Journal articles on the topic 'Crustal tomography'
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1
Hable, Sarah, Karin Sigloch, Eléonore Stutzmann, Sergey Kiselev, and Guilhem Barruol. "Tomography of crust and lithosphere in the western Indian Ocean from noise cross-correlations of land and ocean bottom seismometers." Geophysical Journal International 219, no. 2 (July 26, 2019): 924–44. http://dx.doi.org/10.1093/gji/ggz333.
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SUMMARY We use seismic noise cross-correlations to obtain a 3-D tomography model of SV-wave velocities beneath the western Indian Ocean, in the depth range of the oceanic crust and uppermost mantle. The study area covers 2000 × 2000 km2 between Madagascar and the three spreading ridges of the Indian Ocean, centred on the volcanic hotspot of La Réunion. We use seismograms from 38 ocean bottom seismometers (OBSs) deployed by the RHUM-RUM project and 10 island stations on La Réunion, Madagascar, Mauritius, Rodrigues, and Tromelin. Phase cross-correlations are calculated for 1119 OBS-to-OBS, land-to-OBS, and land-to-land station pairs, and a phase-weighted stacking algorithm yields robust group velocity measurements in the period range of 3–50 s. We demonstrate that OBS correlations across large interstation distances of >2000 km are of sufficiently high quality for large-scale tomography of ocean basins. Many OBSs yielded similarly good group velocity measurements as land stations. Besides Rayleigh waves, the noise correlations contain a low-velocity wave type propagating at 0.8–1.5 km s−1 over distances exceeding 1000 km, presumably Scholte waves travelling through seafloor sediments. The 100 highest-quality group velocity curves are selected for tomographic inversion at crustal and lithospheric depths. The inversion is executed jointly with a data set of longer-period, Rayleigh-wave phase and group velocity measurements from earthquakes, which had previously yielded a 3-D model of Indian Ocean lithosphere and asthenosphere. Robust resolution tests and plausible structural findings in the upper 30 km validate the use of noise-derived OBS correlations for adding crustal structure to earthquake-derived tomography of the oceanic mantle. Relative to crustal reference model CRUST1.0, our new shear-velocity model tends to enhance both slow and fast anomalies. It reveals slow anomalies at 20 km depth beneath La Réunion, Mauritius, Rodrigues Ridge, Madagascar Rise, and beneath the Central Indian spreading ridge. These structures can clearly be associated with increased crustal thickness and/or volcanic activity. Locally thickened crust beneath La Réunion and Mauritius is probably related to magmatic underplating by the hotspot. In addition, these islands are characterized by a thickened lithosphere that may reflect the depleted, dehydrated mantle regions from which the crustal melts where sourced. Our tomography model is available as electronic supplement.
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Crowder, E., N. Rawlinson, D. G. Cornwell, C. Sammarco, E. Galetti, and A. Curtis. "New insights into North Sea deep crustal structure and extension from transdimensional ambient noise tomography." Geophysical Journal International 224, no. 2 (October 10, 2020): 1197–210. http://dx.doi.org/10.1093/gji/ggaa475.
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SUMMARY The deep crustal structure beneath the North Sea is poorly understood since it is constrained by only a few seismic reflection and refraction profiles. However, it is widely acknowledged that the mid to lower crust plays important roles in rift initiation and evolution, particularly when large-scale sutures and/or terrane boundaries are present, since these inherited features can focus strain or act as inhibitors to extensional deformation. Ancient tectonic features are known to exist beneath the iconic failed rift system of the North Sea, making it an ideal location to investigate the complex interplay between pre-existing regional heterogeneity and rifting. To this end, we produce a 3-D shear wave velocity model from transdimensional ambient seismic noise tomography to constrain crustal properties to ∼30 km depth beneath the North Sea and its surrounding landmasses. Major North Sea sedimentary basins appear as low shear wave velocity zones that are a good match to published sediment thickness maps. We constrain relatively thin crust (13–18 km) beneath the Central Graben depocentres that contrasts with crust elsewhere at least 25–30 km thick. Significant variations in crustal structure and rift symmetry are identified along the failed rift system that appears to be related to the locations of Laurentia–Avalonia–Baltica palaeoplate boundaries. We constrain first-order differences in structure between palaeoplates; with strong lateral gradients in crustal velocity related to Laurentia–Avalonia–Baltica plate juxtaposition and reduced lower crustal velocities in the vicinity of the Thor suture, possibly representing the remnants of a Caledonian accretionary complex. Our results provide fresh insight into the pivotal roles that ancient terranes can play in the formation and failure of continental rifts and may help explain the characteristics of other similar continental rifts globally.
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Rezaeifar, M., and E. Kissling. "Regional 3-D lithosphere structure of the northern half of Iran by local earthquake tomography." Geophysical Journal International 223, no. 3 (September 11, 2020): 1956–72. http://dx.doi.org/10.1093/gji/ggaa431.
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SUMMARY The 3-D P-wave velocity structure of the northern half of Iran crust has been determined from the local earthquake tomography using a high-quality data set of semi-automatically re-picked arrival times. The quality and quantity of these re-picked phase data allow the 3-D imaging of large parts of the northern half of Iran lithosphere between 0 and 60 km depth. Our new P-wave tomography model represents a major improvement over existing models in terms of reliability, resolution and consistency. First-order anomalies such as the crustal roots of the Zagros and Alborz Mountains are clearly resolved. In addition, several shallow smaller-scale features like the Central Iran sedimentary basin and volcanic and igneous rocks are visible in the tomographic image. Our results show deep Moho depressions beneath the Central Alborz and Zagros mountain ranges that are part of the Arabia–Iranian–Eurasia continental collision zone and locally this Moho topography agrees very well with existing models of other studies. The observed P-wave velocity structure suggests that compared to the Sanandaj-Sirjan and Zagros mountain ranges there is a minor crustal thickening beneath the Alborz mountain range and Kopeh Dagh region.
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Li, Mengkui, and Tengfei Wu. "Ps-splitting analysis reveals differential crustal deformation beneath the Qinling Orogenic Belt and its surrounding areas." Geophysical Journal International 229, no. 2 (December 17, 2021): 853–61. http://dx.doi.org/10.1093/gji/ggab509.
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SUMMARY Crustal anisotropy parameters beneath the Qinling Orogenic Belt (QOB) and its surrounding areas (including the northeastern Tibetan Plateau) are investigated by harmonic fitting the arrival times of the P-to-S converted phase from the Moho and an intracrustal discontinuity. The measurements reveal strong and spatially varying crustal anisotropy beneath the study region, with an average splitting time of 0.50 ± 0.17 s. The eastern Kunlun Orogen (EKLO), western part of QOB (WWQL) and Longmenshan block (LMB) present relatively larger crustal anisotropy, and the fast orientations changed gradually from NWW–SEE in EKLO and WWQL to NEE–SWW in LMB. The crustal anisotropy measurements, combined with the results from ambient-noise tomography and gravity inversion, suggest that the middle-lower crustal flow induced by the inhomogeneous crustal thickening during the early stage of plateau growth exists beneath these areas. The fast orientations beneath the eastern part of the QOB are predominantly NNE–SSW, nearly orthogonal to that from local shear wave splitting and teleseismic XKS splitting. The crustal anisotropy measurements suggest a layered deformation beneath the eastern QOB. The upper crust retains the fossil deformation formed during the main orogeny, the middle-to-lower crust is dominated by the N–S oriented subduction, collision and continued convergence between the North China Block, South China Block and Qinling microblocks; the upper mantle is decoupled from the crust and mainly controlled by the mantle flow from the Tibetan Plateau.
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Acevedo, Jorge, Gabriela Fernández-Viejo, Sergio Llana-Fúnez, Carlos López-Fernández, and Javier Olona. "Ambient noise tomography of the southern sector of the Cantabrian Mountains, NW Spain." Geophysical Journal International 219, no. 1 (July 8, 2019): 479–95. http://dx.doi.org/10.1093/gji/ggz308.
