Liem alerted me to the local seismic network and sent the Mach 22 seismic plot from the Jim Creek Station. A seismic signal was also recorded at the Cultus Station and Stanwood Station.
Seismic network from the Pacific Northwest Seismic Network
(Stanwood seems a bit far from Stanwood proper)
Jim Creek
Cultus Mountain
Stanwood
All three stations record two events at approximately the same time and at the same time that the landslide event was reported to have happened. If this signal is from the slide, it indicates that two separate events took place a few minutes apart.
I checked other stations a bit further afield and found no signal, further suggesting that the slide was the source of the seismic signal.
7 comments:
Dan: Visually, it looks like there are three signals in the Cultus and Jim Creek stations and a weaker recording of that a longer "third" signal in Stanwood. There is 'noise' in the Jim Creek and Stanwood station that is not recorded in the Cultus station. What is the chance that a minor seismic event preceded the slide?
Thank you for your awesome explanations. I figured the area was unstable, just didn't know its history. So senseless to allow any building of homes in such areas. Simply senseless!
What do you make of the 1.1 magnitude quake just adjacent to the slide area on March 10?
http://pnsn.org/earthquakes/historic?evid=60715651
Much more informative then the news.
Also in keeping with what I had heard over flying the area ten years ago with salmon biologists.
Frs
Is there a cumulative rainfall threshold for landslides that includes the Oso area. The USGS has rainfall thresholds for the Seattle area. Available rainfall data for Darrington near Oso would have exceeded the Seattle cumulative rainfall threshold. Such a threshold might identify times for monitoring and actions that could possibly prevent such disasters.
Thanks for the comments and attention to this difficult event. The plots are a bit rough and Jim Creek signal is noisy. As for an earlier seismic event as a trigger, I do not think so. A 1.1 just would not generate much forcing.
Anon: You are correct that threshold water levels may warrant more diligent monitoring; however, a challenge of deep-seated slide is that they do nor correspond to the same threshold monitoring as there is typically a lag time for the water to penetrate so deep.
Your blog is fascinating--I'm learning so much that I wish I could've learned in my college geology class (where I earned my all-time lowest grade EVER, a testament to a truly uninspiring instructor).
Here's a question for you: I live practically on top of the Jim Creek seismic station (judging by the map...I'm not sure exactly where it is IRL, probably closer to the Navy's towers?) This landslide business is making me a bit, err, morbidly curious about the stability of Ebey Mountain. One would suppose it is comprised of the same stuff as the material above Steelhead Drive, but lacking LIDAR and a drill, how would I know?
I suppose that not having a river at the base of the the slope makes a significant difference--but remember that dreadful grade in Geology 101, I could be completely wrong!
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