Mexico: Earthquake Early Warning Worked

Mexico City’s earthquake early warning system worked well on Friday, giving residents more than a minute’s warning before shaking from the magnitude eight Chiapas earthquake reached the city.

Wednesday, 13th September 2017

Residents of Mexico City (population 21 million) were alerted by sirens and news broadcasts before the city was shaken by last Friday’s magnitude 8.1 quake centred off the coast of Oaxaca. The alert was generated by an early warning system installed after the devastating 1985 earthquake.

At 7:19 a.m. on 19 September 1885, Mexico City was badly shaken by energy released from a magnitude 8.0 earthquake centred 350 km away two minutes earlier. Buildings collapsed or fell over, power and other services failed. The death toll could have been as high as 45,000 with citizens’ groups claiming about 30,000, but only 5,000 bodies were recovered from the debris leaving many missing and never recovered. According to Wikipedia, 2831 buildings were damaged in Mexico City, 880 of those were completely ruined.

The 1985 earthquake brought home the peculiar risk that Mexico City faces. The fledgling city was established on a swampy lakebed which was progressively drained by the Spanish conquistadors. The uncompacted soils are prone to liquefaction during intense shaking and the lakebed resonates, causing amplification of the seismic waves generated by distant earthquakes.

Within six years, the first stage of an earthquake early warning system for Mexico City was being established. The system monitors for earthquake activity off the coast of Mexico, generating an alert for earthquakes over magnitude six which could cause serious shaking in the city. The system has proved unreliable at times, but the sensor network has been expanded from the Guerrero subduction zone to include the coastal area of Oaxaca.

The system is triggered by the detection of a fast-moving p-wave generated by an earthquake. As the slower s-waves reach seismic instruments, calculation of the size of the quake begins. As more instruments contribute their findings an alert can be issued if the system’s threshold of magnitude six is exceeded.

Since 1991, larger groups such as colleges, councils, hospitals etc. have been able to connect to the system known as SASMEX (Sistema de Alerta Sismica Mexicano) to obtain their own alerts. Public warnings are issued by media and by triggering sirens and on-street megaphones in Mexico City. A phone app was launched in 2013.

This video captured on the 8th of May 2014 shows SASMEX in real time calculating the size of a magnitude 6.4 earthquake centred on the coast near Tecpan de Galeana. Within a few seconds it has determined that a public seismic alert needs to be issued. The expanding red circle shows the energy radiating outward from the epicentre, and the text scrolling along the top shows the estimates of earthquake size as various instruments across the country report in.

Last Friday’s earthquake was centred further from Mexico City than the 1985 event and at a greater depth. The 1985 Michoacan quake of magnitude 8.0 was centred near the coast 350 km southwest of Mexico City at a depth of 20 km and generated shaking of Mercalli intensity IX (violent) in the city. Friday’s quake of magnitude 8.1 (known as the Chiapas earthquake) was centred off the coast of Oaxaca 1,000 km south-east of Mexico City at a depth of 70 km and caused shaking of Mercalli intensity of VI (strong) [U.S. Geological Survey figure] in the city.

Friday’s earthquake struck at 11:49 p.m. local time and sirens in the city were sounded waking those already asleep to warn them of the shaking which would soon be felt.

This video of SASMEX shows the system capturing data from instruments as shaking from Friday’s earthquake spreads out from the epicentre off the coast of Oaxaca. Note that the system quickly colours instruments in Oaxaca red as they detect shaking that is determined to be fuerte (strong).

I had hoped to include here some video showing how Mexican television reported the SASMEX alert to the public during the warning period before the shaking began in Mexico City last Friday. Shortly after the quake, news media featured two video clips of television anchors in the studio reporting what appeared to be the quiet moments before the shaking began in Mexico City. However, one clip clearly did not relate to Friday’s quake, and seems to date from the magnitude seven earthquake of April 2014. The other could not be verified.

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