Last week’s magnitude 7.1 earthquake caught me, and everyone else who lives in Mexico City, by surprise. The problem was that everyone here is accustomed to getting plenty of warning before an earthquake – and this time we didn’t.
Since 1993, Mexico City has had a fully operational earthquake early warning system. When a magnitude 8.1 quake struck , the city got over a minute of warning.
But showed that the system can’t respond to all types of events. Its failure to warn the city may hold lessons, not just for Mexico, but also for other countries working to give their citizens a head start.
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“This event in Mexico City is critical,” says at the University of California, Berkeley.
Quake alert
Mexico’s early warning system was born in the aftermath of a 1985 earthquake that levelled parts of the city and took thousands of lives. The unique kind of earthquakes that affect Mexico City meant that such a system was particularly viable.
The quakes are generated by two continental plates that meet along much of Mexico’s west coast. Mexico City lies hundreds of kilometres from this source of quakes, so it ought to be safe. But the city was built on a loose lakebed topped with even looser landfill. This loose soil picks up tremors from far away. In 1985, shifting soil went on toppling buildings long after the earthquake had petered out.
This threat was also an opportunity. With such a long distance between the epicentre and the danger, scientists could create an early warning system. Today, a standard 1985-style earthquake on the coast will trigger alarms in Mexico City 60 or even 90 seconds before it arrives.
“If you have minutes, you can empty any school practically anywhere,” says seismologist at the National Autonomous University of Mexico. Suarez is an advisor to , which issues the warnings.
But the warning on 19 September was more like 0 to 5 seconds.
Little or no warning
That’s because the quake began in a different part of the plate boundary, deeper and closer to the city, which we previously believed was relatively safe. It was more like the sort of tremor that could strike California. This is what has Allen’s attention.
For decades, Allen struggled to persuade California to create a warning system. In 2014, he finally got federal funding and in 2018 a prototype will start issuing limited alerts. Unlike the Mexican system, it will only give about 10 seconds of warning.
There are many potential pitfalls. What if the system gives too many false alarms, so people start ignoring them and are caught out when a big quake comes? What if the system misses the big one? And crucially, what if 10 seconds just isn’t enough time? Understanding how Mexicans responded to such a short window might help the US understand what to do.
It won’t be easy. In my house, the warning arrived at the same time as the first shocks. Others say they ended up in a pile of people slipping and falling down the stairwell.
“What are you going to do with a 5, 8, 10-second warning?” Suarez wonders. His wife was able to leave the store she was shopping in and get to the street, but she was lucky. He says most people won’t be able to do much except duck under their desks.
However, even if people cannot do much in a handful of seconds, computers can do plenty. Suarez says many responses could be automated. For instance, computers could halt trains, stop elevators at the nearest floor, and warn surgeons in operating rooms. He also suggests that Mexico broadcast a countdown, not just a warning, so people know how long they have.
Mere seconds
It is possible to stretch the warning time somewhat. At the moment, Mexico’s warning system relies on S-waves, which offer definitive evidence of an oncoming quake but move slowly through the ground. Suarez says Mexico is now looking at incorporating P-waves, which move faster but could give more false alarms.
California, which does not have the luxury of distant quakes, has always planned to use P-waves. It needs every second it can get.
But there are going to be limits. A study published on 21 September found that large and small quakes are indistinguishable when they start, so there is no way to tell if a quake is going to be huge until it’s too late.
In other words, seismology can only give so much warning. The trick will be to make the best use of those few seconds to save as many lives as possible.
“I personally believe that no children should die below the rubble of their own school,” Suarez says.