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SUMMARY This study presents the first detailed analysis of ambient noise tomography in an area of the continental upper crust in the Cantabrian Mountains (NW Spain), where a confluence of crustal scale faults occurs at depth. Ambient noise data from two different seismic networks have been analysed. In one side, a 10-short-period station network was set recording continuously for 19 months. A second set of data from 13 broad-band stations was used to extend at depth the models. The phase cross-correlation processing technique was used to compute in total more than 34 000 cross-correlations from 123 station pairs. The empirical Green's functions were obtained by applying the time–frequency, phase-weighted stacking methodology and provided the emergence of Rayleigh waves. After measuring group velocities, Rayleigh-wave group velocity tomographic maps were computed at different periods and then they were inverted in order to calculate S-wave velocities as a function of depth, reaching the first 12 km of the crust. The results show that shallow velocity patterns are dominated by geological features that can be observed at the surface, particularly bedding and/or lithology and fracturing associated with faults. In contrast, velocity patterns below 4 km depth seem to be segmented by large structures, which show a velocity reduction along fault zones. The best example is the visualization in the tomography of the frontal thrust of the Cantabrian Mountains at depth, which places higher velocity Palaeozoic rocks over Cenozoic sediments of the foreland Duero basin. One of the major findings in the tomographic images is the reduction of seismic velocities above the area in the crust where one seismicity cluster is nucleated within the otherwise quiet seismic area of the range. The noise tomography reveals itself as a valuable technique to identify shear zones associated with crustal scale fractures and hence, lower strain areas favourable to seismicity.
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Bozdağ, Ebru, and Jeannot Trampert. "On crustal corrections in surface wave tomography." Geophysical Journal International 172, no. 3 (March 2008): 1066–82. http://dx.doi.org/10.1111/j.1365-246x.2007.03690.x.
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Hearn, Thomas M. "Crustal attenuation from USArray ML amplitude tomography." Geophysical Journal International 224, no. 1 (September 19, 2020): 199–206. http://dx.doi.org/10.1093/gji/ggaa445.
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SUMMARY Seismic attenuation across the US is estimated using station ML magnitude data from the USArray. Station magnitudes are recalibrated back to amplitude and back projected in a 2-D tomography. Data represent the amplitudes of the horizontal components of the Lg phase. The western US shows regions of very high attenuation and contrasts with the lesser attenuation of the eastern US. Individual attenuation anomalies can be clearly tied to regional geology. Station gains show broad regional variations that match geographic regions. Most of the high-attenuation areas are regions of high geothermal activity suggesting that intrinsic attenuation dominates over scattering attenuation. An exception is the central San Andreas Fault zone because it lacks any localized heat-flow anomaly. The US east of the Rocky Mountains is bland and contains none of the high-attenuation regions of the western US. Instead, the central US has low-attenuation patches that do not obviously correspond to geologic province. Sediments of the Gulf Coast Plain, Willison Basin and Michigan Basin do show up as intermediate attenuation while the Illinois Basin, Appalachian Basin and other basins are not apparent. In Alaska, attenuation is generally less than the western US, but still much greater than the eastern US. In southeast Alaska, the Wrangell Volcanic Field causes a sizeable high-attenuation zone. The volcanic Aleutian Mountains also have high attenuation. However, moderate to high attenuation also correlates with the tertiary sedimentary basins in Alaska. The North Slope Basin does not seem to attenuate. Thicker crust and mountain roots tend to show less attenuation, if anything, but this correspondence is most likely due to differences in temperature and seismic velocity. Heat, scattering and young sedimentary basins create seismic attenuation in the continental crust.
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Got, Jean Luc, Vadim Monteiller, Jean Virieux, and Paul Okubo. "Estimating Crustal Heterogeneity from Double-difference Tomography." Pure and Applied Geophysics 163, no. 2-3 (March 2006): 405–30. http://dx.doi.org/10.1007/s00024-005-0022-x.
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Ritter, J. R. R., and U. Achauer. "Crustal tomography of the central kenya rift." Tectonophysics 236, no. 1-4 (September 1994): 291–304. http://dx.doi.org/10.1016/0040-1951(94)90181-3.
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Koulakov, I., I. Zabelina, I. Amanatashvili, and V. Meskhia. "Nature of orogenesis and volcanism in the Caucasus region based on results of regional tomography." Solid Earth 3, no. 2 (October 17, 2012): 327–37. http://dx.doi.org/10.5194/se-3-327-2012.
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Abstract. In the paper, we discuss the problem of continental collision and related volcanism in the Caucasus and surrounding areas based on the analysis of the upper mantle seismic structure in a recently derived model by Koulakov (2011). This model, which includes P and S-velocity anomalies down to 1000 km depth, was obtained from tomographic inversion of worldwide travel time data from the catalogue of the International Seismological Center. It can be seen that the Caucasus region is squeezed between two continental plates, Arabian to the south and European to the north, which are displayed in the tomographic model as high-velocity bodies down to about 200–250 km depth. On the contrary, a very bright low-velocity anomaly beneath the collision area implies that the lithosphere in this zone is very thin, which is also supported by strong horizontal deformations and crustal thickening indicating weak properties of the lithosphere. In the contact between stable continental and collision zones, we observe a rather complex alternation of seismic anomalies having the shapes of sinking drops. We propose that the convergence process causes crustal thickening and transformation of the lower crust material into the dense eclogite. When achieving a critical mass, the dense eclogitic drops trigger detachment of the mantle lithosphere and its delamination. The observed high-velocity bodies in the upper mantle may indicate the parts of the descending mantle lithosphere which were detached from the edges of the continental lithosphere plates. Very thin, or even absent, mantle parts of the lithosphere leads to the presence of hot asthenosphere just below the crust. The crustal shortening and eclogitisation of the lower crustal layer leads to the dominantly felsic composition of the crust which is favourable for the upward heat transport from the mantle. This, and also the factors of frictional heating and the radioactivity of felsic rocks, may be the origin of volcanic centres in the Caucasus and surrounding collisional areas.
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Rost, S., G. A. Houseman, A. W. Frederiksen, D. G. Cornwell, M. Kahraman, S. Altuncu Poyraz, U. M. Teoman, et al. "Structure of the northwestern North Anatolian Fault Zone imaged via teleseismic scattering tomography." Geophysical Journal International 227, no. 2 (July 10, 2021): 922–40. http://dx.doi.org/10.1093/gji/ggab265.
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SUMMARY Information on fault zone structure is essential for our understanding of earthquake mechanics, continental deformation and seismic hazard. We use the scattered seismic wavefield to study the subsurface structure of the North Anatolian Fault Zone (NAFZ) in the region of the 1999 İzmit and Düzce ruptures using data from an 18-month dense deployment of seismometers with a nominal station spacing of 7 km. Using the forward- and back-scattered energy that follows the direct P-wave arrival from teleseismic earthquakes, we apply a scattered wave inversion approach and are able to resolve changes in lithospheric structure on a scale of 10 km or less in an area of about 130 km by 100 km across the NAFZ. We find several crustal interfaces that are laterally incoherent beneath the surface strands of the NAFZ and evidence for contrasting crustal structures either side of the NAFZ, consistent with the presence of juxtaposed crustal blocks and ancient suture zones. Although the two strands of the NAFZ in the study region strike roughly east–west, we detect strong variations in structure both north–south, across boundaries of the major blocks, and east–west, parallel to the strike of the NAFZ. The surface expression of the two strands of the NAFZ is coincident with changes on main interfaces and interface terminations throughout the crust and into the upper mantle in the tomographic sections. We show that a dense passive network of seismometers is able to capture information from the scattered seismic wavefield and, using a tomographic approach, to resolve the fine scale structure of crust and lithospheric mantle even in geologically complex regions. Our results show that major shear zones exist beneath the NAFZ throughout the crust and into the lithospheric mantle, suggesting a strong coupling of strain at these depths.
