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1.
The Nature of the 660-Kilometer Upper-Mantle Seismic Discontinuity from Precursors to the PP Phase 总被引：2，自引：0，他引：2
Global Seismic Network data were used to image upper-mantle seismic discontinuities. Stacks of phases that precede the PP phase, thought to be underside reflections from the upper-mantle discontinuities at depths of 410 and 660 kilometers, show that the reflection from 410 kilometers is present, but the reflection from 660 kilometers is not observed. A continuous Lame's constant lambda and seismic parameter at the 660-kilometer discontinuity explain the missing underside P reflections and lead to a P-wave velocity jump of only 2 percent, whereas the S-wave velocity and density remain unchanged with respect to previous global models. The model deemphasizes the role of Lame's constant lambda with regard to the shear modulus and constrains the mineralogical composition across the discontinuity. 相似文献
2.
Bright Spots, Structure, and Magmatism in Southern Tibet from INDEPTH Seismic Reflection Profiling 总被引：8，自引：0，他引：8
LD Brown W Zhao KD Nelson M Hauck D Alsdorf A Ross M Cogan M Clark X Liu J Che 《Science (New York, N.Y.)》1996,274(5293):1688-1690
INDEPTH seismic reflection profiling shows that the decollement beneath which Indian lithosphere underthrusts the Himalaya extends at least 225 kilometers north of the Himalayan deformation front to a depth of approximately 50 kilometers. Prominent reflections appear at depths of 15 to 18 kilometers near where the decollement reflector apparently terminates. These reflections extend north of the Zangbo suture to the Damxung graben of the Tibet Plateau. Some of these reflections have locally anomalous amplitudes (bright spots) and coincident negative polarities implying that they are produced by fluids in the crust. The presence of geothermal activity and high heat flow in the regions of these reflections and the tectonic setting suggest that the bright spots mark granitic magmas derived by partial melting of the tectonically thickened crust. 相似文献
3.
Profiles of repetitive seismnic reflections reveal that the Bering con tinental slope, outer shelf, and rise overlay an acoustically reflective "basement" which extends at least 750 kilometers parallel to the trend of the slope. This acoustic basement is usually covered by several hundred meters of stratified sediments at the top and bottom of the slope; however, it is exposed in sub marine canyons and flanking spurs along the main part of the slope for a distance of at least 550 kilometers northwest of the Pribilof Islands. The lithologic composition and the age of the rocks of the acoustic basement are not known. However, its probable seismic velocity of 3.1 to 3.7 kilometers per second suggests that it is composed of volcanic rocks or lithified sedimentary rocks or both. The regional geology suggests that the acoustic basement is the upper surface of folded late Mesozoic rocks which were locally intruded by granite and serpentine. The structure of the Bering slope, as deduced from the acoustic profiles, suggests that the surface of the basement has been monoclinically flexed and faulted between the shelf edge and the deep Aleutian Basin. 相似文献
4.
Subduction zones play critical roles in the recycling of oceanic lithosphere and the generation of continental crust. Seismic imaging can reveal structures associated with key dynamic processes occurring in the upper-mantle wedge above the sinking oceanic slab. Three-dimensional images of reflecting interfaces throughout the upper-mantle wedge above the subducting Tonga slab were obtained by migration of teleseismic recordings of underside P- and S-wave reflections. Laterally continuous weak reflectors with tens of kilometers of topography were detected at depths near 90, 125, 200, 250, 300, 330, 390, 410, and 450 kilometers. P- and S-wave impedances decreased at the 330-kilometer and 450-kilometer reflectors, and S-wave impedance decreased near 200 kilometers in the vicinity of the slab and near 390 kilometers, just above the global 410-kilometer increase. The pervasive seismic reflectivity results from phase transitions and compositional zonation associated with extensive metasomatism involving slab-derived fluids rising through the wedge. 相似文献
5.
Seismic-reflection signature of cretaceous continental breakup on the wilkes land margin, antarctica
The passive (rifted) continental margin of Wilkes Land, Antarctica, is characterized on seismic reflection records by (i) in the south, a block-faulted sequence of highly stratified continental beds overlain by two distinct unconformities; (ii) a transitional, greatly thinned continental crust overlain by material interpreted to be flood basalt; and (iii) in the north, oceanic crust with a boundary ridge at its edge. The Mohorovici? discontinuity can be followed across the continent-ocean boundary and shows a progressive thinning of continental crust to a minimum of 2.5 kilometers at its northern edge. 相似文献
6.
Lister CR 《Science (New York, N.Y.)》1971,173(3998):719-722
An acoustic reflection profile across the Chile Trench off Valparaiso shows the trench floor to have a substantial sediment accumulation that is far from flat-lying. The morphology is transitional between the flat-lying sediment fill to the south (labeled a type I trench) and the bare V-notch character farther north (labeled type II). A sharp seaward slope break, downbowed reflectors, and a landward downthrown normal fault suggest that the oceanic lithosphere is failing in shear. This mode of failutre is more consistent with the observed features of trenches and Benioff zones than is the concept of bending and underthrusting. 相似文献
7.
