Potent Mexico City earthquake was a rare ‘bending’ quake, study finds

People work to clear debris in Mexico City after the September 2017 earthquake. Image: Getty.

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 16 February, the city mayor said hospitals treated dozens of people for panic attacks.

Seismologists, too, are still studying the 19 September 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 analysing 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 20m.

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 19 September earthquake 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 September 2017 quake that killed 300 in Mexico City. Major earthquakes typically occur along the Pacific coast. Image: D. Melgar/creative commons.

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.

This kind of rare Mexican quake typically has a relatively lower magnitude than the more common Pacific coast variety. But that doesn’t mean the shaking above ground feels weak. Because “bending” quakes strike in Mexico’s densely populated central region, beneath the feet of many millions, the shaking can be very strong indeed.

And when they hit near Mexico City, as September 2017 demonstrated, the consequences can be devastating.

Defining the hazard zones

This same unstable subducted plate runs underneath all central Mexico. And, thanks to previous studies, we know that it is bent across a large, continuous swath of central and southern Mexico.

It is here – from Michoacán state, part way up the Pacific coast, all the way down to southernmost Oaxaca – that bending earthquakes could occur.

But the tectonic plate’s bend, we learned, is only half of the story behind central Mexico’s shaking. The plate’s texture matters, too.

High-resolution images of the ocean bottom off Mexico’s Pacific shore reveal that the seafloor terrain is rugged in a very organised fashion. There, beneath thousands of feet of water, we see high, narrow ridges and deep valleys that run lengthwise in a northwest-to-southeast direction.

This “fabric” was created about 8m years ago, when the rocks first formed – way before tectonic plates collided to give Mexico its subduction zone. Even so, the plate’s texture – marked by this linear fabric of underground mountains and canyons – turns out to be relevant in determining where these rare, bending earthquakes might occur.

High-resoulution images of the Pacific Ocean seafloor off Mexico’s coast reveals that the subducted plate there has a linear texture comprised of ridges, valleys and bumps. This ‘fabric’ continues when the plate slides beneath the Mexican mainland and then angles downward, plunging itself into deep into the Earth’s mantle. ‘Bending’ earthquakes are most likely to occur where the plate bends in the same northwest-to-southeast direction as its ridges and valleys run. Image: Global Multi-Resolution Topography Data Synthesis/creative commons.

Our research found that because its ridges and valleys are oriented uniformly – think of the grain on a sturdy piece of wood – a tectonic plate is far less likely to snap if the force that bends it is at an angle perpendicular to the direction the fabric runs. Like a sheet of plywood, a tectonic plate is more resistant to pressure when bent against the grain.

In other words, large, damaging “bending” earthquakes are most likely to occur where the subducted plate’s own texture aligns with the direction of its downward bend.

This is good news for cities like Morelia, in Michoacán, where we believe the plate’s fabric runs almost perpendicular to the direction of the plate’s break – the wrong setup for a strong earthquake.

But it is bad news for neighboring Puebla and Oaxaca. There, plate texture and plate bend are almost perfectly aligned – off by less than 10 degrees. Under such circumstances, the bent plate can more easily snap and break from continued tectonic movement.

What’s in store for Mexico City?

The part of the plate bend near Mexico City, where the September quake occurred, falls somewhere in between. The alignment between texture and plate is not perfect – but they’re off angle by just 20 to 30 degrees.

That means the capital could see another large quake. And, based on our analysis, the epicenter could actually be closer to the city. This volatile tectonic band extends as far north as the city of Cuernavaca, 30 miles from Mexico City’s southern edge.

These findings are a step forward in understanding Mexico’s complex geology. But we still don’t know how often “bending” earthquakes might happen – whether once a century or every decade. Seismologists worldwide are still far from being able to predict where, when and how the next big one will strike.

The ConversationWhat our new study can do, we hope, is help Mexicans nationwide understand what’s happening beneath their feet.

Diego Melgar, Assistant Professor of Geophysics, University of Oregon and Xyoli Pérez-Campos, Professor, Universidad Nacional Autónoma de México (UNAM).

This article was originally published on The Conversation. Read the original article.

 
 
 
 

There isn’t a war on the motorist. We should start one

These bloody people. Image: Getty.

When should you use the horn on a car? It’s not, and anyone who has been on a road in the UK in living memory will be surprised to hear this, when you are inconvenienced by traffic flow. Nor is it when you are annoyed that you have been very slightly inconvenienced by another driver refusing to break the law in a manner that is objectively dangerous, but which you perceive to be to your advantage.

According to the Highway Code:

“A horn should only be used when warning someone of any danger due to another vehicle or any other kind of danger.”

Let’s be frank: neither you nor I nor anyone we have ever met has ever heard a horn used in such a manner. Even those of us who live in or near places where horns perpetually ring out due to the entitled sociopathy of most drivers. Especially those of us who live in or near such places.

Several roads I frequently find myself pushing a pram up and down in north London are two way traffic, but allow parking on both sides. This being London that means that, in practice, they’re single track road which cars can enter from both ends.

And this being London that means, in practice, that on multiple occasions every day, men – it is literally always men – glower at each other from behind the steering wheels of needlessly big cars, banging their horns in fury that circumstances have, usually through the fault of neither of them, meant they are facing each other on a de facto single track road and now one of them is going to have to reverse for a metre or so.

This, of course, is an unacceptable surrender as far as the drivers’ ego is concerned, and a stalemate seemingly as protracted as the cold war and certainly nosier usually emerges. Occasionally someone will climb out of their beloved vehicle and shout and their opponent in person, which at least has the advantages of being quieter.

I mentioned all this to a friend recently, who suggested that maybe use of car horns should be formally restricted in certain circumstances.

Ha ha ha. Hah.

The Highway Code goes on to say -

“It is illegal to use a horn on a moving vehicle on a restricted road, a road that has street lights and a 30 mph limit, between the times of 11:30 p.m. and 07:00 a.m.”

Is there any UK legal provision more absolutely and comprehensively ignored by those to whom it applies? It might as well not be there. And you can bet that every single person who flouts it considers themselves law abiding. Rather than the perpetual criminal that they in point of fact are.


In the 25 years since I learned to drive I have used a car horn exactly no times, despite having lived in London for more than 20 of them. This is because I have never had occasion to use it appropriately. Neither has anyone else, of course, they’ve just used it inappropriately. Repeatedly.

So here’s my proposal for massively improving all UK  suburban and urban environments at a stroke: ban horns in all new cars and introduce massive, punitive, crippling, life-destroying fines for people caught using them on their old one.

There has never been a war on motorists, despite the persecution fantasies of the kind of middle aged man who thinks owning a book by Jeremy Clarkson is a substitute for a personality. There should be. Let’s start one. Now.

Phase 2 will be mandatory life sentences for people who don’t understand that a green traffic light doesn’t automatically mean you have right of way just because you’re in a car.

Do write in with your suggestions for Phase 3.