By David Jacobson and Ross S. Stein, Temblor

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Mexico City residents received advance warnings of 30-120 seconds in three out of four of large shocks in the past six months, a remarkable 75% success rate. Nevertheless, the warning was late in the one quake that caused damage in the City, the M=7.1 Puebla shock, a shortcoming that is being remedied by strengthening the network and its protocols. But irrespective of that upgrade, Mexico City is uniquely situated to receive long warnings. For the rest of the world, warnings of less than 10 seconds for moderate or stronger shaking are generally possible. That’s all the time we need to ‘drop, cover, and hold on.’ So, let’s get on with it.

Mexico’s Earthquake Early Warning system

Following the devastating 1985 Mexico City earthquake in which left 5,000 people dead and billion of dollars worth of damage, the Mexico City government sponsored the installation of accelerometers around the city in the hope that future earthquake losses could be mitigated. This system, originally named SAS for Sistema de Alerta Sísmica, has been up and running for the safety of the public since 1993. Today, it is known as the Sistema de Alerta Sísmica Mexicano or SASMEX and has expanded to include the entire Oaxaca region, in principle protecting cities such as Acapulco and Puebla. The performance of this system has been now tested by three M=7+ earthquakes and two M=6+ quakes since September 2017.

Friday and Monday’s Oaxaca M=7.2 and M=6.0 warnings succeeded

SASMEX was triggered by the 16 February M=7.2 Oaxaca earthquake near Mexico’s southern coast. According to the preliminary bulletin, Mexico City received a 78 second alert, similar to what we reported from Monica and Evan Freedman on Friday. Fortunately, this earthquake had an epicenter over 220 miles (350 km) from Mexico City, so the city suffered no damage. Close to the epicenter, poorly constructed buildings sustained heavy damage. Nonetheless, there were no earthquake fatalities. Tragically, a helicopter carrying government officials surveying the damage crashed, killing 13 people on the ground, but none in the helicopter because it did not break up, according to NPR. A little more than two days after the mainshock, a M=6.0 aftershock struck. Once again, SASMEX was triggered and gave Mexico City about 30 or more seconds warning.

What it felt like as a weekend tourist in Mexico City during the warnings

Ross’ daughter, Monica Freedman, texted us this following the alert:

“I had been thinking since Friday, would I recognize the siren sound of the earthquake warning? I got my answer at 12:55 am Monday: I popped up in bed. “Evan, Earthquake!” Groggy, we fumbled for shoes and jackets. We couldn’t find our keys, which were needed to open the front door of our Airbnb apartment to get outside. After maybe 30 seconds, I started to feel the sways and called out to Evan that the earthquake had started. It lasted about a minute. We continued outside, some neighbors in robes, others waiting out the shaking to head back into the nearest bar.

I’m pretty sure we did everything wrong—getting out of bed, not taking cover during shaking, going outside after the shaking. Don’t tell my dad! We’ll now be traveling with whistles, and will leave shoes and keys by the door. And I’m going to hound Uncle Barry in Mill Valley to change his entry door lock so he doesn’t need a key to get out of his house in a quake or fire.”

Evan Freedman added this postscript: “While Monica was packing Monday morning, I popped into a coffee shop. As I was waiting in line, I briefly scanned the room. Right next to me was a table was a man, whose profession I couldn’t really determine without being far too intrusive, was explaining something to a husband and wife. What caught my eye was that they were all looking at a Temblor blog post!”

Appraisal of SASMEX’s performance by a national expert

Dr. Gerardo Suárez, professor and former Head of the Institute of Geophysics, National Autonomous University of Mexico (UNAM), wrote us, “The earthquakes in September of 2017 and the two subduction events in the last few days have put SASMEX to the most rigorous and thorough test it has had in its almost 25 years of existence. The M=8.2 Tehuantepec earthquake and the recent M=7.2 and 6.0 Oaxaca events were correctly identified, resulting in early warning of almost 120 s in Mexico City ranged for the Tehuantepec earthquake to about 70 seconds for the February Oaxaca earthquakes. This gave ample time to a large fraction of the population to evacuate schools and other facilities. The city of Oaxaca received an advanced warning of 28 s, Acapulco 44 s,and Puebla 64 s before the arrival of the strong seismic shaking.”

“The hard lesson of the 19 Sep 2017 M=7.1 Puebla quake, in which the warning did not precede the strong shaking, is that earthquakes close to population centers will give a short time alert times. A faster algorithm now under test that makes use of the first 3 s after the onset of the P wave would have given about 10 s of warning to Mexico City. But the public has to understand this intrinsic limitation and procedures should be established to make adequate and effective use of the precious few seconds available before the damaging seismic waves arrive.”

Mexico City: A special case

While Mexico City’s earthquake early warning system has shown its ability to detect earthquakes, the city’s location makes it an ideal site for early warning. This is because the majority of Mexico City’s earthquake hazard comes from the subduction of the Cocos plate at the Middle America Trench several hundreds of kilometers away, and the city is a giant water bed that shakes at least ten times harder and much longer than the surrounding highlands. This means that seismometers on the coast have time to detect the quake long before damaging S-waves arrive, and longer still before they get trapped in the lake bed, reverberating the city like a giant ‘woofer.’ Similar long warnings are unattainable in places like California where the metropolitan areas do not sit atop water-saturated sediments. Warnings will be shorter yet for cities astride active faults. But warnings of several seconds for the possibility of at least weak shaking are possible— worldwide.

What are we waiting for in the U.S?

In the U.S., the USGS, along with U.C. Berkeley, Caltech, the University of Oregon, and the University of Washington are developing an earthquake early warning system known as ShakeAlert. Seismology Professor Douglas Toomey, at the University of Oregon told Temblor that, “ShakeAlert is a West Coast wide system, which includes the Cascadia Subduction zone that has a tectonic setting similar to Mexico. An earthquake early warning system will perform well for megathrust Cascadia events that threaten Northern California, Oregon, Washington, and British Columbia.” The funds to complete and operate ShakeAlert have twice been removed from the President’s budget to Congress, leaving the program in limbo.

As Monica said to us this morning, “If Mexico City has a warning, why don’t we?”

References

Servicio Sismológico Nacional | UNAM, México

USGS

J. M. Espinosa-Aranda, A. Cuéllar, G. Ibarrola, R. Islas & A. García, F. H. Rodríguez & B. Frontana, The Seismic Alert System of Mexico (SASMEX) and their Alert Signals broadcast results, Proceedings of the Fifteenth World Conference on Earthquake Engineering, Lisbon, Portugal, 2012

SASMEX Preliminary Bulletins

NPR

New York Times