Geophysical methods applied to fault characterization and earthquake potential assessment in the Lower Tagus Valley, Portugal     

Joo Carvalho  Joo Cabral  Rui Gonalves  Luís Torres  Luís Mendes-Victor 《Tectonophysics》2006,418(3-4):277-297

The study region is located in the Lower Tagus Valley, central Portugal, and includes a large portion of the densely populated area of Lisbon. It is characterized by a moderate seismicity with a diffuse pattern, with historical earthquakes causing many casualties, serious damage and economic losses. Occurrence of earthquakes in the area indicates the presence of seismogenic structures at depth that are deficiently known due to a thick Cenozoic sedimentary cover. The hidden character of many of the faults in the Lower Tagus Valley requires the use of indirect methodologies for their study. This paper focuses on the application of high-resolution seismic reflection method for the detection of near-surface faulting on two major tectonic structures that are hidden under the recent alluvial cover of the Tagus Valley, and that have been recognized on deep oil-industry seismic reflection profiles and/or inferred from the surface geology. These are a WNW–ESE-trending fault zone located within the Lower Tagus Cenozoic basin, across the Tagus River estuary (Porto Alto fault), and a NNE–SSW-trending reverse fault zone that borders the Cenozoic Basin at the W (Vila Franca de Xira–Lisbon fault). Vertical electrical soundings were also acquired over the seismic profiles and the refraction interpretation of the reflection data was carried out. According to the interpretation of the collected data, a complex fault pattern disrupts the near surface (first 400 m) at Porto Alto, affecting the Upper Neogene and (at least for one fault) the Quaternary, with a normal offset component. The consistency with the previous oil-industry profiles interpretation supports the location and geometry of this fault zone. Concerning the second structure, two major faults were detected north of Vila Franca de Xira, supporting the extension of the Vila Franca de Xira–Lisbon fault zone northwards. One of these faults presents a reverse geometry apparently displacing Holocene alluvium. Vertical offsets of the Holocene sediments detected in the studied geophysical data of Porto Alto and Vila Franca de Xira–Lisbon faults imply minimum slip rates of 0.15–0.30 mm/year, three times larger than previously inferred for active faults in the Lower Tagus Valley and maximum estimates of average return periods of 2000–5000 years for M 6.5–7 co-seismic ruptures.  相似文献   

Discrete numerical modelling of rockfill dams   总被引：1，自引：0，他引：1  

R. Deluzarche  B. Cambou 《国际地质力学数值与分析法杂志》2006,30(11):1075-1096

The aim of this study is to obtain quantitative information on the behaviour of rockfill used in embankment dams, and particularly on the influence of block breakage on the displacement field, from a numerical analysis using the Distinct element method. A methodology is set up to define the resistance of the 2D particles so that the same probability of breaking blocks may be reproduced as in a 3D material. The model uses the discrete element code PFC^2D (Itasca Consulting Group Inc., PFC2D (Particle Flow Code in Two Dimensions), Version 3.0, 2002) and considers breakable clusters of 2D balls. The different parameters are determined from experimental data obtained from laboratory tests performed on rock blocks. The model is validated by comparing the results of the simulation of shearing tests with actual triaxial tests on rockfill material published in the literature. The numerical analysis of block crushing in an actual dam is proposed in the last part of this paper. Copyright © 2006 John Wiley & Sons, Ltd.  相似文献   

Use of electrical resistivity methods for detecting subsurface fresh and saline water and delineating their interfacial configuration: a case study of the eastern Dead Sea coastal aquifers, Jordan     

Awni T. Batayneh 《Hydrogeology Journal》2006,14(7):1277-1283

The alluvial aquifer is the primary source of groundwater along the eastern Dead Sea shoreline, Jordan. Over the last 20 years, salinity has risen in some existing wells and several new wells have encountered brackish water in areas thought to contain fresh water. A good linear correlation exists between the water resistivity and the chloride concentration of groundwater and shows that the salinity is the most important factor controlling resistivity. Two-dimensional electrical tomography (ET) integrated with geoelectrical soundings were employed to delineate different water-bearing formations and the configuration of the interface between them. The present hydrological system and the related brines and interfaces are controlled by the Dead Sea base level, presently at 410 m b.s.l. Resistivity measurements show a dominant trend of decreasing resistivity (thus increasing salinity) with depth and westward towards the Dead Sea. Accordingly, three zones with different resistivity values were detected, corresponding to three different water-bearing formations: (1) strata saturated with fresh to slightly brackish groundwater; (2) a transition zone of brine mixed with fresh to brackish groundwater; (3) a water-bearing formation containing Dead Sea brine. In addition, a low resistivity unit containing brine was detected above the 1955 Dead Sea base level, which was interpreted as having remained unflushed by infiltrating rain.  相似文献   

