首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 468 毫秒
1.
We present a 3-D radially anisotropic S velocity model of the whole mantle (SAW642AN), obtained using a large three component surface and body waveform data set and an iterative inversion for structure and source parameters based on Non-linear Asymptotic Coupling Theory (NACT). The model is parametrized in level 4 spherical splines, which have a spacing of ∼ 8°. The model shows a link between mantle flow and anisotropy in a variety of depth ranges. In the uppermost mantle, we confirm observations of regions with   VSH > VSV   starting at ∼80 km under oceanic regions and ∼200 km under stable continental lithosphere, suggesting horizontal flow beneath the lithosphere. We also observe a   VSV > VSH   signature at ∼150–300 km depth beneath major ridge systems with amplitude correlated with spreading rate for fast-spreading segments. In the transition zone (400–700 km depth), regions of subducted slab material are associated with   VSV > VSH   , while the ridge signal decreases. While the mid-mantle has lower amplitude anisotropy (<1 per cent), we also confirm the observation of radially symmetric   VSH > VSV   in the lowermost 300 km, which appears to be a robust conclusion, despite an error in our previous paper which has been corrected here. The 3-D deviations from this signature are associated with the large-scale low-velocity superplumes under the central Pacific and Africa, suggesting that   VSH > VSV   is generated in the predominant horizontal flow of a mechanical boundary layer, with a change in signature related to transition to upwelling at the superplumes.  相似文献   

2.
Summary. Results from several recent studies suggest that there are lateral heterogeneities of up to a few per cent in the lowermost 150–200 km of the mantle (Bullen's D " region). Inferred anomaly sizes span the range from less than 50 km to greater than 1000 km.
In this study differences in the velocity structure among regions at the base of the mantle were inferred from an analysis of amplitude ratios of PKPAB and PKPDF for given earthquake-station pairs at distances greater than 155° (Sacks, Snoke & Beach). We distinguish two kinds of regions: A (anomalous) regions in which the mean, median and spread in AB/DF amplitude ratios are significantly higher (> 50 per cent) than for a reference radial earth model and N (normal) regions in which the distribution of the amplitude ratios is as expected.
The AB branch has near-grazing incidence to the core and therefore maximum sensitivity to velocity structure compared to the near-normal incident DF phases. Using an iterative, forward-modelling approach, we have determined general characteristics of the velocity structure for regions at the base of the mantle which can produce amplitude-ratio distributions similar to those for an A region. Agreement between model and data is obtained over the period range from 0.5 s to greater than 10 s using a laterally heterogeneous model for the D " region. the model consists of cells which are 200 km in lateral extent with velocity variations of up to ±1 per cent. This structure is modulated by a region-wide (1000km) perturbation which increases smoothly from zero at the edges of the region to a negative 1 per cent at the centre. Small cells (∼40 km) cannot produce anomalously large amplitude, long-period AB arrivals, and larger cells (∼1000km) cannot match the observed scatter. the ∼200 km scale anomalies could be small-scale convection cells confined to the D " region.  相似文献   

3.
Upper mantle shear structure of North America   总被引：5，自引：0，他引：5  
Summary. The waveforms and travel times of S and SS phases in the range 10°–60° have been used to derive upper mantle shear velocity structures for two distinct tectonic provinces in North America. Data from earthquakes on the East Pacific Rise recorded at stations in western North America were used to derive a tectonic upper mantle model. Events on the north-west coast of North America and earthquakes off the coast of Greenland provided the data to investigate the upper mantle under the Canadian shield. All branches from the triplications due to velocity jumps near 400 and 660 km were observed in both areas. Using synthetic seismograms to model these observations placed tight constraints on heterogeneity in the upper mantle and on the details of its structure. SS–S travel-time differences of 30 s along with consistent differences in waveforms between the two data sets require substantial heterogeneity to at least 350 km depth. Velocities in the upper 170 km of the shield are about 10 per cent higher than in the tectonic area. At 250 km depth the shield velocities are still greater by about 4.5 per cent and they gradually merge near 400 km. Below 400 km no evidence for heterogeneity was found. The two models both have first-order discontinuities of 4.5 per cent at 405 km and 7.5 per cent at 695 km. Both models also have lids with lower velocities beneath. In the western model the lid is very thin and of relatively low velocity. In the shield the lid is 170 km thick with very high elocity (4.78 km s-1); below it the velocity decreases to about 4.65 km s-1. Aside from these features the models are relatively smooth, the major difference between them being a larger gradient in the tectonic region from 200 to 400 km.  相似文献   

