The Mexico City Earthquake Of 1985


Section 2


September 19, 1985. At 7:19 in the morning, the Mexico City experienced an earthquake with a magnitude of 8.1. Thirty-six hours later a second earthquake of magnitude of 7.5 occurred. The first earthquake shook buildings in Mexico City for a total of three minutes. People were trapped in poorly constructed buildings that had collapsed on them, and many citizens died as a result. The Mexican government estimated some 5000 people perished, however, international agencies placed the death toll at more than 10,000. Severe damage occurred to about 500 buildings in Mexico City with widespread light to moderate damage in other parts of the country. In the area of greatest damage in downtown Mexico City, some types of structures failed more frequently than others. In the highest damage category were buildings with six or more floors. Resonance frequencies of these buildings were similar to the resonance frequencies of the subsoil. Because of the "inverted pendulum effect" and unusual flexibility of Mexico City structures, upper floors swayed as much as one meter and collapsed. Differential movements of adjacent buildings also resulted in damage. A flexible building often failed if adjacent, more rigid lower buildings held it. Damage or failure often occurred when two swaying buildings came in contact. Corner buildings were also vulnerable to damage. Lessons learned from the patterns of earthquake damage had to be quickly applied to prevent another disaster when an earthquake releases stress that is building up in another area along the Mexican coast between Acapulco and Zihuatanejo. (Arizona.edu)


Although the quake itself killed over 8,000 people, casualties would nave been much higher if the quake had occurred just a couple hours later when the schools and office buildings would have been filled with people. Surrounding areas affected by the earthquake included the Mexican States of Jalisco, Guerrero, and Michoacan. Damage in the epicentral area was restricted to a few tourist resorts and industrial estates along the Mexico Pacific coast. A two-meter tsunami also caused some damage in that area. (johnmartin.com)


The epicenter was located 50 km, approximately 31 miles, off the coast of Mexico. The epicenter is the point on the Earth's surface directly above the point of rupture or focus of the earthquake. This is a region where the Cocos Plate is being sub ducted underneath the North American Plate in Mexico and is the most active subduction thrust fault in the western hemisphere. (guardian.co.uk)


Mexico City has been rocked several times in the past. In this century, Mexico had 42 earthquakes with magnitudes greater than 7. Some earthquakes of magnitude 7 have caused massive damage, but the effect of a quake can be affected by many factors, including its depth and the sort of earth through which it passes as it moves away from the epicenter. Mexico City is built atop a former lakebed in a mountain valley, which acts as an amplifier for the motion of quakes. It is largely built upon soft, poorly consolidated lake sediments from the former Lake Texcoco, which was drained by the Spanish following their occupation of the region. Thick sequences of soft sediments like these tend to amplify seismic waves and cause the ground to shake much more vigorously than the surrounding bedrock. Also, the constant shaking of the ground might explain why residents ignored the foreshock that preceded the earthquake. (seismowatch.com)


Assistance to the injured required the staffing of 131 first-aid stations, and even so a greater number of such installations would have been necessary. Figures finally provided by the Federal District authorities certified that 14,286 injured were attended to. It was necessary to transfer 4,900 people from damaged hospital buildings to other health institutions. It is said that 38,605 additional patients were looked after. Among them, 22,296 for injuries other than physical, 10,188 for minor injuries, and 5,748 for major injuries, with 2,637 requiring hospitalization.


Immediately after the quake, police cars drove slowly through the streets of Mexico City with sirens flashing, asking people over loudspeakers: "Is everything OK?"


They encountered residents gathered in small groups, many shaking with fear. Others raced outside so quickly that they had no shoes to cover their bare feet or had grabbed little more than a blanket to guard against