Cascadia Subduction Zone super-earthquake.

Imagine an 8.5 to 10 mag earthquake, but instead of at one localized point, its basically continuous along about 500 to 1000 miles of a line about 250 miles out in in the Pacific, where one tectonic plate is diving under others, but has been building up friction tension for ~300 years.

And it normally snaps roughly every 250 years, the last time it happened it caused a tsunami that hit Japan, and is the origin of many PNW Native People's flood stories.

So we're ~50 years overdue for that happening again.

When I was a kid, they said it was a 1 in 20 chance happening in this century. Now they say its a 1 in 3 chance.

After this process is over, after everything gets shaken to all hell, and tsunami'd... well, in many areas, the coastal plates actually end up something like 10 to 15 feet lower than it was previously...

So those areas are now just permanently flooded, now under the new default water line.

And if we are all super duper unlucky, this massive of an event could trigger other fault lines along the NA West Coast, in say, California...

... and the Cascade mountain range...

... yeah a lot of them are actually volcanoes, which were formed by this very same plate dynamic that would be snapping in a CSZ rupture... they have just been dormant for a long time... they could potentially become more active or even erupt.

So yeah, that would/could basically destroy most of civilization roughly west of I5, on the West Coast.