The Orphan Tsunami of 1700 [pdf]
pubs.usgs.gov33 points by oliverkwebb 4 days ago
33 points by oliverkwebb 4 days ago
Cascadia has become a little bit of an obsession for me. I had my house retrofitted to help it withstand the inevitable next really big one that is coming because of what I have learned about it (I am also well above the tsunami flood zone). Subduction zones are crazy powerful but it looks like we are finally starting to learn important things about them. The challenge though is getting people to accept that they are real and will happen and entire cities need to move because of them (I'm looking at you Ocean Shores).
Side note, any actual geologists in the room? The recent Philippians 7.6 looks like it may be following a growing pattern of megathrust forshocks to my -deeply- untrained eye. Does someone with actual knowledge and training have a take on that?
Got any recs for contractors in the area to do the work?
What should I get done? (ranch 2 story built in 90s). Cost to expect? Been having bad luck lately with bids and sketch contractors. Takes a lot of effort to sift through.
This was close to DYI with me and the local handyman figuring out a good way to tie my house to its foundation. Not a full retrofit but pretty good for my 100yo house with nothing remotely close to modern design. Here are some resources I found useful though.
https://dnr.wa.gov/sites/default/files/2025-03/ger_homeowner...
Sea wall?
I'm not an expert by any means, but I think the issue with seawalls is they are built to stop waves, not something that acts more like the ocean getting deeper. The water a tsunami brings in is pretty different than a simple wave on the ocean.
The Japanese had long since built immense concrete sea walls (and other barriers like thick clusters of tough, fast-growing trees) throughout the length of their eastern coastlines, and it didn't help enormously during the 2011 earthquake/tsunami. The sheer energy of the waves simply pushed the water over these barriers and killed thousands of people while destroying entire cities.
Another reply here alludes to the fundamental problem with tsunamis by mentioning a phenomenon that's more like the ocean suddenly growing deeper. That's not exactly right but it is close in a way. Tsunamis are vast water displacement events and completely unlike normal large tidal or wind-caused (storm surge) waves because while you might have a storm wave of, say, 15 meters height and a tsunami of the same height, the The tidal/storm wave has much less run-up mass/volume, it's usually just a bit more than what you get right before your eyes. The tsunami on the other hand has a lateral run-up mass behind it that stretches back for as many as dozens of kilometers if I remember correctly, and all of that mass has to keep moving forward until it exhausts itself. Thus when the wave first hits, that's just the very beginning of all the destructive power it brings. A whole vast freight train (so to speak) of surging water mass, with all the displacement energy that caused it built right in, still has to keep moving forward until it expires. This vastly destructive process can take a while to complete itself.
You see why this is also a problem when it comes to sea walls too? If you have a 15 meter concrete sea wall and it gets hit by a 20 meter storm wave, the wave might sort of cross over its top and flood a bit on the other side, but otherwise the sea wall does its basic job. But if that same sea wall is struck by a tsunami of even slightly less than its height, the surging lateral mass of water behind the initial wave just keep pushing forward tremendously until it heavily overflows the wall.
If you watch videos of the 2011 tsunami, and especially videos where the wave actually hits barriers and then overflows them completely, you'll see the above effects in action. Terrifying stuff and very unique to tsunamis, which, I repeat, are completely unlike any ordinary large wave.
Just to clarify a bit more here: Storm surges and tidal waves can be fantastically deadly and destructive too (Hurricane Katrina for example), I don't want to understate their danger. However, part of their destructiveness is the case because of a wider storm surrounding them. On the other hand, compared to the sheer ongoing energetic intensity of any tsunami of comparable or greater wave height, storm waves are the much weaker phenomena and much easier to stop with things like sea walls.
Long article from 2015 in The New Yorker about the Cascadia subduction zone:
https://www.newyorker.com/magazine/2015/07/20/the-really-big...
Just learnt something from the article: It's interesting that the warping of the seafloor is what causes tsunamis, and not the shaking itself. It explains why a shoreline might sometimes recede away before a tsunami's crest strikes: The recession is caused by the seawater dropping with the seafloor, while the forward surge is caused by the ensuing bounce.
Dropping or rising. At the borders of the sea and land plates the sea plates are slipping slowly below the continental crust. Pieces of the land crust get caught and dragged down. Over long periods of time you'll see forested land get dragged down below sea level and flooded to die.
The a rupture will occur, and in the biggest earthquakes you can get a fault that can rise 20 meters almost instantly causing trillions of tons of ocean to suddenly have to go somewhere. After the quake and tsunami you'll see the flooded forests can be many meters above land were new forest will grow and slowly start sinking again.
The largest tsunami on record came from a landslide in a bay: https://en.wikipedia.org/wiki/1958_Lituya_Bay_earthquake_and...
Yeah that'll happen when a good chunk of a mountain basically drops into your body of water, lol:
"The large mass of rock, acting as a monolith (thus resembling high-angle asteroid impact), struck with great force the sediments at bottom of Gilbert Inlet at the head of the bay. The impact created a large crater and displaced and folded recent and Tertiary deposits and sedimentary layers to an unknown depth."
With updated modeling showing that impact triggering the glacier to lift and subsequently release even more material, it's shocking anyone in the bay survived at all.
Edit - found a video with said papers modeling implemented, pretty neat: https://www.youtube.com/watch?v=B1axr5YGRwQ
Much of the literature references this as the biggest ever tsunami at 500+ meters, but an account from one of the survivors who was there on his fishing boat (with his 7-year-old son!) said this specific thing:
"The wave definitely started in Gilbert Inlet, just before the end of the quake. It was not a wave at first. It was like an explosion, or a glacier sluff. The wave came out of the lower part, and looked like the smallest part of the whole thing. The wave did not go up 1,800 feet, the water splashed there."
Still insane, but it was the immediate splash that scoured away trees and soil cover up to 527 meters up the mountain face, not a proper tsunami.
Both the fisherman and his son survived btw.
Yeah I personally agree it’s not quite the same as if the pacific ocean was coming up 527 meters before hitting the shore :)
I'd absolutely love to see something like this live, in the flesh, obviously from far enough to survive it. It would be one of those spectacles for completely flabbergasting one's sense of importance in a self-remaking world.