from the inverted one-dimensional models at 138 nodes and a reliable resolution was spatially resolved using grid cells of 0.2° expanded over both latitude and longitude. Images obtained from this study improve our understanding of the tectonic development as influenced by magma upwelling beneath the Arabian Shield. Results obtained from this study revealed two low-velocity zones in the upper crust at depths of 15 km west and shallow 10 km east, respectively. This suggests that high conductivity is present as a result of upwelling magma that flows into the upper crust of Arabian Shield coming from plausible deep sources such as the Red Sea Rift or the Afar plume, which are located to the west of the studied area.

Structural information derived from ambient noise tomography over a hydrocarbon‐producing region in the Cachar fold belt, lower Assam, northeast India

AbstractAmbient noise tomography is a powerful tool that has found increasing application in reservoir analysis and imaging. The Cachar fold belt in lower Assam, northeast India encompasses several wells under active hydrocarbon production, along with several dry wells. To overcome the lack of active seismic data over the entire fold belt, a passive seismic study was carried out to image the concealed three‐dimensional sub‐surface structures. The data were recorded from February to November 2011 by a network of 65 wideband seismometers spanning an area of about 40 × 60 km2. The data are crosscorrelated in the 2–5 s band, followed by phase‐weighted stacking to estimate noise correlation functions with surface wave signatures. The traveltimes picked from the frequency‐time analysis are utilized in a tomographic inversion for Rayleigh wave group velocities. The group velocity anomalies have a lateral resolution of ~ 3.5 × 5.5 km2 and variations of up to for each period. The group velocities are in turn inverted for S‐wave velocity distribution as a function of depth. The three‐dimensional S‐wave velocity tomograms reveal the tight anticlines and broad synclines, with high‐ and low‐velocity zones corresponding to structural highs and lows, respectively. The structural interpretation is supported for the part of the region with producing wells and covered by active seismic data, wherein the post‐stack time migrated seismic section shows anticlinal and synclinal features similar to those obtained from ambient noise tomography. The structures revealed by ambient noise tomography can help identify zones of interest to be targeted by active seismic surveys in the Cachar fold belt.

Thermo‐Compositional Structure of the South American Platform Lithosphere: Evidence of Stability, Modification, and Erosion

AbstractConstraints on the structure of cratonic lithosphere are essential to improve our understanding of craton formation, evolution and long‐term stability. Here, we perform a joint inversion for the thermal and compositional structure of the mantle lithosphere below the South America Platform, using Rayleigh wave group velocities, elevation, and geoid height. Thick thermal lithosphere (200–300 km) is found below the southern Amazonian and São Francisco Cratons and adjoining Parecis and eastern Paraná Basins. The southern Rio de la Plata Craton also retains a 200–250 km thick keel. Compositionally, Amazonian, São Francisco, and Rio de la Plata lithosphere has a metasomatic and possibly eclogite signature similar to that of North American Proterozoic collision belts. Parecis and eastern Paraná lithosphere has likely been altered by Mesozoic plume activity throughout most of its depth, while the rest of the Paraná Basin and the Chaco and Pantanal Basins appear to have lost the lithospheric root below ∼100 km depth that was there during intracratonic basin formation. The low elevation and high geoid of the western Paraná Basin requires a dense (eclogite) layer within the crust/shallow lithosphere, possibly associated with the NeoProterozoic Western Paraná Suture Zone and/or Mesozoic plume activity, while topography and geoid of the basins further west and of the western Rio de la Plata craton seem affected by dynamic (subduction‐related) topography. Thus, the variable geophysical structure of the platform lithosphere reflects a history that involves besides some stable keels, significant modification and thinning.

High‐Resolution Lithospheric Structure of the Zagros Collision Zone and Iranian Plateau

AbstractFrom a joint analysis of fundamental mode Rayleigh wave group velocities and P‐wave receiver functions, we derive a new, high‐resolution 3D shear‐wave velocity (Vs) model for the crust and uppermost mantle of the Iranian Plateau. The thickest crust (>55 km) is located beneath the Sanandaj‐Sirjan Zone and the deforming belts of Alborz‐Binalud‐Kopet Dag Mountains, whereas regions of lower topography/deformation (e.g., central Iran and the Lut Block), and the regions of younger deformation such as the Makran Accretionary Wedge and the Zagros Simply Folded Belt (SFB) have a thinner (<45 km) crust. Our model reveals a low‐Vs tongue‐shaped feature, indicating the underthrusting of the Arabian crust beneath central Iran. In the central Zagros, the underthrusting of the Arabian crust is steeper, resulting in a narrower deforming zone (∼150 km) and a thicker crust (∼60–65 km), compared to the NW or SE Zagros where the deforming zone is broader (∼250 km) and the crust is thinner (∼55–60 km). Regions of low‐Vs in the upper crust correspond to regions of thick sediments; the South Caspian Basin, the Zagros SFB and foreland basin of the Zagros, and the Makran Subduction Wedge. The subcrustal Rayleigh wave azimuthal anisotropy of the Plateau shows a rather uniform and smoothly‐varying pattern. In the NW Zagros the crustal and subcrustal pattern of anisotropy agrees with that previously estimated from the shear‐wave core phases, implying that the whole lithosphere deforms coherently, but for other regions (e.g., the western Alborz and Kopet Dag), the anisotropic pattern does not support a coherent deformational fabric throughout the lithosphere.

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.

Determination of the Earth’s structure based on intermediate-period surface wave recordings of tidal gravimeters: a case study

Tidal gravimeters can detect intermediate-period surface waves with high accuracy. Three gravimetric stations with estimated transfer functions and co-located with seismic stations were selected: two in Belgium (Membach and Rochefort, in Western Europe) and one in Germany (Black Forest, in Central Europe). The compatibility of gravimetric and seismic recordings of earthquakes in the period range of 10-180 s has been presented. The series of monochromatic signals separated from surface waves for selected events have been calculated using the multiple-filtering procedure, and averaged fundamental-mode Rayleigh wave group-velocity curves have been estimated on a regional scale. Next, averaged dispersion curves for three regions (Italy, Greece, and Western Turkey) were inverted by weighted linear inversion methods. Additionally, a quantitative analysis of resolution tests of inverted models was presented to show the capabilities of the gravimetric data to retrieve a shear-wave velocity distribution with a depth. In particular, a method for determining the depth range of the inverted model has been proposed. Results obtained based on gravimetric data were verified by results from seismic data obtained by applying the same procedure. This study shows the novel application of tidal gravimetric data in the determination of the regional Earth’s structure based on intermediate-period surface waves recordings, as well as a comprehensive approach to the quantitative estimation of a final model resolution.Graphical