I wasn't involved in this study, but I wrote the study that estimated the magnitude of this earthquake[0]. In case anyone is interested, usually the magnitudes of 'paleoearthquakes' (historic/prehistoric earthquakes discovered by finding evidence of old ground deformation) are estimated by relating the measured offset of the earth's surface or a rock/dirt layer across the fault line to the earthquake magnitude through empirical 'scaling relationships'; larger offsets are of course indicative of larger earthquakes. These are simply functions relating a measurable attribute of the earthquake to its magnitude. In the study I did, we combined the measurements of the offsets of a number of paleoearthquakes with estimates of the map length of the fault lines involved and used length-magnitude scaling relations to further refine the final magnitudes. There are some corrections for sampling bias that are included in there and it's all nice and Bayesian if anyone wants to nerd out on the stats.
When we did the study, it was speculated that two of the paleoearthquakes, one on the Seattle Fault and one on another fault on the Olympic Peninsula, could have actually occurred in a single event, but there wasn't much evidence to support this; we consider the magnitude of it on a paragraph at the top of page 1149 but not in the rest of the paper. The recent study (TFA) makes it highly likely that they were part of the same earthquake, but they could be separate earthquakes spaced a few minutes to a few months in time (think of the 7.8 and 7.7 earthquakes in Turkiye this spring, separated by a few hours).
A bit of context about the earthquakes in the Seattle region as well as Cascadia and other areas:
- The earthquakes in the Puget Lowlands and vicinity are relatively infrequent; there are about 15 known earthquakes over the past 17,000 years, and many of them are relatively small (M 6-7). However, they are spatiotemporally clustered[1]: There was a big cluster about 900 AD, and things have been mostly quiescent since then. It can be also shown from the geologic data that at the measurement sites ('paleoseismic trenches'), there haven't been any earthquakes since 17,000 years ago (when the Puget ice sheet retreated) on many of the faults, although the Seattle fault has had a number of earthquakes before.
- The big Cascadia subduction zone events are more frequent (perhaps every 500 years?) and larger, but they may not all be M 9 events, unlike what has been discussed in the famous New Yorker article. That article is based largely on the research of Chris Goldfinger, a scientist at Oregon State University, whose views are credible but on the high side of credible, in the eyes of many other scientists in the region. Many of the earthquakes suggested by the geologic data could be smaller earthquakes (M 7.5-8.5) which won't cause as much ground shaking over such a wide region.
- Earthquakes cause seismic waves at the fault surface, and these attenuate as they travel through the earth towards the surface. The initial magnitude of the waves as the earthquake occurs can be different for subduction zone earthquakes than for shallow earthquakes in the crust, and the attenuation is different for these as well. But importantly, not only are subduction zone earthquakes far off shore, but much of the seismic energy is released deeper in the earth as well, which means more attenuation of ground shaking by the time the waves make it to Seattle.
- A Cascadia earthquake will cause widespread but perhaps moderate damage across the PNW with perhaps, but a strong Seattle fault earthquake will absolutely destroy central Seattle, particularly Pioneer Square and Sodo. The fault comes ashore at Alki Point, for reference. However areas farther away (Edmonds, Tacoma, etc.) will not see nearly as much damage.
- SF and LA both have higher seismic hazard than Seattle[2], considering all earthquake sources, the frequency and magnitudes of earthquakes from the sources, and the seismic ground motions emanating from all of these earthquakes to a site within any of the cities, according to the most recent USGS national seismic hazard model. (See Figure 12 for hazard curves for major US cities).
Thanks for jumping in with your experience - every time the New Yorker article makes the rounds, I do a little light digging to try and find some sources the author based it on. Reading through some of Goldfinger's publications will help unlock a bit more, thank you!
In the late 90s, I took a geology course at Seattle Central, and the teacher told us the reason Alki Point has such a nice flat area all along the beach is because that area used to be the tide flats, but during an ancient earthquake, that entire chunk of West Seattle rose something like 10m in less than a minute and retained that elevation from then on. Is that still the current understanding?
To what extent do you think about micro-quakes are effecting the subduction system? I've heard that we see the land deforming, building tension from the subduction, but there is also a regular cadence where the land deformation reverses temporarily. When I heard about it (Nick Zentner popular lecture), it was suggested that it might be relieving the stress from subduction more evenly, and not allowing it to build up.
