| Summary: | Recently, low-cost accelerograph units have been proposed as earthquake alert devices for Mexico City. These units would trigger when the acceleration reaches a pre-established level, presumably 4 gal. For significant earthquakes, this would occur during P wave or in the early part of its coda. We test the performance of such a unit on accelerograms recorded in the period 1985 - 2008 at SCT and CDAO, two lake-bed sites in Mexico City. As expected, the alert time (the time of arrival of intense ground motion minus the trigger time), TA, for regional earthquakes is found to increase with the PGA. TA of the recorded accelerograms (with PGA > 4 gal) ranges between about -20 and 45 sec; the largest values of TA correspond to the 1985 Michoacan earthquake. With some exceptions, TA is greater than ~15 sec for earthquakes which produced PGA > 20 gal. The device may be useful in mitigating injuries and loss of lives if an alert time of 15 to 45 sec is sufficient to put in to effect pre-established safety measures. This may be practical for one- to three-storey public school buildings. The device would be useful for shutting critical facilities before the arrival of large- amplitude ground motion.
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