Earthquakes and Avalanches

[mtnjam]

Since the 6.5 earthquake that happened on 3/31/2020 and the resulting aftermath of rockfall and widespread avalanches I've been wondering about how seismic waves propagate throught the snowpack.

Of course, with the earthquake that happened last Tuesday in the midst of a powerful storm that deposited over 2 feet of new snow with strong winds, it's going to be hard to tell if the new snow loading with the wind was responsible for the widespread avalanches seen or was it due to the earthquake potentially triggering a PWL that's been mostly buried since late December.

One of the reasons I ask about the seismic waves propagating through the snowpack is, I was skinning up Shaffer Butte at Bogus Basin when the earthquake happened. I find it odd that I didn't feel a thing, nor did I see any sign that an earthquake had occurred. Perhaps it was the location, geology, OR was it due to possibility that the snowpack or new snow, somehow attenuates the effects of seismic activity?

[Bunion 2020]

I am waiting on some pictures from a friend in Stanley. Supposedly lots of activity with the quake.

Good questions and interesting that you were actually on snow during the event and did not feel it.

Good stuff here.

https://www.sawtoothavalanche.com/media/#&gid=1&pid=8

[homemadesalsa]

Hey Bunion I am currently away from computer. Would you excerpt and post an article from TAR 33.3 about an earthquake-triggered avalanche cycle in New Zealand? I'm sure you know this, but the pages need to be jpgs not.pdfs.

Thanks!

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[Bunion 2020]

Here are links to the TAR 33.3 & 29.2

33.3 is a particularly strong issue but I didn't find the article you cited.

https://static1.squarespace.com/stat...nal_LowRes.pdf

29.2 has 2 articles , one from the Brainiacs-Birkland/Hendrix and one from a punter name of Carpenter, no relation to Don/Sarah.

https://avalanche.org/wp-content/upl...nBirkeland.pdf

[mtnjam]

Thanks for posting up those pdf's of the TAR, I'll read through and see if I can find any good info. Seems like TAR always has good information.

[homemadesalsa]

Aha, 29.2 is the one I was referring to, also with a great photo from Laura Adams. Thanks.

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[whipski]

https://www.forbes.com/sites/davidbr.../#2eed30b864da

[eagleskier]

Here is an excellent write up from the perspective of Chris Lundy, who guides for SMG. (He also is one of the avalanche forecasters with the Sawtooth Avy center). One of the SMG guides also took a bunch of photos that they posted with the article. https://sawtoothguides.com/2020/04/0...fODA9r60bt_U0g

Regarding the experience of not feeling anything on Shaffer Butte, I found it interesting that the media (Channel 7) was reporting that the length of time that the quake was felt in the valley was "only" 10 seconds. I was convinced that I felt it much longer than that in my home in Eagle. My experience was corroborated by a video of a live Zoom type session that was going on by some lady, also in Eagle. The video shows the shaking and rolling for a full 50 seconds. The point here is that the length of time may have varied dramatically, depending on where one was standing. If it was only "shaking" for 10 seconds where you were on Shaffer, and the snowpack was acting as an insulator, that could explain the reason why you didn't feel anything???

[Bunion 2020]

^^^ I have actually only felt what was later said to be like a 4.6 while laying in bed one night. The first 10 seconds were, "wtf is going on", followed by maybe 5 seconds of "Oh, this is an earthquake, its kind of cool". The rest of the time was "What if it gets worse? way worse". It stopped after another 5 seconds. 50 seconds sounds terrifying.

[wolfy]

I have been out on skis for several Cali quakes over the years- the only thing we really picked up on was rock and icefall in a fairly large area. We figured the lack of shaking noticeable in the pack was from the elasticity through the layers of snow- kind of like the structural teflon pads used under really tall bldgs.

We were in Mammoth on May 3, 1983 for the 6.5 Coalinga quake- which was centered almost due West of Mammoth- across the Sierra. It was beer 30 in town- but a very obvious shake and roll. The next day, from the top of Mammoth you could see monstrous climax slabs that extended the entire South facing sides of several drainages West of the mtn. and dammed up water flow. Fallon navy helos flew the backcountry but didn't locate anyone, as I recall. Not sure if there was any/much documentation of the event.