# Farallon plate subducting under North American plate: is it a factor in "the next big one"?

I am a writer, not a scientist of any kind. I live in the SW AZ desert (USA), so we get earthquakes from time to time. I grew up in southern CA, right next to the San Andreas fault, so I have a cursory "high school geology" understanding of tectonic plates and earthquake forces. (The schools made sure we understood what was going on and how to prepare for The Big One.)

In doing some more research for a writing project, I came across some web sites that showed the progressive subduction of the Farallon plate (for instance: https://pubs.usgs.gov/gip/dynamic/Farallon.html). I'd never known about that one! These show that all that remains is the Juan de Fuca plate at the north end, and the Rivera and Cocos plates at the south.

So now I'm wondering what has happened to the middle? One web site (and I'm sorry I can't find the link right now) suggested it "folded" and was pushed edge-downward toward the center core. Other sources seem to suggest it broke apart and the remnants fused with the North American plate.

Al of this is in concert with the idea of "the next big one" on the West Coast, where over a hundred years of pent-up friction suddenly releases and the North American and Pacific plates move up to 20 feet. (Common theories bantered about, based on about 2 inches of movement per year, but unable to move that much for over a hundred years. Feel free to correct, please!!)

If the Farallon plate was still solid underneath the NA plate, would it also push the Juan de Fuca, Rivera, and Cocos plates as well? If the three remnant plates are indeed separate and now caught in between the two larger plates moving opposite directions, would they shear apart? Or just individually subduct more (which in itself would be quite a violent movement)?

If the answers are too complex and what I really need is a better basic understanding of the dynamics of plate movements, I would appreciate any resources you can point me towards.

(One terminology question: I keep seeing references to a fault causing earthquakes. It's actually the other way around, isn't it? Plate movement causes earthquakes, which creates faults in the crust up where we are?)

Thank you!!

• "...was still solid..." - Common misconception. Earth's mantle is overwhelmingly solid. There is very little magma down there. Mar 24, 2021 at 12:38
• Geologist Nick Zentner recently uploaded a YT lecture called How did the Rocky Mountains Form, and he said that recent data shows an even older ocean crust than the Farallon plate that was broken off long ago and the most ancient part of it now sits as a large folded 'ribbon candy' formation slowly sinking below the East coast of the USA. Great video highly recommended watch for anyone interested in the subject! Aug 15, 2023 at 1:45
• lets really blow your mind and see how much of the plate is still attached under NA researchgate.net/figure/…
– John
Aug 15, 2023 at 12:32
• San Andreas Fault is a transform fault. There is lateral movement going on. Though an earthquake in a nearby subduction zone might trigger something along the San Adreas fault. Farallon plate is done with, but has some fragments left (Cascadia subduction zone). Might help for further serach, but I am not confident enough for an answer.
– user29219
Sep 15, 2023 at 21:28
• Transform fault also means it exists without subduction taking place. Just like the Azores Gibraltar transform fault which was the cause of devastating earthquakes in history but has no active subduction around. So, while we can't exclude it, there'd probably be a long way to go to postulate that subduction triggers "the big one" along the transform fault. After it's happened is another thing, though.
– user29219
Sep 15, 2023 at 22:01