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.

 
 
 
 

Mayor Marvin Rees' hope for Bristol: A more equitable city that can 'live with difference'

“I call on everyone to challenge racism and inequality in every corner of our city," Bristol Mayor Marvin Rees says. (Matt Cardy/Getty Images)

When the statue of 18th century slave trader Edward Colston was torn from its plinth and dumped in Bristol’s harbour during the city’s Black Lives Matter protests on 7 June, mayor Marvin Rees was thrust into the spotlight. 

Refraining from direct support of the statue’s removal, the city’s first black mayor shared a different perspective on what UK home secretary Priti Patel called “sheer vandalism”:

“It is important to listen to those who found the statue to represent an affront to humanity,” he said in a statement at the time. “I call on everyone to challenge racism and inequality in every corner of our city and wherever we see it.”

48 year-old Rees, who grew up in the city, has since expanded on his approach to the issue in an interview with CityMetric, saying “wherever you stand on that spectrum, the city needs to be a home for all of those people with all of those perspectives, even if you disagree with them.”

“We need to have the ability to live with difference, and that is the ethnic difference, racial difference, gender difference, but also different political perspectives,” he added. “I have been making that point repeatedly – and I hope that by making it, it becomes real.” 


What making that point means, in practice, for Rees is perhaps best illustrated by his approach to city governance.

Weeks after the toppling of Colston’s statue, a new installation was erected at the same spot featuring Jen Reid, a protester of Black Lives Matter. However, the installation was removed, as “it was the work and decision of a London-based artist, and it was not requested and permission was not given for it to be installed”, Rees said in a statement.

Bristol may appear a prosperous city, logging the highest employment rate among the UK’s “core cities” in the second quarter of 2019. But it is still home to many areas that suffer from social and economic problems: over 70,000 people, about 15 percent of Bristol’s population, live in what are considered the top 10 percent most disadvantaged areas in England. 

In an attempt to combat this inequality, Rees has been involved in a number of projects. He has established Bristol Works, where more than 3,000 young people from economically disadvantaged backgrounds are given work experience opportunities. And is now setting up a commission on social mobility. “Launching a Bristol commission on social mobility is not only about social justice; it [should not be] possible for a modern city to leave millions of pounds worth of talent on the shelf, just because the talent was born into poverty,” he says.

The mayor is also a strong supporter of the UN’s Sustainable Development Goals (SDGs), explaining that SDGs offer a way to talk about sustainability within a framework of many issues, ranging from climate change and biodiversity to women’s issues, domestic violence, poverty and hunger.

“What we want to achieve as a city cannot be done as a city working alone,” he insists. “We don’t want to benefit only people inside Bristol, we want to benefit the planet, and the SDGs offer a framework for a global conversation,” suggesting that a vehicle should be launched that allows cities to work together, ideally with organisations such as the UN, the World Bank and the International Monetary Fund involved. 

Greater collaboration between cities would be “beneficial in terms of economies of scale,” he argues, “as cities could get more competitive prices when buying materials for building houses or ordering buses, rather than each city acquiring a few of them at a higher price.”

In an attempt to focus on the long term, Rees launched One City Plan in January 2019, setting out a number of goals for Bristol to achieve by 2050.

Investing in green infrastructure to meet 2030 carbon emission targets spelled out in the SDGs is a key area here, with the mayor noting that transport, mass transit and energy are important sectors looking for further investment and government funding: “The sooner we meet our targets, the sooner we will benefit from them, and invest in sectors that will provide people with jobs.”

Jobs, especially following the outbreak of Covid-19, are of paramount importance to Rees. Bristol’s council wants to ensure that any government money given to the city will be quickly passed on to businesses to help prevent redundancies, he says, though given that mass job losses seem inevitable, reskilling options are also being looked into, such as through a zero-carbon smart energy project called City Leap.

Another important area for investment in Bristol is affordable housing, with 9,000 homes already built under Rees’s term of office. “People could build a base for life with affordable housing, [and this would mean] their mental health would be better because they have a safe place,” he explains. “Children in families that have a home that is affordable are more likely to able to eat and to heat, [and they are more likely to enjoy a] better education.”

Taken in the round, Rees’s agenda for Bristol is its own blueprint for shaping history. The Colston statue now lies in safe storage, with a local museum likely to play host to the controversial monument. But the Black Lives Matters protestors were fighting for a fairer, more equal future, and it is here where Rees is determined to deliver.

Sofia Karadima is a senior editor at NS Media Group.