4.
Teleseismic P waves passing through low-wave-speed bodies in the mantle are refracted, causing anomalies in their propagation directions that can be measured by seismometer arrays. Waves from earthquakes in the eastern Pacific and western North America arriving at the NORSAR array in Norway and at seismic stations in Scotland pass beneath the Iceland region at depths of ∼ 1000–2000 km. Waves arriving at NORSAR have anomalous arrival azimuths consistent with a low-wave-speed body at a depth of ∼ 1500 km beneath the Iceland–Faeroe ridge with a maximum diameter of ∼250 km and a maximum wave-speed contrast of ∼ 1.5 per cent. This agrees well with whole-mantle tomography results, which image a low-wave-speed body at this location with a diameter of ∼ 500 km and a wave-speed anomaly of ∼ 0.5 per cent, bearing in mind that whole-mantle tomography, because of its limited resolution, broadens and weakens small anomalies. The observations cannot resolve the location of the body, and the anomaly could be caused in whole or in part by larger bodies farther away, for example by a body imaged beneath Greenland by whole-mantle tomography.  相似文献   

5.
Summary. An inversion of ISC travel-time data from selected earthquakes in the distance range 30°-90° to 53 stations in Central Europe has been used to model velocity down to 600 km depth. The model explains 0.1–0.2s of the residuals, as for other array studies, leaving 0.5 s unexplained as noise. The uppermost 100 km of the mantle and crust contains inhomogeneities that correlate remarkably well with the geology. This may be due to deep-seated thermal anomalies or, in some areas, to delays introduced by passage of the rays through sedimentary cover. The deeper anomalies are smaller and unrelated to those in the lithosphere, which suggests that the asthenosphere is decoupled from the rigid lithosphere. The structure at 600 km depth is again quite inhomogeneous and might be due to undulations of the 650 km discontinuity. The models show some suggestion of a high velocity slab trending from east to west beneath the Alps.  相似文献   

6.
We have used the S wave receiver function (SRF) technique to investigate the crustal thickness beneath two seismic profiles from the CHARGE project in the southern central Andes. A previous study employing the P wave receiver function method has observed the Moho interface beneath much of the profiles. They found, however, that the amplitude of the P to S conversion was diminished in the western part of the profiles and have attributed it to a reduction of the impedance contrast at the Moho due to lower crustal ecologitization. With SRF, we have successfully detected S to P converted waves from the Moho as well as possible conversions from other lithospheric boundaries. The continental South American crust reaches its maximum thickness of ∼70 km (along 30°S between 70°W and 68.5°W) beneath the Principal Cordillera and the Famatina system and becomes thinner towards the Sierras Pampeanas with a thickness of ∼40 km. Negative phases, possibly related to the base of the continental and oceanic lithosphere, can be recognized in the summation traces at different depths. By comparing our results with data obtained from previous investigations, we are able to further constrain the thickness of the crust and lithosphere beneath the central Andes.  相似文献   

