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G F Panza - One of the best experts on this subject based on the ideXlab platform.
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high resolution rayleigh Wave Group velocity tomography in north china from ambient seismic noise
Geophysical Journal International, 2010Co-Authors: Lihua Fang, Zhifeng Ding, G F PanzaAbstract:Abstract This study presents the results of the Rayleigh Wave Group velocity tomography in North-China performed using ambient seismic noise observed at 190 broadband and 10 very broadband stations of the North-China Seismic Array. All available vertical component time-series for the 14 months span between January, 2007 and February, 2008 are cross-correlated to obtain empirical Rayleigh Wave Green functions that are subsequently processed, with the multiple filter method, to isolate the Group velocity dispersion curves of the fundamental mode of Rayleigh Wave. Tomographic maps, with a grid spacing of 0.25o×0.25o, are computed at the periods of 4.5s, 12s, 20s, 28s. The maps at short periods reveal an evident lateral heterogeneity in the curst of North-China, quite well in agreement with known geological and tectonic features. The North China Basin is imaged as a broad low velocity area, while the Taihangshan and Yanshan uplifts and Ordos block are imaged as high velocity zones, and the Quaternary intermountain basins show up as small low-velocity anomalies. The Group velocity contours at 4.5s, 12s and 20s are consistent with the Bouguer gravity anomalies measured in the area of the Taihangshan fault, that cuts through the lower crust at least. Most of the historical strong earthquakes (M≥6.0) are located where the tomographic maps show zones with moderate velocity gradient.
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rayleigh Wave Group velocity tomography in the aegean area
Tectonophysics, 2002Co-Authors: E Karagianni, G F Panza, D G Panagiotopoulos, Peter Suhadolc, C B Papazachos, B C Papazachos, A Kiratzi, D Hatzfeld, K Makropoulos, Keith PriestleyAbstract:Data from a large-scale experiment which took place in Greece during the period January–July 1997 have been used to investigate the structure of the Aegean area using surface Waves. During this experiment, 30 seismic broadband instruments were deployed throughout the whole Greek area. Additional data during the period 1996–2000 from other temporary networks have been included in the dataset. One hundred eighty-five events with magnitudes 4.0VMwV5.5 recorded by these stations have been collected and processed. The individual dispersion curves of the Group velocity of Rayleigh Waves for each source-station path have been calculated, producing more than 700 paths covering the studied region. These curves have been used to determine Rayleigh Group velocity maps using a 2D-tomography method. On the basis of a regionalization of the dispersion measurements, local averaged dispersion curves have been obtained and non-linearly inverted to obtain models of shear-Wave velocity versus depth. Since the dispersion curves in the period range 5 sVTV30 s are mostly affected by the crustal structure, the model velocities are estimated down to a depth of approximately 35–45 km. The results from the non-linear Hedhehog inversion as applied to a few local dispersion curves show a crustal thickness of approximately 32 km for the Northern Aegean Sea, and a relatively thin crust of approximately 22–24 km for the Southern Aegean Sea. D 2002 Elsevier Science B.V. All rights reserved.
Jose M Alsina - One of the best experts on this subject based on the ideXlab platform.
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the influence of Wave Groups and Wave swash interactions on sediment transport and bed evolution in the swash zone
Coastal Engineering, 2018Co-Authors: Jose M Alsina, Ivan Caceres, Joep Van Der Zanden, Jan S RibberinkAbstract:Abstract Large scale laboratory measurements of sediment dynamics in the swash zone are presented. Two bichromatic Wave Group conditions were generated, having the same energy content but different Wave Group period ( T g = 15.0 s and 27.7 s). For the shortest Wave Group, due to bore focusing, the shoreline fluctuates predominantly at the T g time scale, showing a large runup and the presence of Wave–swash interactions with strong momentum exchange. In contrast, for the longer Wave Groups, the swash excursion is dominated by the individual Waves. The uprush generally promotes onshore sediment advection with consequent erosion at the rundown location but accretion close to the runup. On the contrary, the backwash promotes seaward sediment advection and accretion at the rundown location. The presence of repeated Wave–swash interactions modifies these patterns slightly. A Wave overrunning a previous uprush promotes a reduction in onshore sediment advection while weak Wave–backwash interactions reduce seaward advection. Consequently, the measured sediment dynamics shows stronger intra–swash cross–shore sediment advection for the swash events produced by the short Wave Groups. Measurements of the sheet flow layer near the shoreline show that for the shortest Wave Groups the vertical structure of the concentration is influenced by horizontal advection, leading to large sheet-flow layer thickness. However, for the longer Wave Groups, the local vertical exchange of sediment in the sheet–flow layer is dominant, with the presence of a pick–up and mirroring upper layer similar to oscillatory sheet–flow measurements. These results reaffirm the important effects of the Wave Group structure and the Wave–swash interactions on the swash zone sediment dynamics and beach face evolution.