Out of curiosity, do you have any insight as to the Portland, OR area? We always seem to get glossed over in discussions about the PNW and the West Coast in general.
Portland is probably similarly affected by the subduction zone, but the inland fault system is not as active as in Seattle. Nonetheless, there is a fault that is underneath the Portland Hills that could do some damage to the city.
The seismic risk faced by a city or an individual is really the product of the hazard (formally, the probability of a certain level of ground shaking) and the response of the building stock or other infrastructure. Fortunately, both cities have a large amount of wood-framed housing, which performs quite well in earthquakes (as do modern apartment buildings). However there are a lot of old masonry buildings used for schools, offices, etc. If these aren't properly reinforced, they can be deadly, not only because they are fragile but because they are heavy when they do collapse. But if you live in and work in a modern structure or an old wooden one, you'll probably make it through alright, as long as you have some food and water at home.
The big concern in both cities is actually liquefaction, where some water-saturated soils lose their strength during an earthquake. In both cities, there is some amount of housing stock built on liquefiable soils, but there is a huge amount of commercial and industrial building stock built out on fill dirt on the respective waterfronts (which is where the old masonry buildings are as well). One of the scariests scenarios is the Critical Energy Infrastructure in NW Portland, where massive amounts of oil are in the huge tanks built on fill into the Willamette, liquefying and then spilling into the Willamette/Columbia: https://www.multco.us/sustainability/cei-hub-seismic-risk-an...
Typically the analysis is done by digging trenches across the fault and dating pieces of charcoal found in the sediment with radiocarbon. This study is rare but not unique in using cored trees, or slices of stumps if possible, and dating the event with tree ring dating (dendrochronology), which is vastly superior when it's possible (i.e. when it is demonstrable that an earthquake kills trees). In both cases you want as much as you can get--ideally 5-10 ages at a minimum I think?
To date these Puget Sound samples with a second, independent method, we analyzed individual tree-ring sequences for a jump in annual radiocarbon concentrations associated with an extreme solar proton event (30, 31). This rapid, large magnitude (~10‰) radiocarbon excursion between the years 774 and 775 CE is recorded globally in tree cellulose and can therefore be used as an exact geochronological anchor point (30, 32, 33)
They found more exact dates because a massive solar storm in the past changed carbon isotopes worldwide, which is reflected in tree rings that grew during the storm. That's incredibly cool.
A particularly crazy thing I learned from some friends who I know who work in this field is that they use underwater hydraulic chainsaws (!) to sample some of these trees.
Harvesting timber from trees submerged by the creation of reservoirs is a viable business, I believe, on account of rising prices, particularly for high-quality lumber that is becoming scarce.
It certainly can pay off. My dad was involved in a project removing old logging roads and in doing so they salvaged a lot of old growth timber that had been buried in swampy ground to make corduroy roads, beautiful straight grain, knot free wood. Buried as it was it was very well preserved, and he stockpiled some off to the side in our shed.
Now one piece adorns my parents new house as the lintel board above the fireplace, and its absolutely gorgeous. I think a some chunks were gifted to an acquaintance who builds guitars, and some more were given to a Salish descended carver.
I suspect there's still plenty of much, much easier to access trees. The macro-economic effects of the wars, and covid, often causing gasoline prices increases (which is used a lot in forestry) that are still adding pressure to almost all commodity prices, including lumber
It's not "access to trees" so much as "access to large, old growth trees". The old ones with tight growth rings, large widths and long straight trunks are actually kinda rare these days. The farmed stuff is not the same, compare the growth ring per-inch
There is a very noticeable difference between the lumber of our house (not to mention my in-laws' home) and that being sold by even specialist lumber yards.
Cascadia Subduction Zone[1] is why I never considered living in Seattle/Portland once I found out about it. Funny, cause I live in the SF Bay Area, where the earthquakes are frequent, but they can never reach the magnitude of what's possible by the Cascadia Subduction Zone.
My mom was a Red Cross nurse who worked in the relief effort after the 1964 Good Friday earthquake and tsunami that devastated Anchorage and areas as far south as Crescent City, California.
It was magnitude 9.2, the second largest in recorded history.