7.
The earthquakes in the seismicity belt extending through Indonesia, New Guinea, Vanuatu and Fiji to the Tonga–Kermadec subduction zone recorded at the 65 portable broad-band stations deployed during the Skippy experiment from 1993–1996 provide good coverage of the lithosphere and mantle under the Australian continent, Coral Sea and Tasman Sea.
The variation in structure in the upper part of the mantle is characterized by deter-mining a suite of 1-D structures from stacked record sections utilizing clear P and S arrivals, prepared for all propagation paths lying within a 10° azimuth band. The azimuth of these bands is rotated by 20° steps with four parallel corridors for each azimuth. This gives 26 separate azimuthal corridors for which 15 independent 1-D seismic velocity structures have been derived, which show significant variation in P and S structure.
The set of 1-D structures is combined to produce a 3-D representation by projecting the velocity values along the ray path using a turning point approximation and stacking into 3-D cells (5° by 50 km in depth). Even though this procedure will tend to underestimate wave-speed perturbations, S -velocity deviations from the ak135 reference model exceed 6 per cent in the lithosphere.
In the uppermost mantle the results display complex features and very high S -wave speeds beneath the Precambrian shields with a significant low-velocity zone beneath. High velocities are also found towards the base of the transition zone, with high S -wave speeds beneath the continent and high P -wave speeds beneath the ocean. The wave-speed patterns agree well with independent surface wave studies and delay time tomography studies in the zones of common coverage.  相似文献   

8.
Summary. Travel times and waveforms of long-period SH -waves recorded at distances of 10–30° and some SS waveforms are used to constrain the upper mantle velocities down to a depth of 400km beneath both the Indian Shield and the Tibetan Plateau. the shear velocity in the uppermost mantle beneath both the Indian Shield and the Tibetan Plateau is high and close to 4.7 km s−1. the Indian Shield has a fairly thick high velocity lid, and the mean velocity between 40 and 250 km is between 4.58 and 4.68 km s−1. In contrast, S -wave travel times and waveforms of S -waves, as well as a few for SS , show that the mean velocity between 70 and 250km beneath the central and northern part of the Tibetan Plateau is slower by 4 per cent or more than that beneath the Indian Shield and probably is between 4.4 and 4.5km s−1. No large differences in the structure of the two areas below 250 km are required to explain both the arrival times and the waveforms of SH phases crossing Tibet or the Indian Shield. These results show that the structure of Tibet is not that of a shield and imply that the Indian plate is not underthrusting the whole of the Tibetan Plateau at the present time.  相似文献   

9.
The Canary Islands swell: a coherence analysis of bathymetry and gravity   总被引：2，自引：0，他引：2  
The Canary Archipelago is an intraplate volcanic chain, located near the West African continental margin, emplaced on old oceanic lithosphere of Jurassic age, with an extended volcanic activity since Middle Miocene. The adjacent seafloor does not show the broad oceanic swell usually observed in hotspot-generated oceanic islands. However, the observation of a noticeable depth anomaly in the basement west of the Canaries might indicate that the swell is masked by a thick sedimentary cover and the influence of the Canarian volcanism. We use a spectral approach, based on coherence analysis, to determine the swell and its compensation mechanism. The coherence between gravity and topography indicates that the swell is caused by a subsurface load correlated with the surface volcanic load. The residual gravity/geoid anomaly indicates that the subsurface load extends 600 km SSW and 800 km N and NNE of the islands. We used computed depth anomalies from available deep seismic profiles to constrain the extent and amplitude of the basement uplift caused by a relatively low-density anomaly within the lithospheric mantle, and coherence analysis to constrain the elastic thickness of the lithosphere ( Te ) and the compensation depth of the swell. Depth anomalies and coherence are well simulated with Te =28–36 km, compensation depth of 40–65 km, and a negative density contrast within the lithosphere of ∼33 kg m−3. The density contrast corresponds to a temperature increment of ∼325°C, which we interpret to be partially maintained by a low-viscosity convective layer in the lowermost lithosphere, and which probably involves the shallower parts of the asthenosphere. This interpretation does not require a significant rejuvenation of the mechanical properties of the lithosphere.  相似文献   

