A Serious Discussion On The Cascadia Subduction Zone and Latest Studies

reply to post by Olivine


Ugh...wish I knew how to read those. I can clearly see where the little squares all dip down and rise back at, seemingly all at the same time on all the stations...so it would appear there was definitely movement vs just a glitch...but how much and what that means I haven't a clue.

Thanks for the links though....maybe someone with a bit more knowledge can chime in on this one????

reply to post by Olivine

Does anyone else think this movement is large? Looking at the graphs prior to the end of October, the GPS positions seem fairly constant. Not stationary, but relatively calm. But to my eye, November and December look like a rollercoaster across the entire region.

Just an observation of course and maybe wrong but......

Episodic Tremor and Slip (ETS) is the name given to a process that occurs deep below the Earth's surface, along faults that form the boundaries of tectonic plates. It involves repeated episodes of slow sliding, one plate over the other, of a few centimetres over a period of several weeks, accompanied by energetic seismic noise, called tremor. Tremor is distinctly different from the seismic signals generated by earthquakes.

www.nrcan.gc.ca...

The simple fact that the slow slip is occurring indicates that the plates are moving. Perhaps you cannot apply logic to this, but to me that says all is well and that while there is episodic tremor there will not be a mega-quake.

I am finding it a little difficult to get hold of tremor readings for the 100 years before the last big Cascadia quake, but I would not mind betting that they were absent.

Originally posted by PuterMan

Just an observation of course and maybe wrong but......

Episodic Tremor and Slip (ETS) is the name given to a process that occurs deep below the Earth's surface, along faults that form the boundaries of tectonic plates. It involves repeated episodes of slow sliding, one plate over the other, of a few centimetres over a period of several weeks, accompanied by energetic seismic noise, called tremor. Tremor is distinctly different from the seismic signals generated by earthquakes.

www.nrcan.gc.ca...

The simple fact that the slow slip is occurring indicates that the plates are moving. Perhaps you cannot apply logic to this, but to me that says all is well and that while there is episodic tremor there will not be a mega-quake.

I am finding it a little difficult to get hold of tremor readings for the 100 years before the last big Cascadia quake, but I would not mind betting that they were absent.

So are you saying that the back and forth GPS readings over the past week are residual movement from the last ETS event in the area that ended in mid-October?

Oct 12, 2012 - Officially this ETS is over!

source
When I check the PNSN tremor map for the past month, the Vancouver Island/Puget Sound/Olympic Mountains region has been very quiet.

This is the movement that I posted about. Maybe it isn't significant. I was just asking what others thought.

(Thumbnail is a sample of PNW stations (showing N-S movement) found at nrcan.gc.ca)
(farthest left blue line shows end-date of ETS, 2nd blue line shows date of Mag 7.8 some 900km north of area)

I agree that the plates are moving, and the long-term GPS readings show that.

I was just curious about the 1.5-2.0 centimeter moves late in December.
Seems strange for such a large area of land to move so quickly without active ETS or earthquakes. I doubt the entire GPS satellite array experienced a positioning glitch of some kind.

I'd love to see tremor data for the 100 years proceeding the last Mag 9, too! Heck, I'd love to understand the mechanism for the tremor: fluid migration, miniscule stick-slip at the plate interface, something.

As far as applying logic to this, yes, I suppose my thought processes can be logically-handicapped at times. But I disagree with you that ETS leads to an absence of mega-thrust danger.
I think we don't know.

and Happy New Year.

I found this model of subducting plate depths for Cascadia. I believe it is fairly new (September 2012). It was new to me, so I thought I would share it here.

Source

The main page this model is from contains models for most all of the world's subduction zones. If you click the Slab 1.0 Interactive Map link, you can zoom in to many of the subduction zones and view them by plate depths, dips or strike. It helps visualize what may be going on down below.

Just doing a little more reading, I found some new informaton regarding megathrust earthquakes.

In the article titled Megathrust Surprises, author Kelin Wang states:

Earthquake rupture processes reconstructed from seismic waves, including those measured thousands of kilometres away, show that much of the high-frequency seismic energy that caused violent ground-shaking during the earthquakes in Sumatra in 2004, in Chile in 2010, and most evidently in Japan in 2011 came from way down in the fault zone, at depths of 30 to 50 km. So whereas the shallow, large slip led to tsunami generation and related damage along the coast, local shaking could be attributed to a deeper source.