Velocity structure of a gas hydrate reflector 总被引：3，自引：0，他引：3
Seismic reflection profiles across many continental margins have imaged bottom-simulating reflectors (BSRs) parallel to the seabed; these are often interpreted as the base of a zone in which methane hydrate "ice" is stable. Waveform inversion of seismic reflection data can be used to estimate from seismic data worldwide the velocity structure of a BSR and its thickness. A test of this method at a drill site of the Ocean Drilling Program predicts that sediment pores beneath the BSR contain free methane for approximately 30 meters. The hydrate and underlying gas represent a large global reservoir of methane, which may have economic importance and may influence global climate. 相似文献
8.
Y Makovsky SL Klemperer L Ratschbacher LD Brown M Li W Zhao F Meng 《Science (New York, N.Y.)》1996,274(5293):1690-1691
Three-component wide-angle seismic data acquired in southern Tibet during Project INDEPTH show strong P-to-S converted reflections from reflectors that are aligned at a depth of approximately 15 kilometers beneath the northern Yadong-Gulu rift. These converted reflections are locally higher in amplitude than the corresponding P-wave reflections. Modeling of reflection mode conversion as a function of incidence angle indicates that this condition obtains for a reflector that is a solid over fluid interface; it is not typical of a solid-solid interface. The likely candidates for a fluid trapped within the crystalline crust of southern Tibet are granitic magma and water (brine). 相似文献
9.
A seismic reflection, gravity, and aeromagnetic reconnaissance was made in the Pensacola Mountains, Antarctica, during the 1965-66 austral summer. Prominent ice streams located between the Neptune and Patuxent Ranges and east of the Forrestal Range overlie channels in the rock surface 2000 meters below sea level which are probably of glacial origin. Seismic reflections show that the Filchner Ice Shelf is 1270 meters thick near its southern margin. Along the boundary between West and East Antarctica, Bouguer anomalies decrease from +60 milligals in West Antarctica to -80 milligals in East Antarctica. An abrupt change in crustal structure across this boundary is required to explainl the 2 milligals per kilometer gradient. This may indicate a fault extending through the crust into the mantle. Aeromagnetic profiles delineate anomalies up to 1800 gamma associated with the basic stratiform intrusion which comprises the Dufek and Forrestal ranges. A probable minimum area of 9500 square kilometers is calculated for the intrusive body on the basis of the magnetic anomalies, making it one of the largest bodies of its type. The extension of this magnetic anominaly across a fault forming the north border of the Pensacola Mountains probably precludes transcurrent movement. 相似文献
10.
Joos F Plattner GK Stocker TF Marchal O Schmittner A 《Science (New York, N.Y.)》1999,284(5413):464-467
A low-order physical-biogeochemical climate model was used to project atmospheric carbon dioxide and global warming for scenarios developed by the Intergovernmental Panel on Climate Change. The North Atlantic thermohaline circulation weakens in all global warming simulations and collapses at high levels of carbon dioxide. Projected changes in the marine carbon cycle have a modest impact on atmospheric carbon dioxide. Compared with the control, atmospheric carbon dioxide increased by 4 percent at year 2100 and 20 percent at year 2500. The reduction in ocean carbon uptake can be mainly explained by sea surface warming. The projected changes of the marine biological cycle compensate the reduction in downward mixing of anthropogenic carbon, except when the North Atlantic thermohaline circulation collapses. 相似文献
11.
Using an inverse mantle convection model that assimilates seismic structure and plate motions, we reconstruct Farallon plate subduction back to 100 million years ago. Models consistent with stratigraphy constrain the depth dependence of mantle viscosity and buoyancy, requiring that the Farallon slab was flat lying in the Late Cretaceous, consistent with geological reconstructions. The simulation predicts that an extensive zone of shallow-dipping subduction extended beyond the flat-lying slab farther east and north by up to 1000 kilometers. The limited region of flat subduction is consistent with the notion that subduction of an oceanic plateau caused the slab to flatten. The results imply that seismic images of the current mantle provide more constraints on past tectonic events than previously recognized. 相似文献
12.
From the spacing of internal wave packets generated by tidal flow over topography, one can determine their propagation speed. The propagation speed depends upon the density anomaly and depth of the upper mixed layer. Attributing the density anomaly to temperature only, one can calculate the heat storage in the upper oceanic layer. On the basis of Landsat images of the New England continental shelf, the heat storage calculated from satellite data has been compared with available in situ observations. The data show that the method may have merit and is deserving of further refinement. 相似文献
13.
