Multifault earthquake threat for Seattle region revealed by mass tree mortality • Hacker News Board

Re: Multifault earthquake threat for Seattle region revealed by mass tree mortality

#111

Post by cossatot » Wed, Oct 11, 2023, 1:15 PM UTC

23B1 wrote: post #104

cossatot wrote: post #78

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 throug…

Thank you for this fascinating comment. 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...

Re: Multifault earthquake threat for Seattle region revealed by mass tree mortality

#112

Post by cossatot » Wed, Oct 11, 2023, 1:20 PM UTC

soultrees wrote: post #91

cossatot wrote: post #78

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 throug…

How do you go about collecting the data for these studies? Is it core sampling? And if so how many cores would you need to get a good reading?

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?

Re: Multifault earthquake threat for Seattle region revealed by mass tree mortality

#113

Post by cossatot » Wed, Oct 11, 2023, 1:22 PM UTC

adrianpike wrote: post #87

cossatot wrote: post #78

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 throug…

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!

This is the most recent review paper of Cascadian seismicity: https://www.annualreviews.org/doi/abs/10.1146/annurev-earth-.... I would start here instead of Goldfinger's work, as there is a lot of other science to consider.

Re: Multifault earthquake threat for Seattle region revealed by mass tree mortality

#114

Post by e40 » Wed, Oct 11, 2023, 1:25 PM UTC

cc101 wrote: post #34

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.

Re: Multifault earthquake threat for Seattle region revealed by mass tree mortality

#115

Post by mannykannot » Wed, Oct 11, 2023, 1:40 PM UTC

tony_cannistra wrote: post #23

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. https://www.stanleyinfrastructure.com/products/underwater-ch... The field work in general seems very wet for this kind of research. (my partner did lots of it for her graduate work)

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.

Re: Multifault earthquake threat for Seattle region revealed by mass tree mortality

#116

Post by sparrowInHand » Wed, Oct 11, 2023, 2:07 PM UTC

mnky9800n wrote: post #93

DoreenMichele wrote: post #82

Earlier quoted context omitted.

I can't readily find a source, but there was another one some weeks back of around 4.0. I can't decide if these are good news making a big quake less likely or foreshadowing of a big quake coming.

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.

There is a lot of research on people not taking possible disasters serious: https://en.wikipedia.org/wiki/Normalcy_bias

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.

Re: Multifault earthquake threat for Seattle region revealed by mass tree mortality

#117

Post by 23B1 » Wed, Oct 11, 2023, 2:21 PM UTC

cossatot wrote: post #111

23B1 wrote: post #104

Earlier quoted context omitted.

Thank you for this fascinating comment. 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…

Fascinating. Thank you.

Re: Multifault earthquake threat for Seattle region revealed by mass tree mortality

#118

Post by [deleted] » Wed, Oct 11, 2023, 2:27 PM UTC

cossatot wrote: post #78

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 throug…

[deleted]

Re: Multifault earthquake threat for Seattle region revealed by mass tree mortality

#119

Post by btilly » Wed, Oct 11, 2023, 2:46 PM UTC

seabrookmx wrote: post #89

btilly wrote: post #51

Earlier quoted context omitted.

Let's see. 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.

It's not like the island will just drop into the earth. Unless you were referring the tsunami risk, which is my big fear as a fellow Victorian.

The island does subside in these earthquakes.

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.

However recent articles like https://www.mdpi.com/2624-795X/4/3/13 suggest a much milder risk than I had thought.

Re: Multifault earthquake threat for Seattle region revealed by mass tree mortality

#120

Post by btilly » Wed, Oct 11, 2023, 2:52 PM UTC

interloxia wrote: post #101

djmips wrote: post #97

Earlier quoted context omitted.

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…

It doesn't take a lot of meters to pose a big problem for Victoria, BC. Look at a topographical map: https://en-ca.topographic-map.com/map-1926m2/Victoria/?cente...

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.)