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transfer and dissipation of energy during Wave Group propagation on a gentle beach slope
Journal of Geophysical Research, 2017Co-Authors: Enrique M Padilla, Jose M AlsinaAbstract:The propagation of bichromatic Wave Groups over a constant 1:100 beach slope and the influence of the Group modulation is presented. The modulation is controlled by varying the Group frequency, fg, which is shown to remarkably affect the energy transfer to high and low frequency components. The growth of the high frequency (hf) Wave skewness increases when fg decreases. This is explained by nonlinear coupling between the primary frequencies, which results in a larger growth of hf components as fg decreases, causing the hf Waves to break earlier. Due to high spatial resolution, Wave tracking has provided an accurate measurement of the varying breakpoint. These breaking locations are very well described ( R2>0.91) by the Wave-height to effective-depth ratio (γ). However, for any given Iribarren number, this γ is shown to increase with fg. Therefore, a modified Iribarren number is proposed to include the Grouping structure, leading to a considerable improvement in reproducing the measured γ-values. Within the surf zone, the behavior of the Incident Long Wave also depends on the Group modulation. For low fg conditions, the lf Wave decays only slightly by transferring energy back to the hf Wave components. However, for high fg Wave conditions, strong dissipation of low frequency (lf) components occurs close to the shoreline associated with lf Wave breaking. This mechanism is explained by the growth of the lf Wave height, induced partly by the self-self interaction of fg, and partly by the nonlinear coupling between the primary frequencies and fg.
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sediment transport and beach profile evolution induced by bi chromatic Wave Groups with different Group periods
Coastal Engineering, 2016Co-Authors: Jose M Alsina, Enrique M Padilla, Ivan CaceresAbstract:Abstract In this paper, large-scale experimental data are presented showing the beach profile morphological evolution induced by four different bi-chromatic Wave conditions characterized by very similar energy content between them but varying the modulation period. Important differences were observed in the resultant beach profiles as a function of the Wave Group periods. Larger variability in the profile evolution is generally observed for larger Wave Group periods and, more importantly, as the Wave Group period increases the distance between the generated breaker bar and the shoreline increases. The measured primary Wave height to depth ratio (γ) increases with the Wave Group period, which is consistent with the observed larger Wave height at the breaking location. The primary Wave breaking location is also observed at increasing distances with respect to the initial shoreline as the Wave Group period increases. The variation in γ with Wave Group period is related to the selective energy dissipation of the higher primary frequency component (f1) during the Wave Group shoaling. Broad bandwith conditions (reduced Wave Group period) lead to larger dissipation of Wave heights at the f1 component relative to f2 resulting in a reduction in the Wave modulation and primary Wave height at the breaking location. Suspended sediment fluxes obtained from collocated velocity and sediment concentration measurements in the surf zone showed a consistently larger contribution of the mean return flow to the suspended sediment fluxes compared with the Wave Group and primary Wave components. The distinct beach profile evolution in terms of bar location is interpreted from an increasing distance of the mean breakpoint location and the location of maximum return flow with respect to the shoreline as the Wave Group period increases.
Zaher Hossein Shomali - One of the best experts on this subject based on the ideXlab platform.
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3D joint inversion of gravity data and Rayleigh Wave Group velocities to resolve shear-Wave velocity and density structure in the Makran subduction zone, south-east Iran
Journal of Asian Earth Sciences, 2019Co-Authors: Somayeh Abdollahi, Hermann Zeyen, Vahid Ebrahimzadeh Ardestani, Zaher Hossein ShomaliAbstract:In this study, we developed a method to invert jointly Rayleigh Wave Group velocities and gravity anomalies for velocity and density structure of the lithosphere. We applied the method to the Makran accretionary prism, SE Iran. The reason for using different data sets is that each of these data sets is sensitive to different parameters. Surface Wave Group velocities are sensitive mainly to shear Wave velocity distribution in depth but do not well resolve density variations. Therefore, joint inversion with gravity data increases the resolution of density distribution. Our approach differs from others mainly in the model parameterization: Instead of subdividing the model into a large number of thin layers, we invert for the properties of only four layers: thickness, P- and S-Wave velocities and densities and their vertical gradients in sediments, upper-crust, lower-crust and upper mantle. The method is applied first to synthetic models in order to demonstrate its usefulness. We then applied the method to real data to investigate the lithosphere structure beneath the Makran. The resulting model shows that Moho depth increases from Oman Sea (18–33 km) and Makran fore-arc (33–37 km) to the volcanic-arc (44–46 km). The crustal density is high in the Oman Sea as should be expected for the oceanic crust. We also find a high-velocity anomaly in the upper mantle under the Oman Sea corresponding to the subducting slab. The crust under the fore-arc, volcanic-arc and back-arc settings of Makran subduction zone is characterized by low-velocity zones.