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Kolstrup, M. L., and V. Maupin. "Measuring and crust-correcting finite-frequency travel time residuals – application to southwestern Scandinavia." Solid Earth Discussions 7, no. 3 (July 1, 2015): 1909–39. http://dx.doi.org/10.5194/sed-7-1909-2015.
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Abstract. We present a data processing routine to compute relative finite-frequency travel time residuals using a combination of the Iterative Cross-Correlation and Stack (ICCS) algorithm and the MultiChannel Cross-Correlation method (MCCC). The routine has been tailored for robust measurement of P and S wave travel times in several frequency bands and for avoiding cycle-skipping problems at the shortest periods. We also investigate the adequacy of ray theory to calculate crustal corrections for finite-frequency regional tomography in normal continental settings with non-thinned crust. We find that ray theory is valid for both P and S waves at all relevant frequencies as long as the crust does not contain low-velocity layers associated with sediments at the surface. Reverberations in the sediments perturb the arrival times of the S waves and the long-period P waves significantly, and need to be accounted for in crustal corrections. The data processing routine and crustal corrections are illustated using data from a network in southwestern Scandinavia.
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Kvapil, Jiří, Jaroslava Plomerová, Hana Kampfová Exnerová, Vladislav Babuška, and György Hetényi. "Transversely isotropic lower crust of Variscan central Europe imaged by ambient noise tomography of the Bohemian Massif." Solid Earth 12, no. 5 (May 11, 2021): 1051–74. http://dx.doi.org/10.5194/se-12-1051-2021.
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Abstract. The recent development of ambient noise tomography, in combination with the increasing number of permanent seismic stations and dense networks of temporary stations operated during passive seismic experiments, provides a unique opportunity to build the first high-resolution 3-D shear wave velocity (vS) model of the entire crust of the Bohemian Massif (BM). This paper provides a regional-scale model of velocity distribution in the BM crust. The velocity model with a cell size of 22 km is built using a conventional two-step inversion approach from Rayleigh wave group velocity dispersion curves measured at more than 400 stations. The shear velocities within the upper crust of the BM are ∼0.2 km s−1 higher than those in its surroundings. The highest crustal velocities appear in its southern part, the Moldanubian unit. The Cadomian part of the region has a thinner crust, whereas the crust assembled, or tectonically transformed in the Variscan period, is thicker. The sharp Moho discontinuity preserves traces of its dynamic development expressed in remnants of Variscan subductions imprinted in bands of crustal thickening. A significant feature of the presented model is the velocity-drop interface (VDI) modelled in the lower part of the crust. We explain this feature by the anisotropic fabric of the lower crust, which is characterised as vertical transverse isotropy with the low velocity being the symmetry axis. The VDI is often interrupted around the boundaries of the crustal units, usually above locally increased velocities in the lowermost crust. Due to the north-west–south-east shortening of the crust and the late-Variscan strike-slip movements along the north-east–south-west oriented sutures preserved in the BM lithosphere, the anisotropic fabric of the lower crust was partly or fully erased along the boundaries of original microplates. These weakened zones accompanied by a velocity increase above the Moho (which indicate an emplacement of mantle rocks into the lower crust) can represent channels through which portions of subducted and later molten rocks have percolated upwards providing magma to subsequently form granitoid plutons.
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Liu, Yongsheng, and Ping Tong. "Eikonal equation-based P-wave seismic azimuthal anisotropy tomography of the crustal structure beneath northern California." Geophysical Journal International 226, no. 1 (March 13, 2021): 287–301. http://dx.doi.org/10.1093/gji/ggab103.
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SUMMARY Delineating spatial variations of seismic anisotropy in the crust is of great importance for the understanding of structural heterogeneities, regional stress regime and ongoing crustal dynamics. In this study, we present a 3-D anisotropic P-wave velocity model of the crust beneath northern California by using the eikonal equation-based seismic azimuthal anisotropy tomography method. The velocity heterogeneities under different geological units are well resolved. The thickness of the low-velocity sediment at the Great Valley Sequence is estimated to be about 10 km. The high-velocity anomaly underlying Great Valley probably indicates the existence of ophiolite bodies. Strong velocity contrasts are revealed across the Hayward Fault (2–9 km) and San Andreas Fault (2–12 km). In the upper crust (2–9 km), the fast velocity directions (FVDs) are generally fault-parallel in the northern Coast Range, which may be caused by geological structure; while the FVDs are mainly NE–SW in Great Valley and the northern Sierra Nevada possibly due to the regional maximum horizontal compressive stress. In contrast, seismic anisotropy in the mid-lower crust (12–22 km) may be attributed to the alignment of mica schists. The anisotropy contrast across the San Andreas Fault may imply different mechanisms of crustal deformation on the two sides of the fault. Both the strong velocity contrasts and the high angle (∼45° or above) between the FVDs and the strikes of faults suggest that the faults are mechanically weak in the San Francisco bay area (2–6 km). This study suggests that the eikonal equation-based seismic azimuthal anisotropy tomography is a valuable tool to investigate crustal heterogeneities and tectonic deformation.
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Adimah, N. I., and S. Padhy. "Ambient noise Rayleigh wave tomography across the Madagascar island." Geophysical Journal International 220, no. 3 (November 29, 2019): 1657–76. http://dx.doi.org/10.1093/gji/ggz542.
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SUMMARY The unusual complex lithospheric structure of Madagascar is a product of a number of important geological events, including: the Pan-African Orogeny, episodes of Late Cenozoic intraplate volcanism and several phases of deformation and metamorphism. Despite this rich history, its detailed crustal structure remains largely underexplored. Here, we take advantage of the recently obtained data set of the RHUM-RUM (Réunion Hotspot and Upper Mantle–Réunions Unterer Mantel) seismological experiment, in addition to previously available data sets to generate the first Rayleigh wave group velocity maps across the entire island at periods between 5 and 30 s using the ambient noise tomography technique. Prior to preliminary data preparation, data from Ocean Bottom Seismometers are cleaned of compliance and tilt noise. Cross-correlating noise records yielded over 1900 Rayleigh wave cross-correlation functions from which group velocities were measured to perform surface wave tomography. Dispersion curves extracted from group velocity tomographic maps are inverted to compute a 3-D shear velocity model of the region. Our velocity maps have shown relative improvement in imaging the three sedimentary basins in the western third of the island compared to those of previous studies. The Morondava basin southwest of the island is the broadest and contains the thickest sedimentary rocks while the Antsirinana basin at the northern tip is narrowest and thinnest. The lithosphere beneath the island is characterized by a heterogeneous crust which appears thickest at the centre but thins away towards the margins. A combined effect of uneven erosion of the crust and rifting accommodates our observations along the east coast. Average 1-D shear velocity models in six different tectonic units, support the causes of low velocity zones observed in the west coast of the island and reveal an intermediate-to-felsic Precambrian upper and middle crust consistent with findings of previous seismic studies. Our findings, especially at short periods provide new constraints on shallow crustal structure of the main island region.
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Qorbani, Ehsan, Dimitri Zigone, Mark R. Handy, and Götz Bokelmann. "Crustal structures beneath the Eastern and Southern Alps from ambient noise tomography." Solid Earth 11, no. 5 (October 29, 2020): 1947–68. http://dx.doi.org/10.5194/se-11-1947-2020.
