A Serious Discussion On The Cascadia Subduction Zone and Latest Studies

reply to post by geotrician


Don't let that distract you from the Thread - Many chime in - It is up to YOU to read between the lines posted.. WestCoast is above top notch in her thinking and her subject, so let us keep it there and don't get distracted.

M2.8 60km under the Puget Sound a couple hours ago.

pnsn.org...

reply to post by JohnVidale


This one seems a bit unique in that it is quite deep (58 km). Does that mean anything special for this location?

Nice to see so many of ya here! I hope to have a good exchange of info. I agree with the idea to include to off-shore volcanos. I don't have a map handy, but if I get time this week and no one else has found one, I'll see what I can turn up. It would only make sense that their activity ties into the CSZ, so knowing where they are and what they are doing seems rather important. I know that John has pushed the need of off-shore mapping and GPS as well as monitoring the deep tremors there. If you haven't read his blog that I linked in the OP...do so. It is a very informative read.

John...I read that you've determined that off-shore quake near oregon was probably from the interface zone. How about the 2.8 today in the sound? Is the interface zone a bit deeper there?

Originally posted by JohnVidale
M2.8 60km under the Puget Sound a couple hours ago.

pnsn.org...

John, I guess you are referring to this one just south of Port Townsend?:
(And is it OK with you and PNSN to use such images? If not I will delete it.)

reply to post by geotrician


Posting the graphic is fine. This event was likely on the interior of the subducting slab - interface events go no deeper than 30-40 km. Nisqually 2001 was a similar event just a few km shallower. But the interface is deep there, just perhaps 10 km above the earthquake. The event is somewhat uncommon, but not alarming, and earthquakes go as deep as 100km under parts of Puget Sound.

Thanks for the opportunity to use some graphical aids. In that vein, it is curious that the M2.8 event was in the area where the tremor pattern of the first three months of 2012 originated and spread laterally from. Also the tremor pattern seems to directly overlap the M2.8 slab event. Here is another graphic to illustrate that:

reply to post by westcoast


Westcoast - I will be following your thread here as I usually do with your threads as well as TA's threads. I am exceedingly un-knowledgeable on this subject and likely will have little to contribute, but since I live in Washington - almost literally in the shadow of Mt. Rainier - your threads are of great interest to me. And I for one have never viewed you or TA as "fear-mongering". I see these threads as very informative and I thank you for your efforts. I wish I could do more than just giving you a star and a flag.

reply to post by geotrician


Neat. This can be a sort of prediction to see whether we now observe tremor around the zone in the next few day.

John, from what I can see the tremor pattern of the first three months started at the very nose of the arch in the plate and spread around the arch both north and south. Of course this was probably preceded by slow slip, but I don't have the data. Nevertheless, the M2.8 yesterday may be an expression of the change in intraslab stresses due to the preceding slip at the interface. So in this scenario there comes forth (1) slow slip (2) tremor (3) change in stress within the subducting plate and readjustment of slip zone stress. More to come if you and others want.

reply to post by geotrician

Oh yes Please. I am learning so much here. I go back and have to read a few times. But you have my attention as I'm sure others.

Again, Thank you for sharing your knowledge and wisdom.

reply to post by geotrician


I'm working on some different things at the moment, but I did write a paper last year on the relation between slow slip episodes and small earthquakes web.me.com...

The hard part is gathering enough objective evidence to separate cause and effect, and separate between coincidences and related phenomenon.

Thanks for the link to your paper, John. But the events you noted are much smaller than the M2.8 yesterday. I don't think there will be any tremor episodes associated with yesterday's event because the stress release started in January and there was likely slow slip all around the arch under the Olympic Mts. which changed the whole stress field, resulting in the M2.8 at the downdip bend. Looking at the tremor patterns in the Klamaths and in the first quarter 2012, I expect there to be migration south to the central Oregon coast and possibly a broad expansion of the tremor under Vancouver Island, as has happened several times in the past ten years. Sorry to be so wordy, but the M2.8 sort of proves my hypothesis. I will try to shut up now and let somebody else have the floor.

reply to post by geotrician


You are hiting right on what I have been hedging around for awhile now on some of my other threads, but I haven't worded it so well. Based on your hypothesis, you should take a look (if you haven't already) at the Vancouver Island region. Perhaps back to the recent 6.4 quake there (sep 9th 2011).

The tremors are again clustering there.....I expect to see another off-shore quake in the region soon. I am trying to stay away from predictions here, but really don't see this as one, but rather a tie-in to your hypothesis. You may not agree with me though, if I am interpreting you wrong.

Originally posted by westcoast
reply to post by geotrician

 

You are hiting right on what I have been hedging around for awhile now on some of my other threads, but I haven't worded it so well.

Based on your hypothesis, you should take a look (if you haven't already) at the Vancouver Island region. Perhaps back to the recent 6.4 quake there (sep 9th 2011).

The tremors are again clustering there.....I expect to see another off-shore quake in the region soon. I am trying to stay away from predictions here, but really don't see this as one, but rather a tie-in to your hypothesis. You may not agree with me though, if I am interpreting you wrong.

I will try to give my answer to this but JV is the guy to ask. And it is pretty quiet now, so I will chime in. The 2011 M6.4 event occurred where the effective northern extremity of the Juan de Fuca plate goes under Vancouver Island as defined by the Nootka fault. This seems to be a repeating pattern with several quakes in the 2.0 to 5.0 range, followed by a M6 at this same point. If this is true (I only have looked at post 2000) then it clearly expresses the stress buildup where the northern margin of the plate passes under the thick exotic terrane of Wrangellia under central Vancouver Island. I don't think it is related to the tremor patterns except that the tremor patterns usually have died out before this part of Vancouver Island is reached. Here is a comparison of two of the central VI/plate boundary earthquake patterns. The one of the left is for 2000-2006 and ends with a M6.4 right up against the VI boundary. And of course the Sept. 2011 M6.4 is in almost the same exact place. So I doubt it means anything in regards to an upcoming megaquake.





And to illustrate the extent of tremor under Vancouver Island (constrained by the sensing instruments) here is a tremor plot for 2005-2012:

Anyone else having issues with the PNSN webis? Or is it just me?

They seem to have stopped updating around 2200UTC on the 13th. Same with the spectros linked from the tremor page. John Vidale posted that they had a power outage at UW yesterday--connected?

reply to post by Olivine


I think they are back now. Yes, it was related to the power outage and cascading issues on Tuesday.

reply to post by JohnVidale


It's all working beautifully again. Thank you!

reply to post by geotrician


Thanks for the reply, love the visualls. I guess my next thought is that there is a a very clear pattern of quakes leading up to the 6.4. So how do the tremors tie into it? Is there also a build up of tremors prior to the 6.4's? This is the pattern I have been noticing just by observation without any hard reasearch to back it up.

The next thing to be done (IMO) is to look at Japan's deep tremors leading up the quake last year, for obvious reasons. It is one of the few regions in the world monitored for deep tremors. I have had a hard time finding info on it...most of what I have googled is in Japanese.

reply to post by westcoast


Unfortunately, I think the Japanese earthquake hit a stretch of fault not prone to slow slip, except the episode apparently just between the foreshock and the nucleation of the mainshock.

Westcoast, if you want to read a very comprehensive review of the occurrences, mechanisms, and theories of fault related (subduction zone too of course) non volcanic tremor, take a look at this excellent survey:
ehzftp.wr.usgs.gov...