Check the map in the post above for where the 30-50 km depth zone is in Cascadia.

Another short poster presented in December at the Fall AGU meeting, supports the quote above regarding high-frequency seismic energy radiating from deeper on the plate interface.

What scientists are learning from studying the very well recorded Tohoku earthquake and other recent megathrust events doesn't bode well for the large cities of the Pacific Northwest when Cascadia finally ruptures.

This latest data suggests that the peak accelerations and associated strong ground motions can/will be much closer to Vancouver, Seattle, Tacoma and Portland than previously thought. It's a tough puzzle to crack.
This 2012 USGS hazard workshop PDF by R.D. Hyndman discussing the landward limit of rupture on the CSZ references at least 6 different limiting influences. His final diagrams show rupture 50-100 km west of Seattle, Vancouver, and Portland.

Finally, I remembered the terrifically informative article posted by True American in his big Cascadia thread--here is his post with the link to the article from Central Washington University. An excerpt:

The most important aspect for northern Cascadia is that stronger coupling between 15 and 25 km implies greater coseismic slip near major population centers, and provides an estimate of future coseismic slip along this region. 50% coupling suggests 9 meters of slip should be expected directly up-dip of 25 km. This lies well inland of the coast, directly west of the greater Seattle-Tacoma metropolitan basin.

So coastal communities have to worry about the tsunami, the shaking destroying their escape routes, and the inland urban areas may have to endure much stronger shaking than previously believed. Yikes.

Some sort of door-to-door education effort needs to take place--I just don't think all the residents of the PNW fully realize how bad this will be. (one day far, far in the future--here's hoping )

There looks to be a small/moderate amount of deep tremor happening up and down the subduction zone over the past 24 hours.

Source (click the "24" button)

Look's like it's starting up again.
Slow tremor map. For some reason I have this uneasy feeling again.. With a 6.1 in the south pacific 6.1 usgs,
5.5 off of Craig, Alaska 5.5 usgsand central america 5.1 Nicaragua
being rocked makes the northwest us look like a target for something. I don't buy into the one off of Alaska/Canada "relieving" pressure. Looking for something off the coast of North West Ca soon 3.5-4.5. In my own opinion all these locations tie into the Cascadia Subduction Zone, (besides the 6.1). I tell my friends when I see a 4.0 off the Eureka Ca coast to keep an eye on Oregon, off the coast due west of Eugene and expect a 4.5

Tremor in Cascadia has been hit and miss since mid-November, but it seems to have picked back up over the past 2 days.

A small burst in the north, under Vancouver Island on January 26th.


Then a fairly continuous tremor under northern California today.


Source for both maps.

reply to post by Olivine


I was just going to post that. I find it odd, or maybe I have never noticed it before, but do they bounce back and forth within a day? The ETS (slow moving tremors). I usually see them slowing or dying off and progressing in a certain direction.

reply to post by Isaac (RIP DUSTIN)


Ack! I missed your post Isaac. So sorry.
These ETS outbursts that occur outside of a prolonged (two week +) episode, do seem to jump back and forth between the north and south ends of the subduction zone. I know Westcoast has an interesting yin-yang hypothesis of the energy or stress bouncing from end-to-end.

I hadn't looked at the tremor maps in a few days, but the locations are still jumping up and down.
28 January was in the south. (13 hours, 221 epicenters)
29 January..nothing.
30 January under both ends--Vancouver Island and northern California. (11.5 hours, 206 epicenters)
31 January again under both ends, but very few epicenters making for a strange linear pattern in the south.

Image from www.pnsn.org...

Oddly enough, there was another day of tremor that followed that same linear pattern in Northern California. Glancing at previous images doesn't reveal anything that looks similar.

I wonder if there is a glitch in the software?

reply to post by SkipIntro


You're right, that is really strange two days in a row. It makes me suspect some sort of error in the software too, except that the other tremor on the map looks normal. Maybe Mr. Wech's software is picking something up at the eastern edge of its range, and this is how it shows?

This is me whipping out the dust buster on this thread.

A 5.1 just hit off the coast of Oregon at a depth of 9.9 km.....a little shy of the actual subduction zone, but always a matter of concern in this area.

earthquake.usgs.gov...