10.
P and S receiver functions obtained from a portable array of 34 broad-band stations in east central China provide a detailed image of the crust–mantle and lithosphere–asthenosphere boundaries (LAB) in the Dabie Shan and its adjacent areas. Clear S -to- P converted waves produced at the LAB show a thin lithosphere beneath the whole study area. Based on our results, the thickest lithosphere of 72 km is observed beneath the southern part of the area within the Yangtze craton, whereas beneath the North-China platform, the lithosphere is only 60 km thick. S receiver functions also reveal, in good agreement with P receiver functions, a maximum depth of the Moho beneath the Dabie Shan orogen at approximately 40 km. Furthermore, we interpret the structural difference at 32° latitude as the probable location of the mantle suture formed between the Yangtze and the Sino-Korean cratons.  相似文献   

11.
As a baseline measurement for understanding the Himalayan–Tibetan orogen, a product of continent–continent collision between India and Eurasia, we analyse digital seismic data in order to constrain the seismic anisotropy of the Indian shield. Based on spatially sparse data that are currently available in the public domain, there is little shear-wave birefringence for SKS phases under the Indian shield, even though it is part of a fast-moving plate in the hotspot frame of reference. If most of the northern Indian mantle has little transverse anisotropy, the onset of significant anisotropy under Tibet marks the northern terminus of intact Indian lithosphere that is thrusting under the Himalayan–Tibetan orogen. Beyond this terminus, tectonic fabric such as that associated with the deforming lithospheric mantle of Eurasia must be present in the upper mantle. Along the profile from Yadong to Golmud, the only profile in Tibet where a number of shear-wave birefringence data are available, the amount of birefringence shows two marked increases, near 30° and 33°N, between which a local high in Bouguer gravity anomaly is observed. Such a correlation between patterns of shear-wave birefringence and gravity anomalies is explained by the juxtaposition of Indian lithosphere against the overlying Eurasian lithosphere: while the Eurasian lithospheric mantle appears only to the north of 30°N, the Indian lithospheric mantle extends northwards to near 33°N.  相似文献   

12.
Broad-band data from South American earthquakes recorded by Californian seismic networks are analysed using a newly developed seismic wave migration method—the slowness backazimuth weighted migration (SBWM). Using the SBWM, out-of-plane seismic P -wave reflections have been observed. The reflection locations extend throughout the Earth's lower mantle, down to the core–mantle boundary (CMB) and coincide with the edges of tomographically mapped high seismic velocities. Modelling using synthetic seismograms suggests that a narrow (10–15 km) low- or high-velocity lamella with about 2 per cent velocity contrast can reproduce the observed reflected waveforms, but other explanations may exist. Considering the reflection locations and synthetic modelling, the observed out-of-plane energy is well explained by underside reflections off a sharp reflector at the base of the subducted lithosphere. We also detect weaker reflections corresponding to the tomographically mapped top of the slab, which may arise from the boundary between the Nazca plate and the overlying former basaltic oceanic crust. The joint interpretation of the waveform modelling and geodynamic considerations indicate mass flux of the former oceanic lithosphere and basaltic crust across the 660 km discontinuity, linking processes and structure at the top and bottom of the Earth's mantle, supporting the idea of whole mantle convection.  相似文献   

13.
A 3-D P -velocity map of the crust and upper mantle beneath the southeastern part of India has been reconstructed through the inversion of teleseismic traveltimes. Salient geological features in the study region include the Archean Dharwar Craton and Eastern Ghat metamorphic belt (EGMB), and the Proterozoic Cuddapah and Godavari basins. The Krishna–Godavari basin, on the eastern coastal margin, evolved in response to the Indo–Antarctica breakup. A 24-station temporary network provided 1161 traveltimes, which were used to model 3-D P -velocity variation. The velocity model accounts of 80 per cent of the observed data variance. The velocity picture to a depth of 120 km shows two patterns: a high velocity beneath the interior domain (Dharwar craton and Cuddapah basin), and a lower velocity beneath the eastern margin region (EGMB and coastal basin). Across the array velocity variations of 7–10 per cent in the crust (0–40 km) and 3–5 per cent in the uppermost mantle (40–120 km) are observed. At deeper levels (120–210 km) the upper-mantle velocity differences are insignificant among different geological units. The presence of such a low velocity along the eastern margin suggests significantly thin lithosphere (<100 km) beneath it compared to a thick lithosphere (>200 km) beneath the eastern Dharwar craton. Such lithospheric thinning could be a consequence of Indo–Antarctica break-up.  相似文献   

