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INSTITUTO DE INGENIERÍA, UNAM CIUDAD UNIVERSITARIA, APDO. POSTAL 70-472, COYOACÁN, 04510 MÉXICO, D.F., Mexico
LABORATOIRE DE GÉOPHYSIQUE INTERNE ET TECTONOPHYSIQUE, URA CNRS No. 733, I.R.I.G.M., BP 53X, 38041 GRENOBLE CEDEX, France
LABORATOIRE CENTRAL DES PONTS-ET-CHAUSSÉES, 58, BOULEVARD LEFEBVRE, 75732 PARIS CEDEX 15, France
Abstract
Local site effects caused unprecedented damage at Mexico City during the 19 September 1985, Michoacán, earthquake. Notwithstanding significant efforts, observed long duration strong ground motion in the lake zone has not yet received a truly satisfactory explanation. Following the suggestions of Gilbert (1967) and Lomnitz (1988, 1989), we explore in this paper the possibility of explaining this long duration in terms of gravity perturbed waves in an elastic solid. After a brief review of Gilbert's model and of its limitations, we use a linearization of the free surface boundary condition for nonnegligeable vertical displacements to introduce gravity in our equations. We consider modifications due to gravity in Rayleigh's function and in free surface reflection coefficients, and we find that Rayleigh's pole is significantly affected by gravity for very high Poisson's ratios. Finally, we present results for the problem of ground-motion simulation on the surface of an irregular, thin layer of very soft clay. Our results show that gravity may affect Rayleigh's wave velocity by a factor larger than 2 only in the case of an extremely soft surface layer (Poisson's ratio larger than 0.499999), but that the transition between Rayleigh waves in solids and gravity waves in fluids suggested by Lomnitz (1991) does not exist. Regarding Mexico City, we show that gravity perturbations in an elastic solid do not affect either the character of surface motion or amplitudes or duration of ground motion, and thus do not provide an explanation for the long duration of strong motion in Mexico City.
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Strong Motion in Central Mexico: A Model Based on Data Analysis and Simple Modeling Bulletin of the Seismological Society of America, December 1, 2002; 92(8): 3087 - 3101.
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A Systematic Method for Analyzing Borehole Recordings to Estimate the Wavefield in the Lakebed Zone of Mexico City Bulletin of the Seismological Society of America, October 1, 2000; 90(5): 1268 - 1280.
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F. J. Chavez-Garcia, J. Ramos-Martinez, and E. Romero-Jimenez Surface-wave dispersion analysis in Mexico City Bulletin of the Seismological Society of America, August 1, 1995; 85(4): 1116 - 1126. [Abstract] [PDF]
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C. Lomnitz Comment on two articles on gravity waves by Chavez-Garcia and Bard (1993a, b) Bulletin of the Seismological Society of America, August 1, 1995; 85(4): 1271 - 1271. [PDF]
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F. J. Chavez-Garcia and P.-Y. Bard Reply to comment on "Comment on two articles on gravity waves by Chavez-Garcia and Bard (1993a, b)" by c. lomnitz Bulletin of the Seismological Society of America, August 1, 1995; 85(4): 1272 - 1274. [PDF]
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