The 1985 (M8.1) Michoacán Earthquake and Its Effects in Mexico City

Abstract

The September 19, 1985 M8.1 earthquake west of Michoacán, Mexico, due to shallow subduction of Cocos plate beneath the North American plate. This mega-thrust inverse faulting event broke about 200 km along the boundary of the Mexican subduction zone in the Pacific coast of Michoacán. This earthquake consisted of two main sub-events at 16 km depth, separated 27s in time and about 95 km SE in distance. Thirty-six hours later, a major M7.5 aftershock occurred in the SE edge of the same area. The 1985 earthquake caused enormous damages, even at distances of up to 300 km from the epicenter. This was the case of Mexico City in which the maximum accelerations recorded reached 0.17 g, exceeding building codes limits (Anderson et al. in Science 233:1043–1049, 1986), and spectral amplifications and duration were unprecedented (Singh et al. in Bull Seism Soc Am 78:451–477, 1988). The official account reported more than six thousand buildings destroyed or seriously damaged and estimated between 10,000 and 35,000 persons missing or dead. After this earthquake, seismic monitoring systems have been developed and improved. The building codes are often revised and regular monitoring of structures is currently done after earthquakes to keep safer standards. In the same vein, a number of educational programs were developed to promote a culture of prevention. It is claimed that the 1985 earthquake generated a widespread solidarity and awareness among city dwellers. On September 19th, 2017, an intraplate earthquake M7.1, hit Central Mexico again, causing extensive building damage, testing the effectiveness of developed programs implemented after the 1985 earthquake.