14.
A whole mantle SH velocity model is obtained by using a unique data set and techniques. Body and surface waveforms including major and multi-orbit phases are used as a data set and are inverted by using 3-D Born kernels. The resultant model, SH18CE, reveals the different natures of the two major upwelling systems: the strong low velocity anomalies beneath Africa extend for more than 1000 km from the core–mantle boundary (CMB), whereas those beneath the Pacific are restricted to 300–400 km from the CMB. The results also show the variable natures of stagnant slabs on the 670 discontinuity around Japan: the depths of the strongest high velocity anomalies within the stagnant slabs are different region by region, which is consistent with the detailed delay time tomography model in this area.  相似文献   

15.
S receiver functions from 67 broad-band seismic stations in the western United States clearly reveal the existence of a mantle discontinuity with velocity reduction downward, which we interpret as the lithosphere–asthenosphere boundary (LAB). The average depth of the LAB is ∼70 km. The boundary is relatively sharp with an overall sharpness of less than 20 km. The boundary is more prominent south of the Mendocino Triple Junction, where the Farallon Plate has completely subducted. This may indicate partial melts at the base of the lithosphere caused by the upwelling of the asthenospheric flow through the slab window. A double low velocity zone is observed at base of the lithosphere beneath southern Sierra Nevada, implying a second melting zone at a depth of ∼100 km, well correlated with previous studies of lithospheric delamination in the area.  相似文献   

16.
We present a new technique for the efficient measurement of the traveltimes of long period body wave phases. The technique is based on the fact that all arrivals of a particular seismic phase are remarkably similar in shape for a single event. This allows the application of cross-correlation techniques that are usually used in a regional context to measure precise global differential times. The analysis is enhanced by the inclusion of a clustering algorithm that automatically clusters waveforms by their degree of similarity. This allows the algorithm to discriminate against unusual or distorted waveforms and makes for an extremely efficient measurement technique.
This technique can be applied to any seismic phase that is observed over a reasonably large distance range. Here, we present the results of applying the algorithm to the long-period channels of all data archived at the IRIS DMC from 1976 to 2005 for the seismic phases S and P (from 23° to 100°) and SS and PP (from 50° to 170°). The resulting large data sets are inverted along with existing surface wave and updated differential traveltime measurements for new mantle models of S and P velocity. The resolution of the new model is enhanced, particularly, in the mid-mantle where SS and PP turn. We find that slow anomalies in the central Pacific and Africa extend from the core–mantle boundary to the upper mantle, but their direct connection to surface hotspots is beyond our resolution. Furthermore, we find that fast anomalies that are likely associated with subducting slabs disappear between 1700 and 2500 km, and thus are not continuous features from the upper to lower mantle despite our extensive coverage and high resolution of the mid-mantle.  相似文献   

