Quantitative analysis of surface-wave propagation in the Mexico City Valley

The Mexico City basin is an extreme example of an alluvial geological structure that induces long duration and large amplification on ground motions. Researchers have established that the seismic wavefield within the Mexico City basin is composed primarily of surface waves. In the present work, we present a quantitative analysis of the surface-wave field propagating in the Mexico City basin and generated by thrust- and normal-faulting events. Surface waves are identified and extracted from three-component ground motion recordings using a technique that exploits the polarization of ground motion. The spatial analysis of the extracted waves clearly shows that the basin response, is strongly dominated by low-frequency (similar to 0.25Hz${\sim} 0.25{\rm{\ Hz}}$ to similar to 0.5Hz$\sim 0.5{\rm{\ Hz}}$) Love waves. For all seismic events considered, we found that the identified Love waves can reach amplitudes three times larger as compared to Rayleigh waves. Our analysis indicates that surface wave amplification (as defined in the present work) increases with the 1D fundamental resonant site-period T-1, and appears to be independent of earthquake magnitude. We also quantify the mean delay time tdr${t}_{dr}$ of the central frequency f(0) of the extracted waves. We found that the tdr${t}_{dr}$ of Love waves is spatially more coherent as compared to that of Rayleigh waves. The parameter T-1 appears to be a good "proxy" of surface wave effects. This is manifested by a reciprocal relation that was established between T-1 and the frequency f(0) of the extracted Love waves. Equally important, the latter reciprocal relation turns out to be independent of earthquake magnitude.