eli5: why is it impossible to get a magnitude 10 earthquake

In: 4021

The explanation I’ve seen is that rock is not strong enough to build up the pressure necessary to produce a 10 on the Richter scale. It crumbles and makes lesser quakes before being able to get to that point. Remember, each number on the scale is also 10 times stronger than the previous one. Exponential progress is incredibly powerful.

The scale for measuring earthquakes is a logarithmic scale. In a logarithmic scale each step *multiplies*, so a 2.0 would be 10x more than a 1.0. Then a 3.0 would be 10x more than a 2.0, which makes it 100x more than a 1.0. Then a 4.0 would be 10x more than a 3.0…etc.

This means that the kind of quake necessary to hit 10.0 would have to be very, very, *very* big. As it turns out, there is no fault line on the planet that’s able to create such a huge quake.

It is technically possible to have a magnitude 10 earthquake. The problem is that it requires a truly massive area to register as such. The fault line would basically have to run the length of the Earth, and no such fault line exists, which is why it is classified as “impossible”. *However*, if you include non-tectonic sources – like an asteroid slamming into Earth – then you could have magnitudes of really any quantity.

The magnitude is on a logarithmic scale *of the amplitude of the waves*. This works out that an earthquake with magnitude 6 has ~~10~~ *31* (see u/bravehamster comment for more) times as much energy as one that has magnitude 5 (on Richter scale, there are several scales).

The energy an earthquake can release is dependent (more or less) on the length of the fault line and the amount the fault line moves during the earthquake.

The largest recorded earthquake had magnitude of 9.5 along a thousand mile long fault line. To get to a magnitude of 10 you’d need the some combination of the fault line length and movement to increase significantly. It’s not certain conditions exist on earth to get to that. If you increase the movement then by the time you’d build up enough of a load along the fault to release that much energy you’d almost certainly have a smaller earthquake first that releases the stress, and it doesn’t appear there are any active faults long enough either to really get up to that level either.

There may be some unknown conditions out there, and it *might* be possible to get to 10 or close to, but it’s extremely unlikely, and virtually impossible to get much over that number – eg to get to 10.5 you’d need a fault that ran around halfway around the world to simultaneously go in a big way.

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