What is the maximum size of an earthquake that can occur on Earth?

On May 22, 1960, southern Chile was hit by a catastrophic earthquake that lasted for 10 minutes and made the ground shake so violently that people were unable to stand. Cracks appeared in roads and buildings crumbled to the ground. A man, quoted in a USGS report, initially thought the Cold War had escalated into nuclear Armageddon after experiencing the quake and its subsequent tsunami. The earthquake was named after the town closest to its epicenter, Valdivia, and had a magnitude of 9.5, the largest ever recorded.

Geoscientists say that it is possible for earthquakes to be larger, but the chances of a much larger quake are low. A quake larger than 9.5 would require an enormous chunk of crust to break all at once, but there aren’t many places on Earth where this can occur. Wendy Bohon, an earthquake geologist and science communicator, says that a 9.5 magnitude quake is probably the upper limit for what the planet can generate and a magnitude 10 is extremely unlikely.

The magnitude of an earthquake is a measurement of the amount of energy released, but it’s different from how intense the earthquake feels, which can be influenced by distance from the epicenter and ground conditions. The same quake will feel stronger on loose soil and sand than on firm bedrock. The magnitude of an earthquake depends on the total area of a fault that breaks, which in turn depends on the depth and length of the fault.

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The deepest faults are at subduction zones, where one tectonic plate pushes under another. Although quakes can sometimes occur as deep as 500 miles below the surface of the Earth, most deep quakes don’t cause much shaking at the surface. The faults most capable of causing large, damaging earthquakes are dipping faults in subduction zones, according to Heidi Houston, an earthquake geologist at USC. These dipping faults have the largest areas of rocks that can get stuck against each other and build up stress before breaking.

The magnitude of an earthquake is also dependent on how much the fault moves or slips. As a rule, smaller areas of breaking fault slip less than larger ones. For example, a magnitude 5 quake may slip a few centimeters, but a magnitude 9 may slip 66 feet or more. In the case of the 1960 Chile quake, the ground stretched and actually increased the area of the country, according to Sergio Barrientos, a seismologist at the University of Chile.

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The earthquake magnitude scale is logarithmic, meaning that each unit increase in magnitude results in a 10 times increase in ground motion and a 32 times increase in energy released. For example, the difference in energy released between a magnitude 8 and 9 quake is much greater than the difference between a magnitude 5 and 6. There is still scientific debate about the exact magnitude of the 1960 Chilean quake, but to illustrate the massive differences in energy released at the upper end of the magnitude scale, a magnitude 9 earthquake is like breaking 1,048,576 strands of spaghetti, while a magnitude 5 earthquake is like breaking one strand.

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