17.
Summary. Two localized regions of velocity heterogeneity in the lower mantle with scale lengths of 1000–2000 km and 2 per cent velocity contrasts are detected and isolated through comparison of S, ScS, P and PcP travel times and amplitudes from deep earthquakes in Peru, Bolivia, Argentina and the Sea of Okhotsk. Comparison of the relative patterns of ScS-S differential travel times and S travel-time residuals across North American WWSSN and CSN stations for the different source regions provides baselines for interpreting which phases have anomalous times. A region of low S and P velocities is located beneath Northern Brazil and Venezuela at depths of 1700–2700 km. This region produces S -wave delays of up to 4 s for signals from deep Argentine events recorded at eastern North American stations. The localized nature of the anomaly is indicated by the narrow bounds in azimuth (15°) and take-off angle (13°) of the arrivals affected by it. The long period S -waves encountering this anomaly generally show 30–100 per cent amplitude enhancement, while the short-period amplitudes show no obvious effect. The second anomaly is a high-velocity region beneath the Caribbean originally detected by Jordan and Lynn, who used travel times from deep Peruvian events. The data from Argentine and Bolivian events presented here constrain the location of the anomaly quite well, and indicate a possible short- and long-period S -wave amplitude diminution associated with it. When the travel-time data are corrected for the estimated effects of these two anomalies, a systematic regional variation in ScS-S station residuals is apparent between stations east of and west of the Rocky Mountains. One possible explanation of this is a long wavelength lateral variation in the shear velocity structure of the lower mantle at depths greater than 2000 km beneath North America.  相似文献   

18.
Shear-wave splitting from local deep earthquakes is investigated to clarify the volume and the location of two anisotropic bodies in the mantle wedge beneath central Honshu, Japan. We observe a spatial variation in splitting parameters depending on the combination of sources and receivers, nearly N–S fast in the northern region, nearly E–W fast in the southern region and small time delays in the eastern region. Using forward modelling, two models with 30 and 10 per cent anisotropy are tested by means of a global search for the locations of anisotropic bodies with various volumes. The optimum model is obtained for 30 per cent anisotropy, which means a 5 per cent velocity difference between fast and slow polarized waves. The northern anisotropic body has a volume of 1.00° (longitude) × 0.5° (latitude) × 75 km (depth), with the orientation of the symmetry axis being N20°E. The southern anisotropic body has a volume of 1.25° × 1.25° × 100 km with the symmetry axis along N95°E. Our results show that the anisotropic bodies are located in low-velocity and low- Q regions of the mantle. This, together with petrological data and the location of volcanoes in the arc, suggests that the possible cause of the anisotropy is the preferred alignment of cracks filled with melt.  相似文献   

19.
Summary. A series of long-range explosion seismological experiments has been conducted by the use of specially designed ocean bottom seismographs (OBSs) in the Western Pacific. OBS studies of apparent velocity measurements by the use of natural earthquakes have also been made. The experiments have made clear that large-scale P -wave anisotropy exists in the entire thickness of the oceanic lithosphere. The existence of the large-scale anisotropy in the oceanic lithosphere has been demonstrated for the first time by seismic body-wave studies. Previously, anisotropy had been found only in the uppermost oceanic mantle in the Eastern Pacific.
The azimuth of the maximum velocity, 8.6 km s-1, is about 155° clock-wise from north. The direction is perpendicular to the magnetic lineation of the region, however, the direction differs from the direction of the present plate motion by about 30°. So it appears that the anisotropy has been 'frozen' at least since the change of the plate motion that occurred 40 Myr ago. The frozen anisotropy should set important constraints on the mechanical properties of the lithosphere such as the viscosity and temperature of the lower lithosphere.  相似文献   

20.
Polarization anomaly of Love waves caused by lateral heterogeneity   总被引：1，自引：0，他引：1  
We calculate surface waves propagating in a laterally heterogeneous structure beneath the Kuril trench, where significant Love-wave polarization anomalies, called quasi-Love waves, are generated. Since 3-D wave propagation in the two-dimensionally heterogeneous structure can be assumed, we apply the 2.5-D finite difference method to the surface-wave calculations. The calculations show that a velocity contrast of 7 per cent at depths of less than 210 km beneath the Kuril trench cannot generate quasi-Love waves, and that an unlikely contrast of 20 per cent is required to generate clear quasi-Love waves. The possible cause of the quasi-Love waves inferred from previous studies on coupled free oscillations is a lateral variation in azimuthal anisotropy. The lateral variation in azimuthal anisotropy beneath the Kuril trench suggests a change in the mantle flow induced by the subducting slab.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号