The photos she brought back were quite something. I may still have them somewhere, but in the meantime there are plenty online. LIFE.com has a remarkable collection:
> As a result of the earthquake, 131 people are believed to have died: Nine died as a result of the earthquake itself and another 122 died from the subsequent tsunamis all over the world. Five died from the tsunami in Oregon, and 12 died from the tsunami in Crescent City, California.
My mom was a schoolgirl in anchorage during the earthquake. We have a picture of my grandma and her trailer being 20 feet down from the rest of the ground. Haven’t digitized it yet.
Anchorage was 78 miles from the earthquake and still saw mass building collapses. The death toll was low due to the low population in the area. 128,026 in 1970 presumably less in 1964.
The Cascadia quake will be about same damage to Seattle and Portland as Bay Area quake. It could be up to magnitude 9 but the fault is 60 mi away from the cities. The big difference is that it will affect the whole area. And that PNW is way behind in infrastructure.
That New Yorker article goes viral every few years - it's a well written piece and spins a great yarn, but there's some significant hyperbole and it's sources don't quite back up the claims. Yes, a 9.x will be devastating. No, everything west of i-5 will not be toast.
I'm 10 blocks east, should be fine too. Even if the earthquake somehow gets a couple blocks past I-5 (can't see how that could happen) I'd still have 7-8 blocks as a safety cushion.
One issue though is all the Seattle hospitals are right along I5 so your house might be fine but it’s going to be a disaster for anyone who needs medical care.
Not where I am. Mt Rainier can't reach me but maybe the mountains of Mordor could spring up? Maybe a new volcano, would take a while to build up. An earthquake can get me, I'm by one of the freshwater lakes, so there's possible water action that could get me. But probably the land is terrible by the lake, could subside, and then a wave of water will crush and then drown me.
I'll admit that FEMA isn't exactly cited in a way that's possible to corroborate, but presumably they're estimating a worst-case scenario and not a median- or average-case scenario. That might explain some of the large divorce in figures.
I'd love to find any actual FEMA publications that back up some of the numbers and claims - so far to the best I can tell is it's mostly from interviews with the regional director, which, while valuable, isn't quite at the level of citation I'd like. If anyone's got a PDF I'd love to retract all my critiques though. :)
The news articles exaggerate things either for effect or didn’t realize quakes diminish with distance. The shaking for mag 9 at Portland is similar to mag 7 nearby. One difference is that quake will go on for minutes making liquefaction worse.
There will be lots of damage in Portland. There are lots of unreinforced masonry that will collapse and kill occupants. Most of the bridges, big and small won’t survive cutting off from outside. Infrastructure will be destroyed and utilities will be out of months. All of those could be upgraded which is why Bay Area would be in better shape.
Also, this is talking about Portland and Seattle. Most of the destruction will be on the coast which is closer to the fault. The tsunami will cause lots of damage, destroying most of the towns and killi;g everyone that doesn’t evacuate to high ground.
The 2011 Japan quake is the best comparison but Portland/Seattle aren’t prepared like Tokyo, and there are minimal tsunami defenses.
The fault is at least 150 miles from Seattle proper, the city will not receive the full brunt of the earthquake. Tsunamis are strongly mitigated by the geography of Puget Sound — the biggest tsunami threat is actually the Seattle fault.
By contrast, the San Andreas fault literally runs through the middle of the Bay Area.
The Cascadia quake is 100x as big. I'm at 1/3 the distance from the San Andreas fault. Guess who gets the larger earthquake?
Add to that the fact that I grew up in Victoria. All of Vancouver Island is a subduction zone during a Cascadia earthquake. Sadly, that means that a good chunk of what I knew growing up winds up under water. Seattle may fare better, but my home town does not.
Reading up now, it looks like it is 1-2 meters, mostly on the west coast of the island. But my memory of the early research when I still lived there in the early 1990s was that it was about a 3 meter drop. Between how low much of Victoria is, widespread poorly build older houses, and lots of housing built on unstable landfill in places like James Bay, my belief was that it was going to be pretty devastating to a lot of parts that I knew.
Can you back that up with some references? Very curious, I've never heard any claims that Victoria will end up under water in a big Cascadia earthquake.
An Ask an Earth-Scientist answer by Dr. Gerard Fryerm suggests a few meters is possible.
