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| Gérault et al. (2015) Flat-slab subduction, topography, and mantle dynamics in southwestern Mexico (labels added) |
On Monday, September 19, an earthquake centered 110 km (70 mi) southeast of Mexico City struck with devastating force. Its 7.1 moment magnitude does not reveal the full story of its effects. As soon as I read that the earthquake was focused 51 km (32 mi) deep in the Earth, it struck me that the earthquake may be related to the unusual way in which the Cocos Plate is subducting beneath the Mexican continent.
Sure enough. The consensus so far is that the earthquake occurred within the Cocos Plate, right where the plate bends to dive steeply into the mantle.
As mentioned in a previous post, the Cocos Plate in this area is undergoing flat-slab subduction, also known as shallow-slab subduction. Between the subduction zone along the coast, where the plate first ducks beneath the leading edge of the continent, and where it makes its bend to dive into the mantle, the Cocos plate moves horizontally to the north-northeast beneath Mexico, in a flat-slab configuration. This is shown in the cross-sections in the accompanying diagrams, where the Cocos Plate is flat between its initial bend starting at the coastline, and its final, sharper bend beneath central Mexico.
The place where the Cocos Plate bends down steeply into the mantle and leaves the North American plate behind is along the southern margin of the Trans-Mexican Volcanic Belt. The earthquake occurred right in that bend, as illustrated by the red dot in the cross-section in the figure below.
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| USGS Earthquake Hazards Program, M 7.1 - 5km ENE of Raboso, Mexico |
And that makes sense, because it was a normal-fault earthquake, and it took place within the subducting slab. Normal fault, intraslab earthquakes like this one tend to happen within subducting plates where those plates bend, a way for the plate to accommodate the stress of bending.
Jay Patton, another blogger I discovered while looking into this earthquake, goes into much more detail about the seismology than I do here. (Seismology is the scientific study of earthquakes, and is usually based on analyzing the seismic waves). Should you care to dig deeper, his blog is Jay Patton Online.
Just as a reminder, here is a simplified map of the world's tectonic plates.
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| http://eepb.tk/map-world-tectonic-plates/ |
I titled this post "It Gets Worse" because of the greater amount of wreckage and loss of life from the central Mexican earthquake on Monday than from the M8.1 earthquake that took place in the subduction zone off the Chiapas coast 11 days earlier. In terms of what humans value, starting with human life itself, both earthquakes had tragic effects. As of this writing, there are reported to be about 100 dead in the first earthquake, in the south of Mexico, and about 250 dead as a result of the more recent earthquake that struck in central Mexico.
(It's early; as more bodies are found and identified, the number of lost lives may rise.)
The earlier earthquake was M8.1, the second earthquake M7.1. Both occurred on normal faults within the Cocos Plate, in zones where the plate bends.
So why was the second earthquake, of lesser magnitude, the cause of a greater number of fatalities? Two reasons: where its epicenter is located, and its depth.
Because it was centered near the greatest concentration of people in Mexico, which is Mexico City, and also near other large cities including Puebla, there were a lot more people, and buildings, near the second earthquake.
And because the central Mexican earthquake was shallower (51 compared to 70 km), the seismic waves hit the earth's surface with more force.
And then there is the fact that Mexico City is especially vulnerable to amplified shaking from seismic waves, probably more so than any other major city in the world. That's because this great city is built on a drained and filled-in lake, Lake Texcoco. The lake bed, and the Aztec capitol city of Tenochtitlan on which Mexico City is built, have been covered mostly by artificial fill. Much of modern Mexico City is built on this weak substrate.
Mexico City, sitting in its bowl-shaped geologic basin, is at the center of a "perfect storm" of factors that can add up to bad results from earthquakes. When vibrated by seismic waves, the thick pile of wet, fine-grained sediment beneath the city tends to shake like jello and undergo liquefaction in some places. At the same time, the weak substrate amplifies the height and slows the frequency of the seismic waves. Measurements indicate that the heights of earthquake waves moving across the ground of Mexico City are amplified many times higher than their height in surrounding areas.
During earthquakes, the frequency of seismic waves passing beneath Mexico City - the number of waves passing by per second - is slowed down to a range in which the waves are likely to be in resonance with buildings of a certain height. That means buildings in a certain height range will sway more and more as each subsequent wave strikes it, reinforcing rather than canceling out or damping the vibrations. If the earthquake continues long enough, this can build up to the point that the building exceeds its breaking point.
Resonance with surface waves is thought to be the reason why dozens of buildings between 10 and 25 stories high collapsed during the 1985 earthquake that shook Mexico City - even though the epicenter of that earthquake was 400 km (250 mi) away, near the city of Lázaro Cárdenas on the Michoacán State coast, where the Cocos Plate enters the subduction zone.
