Microtremors Studies Using SPAC Method: Experiences and Applications in Mexico.

The study of Microtremors (environmental vibration) has become one of the key parts for the evaluation of seismic risk. That is because they can be used to estimate the site effects. The first approach is by obtaining the dominant period of the site through the H/V spectral ratio (Nakamura method). However there are parts of the body and surface waves that are not entirely known. The spectral H/V ratio is mainly influenced by SH resonance in the superficial layers. But if we are working with surface waves, like Rayleigh waves, they should be represented by the ellipticity coefficient of theoretical Rayleigh waves, whereas the absolute magnitude of the H/V spectral ratio can not be directly compared with the transfer function. The shear wave velocity of the shallow structure is a basic element in the studies of the ground amplification and for the site response of sedimentary basins. The SPAC method (Spatial Auto Correlations Method) was proposed by Aki in 1957, based on microtremors recorded in instrumental arrays. This method allows us to obtain the dispersion curve of the Rayleigh waves, from which we can estimate the velocity structure. In Mexico we have used this method in geotechnical applications, engineering and seismic hazard studies, to characterize the site effect. Since in some places we do not have enough information to validate our results, we use ellipticity curves to interpret the H/V spectral ratios and compare with our SPAC results. This was implemented in cities with substantial urban density as the city of Monterrey, Nuevo Leon and in areas where the site effect has had great impact as Mexico City and in areas with important seismicity as some parts of the state of Michoacan, There, it has been possible to estimate the shear wave velocity of the soil through this type of study.