Low-cost accelerograph units as earthquake alert devices for Mexico City: how well would they work?

摘要

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), T_A, for regional earthquakes is found to increase with the PGA. T_A of the recorded accelerograms (with PGA > 4 gal) ranges between about -20 and 45 sec; the largest values of T_A correspond to the 1985 Michoacan earthquake. With some exceptions, T_A 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.