Performance Evaluation of the Seismic Alert System (SAS) in Mexico City: A Seismological and a Social Perspective

Mexico City's Seismic Alert System—Sistema de Alerta Sismica, in Spanish (SAS)—was conceived after the disastrous Michoacan earthquake of 19 September 1985, which severely damaged Mexico City. The Michoacan earthquake demonstrated that earthquakes located at distances of approximately 300 to 450 km from Mexico City were capable of inflicting considerable damage to the city, with the subsequent loss of lives and major losses in the city's infrastructure. The SAS is based on the simple observation that the epicenters of the large earthquakes along the Mexican subduction zone that historically have caused damage to Mexico City, and which could inflict similar damage in the future, are located at distances of 300 to 450 km. As a result, the destructive seismic waves generated by these great earthquakes along the Pacific coast take approximately 60 to 80 seconds to reach Mexico City. Under normal circumstances, seismic waves would attenuate and would not produce damage in cities and towns located at these relatively far distances. This is the case with many of the cities and towns located in the vicinity of Mexico City, which normally are not damaged by large earthquakes that take place on the subduction zone. In the case of Mexico City, however, seismic waves arriving from the coast are amplified by a factor of 100 to 500 in the frequency band of 0.2 to 0.7 Hz ( e.g., Ordaz and Singh 1992). This amplification is the result of the interaction of the incoming seismic waves with the soft, water-saturated clay soils, on which a large part of the city is built. These clay soils are remnants of the lake that existed in the Valley of Mexico, which has been gradually drained to accommodate the growing urban sprawl. Therefore, Mexico City occupies a unique and advantageous geographical situation for the design and operation of …