Geotechnical Engineering Aspects of the June 1999 Central Mexico Earthquakes

An earthquake of moment magnitude (M_w) 7.0 struck the central region of Mexico on 15 June 1999 between the states of Puebla and Oaxaca. A second event with a moment magnitude 6.3 occurred on June 21, also affecting the central region of Mexico but with minor consequences. Attenuation relationships of peak ground acceleration with hypocentral distance for the June 15th event were compared with strong-motion recordings over a variety of geologic site conditions. Significant site amplification was observed and was correlated with deep soil conditions. The June 15th event caused significant damage of unreinforced masonry structures, such as churches and houses, including more than 500 historical buildings. Puebla City and the towns of Tehuacán and Acatlán de Osorio were the urban areas hardest hit by the earthquake. Although the earthquake was felt in Mexico City, the damage was light and mostly restricted to nonstructural elements. Comparison with observations obtained during the September 1985 earthquake suggest that significant soil nonlinearity, resulting in increased amplification at larger periods, can be observed for soft soil sites.