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.

 
 
 
 

The IPPC report on the melting ice caps makes for terrifying reading

A Greeland iceberg, 2007. Image: Getty.

Earlier this year, the Intergovernmental Panel on Climate Change (IPCC) – the UN body responsible for communicating the science of climate breakdown – released its long-awaited Special Report on the Ocean and Cryosphere in a Changing Climate.

Based on almost 7,000 peer-reviewed research articles, the report is a cutting-edge crash course in how human-caused climate breakdown is changing our ice and oceans and what it means for humanity and the living planet. In a nutshell, the news isn’t good.

Cryosphere in decline

Most of us rarely come into contact with the cryosphere, but it is a critical part of our climate system. The term refers to the frozen parts of our planet – the great ice sheets of Greenland and Antarctica, the icebergs that break off and drift in the oceans, the glaciers on our high mountain ranges, our winter snow, the ice on lakes and the polar oceans, and the frozen ground in much of the Arctic landscape called permafrost.

The cryosphere is shrinking. Snow cover is reducing, glaciers and ice sheets are melting and permafrost is thawing. We’ve known this for most of my 25-year career, but the report highlights that melting is accelerating, with potentially disastrous consequences for humanity and marine and high mountain ecosystems.

At the moment, we’re on track to lose more than half of all the permafrost by the end of the century. Thousands of roads and buildings sit on this frozen soil – and their foundations are slowly transitioning to mud. Permafrost also stores almost twice the amount of carbon as is present in the atmosphere. While increased plant growth may be able to offset some of the release of carbon from newly thawed soils, much will be released to the atmosphere, significantly accelerating the pace of global heating.

Sea ice is declining rapidly, and an ice-free Arctic ocean will become a regular summer occurrence as things stand. Indigenous peoples who live in the Arctic are already having to change how they hunt and travel, and some coastal communities are already planning for relocation. Populations of seals, walruses, polar bears, whales and other mammals and sea birds who depend on the ice may crash if sea ice is regularly absent. And as water in its bright-white solid form is much more effective at reflecting heat from the sun, its rapid loss is also accelerating global heating.

Glaciers are also melting. If emissions continue on their current trajectory, smaller glaciers will shrink by more than 80 per cent by the end of the century. This retreat will place increasing strain on the hundreds of millions of people globally who rely on glaciers for water, agriculture, and power. Dangerous landslides, avalanches, rockfalls and floods will become increasingly normal in mountain areas.


Rising oceans, rising problems

All this melting ice means that sea levels are rising. While seas rose globally by around 15cm during the 20th century, they’re now rising more than twice as fast –- and this rate is accelerating.

Thanks to research from myself and others, we now better understand how Antarctica and Greenland’s ice sheets interact with the oceans. As a result, the latest report has upgraded its long-term estimates for how much sea level is expected to rise. Uncertainties still remain, but we’re headed for a rise of between 60 and 110cm by 2100.

Of course, sea level isn’t static. Intense rainfall and cyclones – themselves exacerbated by climate breakdown – can cause water to surge metres above the normal level. The IPCC’s report is very clear: these extreme storm surges we used to expect once per century will now be expected every year by mid-century. In addition to rapidly curbing emissions, we must invest millions to protect at-risk coastal and low-lying areas from flooding and loss of life.

Ocean ecosystems

Up to now, the ocean has taken up more than 90 per cent of the excess heat in the global climate system. Warming to date has already reduced the mixing between water layers and, as a consequence, has reduced the supply of oxygen and nutrients for marine life. By 2100 the ocean will take up five to seven times more heat than it has done in the past 50 years if we don’t change our emissions trajectory. Marine heatwaves are also projected to be more intense, last longer and occur 50 times more often. To top it off, the ocean is becoming more acidic as it continues to absorb a proportion of the carbon dioxide we emit.

Collectively, these pressures place marine life across the globe under unprecedented threat. Some species may move to new waters, but others less able to adapt will decline or even die out. This could cause major problems for communities that depend on local seafood. As it stands, coral reefs – beautiful ecosystems that support thousands of species – will be nearly totally wiped out by the end of the century.

Between the lines

While the document makes some striking statements, it is actually relatively conservative with its conclusions – perhaps because it had to be approved by the 195 nations that ratify the IPCC’s reports. Right now, I would expect that sea level rise and ice melt will occur faster than the report predicts. Ten years ago, I might have said the opposite. But the latest science is painting an increasingly grave picture for the future of our oceans and cryosphere – particularly if we carry on with “business as usual”.

The difference between 1.5°C and 2°C of heating is especially important for the icy poles, which warm much faster than the global average. At 1.5°C of warming, the probability of an ice-free September in the Arctic ocean is one in 100. But at 2°C, we’d expect to see this happening about one-third of the time. Rising sea levels, ocean warming and acidification, melting glaciers, and permafrost also will also happen faster – and with it, the risks to humanity and the living planet increase. It’s up to us and the leaders we choose to stem the rising tide of climate and ecological breakdown.

Mark Brandon, Professor of Polar Oceanography, The Open University.

This article is republished from The Conversation under a Creative Commons license. Read the original article.