A hybrid technique for the estimation of strong ground motion in sedimentary basins

A computational hybrid technique is presented to estimate ground motion in two-dimensional, laterally heterogeneous media, with special emphasis given to sedimentary basins, The technique combines modal summation and the finite difference method to describe wave propagation in realistic two-dimensional structures that are anelastic. It can take into account the source, path and local soil effects to calculate the local wavefield due to a seismic event. Propagation of the waves from source position to the sedimentary basin is computed by the modal summation method for P-SV and SH waves, with the treatment of P-SV waves being based on Schwab's (1970) optimization of Knopoff's (1964b) method, and the handling of SH waves being based on Haskell's (1953) formulation; these computations inelude the "mode-follower" procedure and structure minimization described by Panza and Suhadolc (1987). Explicit finite difference schemes are then used to simulate the propagation of seismic waves in the sedimentary basin. These schemes are based on the formulation of Korn and Stöckl (1982) for SH waves, and on the velocitystress, finite difference method for P-SV waves (Virieux, 1986). The schemes are stable for materials with high, as weIl as normal values of Poisson's ratio. Intrinsic attenuation in soft sediments is an important process and is taken into account in the computations to prevent serious errors in the estimation of seismic hazards. In the mode summation method, anelasticity is ineluded by means of the variational method, while in the finite difference computations, it is ineluded by using a method based on the rheological model of the generalized Maxwell body. To define the limits and possibilities of this hybrid technique, three computational examples are presented: one for sites that are far from the source (Mexico City during the September 19, 1985, Michoacan earthquake), one at an intermediate distance (Rome during the January 13, 1915, Fucino earthquake), and one for sites elose to the source (a sedimentary basin in the Friuli region during the September 11, 1976, Friuli aftershock at 16h35 m 04 S ) . Special emphasis is given to understanding the different features of ground motion in sedimentary basins. The most important effects that can be observed are the excitation of local surface waves at lateral heterogeneities, the focusing effects, and local resonances. In one-dimensional structural models, local surface waves are never generated, and therefore, such models are not suitable for the prediction of seismic ground motion in sedimentary basins. In the numerical simulation of accelerograms for the September 11, 1976, Friuli earthquake, the coda of the transverse component is mainly