Two‐dimensional Scattering of P, Sv and Rayleigh Waves: Preliminary Results for the Valley of Mexico

Abstract

SUMMARYA direct boundary element method for calculating the two-dimensional scattering of seismic waves from irregular topographies and buried valleys due to incident P-, SV-and Rayleigh waves is employed to model a section of the Mexico City Valley. The method has been formulated with isoparametric quadratic boundary elements and contains, with respect to previous works in the ÿeld, some improvements that are brie y presented.Because the Mexico City Valley is relatively at and shallow and the contrast of S-waves between the clays and the basement rock is very high, it is believed that the one-dimensional theory is enough to explain the ampliÿcation patterns. Although this is true for most sites, results from recent accelerometric data suggest that two-and three-dimensional models are needed to explain the ampliÿcation behaviour at some places.In this work, two accelerometric sites have been chosen: Site 84 to probe that the one-dimensional model works well for most sites, and Site TB, as an example of irregular response. The two-dimensional method presented here was used to model a section of the valley where site TB is located, showing that this method yields results closer to the observations than the one-dimensional approach. c

Comment on ‘evidence of the dominance of higher-mode surface waves in the lake-bed zone of the valley of Mexico by Shapiroet al.(2001)’

“…It has long been recognized that shallow layers (<100 m) of highly compressible, large‐water content clays in Mexico City are the most important factors in the seismic response and therefore the cause of the observed damage (i.e. Ordaz et al 1988; Seed et al 1988; Singh et al 1988a,b; Sánchez‐Sesma et al 1993; Chávez‐García & Bard 1994; Reinoso et al 1997; Reinoso & Ordaz 1999). These site effects vary enormously in form and amplitude within a few kilometres mainly according to the thickness of these layers.…”

“…To try to find out the answer to this question we have used SSAA's stratigraphy for site RM to evaluate the seismic response of two other sites, SC and 84 (see Fig. 1), where the shallow stratigraphy is also known (for SC we used the values given by Seed et al 1988 and for site 84 we used those of Reinoso et al 1997). Table 1 shows the deep structure proposed by SSAA and used for the three sites.…”

“…Ordaz et al 1988;Seed et al 1988; * Present address: Facultad de Ciencias de la Tierra, UANL, Ex-Hacienda de Guadalupe, Apartado Postal 104, Linares N. L. 67700, Mexico. Singh et al 1988a,b;Sánchez-Sesma et al 1993;Chávez-García & Bard 1994;Reinoso et al 1997;Reinoso & Ordaz 1999). These site effects vary enormously in form and amplitude within a few kilometres mainly according to the thickness of these layers.…”

“…Geological model for site SC (taken from Seed et al 1988;Reinoso & Ordaz 1999). Reinoso et al 1997).…”

Duration of strong ground motion during Mexican earthquakes in terms of magnitude, distance to the rupture area and dominant site period

“…For an earthquake with M = 5 motion is 18 s longer for T s = 5 s than for T s = 0:5 s, and for the same periods duration is 72 s longer for M = 8:1 than for M = 5. This considerable increase in duration is most probably present in the input motion [25] rather than in complicated 2D and 3D local scattering [26]. Figure 10(b) shows two earthquakes with di erent distances to the rupture area ( Figure 1) than those shown in Figure 10(a).…”