Potent Mexico City earthquake was a rare ‘bending’ quake, study finds – and it could happen again

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Six months have passed since a magnitude 7.1 earthquake struck Mexico City, toppling 40 buildings and killing over 300, but the memory remains fresh. Condemned structures dot many neighborhoods, their facades crumbling. And after an earthquake 225 miles away in Oaxaca state shook the capital city again on Feb. 16, 2018, the city mayor said hospitals treated dozens of people for panic attacks.

Seismologists, too, are still studying the Sept. 19 earthquake, trying to better understand what’s happening underneath Mexico City. Our new paper in Geophysical Research Letters brings critical findings to light.

Since the damaging quake, we have been analyzing data from the national network of seismological sensors, as well as high-quality GPS stations around the country. Together, these instruments measure shaking across Mexico. We wanted to know what caused this magnitude 7.1 earthquake and whether a future shock could strike even closer to this city of 20 million.

Here’s what we learned.

The Earth’s trembling surface

People in central Mexico are accustomed to the ground shaking. Since 1980, 40 perceptible earthquakes have hit this region. The Sept. 19 quake actually occurred on the 32nd anniversary of the magnitude 8.1 earthquake that killed at least 10,000 people in and around Mexico City in 1985.

That catastrophe marked an entire generation of Mexicans, including ourselves, back when we were just kids.

Now, as working seismologists, we have discovered that the 2017 earthquake, called Puebla-Morelos, was fundamentally unlike its 1985 predecessor. In fact, it was different than most big Mexican earthquakes, which typically happen along the country’s Pacific coast, where two tectonic plates collide.

The Puebla-Morelos quake occurred well inland – just 70 miles south of Mexico City, in Puebla state. Since the 1920s, only five other large earthquakes have originated in central Mexico.

The zone of potential ‘bending’ earthquakes, where the subducted tectonic plate that runs beneath Mexico juts downward at a sharp angle, is a band spanning the country from center to south. Only five earthquakes have struck this region in the past century, including the deadly Sept. 18, 2017 quake that killed 300 in Mexico City. Major earthquakes typically occur along the Pacific coast. D. Melgar, CC BY-SA


How earthquakes happen

Most major earthquakes worldwide happen along the unstable intersections in the Earth’s crust, where two tectonic plates – that is, the underground slabs that make up the planet’s rocky shell – collide, one plate sliding beneath the other.

These are called subduction zones, and continued plate movements in those areas are responsible for the world’s largest earthquakes – the kinds that occasionally rattle Alaska, Japan, Chile and Indonesia.

At most subduction zones, after one tectonic plate slides beneath a neighboring plate, it continues on a diagonal downward dive and sinks deep into the Earth’s mantle.

Not in Mexico. There, the initial contact between the two tectonic plates – which collide off the country’s southern Pacific coast – starts off normally enough, with the subducted plate sinking diagonally downwards.

But then, just as it begins to jut underneath the Mexican mainland, the plate – which is made of dense, heavy rocks – reverses course. It bends upward, sliding itself horizontally beneath the plate Mexico sits on top of. This setup continues for about 125 miles or so.

Then, underneath Puebla state – just south of Mexico City – at a depth of about 30 miles below ground, the subducted plate abruptly changes direction once more. It dives almost vertically downward, plunging itself deep into the Earth’s mantle.

What is a ‘bending’ quake?

When the plate bends downward, some of the rocks in the plate break. Think of a sturdy piece of wood. Flexed lightly, it bends. But when the flexing becomes too strong, it will splinter violently.

This is what causes “bending” earthquakes like Mexico City’s. After the bent tectonic plate snaps, seismic waves emanate outwards from the breaking point, causing the Earth to tremble. The closer you are to the epicenter, the stronger the shaking.