What is the strongest earthquake possible without wiping out all life?

Question

In the story in question, an extremely powerful earthquake decimates the Earth (but does not destroy it) due to Unforeseen Consequences of Our Actions. Ideally some humans need to survive, so accounting for damages to infrastructure, farming capability, etc., what’s the biggest earthquake that could happen on an Earth-like planet without destroying the planet or wiping out all life? It would have to be strong enough to create huge chasms, level mountains, and destroy most of a civilization, but at least some of humanity has to survive.

The reason for the earthquake can be arbitrary or nonexistent, so long as it causes as much damage as possible without destroying all life along.

Answer 1

As others have said, an earthquake alone is localised and cannot wipe out humanity on a global scale.

There is a related phenomenon that could: volcanic winter. In 1816 a volcanic eruption in Indonesia caused failed harvest in Europe and North America, known as the year without a summer. Multiply that by 10 over a slightly longer period and you have global starvation.

As for unforeseen consequences, it is known that volcanic activity is higher in interglacial periods due to less pressure on the earth's crust. So paradoxically, global warming could be the trigger for a massive volcanic event. Another possible consequence of deglaciation caused by global warming is a sudden release of large quantities of freshwater into the sea. This has happened in the past in the North Atlantic - it cut off the gulf stream circulation in the Atlantic and caused cooling in western Europe

The biggest direct consequence of an earthquake would be a tsunami, but the height of these is limited to a few metres by the max vertical displacement of land that an earthquake achieve. A related phenomenon is a megatsunami which is caused by sudden displacement of a large amount of material into water. This may or may not be triggered by an earthquake. The Lituya Bay megatsunami washed out trees to a height of 524 metres, but that was local. Landslides at volcanic islands can create megatsunamis. It is said that a megatsunami in the Canary Islands could cause damage to the East coast of the United States, though its height would be reduced to that of a regular tsunami as it crossed the atlantic and spread out from its epicentre. I suppose such a landslide at a volcanic island could trigger or be caused by an eruption.

An exceptionally unfortunate chain reaction of the above events could give the result you are looking for.

Note that dam building can cause geological instabity, or exacerbate the consequences of a landslide or earthquake. Vajont dam (mentioned in the megatsunami link) is an example. A much worse dam failure (caused by bad weather and poor engineering) occured at Banqiao and killed 250,000 people, considered the worst technological disaster of all time.

Answer 2

If you think that even an event like the Chicxulub impact did not wipe the entire life out of the planet, I believe that limit is pretty high.

The impact itself is estimated to have caused an earthquake with magnitude 9-11, and I suspect that an earthquake capable of, as you ask, leveling mountains, will produce similar side effects (tsunami, dust, maybe volcanic eruptions).

However I also suspect that it would be pretty difficult to store that much potential energy in the crust, without having it break and release it at an earlier stage.

Infact, as geologists have said

The Valdivia earthquake, named after the town closest to its epicenter, was roughly a magnitude 9.5, the largest ever recorded before or since. But could quakes get bigger?

The answer, geoscientists say, is yes. However, the chances of a much larger quake are low. While a quake larger in magnitude than 9.5 could occur, it would require an enormous chunk of crust to break all at once — the movement of a fault both enormously deep and extraordinarily long. There aren't many places on Earth where that could happen, said Wendy Bohon, an earthquake geologist and science communicator. A 9.5 magnitude quake is probably right around the upper limit for what the planet can generate, Bohon told Live Science, and a magnitude 10 is extremely unlikely.

Answer 3

Frame Challenge: Earthquakes are localized. Remember that the Virginia earthquake that was felt from Florida to Canada was felt over an enormously large area -- for an earthquake. Also the reason it was felt over such an area was the same reason it caused so little damage, the energy was slowly dissipated.

Even tsunamis, which have much larger impact, are localized by the presence of land masses. There is nowhere on earth where a tsunami would hit all of the earth's land masses -- and even there, it hits only the shores.

If you want large, widespread death tolls from earthquakes, your best bet would be some kind of event in the core, that causes cascading earthquakes. The effect would have to reactivate old faults, in places where earthquakes are uncommon and even tear apart plates. Tsunamis would help.

Individually these would need to be no stronger than typical city wrecking ones. But there would need to be a lot.

Answer 4

Well the only way to disrupt life is really make earth unliving, which is easiest done by just having 'no earth'.

An earthquake magnitude 20 something (which is ridiculously high, considering the richter scale is a log scale) has more energy that the bounding energy of the earth itself. So an earthquake of that size would 'quake' violently enough that it would rip earth apart into dust.

I don't think many organisms would survive space or the forming of a new protoplanet when the dust collects again.