And by the way, the 1985 Michoacán earthquake was a subduction earthquake in the strict sense. It was a thrust fault earthquake, in which the North American Plate suddenly thrust upward along the master fault of the subduction zone. That master fault is the huge thrust fault which forms the interface between the overriding North American Plate and the subducting Cocos Plate.
That is why I am calling the two normal-fault, intraplate earthquakes that have stunned Mexico this month tectonic surprises. They are not thrust-fault earthquakes that took place along the subduction zone plate boundary, as one might expect upon hearing that a big earthquake took place in a subduction zone. Instead, they are normal-fault earthquakes that took place within the Cocos Plate.
True subduction earthquakes, which are also known as megathrust earthquakes, are the type that can reach magnitude 9.0 or higher. That makes them the most powerful type of earthquake the earth can generate, so far as we currently know. The 1985 Michoacán megathrust earthquake had a magnitude of "only" 8.0, yet it killed more than 9,000 people, mainly because of how its seismic waves interacted with the ground beneath Mexico City and destroyed dozens of buildings there.
That historic earthquake hit Mexico on September 19, 1985, exactly thirty-two years before this week's earthquake.
Resonance with seismic waves crossing the Mexico City basin during this week's M7.1 earthquake may be why a number of buildings have been badly damaged, and some collapsed. It looks in the news like they are mostly buildings 3-8 stories high. Today, people are still attempting to rescue the last survivors from the rubble, with signs from a few people inside the wreckage - voices, wriggling fingers - that they are still alive.
Resonance is not the only reason buildings break. The ground may shake enough to break buildings to begin with, without amplification, without resonance. It is just that Mexico City also tends to have those other factors adding on: amplification, resonance, weak soils that may undergo liquefaction, and so on, adding to the city's vulnerability when it comes to earthquakes. Plus it is a huge city, with thousands and thousands of buildings and many millions of people.
The people there are strong and resilient, that much is clear (see "Living in Mexico City is a Perpetual Dare" by Carlos Puig). Since the 1985 earthquake, people who live in Mexico, most of all in Mexico City, have practiced what to do in the event of an earthquake, and it appears that most of them did the right thing when this one struck. The part where they bravely and selflessly seek to help each other in the seconds, hours, and days afterward is not exactly part of the training, but it is what the people there do. The actions of the people make it better.
What point is there to learning about those earthquakes from this great distance away, so far away that, even though our seismometers could detect them, there is no way we could feel the earth shaking? The earthquakes in Mexico hold some key lessons for all of us.
We are all at risk for earthquakes, whether we live in Charleston or Los Angeles, Boston or St. Louis. Earthquakes are unpredictable and can occur anywhere at any time. Most earthquakes do not occur on faults that have been identified on geologic maps.
Some places have earthquakes more often. For example, the San Andreas fault zone in California frequently has earthquakes. Additionally, some places are more at risk for really big earthquakes. For example, only subduction zones, so far as we know, are at risk for M9 earthquakes. The recent earthquakes in Mexico give us some chances to learn about how a subduction zone produces big earthquakes in different ways, both out along a coast, and hundreds of kilometers inland beneath a continent.
When earthquakes occur, aside how the amount of shaking generally falls off with increasing distance from the epicenter, some places will shake more than others because of the geology in the ground underneath those places. The earthquakes in Mexico allow us to gain further insight into how the geology of the Mexico basin, the ground beneath Mexico City, causes seismic waves to get bigger, change their frequency, and liquefy parts of the ground.
When an earthquake shakes a region, some buildings, bridges, and other structures will survive better than others, because of how different types of structures - from how they are designed, to how they are attached to their foundations, to the materials they are build of, and so on - respond differently to seismic shaking and weakened soil. The results of the recent earthquakes in Mexico will be studied closely to learn which types of structures tended to fail during earthquakes, and to assess how much help revised buildings codes were in making newer buildings more safe. Engineers, city planners, and legislatures, not just in Mexico, but in other countries, too, can all learn from what happened and act to save more lives in the future.
Here in the Pacific Northwest, we live at - or here in eastern Washington, we live next door to - the Cascadia subduction zone. No two subduction zones are exactly alike. The subduction zone in Mexico has that flat-slab component reaching from the coast inland to within about 100 km (60 mi) of Mexico City before the subducting plate dives deep. The Cascadia subduction zone does not have flat-slab subduction, but like all subduction zones, it shares the propensity for gigantic subduction earthquakes, out along the coast, up to magnitudes 8, 9, or even 9+.