Jordi Julia - One of the best experts on this subject based on the ideXlab platform.
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structure of the crust beneath cameroon west africa from the joint inversion of rayleigh Wave Group velocities and receiver functions
Geophysical Journal International, 2010Co-Authors: Alainpierre K Tokam, Andrew A Nyblade, Jordi Julia, Charles T Tabod, Douglas A Wiens, Michael E PasyanosAbstract:The Cameroon Volcanic Line (CVL) is a major geologic feature that cuts across Cameroon from the south west to the north east. It is a unique volcanic lineament which has both an oceanic and a continental sector and consists of a chain of Tertiary to Recent, generally alkaline volcanoes stretching from the Atlantic island of Pagalu to the interior of the African continent. The oceanic sector includes the islands of Bioko (formerly Fernando Po) and Sao Tome and Principe while the continental sector includes the Etinde, Cameroon, Manengouba, Bamboutos, Oku and Mandara mountains, as well as the Adamawa and Biu Plateaus. In addition to the CVL, three other major tectonic features characterize the region: the Benue Trough located northwest of the CVL, the Central African Shear Zone (CASZ), trending N70 degrees E, roughly parallel to the CVL, and the Congo Craton in southern Cameroon. The origin of the CVL is still the subject of considerable debate, with both plume and non-plume models invoked by many authors (e.g., Deruelle et al., 2007; Ngako et al, 2006; Ritsema and Allen, 2003; Burke, 2001; Ebinger and Sleep, 1998; Lee et al, 1994; Dorbath et al., 1986; Fairhead and Binks, 1991; King and Ritsema,more » 2000; Reusch et al., 2010). Crustal structure beneath Cameroon has been investigated previously using active (Stuart et al, 1985) and passive (Dorbath et al., 1986; Tabod, 1991; Tabod et al, 1992; Plomerova et al, 1993) source seismic data, revealing a crust about 33 km thick at the south-western end of the continental portion of the CVL (Tabod, 1991) and the Adamawa Plateau, and thinner crust (23 km thick) beneath the Garoua Rift in the north (Stuart et al, 1985) (Figure 1). Estimates of crustal thickness obtained using gravity data show similar variations between the Garoua rift, Adamawa Plateau, and southern part of the CVL (Poudjom et al., 1995; Nnange et al., 2000). In this study, we investigate further crustal structure beneath the CVL and the adjacent regions in Cameroon using 1-D shear Wave velocity models obtained from the joint inversion of Rayleigh Wave Group velocities and P-receiver functions for 32 broadband seismic stations. From the 1-D shear Wave velocity models, we obtain new insights into the composition and structure of the crust and upper mantle across Cameroon. After briefly reviewing the geological framework of Cameroon, we describe the data and the joint inversion method, and then interpret variations in crustal structure found beneath Cameroon in terms of the tectonic history of the region.« less
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deep crustal structure of the indian shield from joint inversion of p Wave receiver functions and rayleigh Wave Group velocities implications for precambrian crustal evolution
Journal of Geophysical Research, 2009Co-Authors: Jordi Julia, S Jagadeesh, Thomas J OwensAbstract:[1] The S Wave velocity structure of the crust and uppermost mantle of the Indian shield has been investigated by jointly inverting P Wave receiver functions and Rayleigh Wave Group velocities at 38 broadband stations in the subcontinent. The Indian shield is an amalgamation of several terranes of Archean and Proterozoic age that were partly flooded by Deccan Trap volcanism during Cenozoic times and that make up a natural laboratory for assessing models of Precambrian crustal evolution. Our results reveal significant variations in crustal thickness and deep crustal velocities: 45―50 km thick under the Archean West Dharwar craton and Southern Granulite Terrane, with lower crustal velocities around 4.1 km/s; 32―35 km thick beneath the Archean East Dharwar and Bundelkhand cratons, with lower crustal velocities around 3.8―3.9 km/s; 50―65 km thick under the Proterozoic Bhandara craton, with lower crustal velocities around 4.2―4.3 km/s; and ∼55 km thick under the Proterozoic Aravalli-Delhi belt, with lower crustal velocities around 4.2 km/s. S velocities in the 4.1―4.3 km/s range in the deep crust can be attributed to mafic lithologies, suggesting there has been no secular change in the Precambrian evolution of the south Indian shield. Moreover, pervasive mafic dike swarming throughout the Indian shield suggests that the layer of mafic cumulates is 2.5―1.6 Ga old and that it delaminated from some Archean terranes. Our interpretation is that mafic underplating of the terranes making up the Indian shield occurred in Proterozoic times and that a refractory root developed under the Archean terranes after the Proterozoic event.