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Abstract. We study the crustal structure under the Eastern and Southern Alps using ambient noise tomography. We use cross-correlations of ambient seismic noise between pairs of 71 permanent stations and 19 stations of the Eastern Alpine Seismic Investigation (EASI) profile to derive new 3D shear velocity models for the crust. Continuous records from 2014 and 2015 are cross-correlated to estimate Green's functions of Rayleigh and Love waves propagating between the station pairs. Group velocities extracted from the cross-correlations are inverted to obtain isotropic 3D Rayleigh- and Love-wave shear-wave velocity models. Our models image several velocity anomalies and contrasts and reveal details of the crustal structure. Velocity variations at short periods correlate very closely with the lithologies of tectonic units at the surface and projected to depth. Low-velocity zones, associated with the Po and Molasse sedimentary basins, are imaged well to the south and north of the Alps, respectively. We find large high-velocity zones associated with the crystalline basement that forms the core of the Tauern Window. Small-scale velocity anomalies are also aligned with geological units of the Austroalpine nappes. Clear velocity contrasts in the Tauern Window along vertical cross sections of the velocity model show the depth extent of the tectonic units and their bounding faults. A mid-crustal velocity contrast is interpreted as a manifestation of intracrustal decoupling in the Eastern Alps that accommodated eastward escape of the Alcapa block.
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Koulakov, I., I. Zabelina, I. Amanatashvili, and V. Meskhia. "Nature of orogenesis and volcanism in the Caucasus region based on results of regional tomography." Solid Earth Discussions 4, no. 1 (June 7, 2012): 641–62. http://dx.doi.org/10.5194/sed-4-641-2012.
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Abstract. In the paper we discuss the problem of continental collision and related volcanism in the Caucasus and surrounding areas based on analysis of the upper mantle seismic structure in a recently derived model by Koulakov (2011). This model, which includes P- and S-velocity anomalies down to 1000 km depth, was obtained from tomographic inversion of worldwide travel time data from the catalogue of the International Seismological Center. It can be seen that the Caucasus region is squeezed between two continental plates, Arabian to the south and European to the north, which are displayed in the tomographic model as high-velocity bodies down to about 200–250 km depth. On the contrary, a very bright low-velocity anomaly beneath the collision area implies that the lithosphere in this zone is very thin, which is also supported by strong deformations indicating weak properties of the lithosphere. In the contact between stable continental and collision zones we observe a rather complex alternation of seismic anomalies having the shapes of sinking drops. We propose that the convergence process causes crustal thickening and transformation of the lower crust material into the dense eclogite. When achieving a critical mass, the dense eclogitic drops trigger detachment of the mantle lithosphere and its delamination. The observed high-velocity bodies in the upper mantle may indicate the parts of the descending mantle lithosphere which were detached from the edges of the continental lithosphere plates. Very thin or even absent mantle part of the lithosphere leads to the presence of hot asthenosphere just below the crust. The crustal shortening and eclogitization of the lower crustal layer leads to the dominantly felsic composition of the crust which is favorable for the upward heat transport from the mantle. This, and also the factor of frictional heating, may cause to the origin of volcanic centers in the Caucasus and surrounding collisional areas.
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Gaite, B., A. Villaseñor, A. Iglesias, M. Herraiz, and I. Jiménez-Munt. "A 3-D shear velocity model of the southern North America and the Caribbean plates from ambient noise and earthquake tomography." Solid Earth Discussions 6, no. 2 (October 13, 2014): 2971–3002. http://dx.doi.org/10.5194/sed-6-2971-2014.
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Abstract. We use group velocities from earthquake tomography together with group and phase velocities from ambient noise tomography (ANT) of Rayleigh-waves to invert for the 3-D shear-wave velocity structure (5–70 km) of the Caribbean (CAR) and southern North American (NAM) plates. The lithospheric model proposed offers a complete image of the crust and uppermost-mantle with imprints of the tectonic evolution. One of the most striking features inferred is the main role of the Ouachita-Marathon-Sonora orogeny front on the crustal seismic structure of NAM plate. A new imaged feature is the low crustal velocities along USA-Mexico border. The model also shows a break of the E-W mantle velocity dichotomy of the NAM and CAR plates beneath the Isthmus of Tehuantepec and Yucatan Block. High upper-mantle velocities along the Mesoamerican Subduction Zone coincide with inactive volcanic areas while the lowest velocities correspond to active volcanic arcs and thin lithospheric mantle regions.
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Gaite, B., A. Villaseñor, A. Iglesias, M. Herraiz, and I. Jiménez-Munt. "A 3-D shear velocity model of the southern North American and Caribbean plates from ambient noise and earthquake tomography." Solid Earth 6, no. 1 (February 20, 2015): 271–84. http://dx.doi.org/10.5194/se-6-271-2015.
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Abstract. We use group velocities from earthquake tomography together with group and phase velocities from ambient noise tomography (ANT) of Rayleigh waves to invert for the 3-D shear-wave velocity structure (5–70 km) of the Caribbean (CAR) and southern North American (NAM) plates. The lithospheric model proposed offers a complete image of the crust and uppermost-mantle with imprints of the tectonic evolution. One of the most striking features inferred is the main role of the Ouachita–Marathon–Sonora orogeny front on the crustal seismic structure of the NAM plate. A new imaged feature is the low crustal velocities along the USA-Mexico border. The model also shows a break of the east–west mantle velocity dichotomy of the NAM and CAR plates beneath the Isthmus of the Tehuantepec and the Yucatan Block. High upper-mantle velocities along the Mesoamerican Subduction Zone coincide with inactive volcanic areas while the lowest velocities correspond to active volcanic arcs and thin lithospheric mantle regions.
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Winardhi, S., and R. F. Mereu. "Crustal velocity structure of the Superior and Grenville provinces of the southeastern Canadian Shield." Canadian Journal of Earth Sciences 34, no. 8 (August 1, 1997): 1167–84. http://dx.doi.org/10.1139/e17-094.
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The 1992 Lithoprobe Abitibi–Grenville Seismic Refraction Experiment was conducted using four profiles across the Grenville and Superior provinces of the southeastern Canadian Shield. Delay-time analysis and tomographic inversion of the data set demonstrate significant lateral and vertical variations in crustal velocities from one terrane to another, with the largest velocity values occurring underneath the Central Gneiss and the Central Metasedimentary belts south of the Grenville Front. The Grenville Front Tectonic Zone is imaged as a southeast-dipping region of anomalous velocity gradients extending to the Moho. The velocity-anomaly maps suggest an Archean crust may extend, horizontally, 140 km beneath the northern Grenville Province. Near-surface velocity anomalies correlate well with the known geology. The most prominent of these is the Sudbury Structure, which is well mapped as a low-velocity basinal structure. The tomography images also suggest underthrusting of the Pontiac and Quetico subprovinces beneath the Abitibi Greenstone Belt. Wide-angle PmP signals, indicate that the Moho varies from a sharp discontinuity south of the Grenville Front to a rather diffuse and flat boundary under the Abitibi Greenstone Belt north of the Grenville Front. A significant crustal thinning near the Grenville Front may indicate post-Grenvillian rebound and (or) the extensional structure of the Ottawa–Bonnechere graben. Crustal thickening resulting from continental collision may explain the tomographic images showing the Moho is 4–5 km deeper south of the Grenville Front.
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Hearn, Thomas M., Suyun Wang, Shunping Pei, Zhonghuai Xu, James F. Ni, and Yanxiang Yu. "Seismic amplitude tomography for crustal attenuation beneath China." Geophysical Journal International 174, no. 1 (July 2008): 223–34. http://dx.doi.org/10.1111/j.1365-246x.2008.03776.x.
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Kolstrup, M. L., and V. Maupin. "Measuring and crust-correcting finite-frequency travel time residuals – application to southwestern Scandinavia." Solid Earth 6, no. 4 (October 9, 2015): 1117–30. http://dx.doi.org/10.5194/se-6-1117-2015.
