Browsing by Author "DeBremaecker, Jean-Claude"
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Item Leaking modes of Love waves and seismogram interpretation(1973) Radovich, Barbara Jean; DeBremaecker, Jean-ClaudeLeaking modes of Love waves are investigated for a simplified continental, crust-mantle model. The scheme of Rosenbaum (196) is used, whereby the and k are complex numbers and the group velocity is purely real. The dispersion curves are plotted, and group velocities well below the Airy phase of the normal modes are found. Group velocities of 1.-3.8 km/sec attenuate the least, but even these decay to such an extent at 1 km. from the epicenter, that leaking modes of Love waves will never make a contribution to the seismic record.Item Mantle convection at marginal stability(1979) Warford, Andrew Craig; DeBremaecker, Jean-Claude; Hellums, Jesse D.; Lallemant, Hans G. AvéThe horizontal extent of convection cells in the earth's mantle can be estimated from the geometry of plate boundaries. The vertical dimensions can perhaps be estimated from the theory of marginal stability in variable viscosity fluids. For viscosity laws symmetrical about mid-depth the aspect ratio increases with increasing viscosity contrast, but the law of variation with depth has little effect. The value of the Rayleigh number is affected by both the viscosity law and the contrast. The aspect ratio for the asymmetric cases studied is much less affected except at very high contrasts (>3) and then only in the case of an exponentially varying viscosity. In all cases studied, the variation of the Rayleigh number with wavelength is smaller as the viscosity contrast increases, thus allowing for a fairly wide range of aspect ratios. The variation of velocity with depth indicates that motion takes place in the entire depth range except in the case of viscosity decreasing exponentially with depth and then only at high viscosity contrast (>2).Item Numerical models of subduction dip angle with variable viscosity(1981) Wong, Peter Kin; DeBremaecker, Jean-Claude; Lallemant, Hans G. Avé; Clark, Howard C.Many models have been used to examine subduction dip angle but none of which is both, dynamic and time dependent. Numerical models are developed to follow the thermal evolution of the subduction zone. The energy equation includes advection, the adiabatic gradient, viscous dissipation, radiogenic sources and variable diffusivity. The pressure is eliminated by combining the two momentum equations into a fourth, order stream-function equation. Both the energy and stream-function equations are solved numerically on a special non-uniform offset grid using the Altemating-Directon Implicit method. In a convection cell with Herring-Nabarro viscosity, circulation will be restricted to the left side of the cell while the descending limh is on the right. Altering the aspect ratio by changing the depth, has a much more pronounced effect on the total circulation and velocities than by changing the width. A convection cell will reorganize its circulation pattern to achieve steady-state when a highly viscous block, is "inserted” into the cell to disrupt normal circulation. Finally, the convective pattern in a system in which the boundary conditions are perpetually changing is determined by both its present boundary conditions and also by those preceding it by several millions of years.