What is the magnitude of the largest possible earthquake?

On May 22, 1960, a devastating earthquake struck southern Chile. For about 10 minutes, the ground shook so violently that people could not stand on their feet. Cracks opened in the roads and buildings collapsed. One man, quoted in a USGS report (Opens in a new tab) about surviving the earthquake and subsequent tsunami, initially thought the Cold War had escalated into nuclear Armageddon.

The Valdivia earthquake, named after the city closest to the epicenter, had a magnitude of 9.5 on the Richter scale, the largest earthquake ever recorded before or since. But can earthquakes get bigger?

Geologists say the answer is yes. However, the chances of a much larger earthquake occurring are low. While an earthquake larger than 9.5 on the Richter scale can occur, it would require an enormous piece of the crust to break all at once – a fault movement that is extremely deep and unusually long. There aren’t many places on Earth where that could happen, said Wendy Bohon, an earthquake geologist and science communicator. A magnitude 9.5 earthquake is probably near the upper limit of what the planet can generate, Bohon told Live Science, and a magnitude 10 is extremely unlikely.

“It’s great for Hollywood, but it’s not realistic for Earth, thank goodness,” Bohon said.

Related: The 20 largest recorded earthquakes in history

Magnitude is a measure of the amount of energy released in an earthquake. It differs slightly from how severe an earthquake is, which can be affected by a person’s distance from the epicenter and ground conditions. Bohun said the quake itself would feel stronger to someone standing on soft soil and sand than to someone standing on a solid footing.

The magnitude of the earthquake depends on the total area of ​​the fault that is fractured. This, in turn, depends on how deep the crack is in the crust and how long this part is horizontally. There are physical limits to how wide an area it can fracture. The deepest faults are in subduction zones, where one tectonic plate pushes under another. Go deep enough, though, and the rocks are so warm they’re hot and sticky; Instead of breaking, it bends. While earthquakes can sometimes occur as deep as 500 miles (800 kilometers) below Earth’s surface, according to the USGS (opens in a new tab), most deep earthquakes don’t generate much vibration at the surface; It is those in the upper tens of kilometers of the crust that are most dangerous for people.

The faults most capable of causing large and destructive earthquakes are subduction faults in subduction zones, said Heidi Houston (Opens in a new tab). These plunging faults, so named because they are at an oblique rather than vertical angle, have the largest expanses of rock that can attach to each other, building up pressure and eventually collapsing.

“It’s really the size of the subduction fault plane that is the greatest control over the maximum magnitude of the earthquake, and these fault planes can get larger in the subduction zone setting,” Houston told Live Science.

A normal slip fault showing the fault plane, or fault area that breaks to cause an earthquake. (Image credit: USGS)

But there are also limits to the length of the fault fragment that can break. Even subduction zone faults don’t break all at once, Bohon said. Usually, something gets in the way – perhaps a seamount (undersea mountain), or a change in rock type or rock geometry that makes one segment of a fault more stress-resistant than its neighbour.

Another factor that fuels the magnitude of the quake, Houston said, is how much the fault has moved or slipped. As a rule, smaller fault fracture zones are smaller than larger ones. So, while a magnitude 5 earthquake could slide a few centimeters—a distance over which it is unlikely to break the ground—a magnitude 9 earthquake could slide 66 feet (about 20 meters) or more. In fact, the 1960 Chilean earthquake increased the country’s area because of the way the land stretched, Sergio Barrientos, a seismologist at the University of Chile who experienced the quake, told NPR in 2016 (opens in a new tab).

Related: Where are Earth’s Most Volcanoes?

Understand the size

An earthquake magnitude scale can inadvertently mask the difference between very large earthquakes. The scale is not linear, but logarithmic: for every unit rise, the Earth’s motion increases 10 times and the energy released 32 times. Bohon likes to use the metaphor of breaking a bundle of spaghetti. If breaking one strand of spaghetti was equivalent to a magnitude 5 earthquake, you would have to break 32 strands to release the energy of a magnitude 6 earthquake. On that measure of spaghetti, measure 7 is like a break of 1,024 strands, measure 8 is like 32,768 strands, and measure 9 is like 1,048,576 strands.

As this example shows, the difference between a magnitude 8 and magnitude 9 earthquake, in terms of the energy released, is much more than the difference between magnitude 5 and magnitude 6. Thus, a magnitude 9.5 to 9.6 earthquake takes up much more area than the fault zone fracture of the transition between 5.5 and 5.6 degrees.

Because of the uncertainty in the measurements, Houston said, there is still scientific debate about whether the 1960 Chilean earthquake of magnitude 9.5 was exactly that. But to underscore the point about the huge differences in magnitude of seemingly small numbers on this end of the magnitude scale, a magnitude 9.5 earthquake is more than twice as powerful as the next largest ever recorded, a magnitude 9.2 that hit the Alaska region. Houston said Prince William Sound in 1964.

There are, of course, planetary cataclysms that could theoretically lead to much larger earthquakes: a collision with an asteroid, for example. (Some scientists believe that the impact of the end-Cretaceous asteroid that killed the non-marine dinosaurs 66 million years ago triggered earthquakes of double magnitude, though quantifying is difficult.) Houston said. But she said the chances of something larger than the mid-90s being in size in a human lifetime are very low. The largest ancient earthquake estimated based on geological evidence was also in Chile, about 3,800 years ago, and it also likely had a magnitude of about 9.5, according to the 2022 research.

Bohon said size isn’t always the most important factor in how dangerous an earthquake is, at least not for humans. Small earthquakes have caused many, many deaths, just because they hit populated areas and buildings are prone to collapse. While a magnitude 9.5 earthquake in Chile killed about 2,000 people, a magnitude 8 earthquake is believed to have killed about 830,000 people in Shaanxi, China, in 1556. In 2005, a magnitude 7.6 earthquake killed an estimated 79,000 people in Kashmir, and in 2010 a 7.0-magnitude earthquake killed nearly 220,000 people in Haiti. Even the 1994 Northridge earthquake, a magnitude 6.7 on a fault that no one had ever noticed, killed 57 people, injured thousands, and caused billions of dollars in damage as it affected Los Angeles.

“A lot of potential faults could have devastating earthquakes,” Bohon said. “But people only think of the big thing.”

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