>"Will part of Vancouver Island "break off and sink," or "split in two at Alberni Inlet?" No. In a big earthquake the seafloor offshore from Vancouver Island will be uplifted and the area along the coast will sink, but at worst that sinking will only be a few meters. If you live very close to the beach and close to sea level then there is a possibility that your house will be flooded, but no big piece of the island is going to break off and disappear. It is possible that there will be submarine landslides along the steepest slopes offshore, but each of those is likely only be small in extent.
That said, https://www.esquimalt.ca/sites/default/files/greater_victori... is built on more up to date information than I had. And suggests that subsidence + tsunami is only 4 meters. Which doesn't pose a serious threat. (If they are wrong and get a 10 meter combination, of course, that would be a far, far worse story.)
"The fault is at least 150 miles from Seattle proper,"
Looking at my geological units map of Washington, Seattle is surrounded by a lot of faults, much like parts of Southern California. Some of those much, MUCH closer than 150 miles, and devastating if they go. I can see one going from Port Angeles, crossing the 101, and going into Port Gamble. It likely continues to run either directly under Seattle or just south of it, in fact that might be the continuation of that reverse thrust fault dead south of Seattle which is shown in the map for the story article. If that one goes, Seattle's not having a good time.
The Seattle fault is a significantly bigger threat to Seattle than the Cascadia Subduction Zone and it doesn’t get nearly as much attention. Parts of the city’s shorelines could subside up to 10-20 feet, and the tsunamis are substantial.
As I understand it, the entire region is on a flat(well, tilted), planar fault because of the way the crust subducts. So you can have earthquakes anywhere in the western PNW from that.
But remember, the bay area is also home to many, many faults, and we don't even know the locations of them all. For instance, the next big bay area quake is likely to come from the Hayward Fault in the E. Bay.
On the other hand, Seattle is one of the best places in the world to ride out climate change, which is 100% happening, and happening already. It’s all a numbers game.
People were saying that about Vermont, then last summer they had massive flooding that put cities under several feet of water.
I knew a lot of, frankly over-confident people who moved to New England because the news told them it was good for Climate Change. I think the “experts” know a lot less about Climate Change impacts to weather than they want to admit.
You do get flooding and at least residual hurricanes in New England. e.g. [1] Depends where your house is of course. Some properties are probably pretty well-situated; others much less so.
Just about a month ago the city next door to me in MA got a freak foot of rain which caused some serious flooding. I had to pump water out of my basement for the first time in years.
Very few places are immune to at least local natural disasters of various types and some of those, like the Vegas area have their own long-term issues.
Are we talking in earnest? Because my complaint is we never do. Instead, like your prior comment, it’s either ‘well, as an individual you can do x’ (which doesn’t solve the actual problem in any real way, and hurts the individual) or ‘this problem doesn’t exist’.
I’ve been wanting to actually talk about actually solving the problem for decades. But get no takers.
And here, we’re even attacking people just trying to avoid getting run over by the steamroller we insist on not stopping.
I'm an Environmental Resource Management major. I've recently learned that wetlands store more carbon than forests and we've lost more than 85 percent of them since the 1700s.
I posted an Ask HN looking for feedback on where to start trying to do the math and did not really get satisfactory replies. I am wondering if "the industrial revolution" is not really the primary cause of human caused climate change but perhaps largely coincidental. The math needs to get done to meaningfully examine that question.
I'm always wondering how one would foster substantial global wetlands restoration.
There are probably a lot of factors. Pumping gigatons/year (literally) of previously geologically sequestered carbon into the atmosphere for a century+ is bound to have some effect. Removing sinks and reversing previous reservoirs also is going to have impacts. No question.
I wouldn’t discount measurable increases in global atmospheric co2 either way. And continuing to pump gigatons/yr of ‘new’ co2 into the atmosphere is so unlikely to help the situation, that I’d hesitate to try to downplay it.
The issue with wetlands and forests in this math is that both, generally, are like ‘capacitors’ - except in rare cases, their storage is temporary. It doesn’t (and can’t, if you do the math on volume of space available!) store large quantities of carbon permanently on a geological timeframe. Unless said wetland or forest gets subducted or buried anyway before it rots. If that’s happened though, it drying up isn’t likely to reverse that sequestration.
