What is the maximum possible magnitude for an earthquake?

A very strong earthquake occurred in 1960 in Chile, which became known as Valdivia, in relation to the city closest to its epicenter, went down in history as the strongest earthquake of all time.

The largest earthquake on record reached 9.5 on the Richter scale. Is a new episode likely to cross the line? Image: Andrey VP/Shutterstock

Since then, no others have surpassed the 9.5 magnitude mark reached in this episode. Here comes the question: this has never happened before because this would be the maximum possible magnitude for an earthquake on our planet?


The answer is: it is not. The chances of a much larger earthquake are there, but very small. Although an earthquake of magnitude greater than 9.5 could occur, it would require a huge chunk of the Earth’s crust to detach immediately – the movement of an extremely deep and extremely large fault.

“There aren’t many places on Earth where this could happen,” Wendy Bohon, an earthquake geologist and communications scientist, said in an interview with the website. living science. “A magnitude 9.5 earthquake is probably right around the upper limit for what the planet can produce, and a magnitude 10 is extremely unlikely.”

Credits: BBC News Brazil

Are magnitude and intensity the same thing?

Magnitude is a unit of measurement that quantifies the energy released in an earthquake. It’s a bit different than intensity, which can be affected by one’s distance from the focus and ground conditions. “The same earthquake will be stronger for someone standing on loose soil and sand than someone standing on solid rock,” Bohon explained.

The magnitude of an earthquake depends on the total extent of a fault that ruptures. This, in turn, depends on how deep the fault goes down through the crust and how long the segment breaks horizontally.

There are natural limits to how large an area can be broken. The deepest faults are found in subduction zones, where one tectonic plate moves beneath another. But going down deep enough, the rocks are so hot and malleable that instead of breaking, they bend.

Although earthquakes can sometimes occur as much as 800 kilometers below the Earth’s surface, according to the United States Geological Survey (USGS), most deeper earthquakes do not cause much surface shaking. Those located in the upper tens of kilometers of the crust are the most dangerous for people.

Type of tectonic fault most prone to earthquakes

The faults most likely to trigger large earthquakes are sinkholes in subduction zones, said Heidi Houston, an earthquake geologist at the University of Southern California.

These dip faults, so named because they are at an oblique rather than a vertical angle, have the largest areas of rock that can be wedged together, building up pressure and eventually fracturing.

“It’s really the size of the subduction fault plane that is the biggest control on the maximum earthquake size, and those fault planes can get bigger in the subduction zone scenario,” Houston said.

A magnitude 7 earthquake struck Lombok, Indonesia in 2018. Credit: Kurniawan Rizqi – Shutterstock

There are even limits to the length a fault thread can break. “Even subduction zone faults don’t all break at the same time,” Bohon said. “Usually something gets in the way — a mountain, perhaps, or a change in rock type or rock geometry that makes one part of a fault more resistant to pressure than its neighbor.”

Houston explains that another factor that drives earthquake size is how much the fault moves, or slides. As a general rule, smaller fault rupture zones slip less than larger ones. “So while a magnitude 5 earthquake might slip a few centimeters—a distance that is unlikely to rupture the ground above—a magnitude 9 earthquake might slip about 20 meters or more.”

“The 1960 earthquake in Chile really increased the area of ​​the country because of the way the ground spread,” Sergio Barrientos, a seismologist at the University of Chile who lived through the earthquake, told NPR in a 2016 interview.

Read more:

What is the Richter scale, which classifies earthquakes

The Richter scale is the basis for determining the magnitude of earthquakes, giving an accurate idea of ​​the potential for earthquakes occurring in the lithosphere – a layer about 100 km thick on average, formed by the Earth’s crust and mantle (learn more for the layers of the planet in this text).

As explained on the UOL Mundo Educação website, the calculation of the Richter scale is usually related to the distance from the epicenter (exact place of the earthquake underground) to the epicenter (point where the earthquake is most strongly felt on the surface), in addition to the time of occurrence and its width.

However, for those cases where earthquakes occur at great depths, there are other means of calculation, since their effects on the surface, as mentioned above, are generally small.

In general, earthquakes above 6 can be considered serious.

Richter Scale:

  • Magnitude less than 2: tremors recorded only by seismographs (a device that records ground movements).
  • Magnitude between 2 and 4: impact similar to the passage of a large, heavy vehicle.
  • Size between 4 and 6: ability to break glass, crack walls and move furniture.
  • Size between 6 and 7: destroys buildings and destroys fragile buildings.
  • Size between 7 and 8: causes severe damage to buildings and large cracks in the ground.
  • Size between 8 and 9: causes the destruction of bridges, viaducts and almost all structures.
  • Magnitude greater than 9: Causes total destruction with visible ripples.

The scale is not linear, but logarithmic: for every unit that goes up, the ground motion increases 10 times and the energy released increases 32 times. Bohon likens it to breaking spaghetti strings.

“If breaking one strand of spaghetti is equivalent to a magnitude 5 earthquake, you would have to break 32 strands to release the energy of a magnitude 6 earthquake,” he said. “On this spaghetti scale, a size 7 is like 1,024 strands breaking, a size 8 is like 32,768 strands, and a size 9 is like 1,048,576 strands.”

As the example given by the scientist shows, the difference between a magnitude 8 and 9 earthquake, in terms of energy released, is much greater than the difference between a magnitude 5 and a magnitude 6 earthquake.

So increasing the magnitude of an earthquake from 9.5 to 9.6 takes much longer to rupture an area fault than it does for it to range between 5.5 and 5.6.

There are, of course, planetary catastrophes that could theoretically lead to very large earthquakes: an asteroid impact, for example – some scientists believe that the asteroid impact that killed the birdless dinosaurs 66 million years ago caused earthquakes of double-digit magnitudes, though it is difficult to certify the size.

“On time scales of billions of years, Earth could certainly see such a catastrophe,” Houston said. “But the odds of anything larger than the average 9 in size in a human lifetime are very low.”

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