An on-/offshore seismic network consisting of 36 three-component stand-alone digital stations was deployed in the area of the Saronikos Gulf, in the vicinity of Athens (Greece), in the fall of 2001. In the present study, from an initial set of more than 1000 micro-earthquakes, 374 were selected and 6666 P- and S-wave arrivals were inverted, based on a 3D linearized tomography algorithm, in order to determine the 3D velocity structure of the region.
The resulting 3D velocity distribution, in agreement to the micro-seismicity distribution, reflects the Saronikos structure down to a depth of 12 km. So, the neotectonic basin of the Saronikos Gulf is divided in two parts by a central platform, which implies the existence of a NNE–SSW-trending rupture zone. This zone is probably the offshore extension of a large thrust belt dominating the adjacent onshore areas. Due to their different structure, the two basins are dominated by different velocity values in comparison to the central platform.
The western part is characterised by higher seismic activity than the eastern one. Furthermore, the western Saronikos Gulf is divided in a northern and a southern part by a well-defined rupture zone trending E–W. This seems to be the extension of the Corinthiakos Gulf fault zone. At the depth of 17 km, the velocity increases considerably and the crustal thickness is restricted down to 20 km. This ‘unexpected’ low thickness in the region of Saronikos Gulf seems to be the result of the extensional stress field, which dominates the region, as well as of the emergence of the mantle material along the volcanic arc, which clearly appears at the depth of 12 km. Yet the lack of deep events and, hence, the poor resolution below the depth of 17 km does not support a definite conclusion about the crust–mantle boundary in this region. 相似文献
It is pointed out that observations of periodic nightglow structures give excellent information on atmospheric gravity waves in the mesosphere and lower thermosphere. The periods, the horizontal wavelengths and the phase speeds of the waves can be determined from airglow images and, using several cameras, the approximate altitude of the luminous layer can also be determined by triangulation. In this paper the possibility of applying tomographic methods for reconstructing the airglow structures is investigated using numerical simulations. A ground-based chain of cameras is assumed, two-dimensional airglow models in the vertical plane above the chain are constructed, and simulated data are calculated by integrating the models along a great number of rays with different elevation angles for each camera. After addition of random noise, these data are then inverted to obtain reconstructions of the models. A tomographic analysis package originally designed for satellite radiotomography is used in the inversion. The package is based on a formulation of stochastic inversion which allows the input of a priori information to the solver in terms of regularization variances. The reconstruction is carried out in two stages. In the first inversion, constant regularization variances are used within a wide altitude range. The results are used in determining the approximate altitude range of the airglow structures. Then, in the second inversion, constant non-zero regularization variances are used inside this region and zero variances outside it. With this method reliable reconstructions of the models are obtained. The number of cameras as well as their separations are varied in order to find out the limitations of the method. 相似文献
Tomographic reconstruction of the three-dimensional auroral are emission is used to obtain vertical and horizontal distributions of the optical auroral emission. Under the given experimental conditions with a very limited angular range and a small number of observers, algebraic reconstruction methods generally yield better results than transform techniques. Different algebraic reconstruction methods are tested with an auroral are model and the best results are obtained with an iterative least-square method adapted from emission-computed tomography. The observation geometry used during a campaign in Norway in 1995 is tested with the are model and root-mean-square errors, to be expected under the given geometrical conditions, are calculated. Although optimum geometry was not used, root-mean-square errors of less than 2% for the images and of the order of 30% for the distribution could be obtained. The method is applied to images from real observations. The correspondence of original pictures and projections of the reconstructed volume is discussed, and emission profiles along magnetic field lines through the three-dimensionally reconstructed arc are calibrated into electron density profiles with additional EISCAT measurements. Including a background profile and the temporal changes of the electron density due to recombination, good agreement can be obtained between measured profiles and the time-sequence of calculated profiles. These profiles are used to estimate the conductivity distribution in the vicinity of the EISCAT site. While the radar can only probe the ionosphere along the radar beam, the three-dimensional tomography enables conductivity estimates in a large area around the radar site.Former address: MPE Garching 相似文献