OPEC currently estimates approximately 70 million barrels of oil are extracted each year (coal far more I believe by volume). Each barrel is 42 gallons. That is just a hair under 3 billion gallons of oil alone. Or 11.3 million cubic meters of oil, per year. Nothing in comparison to the large rivers of the world, but enough to drown most any large city.
It would take a lot of lost wetland by volume to make up for that, especially when you look at cumulative effects over time. Even if the wetlands were pure carbon by volume (hydrocarbons aren’t, but are quite close for a dense liquid!), it isn’t clear there even is or has been that much wetland. That may give you a start though, for the math.
Your comment - ‘“the industrial revolution" is not really the primary cause of human caused climate change but perhaps largely coincidental’ reads to me like downplaying it?
I added some math to my comment, hopefully it helps.
One way to estimate might be a simplistic, volume/density one. How much carbon is there in the applicable geography/land column? In a forest, it would be the trees themselves (above ground) + the soil. In a wetland, the vegetation + mud/sediment.
There should studies with core samples + samples of vegetation that can give good estimates. Then extrapolate. And you’ll have at least a rough estimate of the potential upper and lower bounds, assuming all that carbon instantly turned atmospheric (or at some assumed rate).
Combine that with numbers like I gave above (I’d do some googling for Coal numbers and add that too), and you should be able to estimate the maximum relative contribution of each source. We’re burning Coal in the billions of tons/yr quantities, and each short ton of coal produces well over a ton of co2 - so that is billions of tons/yr of co2 on its own.
Peatlands cover just 3 percent of the planet’s surface but they store about 30 percent of all land-based carbon, or twice as much as all of the world’s forests combined. Coastal wetlands remove atmospheric CO2 up to fifty-five times faster than rain forests.
I'm not downplaying anything. I'm just seeing data that suggests wetlands loss is a really big deal.
Man, I moved from Seattle to the Bay Area and in the intervening 20 years it sure seems like summers in the PNW have gotten hotter than they are down here.
Longtime Bay Area resident here - I haven't looked at the data, but it seems like this area seems to be getting colder over the past ten years. If so, it's an illustration that we're facing not global warming but climate change.
Fresno County has a history of actually increasing its groundwater levels intentionally at times. Anyone interested in water issues could stand to take a look at what they do.
This is not something with a "source" or a real academic answer. There are too many variables and long tail events.
Survival in some of the worst case climate change scenarios involves moving away from the equator where crop failure and extreme heat will occur.
It involves having good access to water, and ideally farmland as wars over resources and refugee migration accelerate.
Martial law of some kind would become a normalized thing in population centers as basic resources and commodities become more expensive. Seattle has a large military presence.
Seattle is positioned geographical well for these things. Being farther north alone is a huge benefit. The mountains of the west due to their proximity to the Pacific enable much more temperate climates than other port cities.
This all sounds reasonable but also anecdote. I'm curious if there are any analyses, studies, or anything other than PNWers' HN comments backing claims like these.
I don’t have sources readily available but I have looked at several climate change projection models for the US in the past to answer similar questions. There are a handful of locales in the US where the local climate doesn’t change much under most models and parameters. They behave almost like fixed points in a mathematical sense. Some of these fixed points are not anywhere you’d likely want to live, so that might not help much.
However, the Seattle region is one, and it already has a pleasant temperate climate. IIRC, the main expected change is that it will get more sun in the winter months, which would address the main criticism of Seattle’s weather.
Of course, these models could be wildly off, but to the extent we have such models Seattle is in an enviable position. While there are some regions that look like they will change for the better e.g. some arid regions will get substantially more rain, it is hard to predict the true impact of that change on those regions — that outcome might come with significant trade offs.
> Funny, cause I live in the SF Bay Area, where the earthquakes are frequent, but they can never reach the magnitude of what's possible by the Cascadia Subduction Zone.
What makes you think a big one ("The Big One") isn't going to hit the Bay Area?
From what I’ve read the faults in the Bay Area can’t really generate above an 8.0, whereas the cascadia zone can produce much larger events (remember a 9.0 is 10x the intensity of 8.0). It has to do with the size and type of the faults.
