GRAVITY-WAVES IN MEXICO-CITY .2. COUPLING BETWEEN AN ANELASTIC SOLID AND A FLUID LAYER

This paper completes a previous study of the role of gravity during th e 19 September 1985, Michoacan earthquake, at Mexico City. It is assum ed, as suggested by Lomnitz (1989, 1990), that nonlinear mechanisms co uld bring the theology of the Mexico City soft clay layer nearer to a fluid than to a solid. In order to test whether gravity waves in a vis cous fluid could provide an explanation of exceedingly long duration o bserved at Mexico City, we have studied the motion on the surface of a n irregular viscous fluid layer overlaying an elastic half-space. Our formulation takes into account vertical displacement at the free surfa ce of the fluid layer, but requires a linearization of the correspondi ng boundary condition. A decomposition of diffracted fields in terms o f plane waves allows the use of Aki-Larner's method. Excitation is giv en by vertically incident SV waves. We present results both for nonvis cous and viscous fluids and show that an adequate free boundary condit ion is essential for this problem. Lacking data, we have performed sev eral simulations for values of viscosity going from 300 to 30,000 Pa.s . In the case of a nonviscous fluid, gravity surface waves are generat ed efficiently by diffraction of elastic waves on the irregular interf ace, but attenuate rapidly when including viscosity and are not signif icant for viscosities above 3000 Pa.s. The results presented in our co mpanion paper and here, allow us to conclude that gravity is not a lik ely explanation to the long duration of ground motion observed at Mexi co City. In a more general context, our model may be useful to study t he interaction of seismic waves with water reservoirs, such as dams.