The Mexico Earthquake of September 1985, A Preliminary Report

On September 19, 1985 a great earthquake occurred near the Pacific coast of Mexico close to the towns of Ixtapa and Lazaro Cardenas. The epicenter was located at 18.1°N and 102.3°W on the Mexico subduction zone (see Figure 1.1). The magnitude was M9=8.1 and the Modified Mercalli intensity in Mexico City was estimated at VIII to IX. A major aftershock (Ms=7.5) occurred on September 20, 1985. Table 1.1 summarizes the data on the two large events.

The southwestern coast of Mexico is a very active seismic region with many large earthquake events through its history. The seismic activity is due to the relative motion of the Cocos and North American plates. The September 19 and 20, 1985 earthquakes appeared to have occurred on the Michoacan Gap of the Mexico subduction zone. No earthquakes are known to have occurred on the gap in the last 185 years even though adjacent areas along the subduction zone have had considerable seismic activity.

Severe damage occurred in Mexico City at a distance of 350 km from the epicenter, while minor damage occurred in Ixtapa and Lazaro Cardenas at a distance of 70 km from the epicenter. The characteristics of the September 19 earthquake are the long period and long duration (-3 min) of the ground motion. These characteristics were some of the factors for severe damage in the soft soil areas of Mexico City instead of in the hard soil area near the epicenter. The peak ground acceleration recorded in Mexico City at one of the soft soil station is 168 gals, while that at a firm ground station is 38 gals (Quass et al. 1985). (Near the epicenter peak ground accelerations of 271 gals were recorded Mena et al. 1985.)

At the present, the total number of buildings known to be damaged is 7,400. Reinforced concrete frame buildings with 6 to 15 stories were mainly damaged in the soft soil area of Mexico City. Table 1.2 summarizes the data currently available on the total deaths, injuries and severely damaged buildings.

In this report we present a summary of the damage observed by Mr. Shigeru Suzuki who visited the devastated areas within Mexico City. A brief review of the seismic history and the tectonic setting of the area is also included in order to provide a better understanding of the hazard in the region. Preliminary estimates of the probability of occurrences of events of the size of the September 19, 1985 event in the same region are also obtained. The damage to buildings observed in Mexico City is mainly described and illustrated with photographs. Based on these observations, the damage in the region is discussed.