But from what I recall reading, the big one that hits Seattle will exceed the big one that hits the Bay. Seattle's big ones are higher magnitude but less frequent. Iirc it's also likely the 9.0 that hits Seattle won't be in most of our lifetimes. The last really big one was before Europeans arrived.
Rainier erupting would devastate slot of small communities between it and Tacoma. Like Helen’s blowing it’s glacial too (liquifying into a muddy huge river) but worse since the area around is more populated.
Yes and a lot of these suburbs are built upon the mudslides from previous (recent) eruptions (few million people). Last I heard it was a 1 in 7 chance in my lifetime. It's incredibly stupid and dangerous. Mudslides can travel up to 50 mph or more, and it doesn't even need to be an eruption that kicks it off.
No, but you would feel it well inland throughout the Mountain West. See this simulated USGS map for a 9.0 quake: https://upload.wikimedia.org/wikipedia/commons/8/80/9.0_Casc... - for scale, the light blues are about the edge of where you'd clearly go "oh, yep, that's an earthquake", although you might notice such a large one at lower ground acceleration because the shaking would be quite prolonged (~minutes, rather than the ~10-20 seconds of a typical minor quake).
Your experience depends quite a bit on where you live and/or work, or, more specifically, what's underneath it. If one has nothing to do with the Millennium Tower, that is probably a plus.
There is little evidence that earthquakes come as precursors to larger earthquakes. Most papers reporting such things have selection bias since they are written after the big earthquake occurred.
A fault is a series of hooks, nooking into the oppossing plate. One of them giving way, in a small event, gives off its energy partially to the surroundings (the quake) - the rest- stays as additional pressure on another rockformation.
Its understandable, but in the end also just noise for those who do and want to discuss mitigiation strategies or statistics. The urge to conjur up security by repeating doubt mighte be huge, but like all prayer, should be kept to oneself.
i don't really think this is a fair conceptual model for a fault nor for how an earthquake nucleates progresses and then ends. it doesn't really account for plastic deformation, fluids, the interface media (a fault has more than just two rocks touching each other, there is typically some amount of ground up particles, etc that sits at hte interface). there are faults that move aseismically (e.g., the subduction zone in mexico) that do not produce earthquakes at all. faults are a rather complex system that isn't really just hooks connected across an interface. i guess this description could be considered an asperity.
It's more the other way around. A large earthquake is often followed by a swarm of aftershocks, some of which can cause significant damage.
The media failed to report that the recent quake in Morocco was followed by tens of smaller quakes over the next few weeks, all in a small-ish area that had been quake-free. (In recent memory, at least.)
An interesting observation is that Cascadia Subduction Zone is potentially less devastating than Seattle Fault despite being capable of a 100x bigger magnitude (9+ vs 7.2) thanks to a longer distance from the city
I can't seem to load this article, but I would like to offer this: I have long thought that the major effect of natural disasters is not the disaster itself, but the social unrest cause by the government not being able to cope. A super quake would be bad enough, but social unrest would make it much worse.
After the 89 quake in the Bay Area the people in Oakland came together and were incredibly helpful to each other. The freeway that collapsed was in a pretty bad part of town, yet there was no social unrest, only unity.
This article actually is loaded with inaccuracies. I once called the local earthquake office where I live, where the local scientists and such work.
They said this article is the bane of their existence because of people calling into their office with false beliefs that have just propagated into fear over time.
I dont even think the sentence got out of my mouth "so about that article I saw...", I was calling to see if my area was in danger of a tsunami, and she gave relieved my fears greatly.
It will be bad, but also there's a lot more nuance to it.
Sadly she didn't give one, however.. if you live near the CSZ like I do, then check your city to see who does their flood maps, and then call that office. For my area specifically they had a whole branch devoted to earthquakes called the earthquake office or something.
I picked her brain for about an hour, asking about all different questions about liklihood of tsunami, how far inland, I asked her stuff about my foundation and my house etc.
Id say it would be worthwhile to try and call your local office.
Some friends I know who work in earthquakes maintain that this article caused a measurable increase in the amount of funding available for earthquake research.
On the contrary, that video probably surfaced for you precisely because this study is making the rounds. More people than usual are probably searching for and watching stuff about Seattle seismology.
As it turns out, the algorithm does not work in such mysterious ways.
