Seismic Alert System of Mexico City

Juan M. Espinosa-Aranda, A. Jimenez, G. Ibarrola, F. Alcantar, A. Aguilar, M. Inostroza, S. Maldonado
Centro de Instrumentacion y Registro Sismico, A.C., Anaxagoras #814, CP 03020 , Mexico City, Mexico.
Tel. : +52-5-5 23 73 94, Fax: +52-5-6 69 25 12,
E-mail: cires@servidor.unam.mx,
Home page: http://www.unam.mx/cires

The Seismic Alert System (SAS) is a public earthquake early warning system developed with the sponsorship of the Mexico City Government authorities. The SAS is operating since August 1991. The system is capable of generating warning signals about 60 sec. average in advantage to the "S" waves first arrivals in Mexico City, when it detects strong earthquakes occurring 280 km faraway, in the "Guerrero Gap". The SAS was programmed to issue a general alert signal when the earthquake magnitude forecasting estimation is strong (M>6), and to issue a restricted one if this evaluation is moderated (5<M<6).

Until now the greatest seismic event detected by the SAS was the September 14, 1995, M7.3 "Copala" earthquake. In a live test that checked the whole system, the SAS was activated and a general warning signal was issued in Mexico City, 72 sec. prior to the arrival of strong ground motion effects. The SAS earthquake warning signal was disseminated via local AM/FM commercial radio stations allowing the operation of the audio alerting and triggering prevention procedures, designed in each public school within the earthquake hazard reduction program applied by the Secretariat of Public Education in the valley of Mexico, since September, 1985. The "Copala" general warning was used too by: government agencies with emergency response functions, key utilities, the Mexico City subway and in one humble big house complex. This earthquake early warning reached about 4 million citizens through the Mexico City's broadcasters invaluable participation in relaying this alert signal.

Since August, 1991, after more than 80 months of continuous operation, the SAS has being capable to detect 656 seismic events in the Guerrero Coast; nine of them so strong to trigger general early warning signals in Mexico City; 32 moderated ones, including one false general early warning signal and one earthquake well detected, but not warned.

However, important facts must be additionally considered: historically not all the guidelines and recommendations for the use of the SAS public service have been carried out or are being applied continuously. The average Mexico City adult resident is not trained. The new government authorities are studying how to promote the system enhancement. Contrary to general speculations, when a SAS false alert was issued on November 16, 1993, 19:11, in rush hours, with an estimated radio audience of 2,000,000 people, many of them not trained, common sense prevailed and nobody died or got injured because of panic; before that, a general argument used against issuing the radio early warning to all public was that, when the earthquake alert would sound, many not trained people could die or get badly injured in public places due to alarm and panic reactions.

The reason for developing the SAS and its results should be reviewed and publicly discussed with the aim to promote performance drills to improve the efficacy and efficiency of the reaction procedures in using the early warning, mainly because until now the big "Guerrero earthquake" has not occurred yet.