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Abstract. We present a data-processing routine to compute relative finite-frequency travel time residuals using a combination of the Iterative Cross-Correlation and Stack (ICCS) algorithm and the Multi-Channel Cross-Correlation method (MCCC). The routine has been tailored for robust measurement of P- and S-wave travel times in several frequency bands and for avoiding cycle-skipping problems at the shortest periods. We also investigate the adequacy of ray theory to calculate crustal corrections for finite-frequency regional tomography in normal continental settings with non-thinned crust. We find that ray theory is valid for both P and S waves at all relevant frequencies as long as the crust does not contain low-velocity layers associated with sediments at the surface. Reverberations in the sediments perturb the arrival times of the S waves and the long-period P waves significantly, and need to be accounted for in crustal corrections. The data-processing routine and crustal corrections are illustrated using data from a~network in southwestern Scandinavia.
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Lu, Y., L. Stehly, R. Brossier, and A. Paul. "Imaging Alpine crust using ambient noise wave-equation tomography." Geophysical Journal International 222, no. 1 (March 24, 2020): 69–85. http://dx.doi.org/10.1093/gji/ggaa145.
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SUMMARY We present an improved crustal Vs model and Moho depth map using ambient noise wave-equation tomography. The so-called ‘ambient noise wave-equation tomography’ is a method to invert seismic ambient noise phase dispersion data based on elastic waveform simulation, which accounts for 3-D and finite-frequency effects. We use cross-correlations of up to 4 yr of continuous vertical-component ambient seismic noise recordings from 304 high-quality broad-band stations in the Alpine region. We use model LSP_Eucrust1.0 obtained from traditional ambient noise tomography as initial model, and we iteratively improve the initial model by minimizing frequency-dependent phase traveltime differences between the observed and synthetic waveforms of Rayleigh waves in the period range 10–50 s. We obtain the final model after 15 iterations with ∼65 per cent total misfit reduction compared to the initial model. At crustal depth, the final model significantly enhances the amplitudes and adjusts the shapes of velocity anomalies. At Moho and upper-mantle depth, the final model corrects an obvious systematic velocity shift of the initial model. The resulting isovelocity Moho map confirms a Moho step along the external side of the external crystalline massifs of the northwestern Alps and reveals underplated gabbroic plutons in the lower most crust of the central and eastern Alps. Ambient noise wave-equation tomography turns out to be a useful tool to refine shear wave velocity models obtained by traditional ambient noise tomography based on ray theory.
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Shirzad, Taghi, Marcelo Assumpcao, and Marcelo Bianchi. "Ambient seismic noise tomography in west-central and Southern Brazil, characterizing the crustal structure of the Chaco-Paraná, Pantanal and Paraná basins." Geophysical Journal International 220, no. 3 (December 3, 2019): 2074–85. http://dx.doi.org/10.1093/gji/ggz548.
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SUMMARY Surface wave analysis provides important information on crustal structure, but it is challenging to obtain accurate/robust models in aseismic regions because of the lack of local earthquake records. In this paper, interstation empirical Green's functions retrieved by ambient seismic noise in 75 broad-band stations from 2016 January to 2018 September were used to study crustal structure in west-central Brazil. Fast marching method was applied to calculate the 2-D surface wave tomographic maps, and local dispersion curves were estimated in the period range of 4–80 s for each geographic cell. 1-D damped least squares inversion method was then conducted to obtained shear wave velocity model. Finally, the average ($\tilde{\rm V}$S) of the calculated VSV and VSH quasi 3-D models were used to characterize the crustal structure. Besides the checkerboard test resolution, a stochastic test with the effect of errors in the dispersion curves and choice of inversion parameters were carried out to better evaluate model uncertainties. Our results show a clear relation between the sedimentary thickness and geological units with the shorter period tomographic maps. Agreement has also been observed in longer periods such as the clear N–S anomaly along the Asuncion and Rio Grande Arches representing the boundary between the Chaco-Paraná and the Paraná basins. A 3-D composite velocity model shows a crustal structure consisting of three main layers. Some differences in lower crustal properties were found between the Paraná and Chaco-Paraná basins, consistent with a recently postulated, gravity-derived Western Paraná suture zone. However, no high velocities along the SW–NE axis of the Paraná basin were found to confirm proposed underplating. At the eastern edge of the Pantanal basin, the thin crust seems to be associated with a very thin (or lack of) lower crustal layer, consistent with a recently proposed crustal delamination hypothesis for the formation of the Pantanal basin.
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McVey, B. G., E. E. E. Hooft, B. A. Heath, D. R. Toomey, M. Paulatto, J. V. Morgan, P. Nomikou, and C. B. Papazachos. "Magma accumulation beneath Santorini volcano, Greece, from P-wave tomography." Geology 48, no. 3 (December 9, 2019): 231–35. http://dx.doi.org/10.1130/g47127.1.
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Abstract Despite multidisciplinary evidence for crustal magma accumulation below Santorini volcano, Greece, the structure and melt content of the shallow magmatic system remain poorly constrained. We use three-dimensional (3-D) velocity models from tomographic inversions of active-source seismic P-wave travel times to identify a pronounced low-velocity anomaly (–21%) from 2.8 km to 5 km depth localized below the northern caldera basin. This anomaly is consistent with depth estimates of pre-eruptive storage and a recent inflation episode, supporting the interpretation of a shallow magma body that causes seismic attenuation and ray bending. A suite of synthetic tests shows that the geometry is well recovered while a range of melt contents (4%–13% to fully molten) are allowable. A thin mush region (2%–7% to 3%–10% melt) extends from the main magma body toward the northeast, observed as low velocities confined by tectono-magmatic lineaments. This anomaly terminates northwest of Kolumbo; little to no melt underlies the seamount from 3 to 5 km depth. These structural constraints suggest that crustal extension and edifice loads control the geometry of magma accumulation and emphasize that the shallow crust remains conducive to melt storage shortly after a caldera-forming eruption.
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Lee, Sungho, Arushi Saxena, Jung-Hun Song, Junkee Rhie, and Eunseo Choi. "Contributions from lithospheric and upper-mantle heterogeneities to upper crustal seismicity in the Korean Peninsula." Geophysical Journal International 229, no. 2 (December 29, 2021): 1175–92. http://dx.doi.org/10.1093/gji/ggab527.
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SUMMARY The Korean Peninsula (KP), located along the eastern margin of the Eurasian and Amurian plates, has experienced continual earthquakes from small to moderate magnitudes. Various models to explain these earthquakes have been proposed, but the origins of the stress responsible for this region's seismicity remain unclear and debated. This study aims to understand the stress field of this region in terms of the contributions from crustal and upper-mantle heterogeneities imaged via seismic tomography using a series of numerical simulations. A crustal seismic velocity model can determine the crustal thickness and density. Upper-mantle seismic velocity anomalies from a regional tomography model were converted to a temperature field, which can determine the structures (e.g. lithospheric thickness, subducting slabs, their gaps, and stagnant features) and density. The heterogeneities in the crustal and upper mantle governed the buoyancy forces and rheology in our models. The modelled surface topography, mantle flow stress, and orientation of maximum horizontal stress, derived from the variations in the crustal thickness, suggest that model with the lithospheric and upper-mantle heterogeneities is required to improve these modelled quantities. The model with upper-mantle thermal anomalies and east–west compression of approximately 50 MPa developed a stress field consistent with the observed seismicity in the KP. However, the modelled and observed orientations of the maximum horizontal stress agree in the western KP but they are inconsistent in the eastern KP. Our analysis, based on the modelled quantities, suggested that compressional stress and mantle heterogeneities may mainly control the seismicity in the western area. In contrast, we found a clear correlation of the relatively thin lithosphere and strong upper-mantle upwelling with the observed seismicity in the Eastern KP, but it is unclear whether stress, driven by these heterogeneities, directly affects the seismicity of the upper crust.
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Shulgin, Alexey, Jan Inge Faleide, Rolf Mjelde, Asbjørn Breivik, and Ritske Huismans. "Crustal domains in the Western Barents Sea." Geophysical Journal International 221, no. 3 (April 24, 2020): 2155–69. http://dx.doi.org/10.1093/gji/ggaa112.