One day I got to work early before anyone else was there and I was daydreaming pondering earthquake scenarios and what I would do if one happened (I was on an upper floor).
It wasn't until hours later when I checked the news and heard conversations around the office that I realized I had felt an earthquake and just didn't realize it. It had never even occurred to me
I think living in a city I'm just accustomed to a car driving by in a parking garage and various other rumblings so I never considered that's what it was. Later in retrospect, I recognized that being on the 7th floor of a concrete building, nowhere near a parking garage or construction, that was the only explanation.
The most memorable part of the experience was realizing that my unconscious brain recognized it long before I processed it consciously.
I've been watching a bunch of Nick Zentner's lectures for the last few weeks and I live nowhere near the PNW, so my case I doubt it was algo based. I have no doubt that in general you are right and the topic is "trending" (for whatever that means these days).
I was originally watching videos from a guy who goes hiking further north in BC. From there, I searched for similar videos in WA. From there I saw a couple of videos about the modern history (1800's to today) and settlement of Seattle and the sound. From there I saw one of Nick Z's geology videos which I really liked and have been watching them since.
Thanks for the Nick Z rec. Start the video linked upthread and already loving it. I recently went on a earth formation / geology binge on YT over the last few weeks and this is right up my interests.
When we did the study, it was speculated that two of the paleoearthquakes, one on the Seattle Fault and one on another fault on the Olympic Peninsula, could have actually occurred in a single event, but there wasn't much evidence to support this; we consider the magnitude of it on a paragraph at the top of page 1149 but not in the rest of the paper. The recent study (TFA) makes it highly likely that they were part of the same earthquake, but they could be separate earthquakes spaced a few minutes to a few months in time (think of the 7.8 and 7.7 earthquakes in Turkiye this spring, separated by a few hours).
A bit of context about the earthquakes in the Seattle region as well as Cascadia and other areas:
- The earthquakes in the Puget Lowlands and vicinity are relatively infrequent; there are about 15 known earthquakes over the past 17,000 years, and many of them are relatively small (M 6-7). However, they are spatiotemporally clustered[1]: There was a big cluster about 900 AD, and things have been mostly quiescent since then. It can be also shown from the geologic data that at the measurement sites ('paleoseismic trenches'), there haven't been any earthquakes since 17,000 years ago (when the Puget ice sheet retreated) on many of the faults, although the Seattle fault has had a number of earthquakes before.
- The big Cascadia subduction zone events are more frequent (perhaps every 500 years?) and larger, but they may not all be M 9 events, unlike what has been discussed in the famous New Yorker article. That article is based largely on the research of Chris Goldfinger, a scientist at Oregon State University, whose views are credible but on the high side of credible, in the eyes of many other scientists in the region. Many of the earthquakes suggested by the geologic data could be smaller earthquakes (M 7.5-8.5) which won't cause as much ground shaking over such a wide region.
- Earthquakes cause seismic waves at the fault surface, and these attenuate as they travel through the earth towards the surface. The initial magnitude of the waves as the earthquake occurs can be different for subduction zone earthquakes than for shallow earthquakes in the crust, and the attenuation is different for these as well. But importantly, not only are subduction zone earthquakes far off shore, but much of the seismic energy is released deeper in the earth as well, which means more attenuation of ground shaking by the time the waves make it to Seattle.
- A Cascadia earthquake will cause widespread but perhaps moderate damage across the PNW with perhaps, but a strong Seattle fault earthquake will absolutely destroy central Seattle, particularly Pioneer Square and Sodo. The fault comes ashore at Alki Point, for reference. However areas farther away (Edmonds, Tacoma, etc.) will not see nearly as much damage.
- SF and LA both have higher seismic hazard than Seattle[2], considering all earthquake sources, the frequency and magnitudes of earthquakes from the sources, and the seismic ground motions emanating from all of these earthquakes to a site within any of the cities, according to the most recent USGS national seismic hazard model. (See Figure 12 for hazard curves for major US cities).
[0]: https://rocksandwater.net/pdfs/styron_sherrod_bssa_puget_eq_...
[1]: https://pubs.geoscienceworld.org/gsa/geosphere/article/10/4/...
[2]: https://journals.sagepub.com/doi/10.1177/8755293019878199