Infrasound generated by the sonic boom from the inbound Concorde supersonic transport is recorded at Palisades, New York (Lamont-Doherty Geological Observatory), as a series of impulses from distances varying from 165 to about 1000 kilometers. Refraction effects determined by temperature and wind conditions return the signal to the surface from both stratospheric (40 to 50 kilometers) and thermospheric (100 to 130 kilometers) levels. The frequency of the recorded signal is a function of the level of reflection; the frequency decreases from impulse stretching as the atmosphere becomes more rarified relative to the sound pressure. The horizontal trace velocity of the signal across the array of instruments is equal to the acoustic velocity at the reflection level. The sonic boom can thus be used to provide temperature-wind parameters at reflection levels estimated from the signal frequency. Daily observed signal variations have indicated significant variations in these parameters. 相似文献
14.
The influence of vegetation-atmosphere-ocean interaction on climate during the mid-holocene 总被引：1，自引：0，他引：1
A Ganopolski C Kubatzki M Claussen V Brovkin V Petoukhov 《Science (New York, N.Y.)》1998,280(5371):1916-1919
Simulations with a synchronously coupled atmosphere-ocean-vegetation model show that changes in vegetation cover during the mid-Holocene, some 6000 years ago, modify and amplify the climate system response to an enhanced seasonal cycle of solar insolation in the Northern Hemisphere both directly (primarily through the changes in surface albedo) and indirectly (through changes in oceanic temperature, sea-ice cover, and oceanic circulation). The model results indicate strong synergistic effects of changes in vegetation cover, ocean temperature, and sea ice at boreal latitudes, but in the subtropics, the atmosphere-vegetation feedback is most important. Moreover, a reduction of the thermohaline circulation in the Atlantic Ocean leads to a warming of the Southern Hemisphere. 相似文献
15.
针对带浮筏隔振装置的机舱段,从声辐射的指向性、声辐射功率以及场点辐射声压、声强的角度出发,采用有限元和边界元相结合的方法,计算分析了船舶结构在水中航行时,由船舶辅机的振动引起的船舶壳体的辐射噪声特性,并考虑流体与船舶结构的流固耦合问题,计算结果表明,船舶结构的振动辐射噪声有着很强的指向性,频率越高,指向性越强,辐射声压随着传播距离的增加而迅速下降,下降的梯度与激振频率、场点的远近有关. 相似文献
16.
Restored pictures of Ganymede have been produced that have some identifiably reliable features and some identifiable artifacts. The latter arise from artifacts in parts of the red image data. Among the presumably reliable features are some mare-like objects (perhaps with some internal structure), and a few rather large, bright rings. Whether the latter are ice, or arise from near-specular reflection from smooth surface features, is left for future investigation. One of the restoring methods used, maximum entropy, has been shown to be applicable to moderately extended images. In view of its short time requirements (30 seconds per picture), the method should be applicable to moderately larger images, for example with twice the given number of data points. 相似文献
17.
Two-dimensional seismic prospecting methods are inadequate to solve the three-dimensional geological problems that arise in petroleum exploration. The three-dimensional seismic technique is a data collection and processing system that samples the subsurface adequately. The structure and lithology of rocks are mapped accurately by imaging their acoustic reflectivity with the scalar wave equation. In oil industry economics, the expense of three-dimensional seismic methods is justified by their usefulness for discovering additional reserves and for reducing the growing costs of developing oil and gas fields. 相似文献
18.
The dispersion of longitudinal acoustic phonons was measured by inelastic x-ray scattering in the hexagonal closed-packed (hcp) structure of iron from 19 to 110 gigapascals. Phonon dispersion curves were recorded on polycrystalline iron compressed in a diamond anvil cell, revealing an increase of the longitudinal wave velocity (VP) from 7000 to 8800 meters per second. We show that hcp iron follows a Birch law for VP, which is used to extrapolate velocities to inner core conditions. Extrapolated longitudinal acoustic wave velocities compared with seismic data suggest an inner core that is 4 to 5% lighter than hcp iron. 相似文献
19.
Kanasewich ER 《Science (New York, N.Y.)》1968,161(3845):1002-1005
Study of deep seismic reflections has detected a Precambrian rift valley below flat-lying sediments in southern Alberta. The anomalous magnetic and gravity trends show that the rift is continuous across Alberta and British Columbia (through the Kimberley lead-zinc field) and possibly the Coeur d'Alene mining district of Idaho. There is evidence that these ore bodies were deposited in a Precambrian rift under conditions similar to those prevailing in the hot-brine areas of the modern Red Sea. 相似文献
20.
Ocean-going ships carry, as ballast, seawater that is taken on in port and released at subsequent ports of call. Plankton samples from Japanese ballast water released in Oregon contained 367 taxa. Most taxa with a planktonic phase in their life cycle were found in ballast water, as were all major marine habitat and trophic groups. Transport of entire coastal planktonic assemblages across oceanic barriers to similar habitats renders bays, estuaries, and inland waters among the most threatened ecosystems in the world. Presence of taxonomically difficult or inconspicuous taxa in these samples suggests that ballast water invasions are already pervasive. 相似文献