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SUMMARY The crustal architecture of the Barents Sea is still enigmatic due to complex evolution during the Timanian and Caledonian orogeny events, further complicated by several rifting episodes. In this study we present the new results on the crustal structure of the Caledonian–Timanian transition zone in the western Barents. We extend the work of Aarseth et al. (2017), by utilizing the seismic tomography approach to model Vp, Vs and Vp/Vs ratio, combined with the reprocessed seismic reflection line, and further complemented with gravity modelling. Based on our models we document in 3-D the position of the Caledonian nappes in the western Barents Sea. We find that the Caledonian domain is characterized by high crustal reflectivity, caused by strong deformation and/or emplacement of mafic intrusions within the crystalline crust. The Timanian domain shows semi-transparent crust with little internal reflectivity, suggesting less deformation. We find, that the eastern branch of the earlier proposed Caledonian suture, cannot be associated with the Caledonian event, but can rather be a relict from the Timanian terrane assemblance, marking one of the crustal microblocks. This crustal block may have an E–W striking southern boundary, along which the Caledonian nappes were offset. A high-velocity/density crustal body, adjacent to the Caledonian–Timanian contact zone, is interpreted as a zone of metamorphosed rocks based on the comparison with global compilations. The orientation of this body correlates with regional gravity maxima zone. Two scenarios for the origin of the body are proposed: mafic emplacement during the Timanian assembly, or massive mafic intrusions associated with the Devonian extension.
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Civilini, F., W. D. Mooney, M. K. Savage, J. Townend, and H. Zahran. "Crustal imaging of northern Harrat Rahat, Saudi Arabia, from ambient noise tomography." Geophysical Journal International 219, no. 3 (August 19, 2019): 1532–49. http://dx.doi.org/10.1093/gji/ggz380.
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SUMMARY Harrat Rahat is a volcanic field located in west-central Saudi Arabia and is the site of the most recent eruption in the country (1256 CE). An earthquake swarm at a nearby volcanic field in 2009 prompted the need for new hazard models for this region, which includes the holy city of Medina. Tomography studies can be used to infer material properties of the subsurface such as partial melt, and are instrumental for volcanic hazard assessment. Regional earthquakes have been used to determine mantle structure, but such crustal models are often hindered by an insufficient number of earthquakes in the plate interior. We use ambient seismic noise to compute Rayleigh and Love surface-wave dispersion maps between 5 and 12 s for northern Harrat Rahat. The surface-wave maps are inverted to produce shear-wave velocities using a neighbourhood algorithm and interpolated into a pseudo-3-D model. The distributions of surface-wave and shear-wave velocities are heterogenous, varying between ±3 and 8 per cent. However, low velocities are not restricted to the Harrat. We observed a difference between Rayleigh- and Love-wave velocities that extends north from the site of the 1256 CE eruption and coincides with a low gravity anomaly. We obtain a shear-wave velocity increase of 10–15 per cent between 15 and 25 km depth consistent with the Conrad discontinuity, the interface between andesitic upper crust and the mafic lower crust of the Arabian Shield. The average velocities of the upper and lower crust are estimated to be 3.64 and 3.95 km s–1 using Rayleigh waves and 3.53 and 4.16 km s–1 using Love waves, which are in good agreement with the results of other geophysical studies of this area. The magnitude of the low-velocity anomalies, their location away from the Harrat, and the lack of reversals in the shear-velocity inversions suggest that the presence of a crustal magma chamber is not likely. If a magma chamber exists, it is smaller than can be imaged with a secondary microseism source (approximately 15 km wavelength), deeper than 30 km, or shallower than 5 km with a small velocity contrast.
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Yang, T., and Y. Shen. "Frequency-Dependent Crustal Correction for Finite-Frequency Seismic Tomography." Bulletin of the Seismological Society of America 96, no. 6 (December 1, 2006): 2441–48. http://dx.doi.org/10.1785/0120060038.
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Yang, Yu, Jianshe Lei, Yinshuang Ai, Guangwei Zhang, Changqing Sun, Enbo Fan, Long Li, et al. "Crustal structure beneath Northeast China from ambient noise tomography." Physics of the Earth and Planetary Interiors 293 (August 2019): 106257. http://dx.doi.org/10.1016/j.pepi.2019.04.008.
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Ojo, Adebayo Oluwaseun, Sidao Ni, and Zhiwei Li. "Crustal radial anisotropy beneath Cameroon from ambient noise tomography." Tectonophysics 696-697 (January 2017): 37–51. http://dx.doi.org/10.1016/j.tecto.2016.12.018.
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Saygin, Erdinc, and B. L. N. Kennett. "Crustal structure of Australia from ambient seismic noise tomography." Journal of Geophysical Research: Solid Earth 117, B1 (January 2012): n/a. http://dx.doi.org/10.1029/2011jb008403.
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Du, Nanqiao, Zhiwei Li, Tianyao Hao, Xin Xia, Yutao Shi, and Ya Xu. "Joint tomographic inversion of crustal structure beneath the eastern Tibetan Plateau with ambient noise and gravity data." Geophysical Journal International 227, no. 3 (July 31, 2021): 1961–79. http://dx.doi.org/10.1093/gji/ggab299.
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SUMMARY We have developed a joint tomographic inversion method with seismic surface wave dispersion and gravity data for obtaining more reliable crustal 3-D shear wave structures. We take the eikonal-based direct surface wave tomographic method and adaptive gravity modelling method in spherical coordinates in the inverse problem. Based on the empirical relations between seismic velocity and density parameters, our method combines surface wave dispersion curves (i.e. surface wave traveltimes at different periods) and Bouguer gravity anomaly data together to invert for 3-D shear wave velocity structures. In our method, off-great-circle propagation of the surface wave and the earth's curvature is considered in the forward modelling. Synthetic tests suggest that the joint tomographic method could improve the reliability and obtain more convincing results than individual seismic surface wave tomography. The gravity data can provide more constraints into the model resolution and help restore the crustal anomalies better. The inversion results in the eastern Tibetan Plateau and Sichuan basin indicate complex distributions of low-velocity zone in the mid-crust of the eastern Tibetan Plateau and a craton-like basement of the Sichuan basin, which supports the crust channel flow model. Although both the 3-D shear wave velocity model from joint inversion and the individual seismic surface wave inversion can fit the surface wave data almost equally well, the joint inversion model can better match the gravity data We also found that the 3-D model from joint inversion in this study shows similar structural characteristics with the surface wave tomographic model, which indicates the icing on the cake effects of gravity data.
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Li, Jun, Hui Li, Hui Chen, Jinrong Su, Yongsheng Liu, and Ping Tong. "Eikonal Equation-Based Seismic Tomography of the Source Areas of the 2008 Mw 7.9 Wenchuan Earthquake and the 2013 Mw 6.6 Lushan Earthquake." Bulletin of the Seismological Society of America 110, no. 2 (February 18, 2020): 886–97. http://dx.doi.org/10.1785/0120190134.
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ABSTRACT We use the eikonal equation-based seismic travel-time tomography method to image the source areas of the 2008 Wenchuan earthquake and the 2013 Lushan earthquake in the Longmenshan fault zone. High-resolution VP and VS models are obtained by inverting 75,686 P-wave and 74,552 S-wave travel times of local earthquakes during the period from 2009 to 2018. The tomographic models reveal strong crustal velocity heterogeneities in the study area. A significant velocity contrast exists across the Longmenshan fault zone: The western Songpan–Ganzi block is a high-velocity body, whereas the eastern Sichuan basin is a low-velocity anomaly. The hypocenter of the 2008 Wenchuan earthquake is between a high-velocity and a low-velocity anomaly. Beneath the Wenchuan mainshock, there is a significant low-velocity structure in the lower crust. The 2013 Lushan earthquake occurred in rocks associated with a high-velocity anomaly. A distinct low-velocity zone with low seismicity is imaged between the 2008 Wenchuan earthquake and the 2013 Lushan earthquake, where the crustal ductile deformation is likely to occur. The Baoxing complex to the northwest of the Lushan hypocenter exhibits as a high-velocity anomaly, which may be a carrier of stress accumulation and more prone to seismic activities in the future.