Cities up to several hundred km (over 200 miles) from a great subduction earthquake may be seriously damaged, if the combination of seismic waves, geology beneath the city, and how the city was built add together to amplify the seismic waves and weaken the ground beneath the city during the earthquake, and buildings and other built structures are not able to withstand the shaking and ground weakening.
Mexico City has the unfortunate honor of a combination of geology and artificial draining and infilling of its ground which makes the city especially vulnerable to damage from earthquakes.
But Mexico City is not the only place that has those problems. If the first two conditions exist where you live - if the geology and ground-infilling history of a place make it weak and extra shaky in the event of an earthquake - then it comes down to how building and infrastructure are constructed.
Just about every city has some zones of weak and extra shaky ground beneath it. Most cities are partly built on drained and filled in wetlands, lakes, or ponds. How well a city survives an earthquake is up to the engineers, scientists, and planners, the legislators who write the laws and building codes, the people who enforce the building codes, and the construction industry. Ultimately, in a democracy, it is up to all of us.
In Mexico City, after 1985, new buildings have been required to be engineered and constructed so that they would not collapse during an earthquake. Experts will assess how much the revised building code helped, and try to apply the lessons learned to preparing, yet again, for future earthquakes. Earthquakes caused by subduction are inevitable. How well we are prepared for an earthquake is under our control.
We can learn a lot from Mexico, about its subduction zone and what that tells us about how plate tectonics works, about how building codes work, about using an instant-warning system for incoming earthquakes (something only Mexico and Japan have), about how different cities, with their different geologies and building histories, are shaken and undermined in a variety of earthquake scenarios.
And we can be inspired by how the people there responded to the recent earthquakes, both the professionals, guided by by their training, and all the people who pitched in to help, driven by their humanitarian propensity.
(It's early; as more bodies are found and identified, the number of lost lives may rise.)
The earlier earthquake was M8.1, the second earthquake M7.1. Both occurred on normal faults within the Cocos Plate, in zones where the plate bends.
So why was the second earthquake, of lesser magnitude, the cause of a greater number of fatalities? Two reasons: where its epicenter is located, and its depth.
Because it was centered near the greatest concentration of people in Mexico, which is Mexico City, and also near other large cities including Puebla, there were a lot more people, and buildings, near the second earthquake.
And because the central Mexican earthquake was shallower (51 compared to 70 km), the seismic waves hit the earth's surface with more force.
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| The Aztec city, México-Tenochtitlan |
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| The modern city, Ciudad de México |
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| Cross-section of Mexico City basin. It is the upper Quaternary lacustrine and alluvial deposits, thick and wet, which amplify seismic surface waves, and tend to fail during earthquakes. Most of Mexico City is built on that substrate, with artificial fill on top. (Concrete Building Coalition) |
Resonance with surface waves is thought to be the reason why dozens of buildings between 10 and 25 stories high collapsed during the 1985 earthquake that shook Mexico City - even though the epicenter of that earthquake was 400 km (250 mi) away, near the city of Lázaro Cárdenas on the Michoacán State coast, where the Cocos Plate enters the subduction zone.
And by the way, the 1985 Michoacán earthquake was a subduction earthquake in the strict sense. It was a thrust fault earthquake, in which the North American Plate suddenly thrust upward along the master fault of the subduction zone. That master fault is the huge thrust fault which forms the interface between the overriding North American Plate and the subducting Cocos Plate.
That is why I am calling the two normal-fault, intraplate earthquakes that have stunned Mexico this month tectonic surprises. They are not thrust-fault earthquakes that took place along the subduction zone plate boundary, as one might expect upon hearing that a big earthquake took place in a subduction zone. Instead, they are normal-fault earthquakes that took place within the Cocos Plate.
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| Geology Cafe |
That historic earthquake hit Mexico on September 19, 1985, exactly thirty-two years before this week's earthquake.
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| Collapsed apartment building, Mexico City, 1985 (Wikipedia) |
Resonance with seismic waves crossing the Mexico City basin during this week's M7.1 earthquake may be why a number of buildings have been badly damaged, and some collapsed. It looks in the news like they are mostly buildings 3-8 stories high. Today, people are still attempting to rescue the last survivors from the rubble, with signs from a few people inside the wreckage - voices, wriggling fingers - that they are still alive.
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| "Volunteers and rescue workers search for children trapped inside..." (Seattle Times) |
Resonance is not the only reason buildings break. The ground may shake enough to break buildings to begin with, without amplification, without resonance. It is just that Mexico City also tends to have those other factors adding on: amplification, resonance, weak soils that may undergo liquefaction, and so on, adding to the city's vulnerability when it comes to earthquakes. Plus it is a huge city, with thousands and thousands of buildings and many millions of people.
