1718 Bulletin of the Seismological Society of America, Vol. 97, No. 5, pp. 1718–1729, October 2007, doi: 10.1785/0120050202 The Seismic Alert System for Mexico City: An Evaluation of Its Performance and a Strategy for Its Improvement by A. Iglesias, S. K. Singh, M. Ordaz, M. A. Santoyo, and J. Pacheco Abstract The seismic alert system (SAS) for Mexico City has now been in opera- tion for about 15 years. The SAS takes advantage of the fact that the city is located more than 300 km from the foci of many of the potentially damaging earthquakes. The system consists of 15 accelerometers located along the coast of the State of Guerrero, above a segment of subduction plate boundary that is a mature seismic gap. An algorithm estimates the magnitudes of earthquakes from the near-source accelerograms and issues public and restricted alerts for earthquakes with M 6 and 5  M  6, respectively. An evaluation of the SAS’s performance during 1991–2004 reveals a surprisingly high rate of failure and false alerts. This poor performance results from an inadequate detection algorithm and a limited areal coverage by the SAS. This renders the alert system of limited use. In this article we propose an alternative strategy for detecting earthquakes poten- tially damaging to Mexico City that differs substantially from the one presently implemented by the SAS. Although our analysis is based on close-to-source acceler- ograms of 45 Mexican earthquakes and the corresponding peak accelerations re- corded at a reference site in Mexico City (CU), there is no restriction on the distance to the field station, except that its location should provide sufficient alert time to Mexico City. Based on these data, an attenuation relation is derived to compute expected peak acceleration at CU (A red ) from root-mean-square acceleration (A rms ) at a field station. The relation permits specification of an A red threshold, given the peak acceleration at CU (A CU ) for which an alert is desired along with prescribed proba- bilities of failure and false alert. We find that the use of bandpass-filtered (0.2–1.0 Hz) accelerograms leads to an improved performance of the SAS. The choice of the filter is guided by the frequency band of amplification of seismic waves in the lake-bed zone of Mexico City. We think that a single level of general public alert may be the best option. A good choice appears to be an alert for A CU  2 gal (for 0.2–1.0 Hz bandpass-filtered accelerograms) with 1% probability of failure. To accomplish this we must set A red  0.8 gal. The data since 1985 suggest that such an alert would occur about once or twice a year and the event will be felt by most persons in the lake-bed zone. The proposed algorithm, along with an array of sensors located 30 to 40 km apart and distributed in a roughly semicircular arc of 310-km radius centered at Mexico City, should considerably improve the areal coverage and performance of the SAS and potentially save thousands of lives. Introduction The metropolitan area of Mexico City is inhabited by about 20 million persons. Although the city is located more than 300 km from the Pacific coast, where most large earth- quakes originate, it nevertheless suffers frequent earthquake disasters. In most other regions of the world the amplitude of seismic waves at such distances is sufficiently diminished so that even large earthquakes do not cause any damage. The principal cause of the unexpected phenomenon in Mex- ico City is well known: an extraordinary amplification of seismic waves in the frequency band of 0.2–1.0 Hz resulting from the soft clays that underlie the lake-bed zone of the Valley of Mexico (e.g., Singh et al., 1988a,b). The most recent example was the Michoacan earthquake of 1985 (M w 8.0), which originated at a distance of about 350 km from the city. The collapse of buildings during the earth- quake killed about 10,000 and injured 30,000 persons.