Résumé On donne l'expression du déplacement dû à la propagation d'une dislocation uniforme partant du centre d'une faille circulaire, avec une vitesse de propagation finie ou infinie. Dans cedernier cas on se trouve dans une situation analogue à celle de la diffraction de Fraunhofer en Optique. Le spectre d'amplitude de l'ondeP est modulé dans son détail par la fonction de Bessel d'ordre1. Quand le rayon du cercle est relativement faible, on retrouve les expressions d'une source ponctuelle, dans lesquelles la densité surfacique de moment intégré est remplacé par le moment sismique total. On met en évidence une fréquence de coupure qui augmente avec la profondeur du foyer et diminue quand le rayon de la faille augmente. On donne enfin une expression du moment sismique total en fonction de l'amplitude spectrale aux basses fréquences.
Summary We have examined the case of the radial propagation of a uniform dislocation and stated in detail the expression of the displacement. The rupture velocity is assumed finite or infinite. The last case shows some analogies with the Fraunhofer diffraction. TheP wave amplitude spectrum is modulated in its detail by theJ1 Bessel function. With small values of the radius, one can obtain the displacement by replacing in the expression corresponding to point sources the integral of the moment density by the total seismic moment. The corner frequency increases with the focal depth, and decreased when the fault radius increases. Finally, we give an expression of the total seismic moment deduced from the low frequency spectral amplitude components.
Several issues relevant to the mobility of long-runout landslides are examined. A central idea developed in this paper is that the apparent coefficient of friction (ratio of the fall height to the runout distance) commonly used to describe landslide mobility is physically meaningless. It is proposed that the runout distance depends primarily on the volume and not on the fall height, which just adds scatter to the correlation. The negative correlation observed between the apparent coefficient of friction and the volume is just due to the fact that, on the gentle slopes on which landslides travel and come to rest, a large increase in runout distance due to a large volume corresponds to a small increase in the total fall height, hence to a decrease in the apparent coefficient of friction.
It is shown that the spreading of a fluid-absent, granular flow is not able to explain the large runout distances of landslides, and in particular does not allow the centre of mass to travel further than expected for a sliding block. This contrasts with the behaviour of natural landslides, for which the centre of mass is shown to travel much further than expected from a simple Coulomb model. The presence of an interstitial fluid which can partly or entirely support the load of particles allows the effective coefficient of solid friction to be reduced or even suppressed. Air is not efficient for fluidising large landslides and a loose debris cannot slide over a basal layer of entrapped and compressed air, as air would rapidly pass through the debris in the form of bubbles during batch sedimentation. Water is much more efficient as a fluidising medium due to its higher density and viscosity, and its incompressibility. As water is known to enhance the mobility of the saturated debris flows, it is proposed that water is also responsible for the long runout of landslides. This is consistent with the fact that the increase in runout with volume is similar for debris flows and landslides. Field evidence suggests that most landslides are unsaturated with water but not dry, even on Mars.
Comparison of the velocity of well-documented landslides with that predicted by fluid-absent, granular models shows that these models predict landslides that are much faster and less responsive to topography than natural ones. The relatively low velocities of landslides suggest that energy dissipation is dominated by a velocity-dependent stress and that the coefficient of solid friction is very low. This is consistent with the physics of fluidised or partly fluidised debris and suggests that landslide velocity may be controlled by local slope and flow thickness rather than by the initial fall height. In the absence of a supply of fluid at the base, fluidisation requires a net downward flux of sediment, implying some deposition at the base of landslides, which may thus progressively run out of material. In such a model, the spreading of the portion of a landslide beyond a certain distance would primarily depend on the volume passing this distance and not on the total volume of the landslide. Landslide deposits may therefore have self-similar shapes, in which the area covered beyond a certain distance is a constant function of the volume beyond that distance. It is shown that the shape of some well-documented landslide deposits is in reasonable agreement with this prediction. One consequence is that, as recently proposed for debris flows, assessment of hazards related to landslides should be based on the correlation between the volume and the area covered by the deposit, rather than on the apparent coefficient of friction. 相似文献
The shape of the Earth and of planets depends on the exciting forces and on the rheology of the medium. From the equilibrium equation, we present the main modelisations of the viscous and inviscid fluids and we essentially describe the characteristics of linear viscoelastic deformations, for the Maxwell viscoelastic model of rheology. We use the elastic, viscoelastic and fluid Love numbers in order to investigate the associated relaxation modes. For these various kinds of rheology of the planets interior, we compute the geoid and the topography induced by an internal mass distribution. Finally, we show the importance of this viscoelastic deformation calculations in the study of the celestial body rotations. 相似文献
Tidal forces acting on the Earth cause deformations and mass redistribution inside the planet involving surface motions and
variation in the gravity field, which may be observed in geodetic experiments. Because for space geodesy it is now necessary
to achieve the mm level in tidal displacements, we take into account the hydrostatic flattening of the Earth in the computation
of the elasto-gravitational deformations. Analytical solutions are derived for the semi-diurnal tides on a slightly elliptical
homogeneous incompressible elastic model. That simple analytical Earth’s model is not a realistic representation of any real
planet, but it is useful to understand the physics of the problem and also to check numerical procedures. We rediscover and
discuss the Love’s solutions and obtain new analytical solutions for the tangential displacement. We extend these analytical
results to some geodetic responses of the Earth to tidal forces such as the perturbation of the surface gravity field, the
tilt and the deviation of the vertical with reference to the Earth’s axis. 相似文献
On the basis of an existing multiparameter borehole fluid sensor (p, T, Cw, pH, Eh), a new downhole tool designed for 200 bars and 75 °C was developed to measure the spontaneous electrical potential in situ with great precision (better than a μV). To this end, the new sensor is based on the use of unpolarizable Pb/PbCl2 electrodes either at the surface or downhole. In situ testing has demonstrated a capacity to identify several subsurface sources of natural electrical potential such as diffusion ones (membrane potential in the presence of clays, Fickian processes due to pore fluid salinity gradients), or else the electrokinetic mechanisms with gradients in pore fluid pressure. As a consequence, Multi-Sensors Electrical Tool (MuSET) might be used as an “electrical flowmeter” sensitive to both horizontal and vertical fluid flow in a vertical borehole. 相似文献
Usually, one can explain the orthogonal distribution of first movement by the dislocation model of seismic sources, namely a uniform slip. However, other representations may be obtained if the normal component of the dislocation is present. When this component is less than 0.6 times the tangential one, we obtain conical distributions. Some models which simulate a volume phenomenon provide analogous results. We show that they could agree with some observations. 相似文献