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| New York Times |
The people there are strong and resilient, that much is clear (see "Living in Mexico City is a Perpetual Dare" by Carlos Puig). Since the 1985 earthquake, people who live in Mexico, most of all in Mexico City, have practiced what to do in the event of an earthquake, and it appears that most of them did the right thing when this one struck. The part where they bravely and selflessly seek to help each other in the seconds, hours, and days afterward is not exactly part of the training, but it is what the people there do. The actions of the people make it better.
What point is there to learning about those earthquakes from this great distance away, so far away that, even though our seismometers could detect them, there is no way we could feel the earth shaking? The earthquakes in Mexico hold some key lessons for all of us.
We are all at risk for earthquakes, whether we live in Charleston or Los Angeles, Boston or St. Louis. Earthquakes are unpredictable and can occur anywhere at any time. Most earthquakes do not occur on faults that have been identified on geologic maps.
Some places have earthquakes more often. For example, the San Andreas fault zone in California frequently has earthquakes. Additionally, some places are more at risk for really big earthquakes. For example, only subduction zones, so far as we know, are at risk for M9 earthquakes. The recent earthquakes in Mexico give us some chances to learn about how a subduction zone produces big earthquakes in different ways, both out along a coast, and hundreds of kilometers inland beneath a continent.
When earthquakes occur, aside how the amount of shaking generally falls off with increasing distance from the epicenter, some places will shake more than others because of the geology in the ground underneath those places. The earthquakes in Mexico allow us to gain further insight into how the geology of the Mexico basin, the ground beneath Mexico City, causes seismic waves to get bigger, change their frequency, and liquefy parts of the ground.
When an earthquake shakes a region, some buildings, bridges, and other structures will survive better than others, because of how different types of structures - from how they are designed, to how they are attached to their foundations, to the materials they are build of, and so on - respond differently to seismic shaking and weakened soil. The results of the recent earthquakes in Mexico will be studied closely to learn which types of structures tended to fail during earthquakes, and to assess how much help revised buildings codes were in making newer buildings more safe. Engineers, city planners, and legislatures, not just in Mexico, but in other countries, too, can all learn from what happened and act to save more lives in the future.
Here in the Pacific Northwest, we live at - or here in eastern Washington, we live next door to - the Cascadia subduction zone. No two subduction zones are exactly alike. The subduction zone in Mexico has that flat-slab component reaching from the coast inland to within about 100 km (60 mi) of Mexico City before the subducting plate dives deep. The Cascadia subduction zone does not have flat-slab subduction, but like all subduction zones, it shares the propensity for gigantic subduction earthquakes, out along the coast, up to magnitudes 8, 9, or even 9+.
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| Mexico City basin. Solid line shows border of filled-in Lake Texcoco. The darker the red, the more intense and damaging the ground-shaking during the September 19 earthquake. (New York Times) |
Cities up to several hundred km (over 200 miles) from a great subduction earthquake may be seriously damaged, if the combination of seismic waves, geology beneath the city, and how the city was built add together to amplify the seismic waves and weaken the ground beneath the city during the earthquake, and buildings and other built structures are not able to withstand the shaking and ground weakening.
Earthquake damage to a city depends on:
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1. the seismic waves (the nature of the earthquake)
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2. the geology beneath the city
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3. how the ground was altered - drained, filled in, etc.
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4. the engineering and construction of the buildings
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Mexico City has the unfortunate honor of a combination of geology and artificial draining and infilling of its ground which makes the city especially vulnerable to damage from earthquakes.
But Mexico City is not the only place that has those problems. If the first two conditions exist where you live - if the geology and ground-infilling history of a place make it weak and extra shaky in the event of an earthquake - then it comes down to how building and infrastructure are constructed.
Just about every city has some zones of weak and extra shaky ground beneath it. Most cities are partly built on drained and filled in wetlands, lakes, or ponds. How well a city survives an earthquake is up to the engineers, scientists, and planners, the legislators who write the laws and building codes, the people who enforce the building codes, and the construction industry. Ultimately, in a democracy, it is up to all of us.
In Mexico City, after 1985, new buildings have been required to be engineered and constructed so that they would not collapse during an earthquake. Experts will assess how much the revised building code helped, and try to apply the lessons learned to preparing, yet again, for future earthquakes. Earthquakes caused by subduction are inevitable. How well we are prepared for an earthquake is under our control.
We can learn a lot from Mexico, about its subduction zone and what that tells us about how plate tectonics works, about how building codes work, about using an instant-warning system for incoming earthquakes (something only Mexico and Japan have), about how different cities, with their different geologies and building histories, are shaken and undermined in a variety of earthquake scenarios.
And we can be inspired by how the people there responded to the recent earthquakes, both the professionals, guided by by their training, and all the people who pitched in to help, driven by their humanitarian propensity.
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