The strong ground motion in Mexico City: Analysis of data recorded by a 3D array

In 2001, we installed 11 new accelerometers in the lake-bed zone of Mexico City, which, together with the existing stations, form a 3D array of small aperture, with 15 surface and six borehole stations. This new array recorded the 10/08/2001 Guerrero event (M=6.1) and the 01/22/2003 Colima (M=7.6) event. The analysis of these two events showed that, for periods longer than 2.5 s, the dominant wavefield is composed of a fundamental mode of surface waves arriving from the epicenter and diffracted surface waves generated at the southern boundary of the Mexican Volcanic Belt. For periods shorter than 2.5 s, the resonance of the superficial clay layer becomes predominant. The wavefield consists in a superposition of diffracted waves propagating from the western part of the valley. Our results show that the ground motion is strongly amplified by the soft clay layer but its duration is controlled by the incident wavefield.