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.

 
 
 
 

Older people need better homes – but then, so does everybody else

Colne, Lancashire. Image: Getty.

Towards the end of last year, I started as an associate director at the Centre for Ageing Better, working particularly on our goal around safe and accessible homes. Before I arrived, Ageing Better had established some ambitious goals for this work: by 2030, we want the number of homes classed as decent to increase by a million, and by the same date to ensure that at least half of all new homes are built to be fully accessible.

We’ve all heard the statistics about the huge growth in the number of households headed by someone over 65, and the exponential growth in the number of households of people over 85. Frustratingly, this is often presented as a problem to be solved rather than a major success story of post war social and health policy. Older people, like everyone else, have ambitions for the future, opportunities to make a full contribution to their communities and to continue to work in fulfilling jobs.

It is also essential that older people, again like everyone else, should live in decent and accessible homes. In the last 50 years we have made real progress in improving the quality of our homes, but we still have a lot to do. Our new research shows that over 4 million homes across England fail to meet the government’s basic standards of decency. And a higher proportion of older people live in these homes than the population more generally, with over a million people over the age of 55 living in conditions that pose a risk to their health or safety.

It shouldn’t be too difficult to ensure all our homes meet a decent standard. A small number of homes require major and expensive remedial work, but the overwhelming majority need less than £3,000 to hit the mark. We know how to do it. We now need the political will to make it a priority. Apart from the benefits to the people living in the homes, investment of this kind is great for the economy, especially when so many of our skilled tradespeople are older. Imagine if they were part of training young people to learn these skills.


At a recent staff away day, we explored where we would ideally want to live in our later lives. This was not a stretch for me, although for some of our younger colleagues it is a long way into the future.

The point at which the conversation really took off for me was when we moved away from government definitions of decency and accessibility and began to explore the principles of what great homes for older people would be like. We agreed they needed light and space (by which we meant real space – our national obsession with number of bedrooms as opposed to space has led to us building the smallest new homes in Europe).

We agreed, too, that they needed to be as flexible as possible so that the space could be used differently as our needs change. We thought access to safe outdoor space was essential and that the homes should be digitally connected and in places that maximise the potential for social connection.

Of course, it took us just a few seconds to realise that this is true for virtually everyone. As a nation we have been dismal at moving away from three-bed boxes to thinking differently about what our homes should look like. In a world of technology and factory building, and as we build the new generation of homes we desperately need, we have a real chance to be bold.

Great, flexible homes with light and space, in the places where people want to live. Surely it’s not too much to ask?

David Orr is associate director – homes at the Centre for Ageing Better.