ANALYSIS OF SITE EFFECTS AND THEIR CORRELATION WITH DAMAGE DISTRIBUTION OBSERVED DURING THE COLIMA ( MEXICO ) EARTHQUAKE OF JANUARY 21 , 2003

In order to evaluate the influence of ground condition on damage distribution in Villa de Alvarez town (Colima, Mexico) during the 2003 Colima earthquake ( MW=7.8), several geophysical surveys were carried out. The structure of shallow sedimentary materials and the depth of the basement have been estimated applying the Spatial Autocorrelation method (SPAC), using several regular pentagonal arrays with radii up to 25m. Seven Swave velocity (VS) profiles have been inverted in Villa de Alvarez by using Rayleigh wave velocity dispersion curves for depths down to 30m. According to the local shallow structures obtained, we find clear lateral variations in the velocity structure for several urban zones that are correlated with different damage level. Microtremor measurements were recorded at 70 sites with a grid of about 100m x 100m interval across damaged area. The horizontal-to-vertical spectral ratios (HVSR) were determined in order to obtain the predominant period for each site. The predominant period range in the studied zone is about 0.1 – 0.6sec. Shorter predominant periods (0.1-0.2sec) were found in damaged zone whereas larger periods (greater than 0.4sec) were obtained in urban areas without damage. The site effects and their correlation with damage distribution on masonry structures observed during the 2003 Colima earthquake were very clear and remarkable. One of the main results has been that masonry houses with one or two storeys located on soils with dominant period around 0.15sec in Villa de Alvarez, showed the most serious damages.

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