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Excitation of Surface Waves in the Valley of Mexico

¹ Earthquake Research Institute
University of Tokyo
1-1-1, Yayoi, Bunkyo-ku
Tokyo 113-0032, Japan

By analyzing the three components of surface and downhole accelerograms obtained at four borehole stations located in three geotechnical zones of the Valley of Mexico, differences in the nature of seismic motions among the different zones are investigated. The target period range is mainly a narrow band including the theoretical primary predominant period (the fundamental resonant period) at each station. The wave types of seismic motions are identified by calculating the theoretical vertical amplitude ratios of various seismic waves and the cross-correlation functions between the surface and downhole recordings. In the calculation of the theoretical amplitude ratios, the damping for body-wave propagation and the damping for the eigenfunctions of surface waves are considered separately. For the two kinds of damping, period-independent and period-dependent quality factors are employed. We found that the seismic motions were dominated by surface waves rather than S waves around the predominant period at the four stations. Based on the theoretical amplitude ratios, we could explain the large vertical amplitude ratios observed at the predominant period at all stations. Our conclusions are summarized as follows: (1) The resonant seismic motions are mainly composed of Love waves and S waves. (2) The Love-wave theoretical amplitude ratios are considerably larger than the S- wave ones at the predominant period at two soft-soil stations. (3) The low values of period-dependent quality factors for the eigenfunctions of surface waves explain the large peak amplitude ratios observed at the predominant period at two soft-soil stations.