The Mexico Earthquake of September 19,1985—The Incident Wavefield in Mexico City during the Great Michoacán Earthquake and Its Interaction with the Deep Basin

We used the vertical displacement records of the ground motion in Mexico City during the great 1985 Michoacan earthquake to identify the nature of the waves responsible for the heavy damage suffered by the town. The records at the different stations exhibit very similar waveshapes. A multichannel phase analysis shows the arrival, from the source zone, of a strong coherent wavetrain which represent the most of the energy incident at periods around 3 sec. We interpreted this arrival as the regional continental phase Lg. The study of other instrumental data in the period range relevant to the present investigation shows the efficiency of Lg propagation between the subduction zone and Mexico City.

We have computed the response to Lg waves of a sedimentary basin whose characteristics correspond to the deep basin beneath Mexico City. The amplification reaches a value of about 5. The results obtained are in good agreement with the response of the basin to a single obliquely incident plane wave. The durations of the theoretical signals are similar to those of actual observations at sites in the hills zone and in the intermediate zone.