Time domain Gauss–Newton seismic waveform inversion in elastic media     

Dong-Hoon Sheen  Kagan Tuncay  Chang-Eob Baag  Peter J. Ortoleva 《Geophysical Journal International》2006,167(3):1373-1384

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The place of individuals in the politics of scale   总被引：1，自引：0，他引：1  

Julie Cidell 《Area》2006,38(2):196-203

Conceptualizations and re-conceptualizations of scale have been powerful tools for explaining spatial processes that transcend traditionally-bounded territories. Examining the role of individuals within a politics of scale can do even more to explain multi-scalar conflicts. This paper does so by examining struggles over airport expansion in the US, showing how an understanding of the various roles of individuals in the politics of scale – as sites of multiple scales, as actors constituting other scales, and as scales in and of themselves – better explains multi-scalar conflicts and offers more opportunities for resolving them.  相似文献   

Branding the design metropole: the case of Montréal, Canada     

Norma M Rantisi  Deborah Leslie 《Area》2006,38(4):364-376

As part of a broader process of inter-urban competition, city governments have increasingly sought to 'position' themselves as centres of creativity. In these branding initiatives, culture is viewed as a tool of urban regeneration and economic development. Our paper examines the case of Commerce Design Montréal, an annual design competition run by the City of Montréal, which aims to brand Montréal as a centre of design. Commerce Design Montréal is an example of a 'fast' policy initiated by the state, but carried out by business owners and citizens. As such, it represents a downloading of the responsibility for economic development to the private or individual scale and adopts only a partial view of the varied actors and uses that are implicated in the design process. The paper considers the opportunities and challenges that this model presents for promoting design as a form of urban regeneration in a neo-liberal context.  相似文献   

Dual simulations of fluid flow and seismic wave propagation in a fractured network: effects of pore pressure on seismic signature   总被引：1，自引：0，他引：1  

S. Vlastos  E. Liu  I. G. Main  M. Schoenberg  C. Narteau  X. Y. Li  B. Maillot 《Geophysical Journal International》2006,166(2):825-838

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Comparison of methods to model the gravitational gradients from topographic data bases   总被引：7，自引：0，他引：7  

Christopher Jekeli  Lizhi Zhu 《Geophysical Journal International》2006,166(3):999-1014

A number of methods have been developed over the last few decades to model the gravitational gradients using digital elevation data. All methods are based on second-order derivatives of the Newtonian mass integral for the gravitational potential. Foremost are algorithms that divide the topographic masses into prisms or more general polyhedra and sum the corresponding gradient contributions. Other methods are designed for computational speed and make use of the fast Fourier transform (FFT), require a regular rectangular grid of data, and yield gradients on the entire grid, but only at constant altitude. We add to these the ordinary numerical integration (in horizontal coordinates) of the gradient integrals. In total we compare two prism, two FFT and two ordinary numerical integration methods using 1" elevation data in two topographic regimes (rough and moderate terrain). Prism methods depend on the type of finite elements that are generated with the elevation data; in particular, alternative triangulations can yield significant differences in the gradients (up to tens of Eötvös). The FFT methods depend on a series development of the topographic heights, requiring terms up to 14th order in rough terrain; and, one popular method has significant bias errors (e.g. 13 Eötvös in the vertical–vertical gradient) embedded in its practical realization. The straightforward numerical integrations, whether on a rectangular or triangulated grid, yield sub-Eötvös differences in the gradients when compared to the other methods (except near the edges of the integration area) and they are as efficient computationally as the finite element methods.  相似文献   

Spherical Slepian functions and the polar gap in geodesy   总被引：4，自引：0，他引：4  

Frederik J. Simons  F. A. Dahlen 《Geophysical Journal International》2006,166(3):1039-1061

The estimation of potential fields such as the gravitational or magnetic potential at the surface of a spherical planet from noisy observations taken at an altitude over an incomplete portion of the globe is a classic example of an ill-posed inverse problem. We show that this potential-field estimation problem has deep-seated connections to Slepian's spatiospectral localization problem which seeks bandlimited spherical functions whose energy is optimally concentrated in some closed portion of the unit sphere. This allows us to formulate an alternative solution to the traditional damped least-squares spherical harmonic approach in geodesy, whereby the source field is now expanded in a truncated Slepian function basis set. We discuss the relative performance of both methods with regard to standard statistical measures such as bias, variance and mean squared error, and pay special attention to the algorithmic efficiency of computing the Slepian functions on the region complementary to the axisymmetric polar gap characteristic of satellite surveys. The ease, speed, and accuracy of our method make the use of spherical Slepian functions in earth and planetary geodesy practical.  相似文献   

Effects of mode coupling and location of rotational axis on glacial induced rotational deformation in a laterally heterogeneous viscoelastic earth     

Patrick Wu  Hansheng Wang 《Geophysical Journal International》2006,167(2):853-859

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