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thin lithosphere beneath the ethiopian plateau revealed by a joint inversion of rayleigh Wave Group velocities and receiver functions
Journal of Geophysical Research, 2007Co-Authors: M T Dugda, Andrew A Nyblade, Jordi JuliaAbstract:[1] The seismic velocity structure of the crust and upper mantle beneath Ethiopia and Djibouti has been investigated by jointly inverting receiver functions and Rayleigh Wave Group velocities to obtain new constraints on the thermal structure of the lithosphere. Most of the data for this study come from the Ethiopia broadband seismic experiment, conducted between 2000 and 2002. Shear velocity models obtained from the joint inversion show crustal structure that is similar to previously published models, with crustal thicknesses of 35 to 44 km beneath the Ethiopian Plateau, and 25 to 35 km beneath the Main Ethiopian Rift (MER) and the Afar. The lithospheric mantle beneath the Ethiopian Plateau has a maximum shear Wave velocity of about 4.3 km/s and extends to a depth of ∼70–80 km. Beneath the MER and Afar, the lithospheric mantle has a maximum shear Wave velocity of 4.1–4.2 km/s and extends to a depth of at most 50 km. In comparison to the lithosphere away from the East African Rift System in Tanzania, where the lid extends to depths of ∼100–125 km and has a maximum shear velocity of 4.6 km/s, the mantle lithosphere under the Ethiopian Plateau appears to have been thinned by ∼30–50 km and the maximum shear Wave velocity reduced by ∼0.3 km/s. Results from a 1D conductive thermal model suggest that the shear velocity structure of the Ethiopian Plateau lithosphere can be explained by a plume model, if a plume rapidly thinned the lithosphere by ∼30–50 km at the time of the flood basalt volcanism (c. 30 Ma), and if warm plume material has remained beneath the lithosphere since then. About 45–65% of the 1–1.5 km of plateau uplift in Ethiopia can be attributed to the thermally perturbed lithospheric structure.
Somayeh Abdollahi - One of the best experts on this subject based on the ideXlab platform.
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3D joint inversion of gravity data and Rayleigh Wave Group velocities to resolve shear-Wave velocity and density structure in the Makran subduction zone, south-east Iran
Journal of Asian Earth Sciences, 2019Co-Authors: Somayeh Abdollahi, Hermann Zeyen, Vahid Ebrahimzadeh Ardestani, Zaher Hossein ShomaliAbstract:In this study, we developed a method to invert jointly Rayleigh Wave Group velocities and gravity anomalies for velocity and density structure of the lithosphere. We applied the method to the Makran accretionary prism, SE Iran. The reason for using different data sets is that each of these data sets is sensitive to different parameters. Surface Wave Group velocities are sensitive mainly to shear Wave velocity distribution in depth but do not well resolve density variations. Therefore, joint inversion with gravity data increases the resolution of density distribution. Our approach differs from others mainly in the model parameterization: Instead of subdividing the model into a large number of thin layers, we invert for the properties of only four layers: thickness, P- and S-Wave velocities and densities and their vertical gradients in sediments, upper-crust, lower-crust and upper mantle. The method is applied first to synthetic models in order to demonstrate its usefulness. We then applied the method to real data to investigate the lithosphere structure beneath the Makran. The resulting model shows that Moho depth increases from Oman Sea (18–33 km) and Makran fore-arc (33–37 km) to the volcanic-arc (44–46 km). The crustal density is high in the Oman Sea as should be expected for the oceanic crust. We also find a high-velocity anomaly in the upper mantle under the Oman Sea corresponding to the subducting slab. The crust under the fore-arc, volcanic-arc and back-arc settings of Makran subduction zone is characterized by low-velocity zones.
