排序方式： 共有43条查询结果，搜索用时 15 毫秒
31.
32.
Two new techniques in surface wave analysis, the modified moving window method and the method of stacking and cleaning, are used to determine average group and phase velocities over the area covering the Adriatic Sea and the Northern and Central Apennines, while avoiding the anomalous region under the Po plain.The data indicate that the lateral homogeneity of the region is sufficiently good to allow for an averaged model of the lithosphere. Backus-Gilbert inversion of the data leads to an averaged model with a thick sedimentary cover (10–11 km), a normal continental Moho depth of about 35 km, and a velocity rise with depth below the crust.The extremely low group velocity data that are encountered both in the Apennine region and in the Adriatic Sea suggest a similarity between the crustal structure of this area and the Eastern Mediterranean....the olive-sandalled Apennine In the South dimly islanded. (Shelley) 相似文献
33.
Simultaneous inversion of teleseismic data results in a model for the western branch of the East African Rift system between 1°S and 10°N characterized by a 35 km crust and thin high-velocity lid, overlying a channel possessing both low S and low P velocities (4.47 and 7.69 km/sec, respectively). A strong reflector at 140 km depth marks the boundary to high-velocity material (4.67 and 8.44 km/sec, respectively).The eastern branch has a crustal thickness of 40 km and is characterized by low S velocities, 4.43 km/sec in the lid to a depth of 78 km, 4.09–4.21 km/sec in a channel which extends to 161 km depth. The S velocities remain relatively low at greater depths, but cannot be precisely determined because of the limited resolution.We have interpreted these and other geophysical data as due to a diapiric intrusion of material with a high pyroxene content and possibly with some low-density eclogite, coming from the mesosphere. A mushroom of this rock has stalled below the western branch at about 55 km below the surface and is cooling. The diapirism in the eastern branch has broken through to the surface. The mantle of the African shield to the west of the Rift shows seismic velocities which indicate that it is depleted in basaltic components. 相似文献
34.
Rayleigh wave group velocity data from paths crossing the Levantine Sea are presented. We have derived a suite of models for the crustal structure of the Levantine Sea for extreme values of data errors and of the data corrections which were applied in order to account for lateral heterogeneity.We conclude that models with a crustal thickness less than 30 km are not consistent with the data. Our preferred models are characterized by a crustal thickness of 35–40 km. These results and the presence of an extremely thick sedimentary sequence point to a passive continental margin type of structure underlying the Levantine Sea. Additional data from the path Sicily-Jerusalem suggest that this type of structure is representative of the whole of the eastern Mediterranean (Levantine Sea and Ionian Sea). 相似文献
35.
36.
37.
38.
The seismic anomaly beneath Iceland extends down to the mantle transition zone and no deeper 总被引：8，自引：0，他引：8
G. R. Foulger M. J. Pritchard B. R. Julian J. R. Evans R. M. Allen G. Nolet W. J. Morgan B. H. Bergsson P. Erlendsson S. Jakobsdóttir S. Ragnarsson R. Stefansson K. Vogfjörd 《Geophysical Journal International》2000,142(3):F1-F5
A 3-D teleseismic tomography image of the upper mantle beneath Iceland of unprecedented resolution reveals a subvertical low wave speed anomaly that is cylindrical in the upper 250 km but tabular below this. Such a morphological transition is expected towards the bottom of a buoyant upwelling. Our observations thus suggest that magmatism at the Iceland hotspot is fed by flow rising from the mantle transition zone. This result contributes to the ongoing debate about whether the upper and lower mantles convect separately or as one. The image also suggests that material flows outwards from Iceland along the Reykjanes Ridge in the upper 200 km, but is blocked in the upper 150 km beneath the Tjornes Fracture Zone. This provides direct observational support for the theory that fracture zones dam lateral flow along ridges. 相似文献
39.
40.
Rayleigh wave group velocities provide a low-cost means for a quick assessment of averaged local properties of the Earth's crust in continental margin regions of the Atlantic type. Sufficiently accurate measurements (with a standard error of 0.3 km/s or less) of group velocities in continental shelf areas at periods between 5 and 30 seconds provide important information about structural parameters. They may resolve the Moho depth to within 4 or 5 km, depending on crustal thickness and give useful estimates of the average velocities in the upper part of the crust. The group velocities of Rayleigh waves in this period range are influenced most by the shear velocity at all depths and the compressional velocity and the density near the surface. For continental rise regions, the dominating influence of the water layer limits the effectiveness of the method. 相似文献