Mexico City: Earthquake Dynamics of Structures

The 8.1 magnitude earthquake that struck Mexico City in 1985 left 10,000 people dead, and over 400 buildings collapsed. The extent of the damage left behind by this powerful quake has been extensively studied to make improvements to engineering and architectural practices in earthquake-prone areas of the world. Thirty-two years later, on the exact anniversary of the devastating earthquake, Mexico City was once again jolted by a 7.1 magnitude earthquake. Although still significant, the 2017 earthquake collapsed only about a tenth of the buildings collapsed by the 1985 Earthquake, and in turn resulted in a lower death toll. Even though these earthquakes struck in the same seismic region, their effects were vastly different. This thesis completes a comparison between the two earthquakes focusing on the structural impacts including background on Mexico City's unique geology, basic concepts necessary to understand the response of structures to earthquake excitation, and structural failure modes observed in both earthquakes. The thesis will also discuss the earthquake's fundamental differences that led to the discrepancy in structural damage and ultimately in lower death tolls. Of those discussed, is the types of buildings that were targeted and collapsed. In 1985, buildings with 6 or more floors had the highest damage category. Resonance frequencies of these buildings were similar to the resonance frequencies of the subsoil, leading to amplified oscillations, and ultimately in failure. The 2017 earthquake did not have as much distance from the epicenter for the high frequency seismic waves to be absorbed. In contrast, the shorter, faster waves that reached the capital affected smaller buildings, and spared most tall buildings.