Seismic Microzonation of the Texcoco Lake Area, Mexico

This paper presents some of the most relevant results obtained from field, laboratory and analytical investigations aimed at characterizing the seismic environment prevailing at the Texcoco lake region, in the Valley of Mexico, with the goal of developing a microzonation. In particular, this study focuses on an area of 19.0 by 5.5 km2, which has been instrumented with four seismological stations: TXSO, TXS1, TXS2 and TXCH that have recorded ground motions for at least 15 years. Field investigations were conducted to define the subsoil conditions underneath each station. Dynamic testing in these high plasticity clays was carried out to establish the variation of shear stiffness and damping with strain level. Ground motion definition was achieved through empirically derived response spectra obtained from sets of earthquake ground motions recorded at a nearby station located in soft soil, which were deconvolved to the base rock. An statistical analysis using random shear wave velocity profiles and an stochastic site response analysis was used to developed sets of response spectra to reduce uncertainties associated with soil properties determination and seismic environment characterization. The final proposed response spectra for each studied point were developed from the envelope plus one standard deviation computed at each ground motion station. These spectra show good agreement both in frequency content and spectral ordinates with those obtained directly from measurements taken at these seismological stations. Finally equations to construct the proposed response spectra were proposed