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Tu, Chenming, Qing Liang, Chunhui Tao, Zhikui Guo, Zhengwang Hu, and Chao Chen. "Gravity Data Reveal New Evidence of an Axial Magma Chamber Beneath Segment 27 in the Southwest Indian Ridge." Minerals 12, no. 10 (September 27, 2022): 1221. http://dx.doi.org/10.3390/min12101221.
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Hydrothermal systems are integral to mid-ocean ridge activity; they form massive seafloor sulfide (SMS) deposits rich in various metallic elements, which are potential mineral resources. Since 2007, many hydrothermal fields have been discovered along the ultraslow-spreading Southwest Indian Ridge (SWIR). The Duanqiao hydrothermal field is located at segment 27’s axis between the Indomed and Gallieni transform faults; tomography models reveal an obvious low-velocity anomaly beneath it, indicating a possible axial magma chamber (AMC). However, confirmation of an AMC’s existence requires further study and evidence. In this study, we first calculated the gravity effect to identify the heterogeneous distribution of crustal density beneath segment 27 and the surrounding area. Next, we used the gravity-inversion method to obtain the crustal density structure beneath the study area. The results indicate that a thickened crust and low-density crustal materials exist beneath segment 27. The low-density anomaly in the lower crust beneath the Duanqiao hydrothermal field suggests the existence of an AMC covered with a cold and dense upper crust. The density results identify several faults, which provide potential channels for magma migration. In addition, the melt migrates westward and redistributes laterally toward the segment’s western end. However, when migrating toward the segment’s eastern end, the melt is affected by a rapid cooling mechanism. Therefore, the segment’s ends present different density features and morphologies of nontransform discontinuities (NTDs).
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Pawlak, Agnieszka, David W. Eaton, Fiona Darbyshire, Sergei Lebedev, and Ian D. Bastow. "Crustal anisotropy beneath Hudson Bay from ambient noise tomography: Evidence for post-orogenic lower-crustal flow?" Journal of Geophysical Research: Solid Earth 117, B8 (August 2012): n/a. http://dx.doi.org/10.1029/2011jb009066.
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Medved, Irina, Gulten Polat, and Ivan Koulakov. "Crustal Structure of the Eastern Anatolia Region (Turkey) Based on Seismic Tomography." Geosciences 11, no. 2 (February 15, 2021): 91. http://dx.doi.org/10.3390/geosciences11020091.
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Here, we investigated the crustal structure beneath eastern Anatolia, an area of high seismicity and critical significance for earthquake hazards in Turkey. The study was based on the local tomography method using data from earthquakes that occurred in the study area provided by the Turkiye Cumhuriyeti Ministry of Interior Disaster and Emergency Management Directorate Earthquake Department Directorate of Turkey. The dataset used for tomography included the travel times of 54,713 P-waves and 38,863 S-waves from 6355 seismic events. The distributions of the resulting seismic velocities (Vp, Vs) down to a depth of 60 km demonstrate significant anomalies associated with the major geologic and tectonic features of the region. The Arabian plate was revealed as a high-velocity anomaly, and the low-velocity patterns north of the Bitlis suture are mostly associated with eastern Anatolia. The upper crust of eastern Anatolia was associated with a ~10 km thick high-velocity anomaly; the lower crust is revealed as a wedge-shaped low-velocity anomaly. This kind of seismic structure under eastern Anatolia corresponded to the hypothesized existence of a lithospheric window beneath this collision zone, through which hot material of the asthenosphere rises. Thus, the presented results help to clarify the deep structure under eastern Anatolia.
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Movaghari, R., and G. Javan Doloei. "3-D crustal structure of the Iran plateau using phase velocity ambient noise tomography." Geophysical Journal International 220, no. 3 (December 17, 2019): 1555–68. http://dx.doi.org/10.1093/gji/ggz537.
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SUMMARY More accurate crustal structure models will help us to better understand the tectonic convergence between Arabian and Eurasian plates in the Iran plateau. In this study, the crustal and uppermost mantle velocity structure of the Iran plateau is investigated using ambient noise tomography. Three years of continuous data are correlated to retrieve Rayleigh wave empirical Green's functions, and phase velocity dispersion curves are extracted using the spectral method. High-resolution Rayleigh wave phase velocity maps are presented at periods of 8–60 s. The tomographic maps show a clear consistency with geological structures such as sedimentary basins and seismotectonic zones, especially at short periods. A quasi-3-D shear wave velocity model is determined from the surface down to 100 km beneath the Iran plateau. A transect of the shear wave velocity model has been considered along with a profile extending across the southern Zagros, the Sanandaj-Sirjan Zone (SSZ), the Urumieh-Dokhtar Magmatic Arc (UDMA) and Central Iran and Kopeh-Dagh (KD). Obvious crustal thinning and thickening are observable along the transect of the shear wave velocity model beneath Central Iran and the SSZ, respectively. The observed shear wave velocities beneath the Iran plateau, specifically Central Iran, support the slab break-off idea in which low density asthenospheric materials drive towards the upper layers, replacing materials in the subcrustal lithosphere.
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Zhou, Hua‐wei. "Multiscale traveltime tomography." GEOPHYSICS 68, no. 5 (September 2003): 1639–49. http://dx.doi.org/10.1190/1.1620638.
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Nonuniqueness in traveltime tomographic velocity analysis is largely the result of insufficiency in model parameterization. Traditional tomography parameterizes the model with nonoverlapping cells. Such single‐scale tomography (SST) from inverting traveltimes only may produce underdeterminacy at places of insufficient ray coverage. To cope with the poor and uneven ray coverage, a multiscale tomography (MST) method is devised, which is a simultaneous application of many overlapping SSTs with different cell sizes. The MST decomposes velocity anomalies into components in a set of submodels of different cell sizes. At each model location the cells containing higher consistency between data contributions gain more from the inversion. The final MST model is a postinversion superposition of all submodel solutions. The MST method is applicable to the determination of interval velocity and interface geometry using turning rays or reflection rays. In comparison with the SST, superior results of the MST are seen in synthetic examples. The MST method is used to construct the 3D crustal velocities of southern California using the first arrivals from local earthquakes. While the SST and MST models may fit the data equally well, the MST model is smoother and geologically more plausible than the SST model.
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Sambolian, S., A. Gorszczyk, S. Operto, A. Ribodetti, and B. Tavakoli F. "Mitigating the ill-posedness of first-arrival traveltime tomography using slopes: application to the eastern Nankai Trough (Japan) OBS data set." Geophysical Journal International 227, no. 2 (July 7, 2021): 898–921. http://dx.doi.org/10.1093/gji/ggab262.
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SUMMARY First-arrival traveltime tomography is one of the most used velocity model building techniques especially in sparse wide-angle acquisitions for deep crustal seismic imaging cases. Relying on the inversion of a picked attribute, the absolute traveltimes, the approach is ill-posed in terms of non-uniqueness of the solution. The latter is remedied by proper regularization or the introduction of prior information. Indeed, since traveltime kernels are vulnerable to the velocity–depth ambiguity, the inversion is stabilized by the introduction of complementary data like reflections and explicit reflectors in the velocity models. Here, we propose to supplement first-arrival traveltimes by their slopes, in other words the horizontal component of the slowness vectors at the sources and/or receivers. Slopes are a crucial attribute in state of the art scattering-based or reflection-based tomographic methods like slope tomography or wavefront tomography where the differential information is needed in order to locate the scattering events position or to parametrize the wavefront. The optional but valuable injection of slopes as an objective measure in first-arrival traveltime tomography stabilizes the problem by constraining the emergence angle or in turn implicitly the turning point depth of the rays. We explain why slopes have a tremendous added value in such a tomographic problem and highlight its remedial effect in cases where the medium is unevenly illuminated. We also show that the contribution of slopes become even more significant when the acquisition is sparse as it is generally the case with ocean-bottom seismometer surveys. The inferred models from such an extended time-attributes tomography will be used as initial guesses in a full-waveform inversion workflow context. The proposed strategy is benchmarked in 2-D media against a dip section of the SEG/EAGE overthrust model and then followed by a revisit of ocean bottom seismometers data from the eastern-Nankai subduction margin as a real deep crustal case study.