I guess the stress from Santa Cruz has to go somewhere

I haven't had much time to check on quakey things lately, so while trying to get caught up, I noticed this nice little bubble of tremor near Portland, Oregon over the past 6 days. I can't remember a recent sustained tremor episode in this area. It looks to be moving to the west over time.


source

If you check out the realtime tremor map, (click the 12 hour button) these deep, tiny quakes look to still be occuring.

@radpetey--Thanks for dusting off this thread, you made it much easier to find.

Olivine, the ets swarms used to occur approximately every 14 months, would you say that we are now seeing a change to that pattern with these swarms that keep occuring?

reply to post by AlexanderM


It's hard to say, AlexanderM.
The "main" ETS that is studied extensively, is the area that moves under Puget Sound and southern Vancouver Island. And yes, 3 of the past 4 rounds of tremor and associated slow-slip, have come 3 months earlier than the 14-month cycle that was first seen.
As with anything on a geologic time scale, I think forcing the recently discovered (a decade ago) ETS into an artificial timetable with only 10 years of data, was probably a bit premature.

The tremor in Oregon and northern California doesn't seem to get the same focus of attention as the northern events do. This area near Portland that is currently showing tremor had nice episodes in Aug-Sept 2009 and Jun-July 2011, with strong associated GPS movement. But that is as far back as I could find information.

The current thinking (as I understand it), is that ETS under Oregon or California is unrelated to the tremor episodes in Washington/Canada. Personally, I don't see how that is possible, it's all related to the same huge thrust fault. Heck--they don't even have an established causal mechanism for the tremor and slip nailed down.

I try to watch all of the ETS burbles along the length of the fault--just trying to understand the relationship between tremor areas, quakes or the lack thereof, and associated GPS movements.
Not very scientific, I admit. I'm just hoping to stumble upon some sort of pattern or precurser.

In the program, "Monster Quake, Are We There Yet, or Are We Next," one of the USGS scientists was saying that the ets swarms could be the mechanism that unleashes the big one. I guess the theory being that the ets releases stress at intervals, but once it ceases to release sufficient stress the fault could simply give out during one of the swarms. I keep my eye on the ets swarms, looking for changes in the pattern, just on the chance that it may signal that the fault is getting ready to rupture. Or, if we see an ets swarm, and then soon after subsequent swarms begin, it could be an indication that the swarms are no longer releasing enough stress and the fault may be getting ready to rupture. I thought there was a subsequent swarm after they had the quake off the southern tip of the queen charlottes. Anyway, I realise that there may be nothing to this theory but I'm keeping an eye on it anyway.

Cheers and thanks for all the great info!

I updated the tremor map to include the past 2 days. Map source.
Looking at the realtime map, tremor has definitely slowed over the past 6 hours. It may pick back up again...or not.



I also use this website from Central Washington University to watch for movement associated with tremor episodes.
Here is a small sample of GPS stations located near the current ETS activity in Oregon. I circled the movement from the past two episodes in this area (August-Sept. 2009 & June-July 2011)--to give an idea of what we may see if this current activity continues.


(click to enlarge thumbnail)
source

It's still snowing (over 13 inches so far)...so opted for a break from shoveling and thought I would give a quick update on the tremor in the PNW.

Here is the map updated through last evening. source


The trend still looks to be toward the south and west, over time.

Here is a screen grab of the ongoing tremor over the past 12 hours (not included on the map above). Activity is being picked up in several more spots up and down the CSZ, potentially putting a little bit more strain on the locked area to the west.

source (set to show last 12 hours)

Originally posted by Olivine
It's still snowing (over 13 inches so far)...so opted for a break from shoveling and thought I would give a quick update on the tremor in the PNW.

Here is the map updated through last evening. source

The trend still looks to be toward the south and west, over time.

Here is a screen grab of the ongoing tremor over the past 12 hours (not included on the map above). Activity is being picked up in several more spots up and down the CSZ, potentially putting a little bit more strain on the locked area to the west.

source (set to show last 12 hours)

edit on 3/6/2013 by Olivine because: add source

Great post....!

Man that is kind of scary how the slip seems to be symmetrically spread out from top to bottom, so to speak.

Could the main rupture be off the coast of Oregon, and then cause uplift in both directions??

That area is sooo scary to me....as I have family on that coast, just west of Salem, Oregon.