SURFACE-WAVE DISPERSION ANALYSIS IN MEXICO-CITY

In this article, we present an observational investigation of ground m otion at Mexico City focused on surface waves. Our purpose is 2-fold; first, to understand incident ground motion during the great Michoacan earthquake of 19 September 1985, and second, to characterize surface waves propagating in the lake-bed zone. To this end we analyze the str ong-motion records obtained at Mexico City for the large (M(s) = 8.1) earthquake of 19 September 1985. It is shown that, in the low-frequenc y range, we observe the Rayleigh fundamental mode in both the vertical and the radial components, and the Love fundamental mode in the trans verse component at all the strong-motion stations. The vertical compon ent also shows the first higher mode of Rayleigh waves. We use a very broadband record obtained at station CU for the smaller (M(s) = 6.7) e arthquake of 14 May 1993 to verify that the dispersion computed from t he model of Campillo et al. (1989) represents well the average surface -wave propagation between the coast and Mexico City in the 7- to 10-se c period range. We use this result to assign absolute times to the str ong-motion records of the Michoacan event. This allowed us to identify additional wave trains that propagate laterally in directions other t han great circle in the 3- to 5-sec period range. These wave trains ar e identified as Love waves. In a second analysis, we study a set of re fraction data obtained during a small-scale (250 m) experiment on the virgin clay of the lake-bed zone. Phase-velocity dispersion curves for several modes of Rayleigh waves are identified in the refraction data and inverted to obtain an S-wave velocity profile. This profile is us ed as the uppermost layering in a 2D model of Mexico City valley. The results of numerical simulation show that surface waves generated by l ateral finiteness of the clay layer suffer large dispersion and attenu ation. We conclude that surface waves generated by the lateral heterog eneity of the uppermost stratigraphy very significantly affect ground motion near the edge of the valley, but their importance is negligible for distances larger than 1.5 km from the edge. Thus, locally generat ed surface waves propagating through the clay layer cannot explain lat e arrivals observed for the 1985 event. We suggest that the long durat ion of strong motion is due to the interaction between lateral propaga tion of waves guided by deep layers (1 to 4 km) and the surficial clay layer. This interaction is possible by the coincidence of the dominan t frequency of the uppermost layers and the frequency of the deeply gu ided waves.