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Dong, Xingpeng, Dinghui Yang, and Hejun Zhu. "Adjoint Tomography of the Lithospheric Structure beneath Northeastern Tibet." Seismological Research Letters 91, no. 6 (September 23, 2020): 3304–12. http://dx.doi.org/10.1785/0220200135.
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Abstract Northeastern Tibet is still in the primary stage of tectonic deformation and is the key area for studying the lateral expansion of the Tibetan plateau. In particular, the existence of lower crustal flow, southward subduction of the Asian lithosphere, and northward subduction of the Indian lithosphere beneath northeastern Tibet remains controversial. To provide insights into these issues, a high-resolution 3D radially anisotropic model of the lithospheric structure of northeastern Tibet is developed based on adjoint tomography. The Tibetan plateau is characterized as a low S-wave velocity lithosphere, in contrast with the relatively high S-wave velocities of the stable Asian blocks. Our tomographic result indicates that the low-velocity zone (LVZ) within the deep crust extends northeastward from Songpan–Ganzi to Qilian, which is interpreted as a channel flow within the crust. The upper mantle of Alxa and Qinling–Qilian are dominated by a rather homogeneous LVZ, which is inconsistent with the hypothesis that the Asian lithospheric mantle is being subducted southward beneath northeastern Tibet. Furthermore, high-velocity regions are observed in the southern Songpan–Ganzi region at depths ranging from 100 to 200 km, indicating that the northward-subducting Indian plate has probably reached the Xianshuihe fault.
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Guo, Gaoshan, Haiqiang Lan, Xiaole Zhou, Youshan Liu, Umair Bin Waheed, and Jingyi Chen. "Topography-dependent eikonal tomography based on the fast-sweeping scheme and the adjoint-state technique." GEOPHYSICS 87, no. 2 (December 27, 2021): U29—U41. http://dx.doi.org/10.1190/geo2021-0116.1.
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First-arrival traveltime tomography has been widely used for upper crustal velocity modeling, but it usually suffers from the problem of complex surface topography. To overcome this problem, we have developed a new topography-dependent eikonal tomography scheme that combines a developed accurate and efficient traveltime modeling method and introduces a flexible and robust adjoint inversion scheme in the presence of irregular topography. A surface-flattening scheme is used to handle the irregular surface, where the real model is discretized by curvilinear grids and the irregular free surface is mathematically flattened through the transformation from Cartesian to curvilinear coordinates. Based on this parameterization, the forward traveltime modeling is conducted by a monotone fast-sweeping method that discretizes the factored topography-dependent eikonal equation with a point-source condition. This algorithm can circumvent the source-singularity problem and decrease the numerical error in the vicinity of a point source in the curvilinear system. Then, the gradient-based inversion is used to minimize the misfit function, which is achieved by a matrix-free adjoint-state method without cumbersome ray tracing and explicit estimation of the Fréchet derivative matrix in the curvilinear coordinate system. The new tomographic scheme is evaluated through numerical examples with different seismic structures with complex topography, and then applied to a wide-angle profile acquired in the northeastern Tibetan Plateau. The results validate the effectiveness and efficiency of our tomography scheme in constructing shallow crustal velocity models with irregular topography.
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Barberi, G., M. T. Cosentino, A. Gervasi, I. Guerra, G. Neri, and B. Orecchio. "Crustal seismic tomography in the Calabrian Arc region, south Italy." Physics of the Earth and Planetary Interiors 147, no. 4 (December 2004): 297–314. http://dx.doi.org/10.1016/j.pepi.2004.04.005.
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Jaiswal, Namrata, Chandrani Singh, and Arun Singh. "Crustal structure of western Tibet revealed by Lg attenuation tomography." Tectonophysics 775 (January 2020): 228245. http://dx.doi.org/10.1016/j.tecto.2019.228245.
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Green, Robert G., Keith F. Priestley, and Robert S. White. "Ambient noise tomography reveals upper crustal structure of Icelandic rifts." Earth and Planetary Science Letters 466 (May 2017): 20–31. http://dx.doi.org/10.1016/j.epsl.2017.02.039.
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Nuñez, E., M. Schimmel, D. Stich, and A. Iglesias. "Crustal Velocity Anomalies in Costa Rica from Ambient Noise Tomography." Pure and Applied Geophysics 177, no. 2 (September 9, 2019): 941–60. http://dx.doi.org/10.1007/s00024-019-02315-z.
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Aben, Franciscus M., Nicolas Brantut, Thomas M. Mitchell, and Emmanuel C. David. "Rupture Energetics in Crustal Rock From Laboratory‐Scale Seismic Tomography." Geophysical Research Letters 46, no. 13 (July 8, 2019): 7337–44. http://dx.doi.org/10.1029/2019gl083040.
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Sun, Youshun, and M. Nafi Toksöz. "Crustal structure of China and surrounding regions fromPwave traveltime tomography." Journal of Geophysical Research: Solid Earth 111, B3 (March 2006): n/a. http://dx.doi.org/10.1029/2005jb003962.
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Mayor, Jessie, Marie Calvet, Ludovic Margerin, Olivier Vanderhaeghe, and Paola Traversa. "Crustal structure of the Alps as seen by attenuation tomography." Earth and Planetary Science Letters 439 (April 2016): 71–80. http://dx.doi.org/10.1016/j.epsl.2016.01.025.
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Tong, P., D. Zhao, D. Yang, X. Yang, J. Chen, and Q. Liu. "Wave-equation based traveltime seismic tomography – Part 2: Application to the 1992 Landers earthquake (<i>M</i><sub>w</sub> 7.3) area." Solid Earth Discussions 6, no. 2 (August 25, 2014): 2567–613. http://dx.doi.org/10.5194/sed-6-2567-2014.
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Abstract. High-resolution 3-D P and S wave crustal velocity and Poisson's ratio models of the 1992 Landers earthquake (Mw 7.3) area are determined iteratively by a wave-equation based traveltime seismic tomography (WETST) technique as developed in the first paper. The details of data selection, synthetic arrival-time determination, and trade-off analysis of damping and smoothing parameters are presented to show the performance of this new tomographic inversion method. A total of 78 523 P wave and 46 999 S wave high-quality arrival-time data from 2041 local earthquakes recorded by 275 stations during the period of 1992–2013 is used to obtain the final tomographic models which costs around 10 000 CPU h. Checkerboard resolution tests are conducted to verify the reliability of inversion results for the chosen seismic data and the wave-equation based traveltime seismic tomography method. Significant structural heterogeneities are revealed in the crust of the 1992 Lander earthquake area which may be closely related to the local seismic activities. Strong variations of velocity and Poisson's ratio exist in the source regions of the Landers and three other strong earthquakes in this area. Most seismicity occurs in areas with high-velocity and low Poisson's ratio, which may be associated with the seismogenic layer. Pronounced low-velocity anomalies revealed in the lower crust along the Elsinore, the San Jacinto and the San Andreas faults may reflect the existence of fluids in the lower crust. The recovery of these strong heterogeneous structures are facilitated by the use of full wave equation solvers and WETST and verifies their ability in generating high-resolution tomographic models.