7.7 Quake Just Hit SE Alaska High on the Cascadia Subduction Zone

Meanwhile, as we wait for USGS... reply to post by TrueAmerican

There's likely to be some aftershocks southwards, just as occurred with the Oct 28, 2012 mag 7.7 event. But as you say, the ones with this biggie are tending to stick closer to the fault line compared to the October event.

Here's a little gif I made up that shows the Oct 28 main shock (biggest blob) and the next four aftershocks. I have 89 images but no way I can make a gif for them all that I can upload here. Just getting these five images in meant a lot of cropping.


To get an idea of scale, the first aftershock was about 100 km Sth, the next was around 200 km SW, the third was about 120 WNW, and the fourth around 80 km WSW. But the fact is, they didn't get a long way north or south of the main event and I appreciate this is what you're referring to.

There was an event about 400 ESE, down in the Nthn section of the Juan de Fuca plate, but I'm guessing that it wouldn't be considered an aftershock of that event anyway, though it might have been remotely triggered by it. (With some delay, though.)

reply to post by silo13

No worries.

RSOE is not bad for picking up sniffs of news about chemical spills, nuclear mishaps and so on, and for that purpose they are not a bad resource. But with quakes, the problem is demonstrated by their feed display that you posted. What most likely happened there is that they picked up from some sources and based these "quakes" times on when they went public, without having any system to check back and see that they all relate to the same, single event.

They also have problems with locations, sometimes placing quakes hundreds or even thousands of km from where they really happened. There was one last week which RSOE reported as being near the south pole, when in fact it was in the 60-degree southern latitude range. That's quite a difference!

This is why it's useful to check seismo sources where possible, as they make it pretty obvious if there is just one big event, or several. If in doubt, just visit QuakeWatch and you'll see what some of our resident quake geeks have found out. They draw on a wide range of resources and they're regularly linked to in the thread.

Originally posted by JustMike
Meanwhile, as we wait for USGS...

At 16 microns/s and 40 microns/s, that quake was a mere fidget, and you might be waiting quite while for the USGS to post that, and they may never, cause it was so small. The fault is still moving, as I monitor in spectrum mode stations AT.CRAG and AT.SIT. Plenty of small fidgets like that still occurring.

I tried to pull quake data in GEE to get a read in microns/sec on the main shock, but no go, so I can't give you a read comparison atm.

reply to post by TrueAmerican

Hi TA. Well I guess that could be the reason why.

I was using a prior event for comparison (which I believed was the 5.1 aftershock) but if I've done a misread on that, then it wouldn't surprise me. (I probably need more coffeeee... )

EDIT: but yeah, thinking back to prior events, the trace should have been a lot more microns/sec to indicate a mag 5 event. Darn... I definitely did a poor comparison this time.

My bad... Sorry about that.

reply to post by JustMike


Hey, this is complicated, and I screw up all the time. So I feel your pain.

But try to pay attention to the scales carefully when an event occurs. You may have just missed that mm/s (for the 5.1) and MICRONS/s for the small stuff.

Reading the event summary from USGS, and looking at the distribution of aftershocks, this seems to have been a strike-slip event, that propagated northwestward, along fault.
Some text from the above link:

The January 5, 2013 M 7.5 earthquake off the west coast of southeastern Alaska occurred as a result of shallow strike-slip faulting on or near the plate boundary between the Pacific and North America plates. At the location of this earthquake, the Pacific plate is moving approximately northwestward with respect to the North America plate at a velocity of 51 mm/yr.

When you look at the centroid moment solution, the 2nd nodal plane solution seems to make the best sense, if the fault ruptured toward the NW.
(If I'm understanding how to read the "beachballs" correctly. Someone please help me if I've got this wrong.)


Ack! I forgot this portion of text from the summary linked above:

The January 5th, 2013 earthquake is related to that Haida Gwaii earthquake three months previously, and is an expression of deformation along the same plate boundary system.

So today's earthquake is associated with the Haida Gwaii event 3 months ago. But we haven't seen similarly sized activity south of here (read Cascadia Subduction Zone). Should we infer that stress is even higher in the northern CSZ, the stress regimes between the Queen Charlotte Fault and CSZ are unrelated, or that these EQ sequences to the north have lessened stress on the CSZ?

What is your thinking quake-watchers of ATS?
I'm leaning toward this movement bringing a large event on the CSZ one step closer...

reply to post by TrueAmerican

That's pretty much what happened, I guess. Basically I was comparing off a previous event on the GEE trace and thinking it was the m 5.1 aftershock they had there, but thinking back on it, the one I was comparing to was too recent to have been that particular event.

I hate messing up like that, but all the same I'd like to thank you for pointing it out. I'd rather know when I've messed up a reading than remain ignorant.

I've edited in amendments in a couple of my posts on the previous page to state that I got it wrong.

Now, if we could just have a high four to low five event to use for comparitive measurements, that would help a great deal. I expect there'll be another one before long. Just need to log it and screenshot the GEE traces and then it's more reliable for comparitive purposes.

reply to post by JustMike


I'm watching too, Mike. I'll try to screen cap if I catch one, for size comparison.

Actually, there is an aftershock coming across now. About 100mic/sec at 15:32:30.

Here it is. A second one popped up at the end. (from GEE):



This turned out to be Mag 4.0 mb, according to USGS.

The second earthquake above was a Mag 4.4 mb. Nearly the same amplitude, but it looks to have lasted longer.

reply to post by Olivine


That's about 150 micons/s at CRAG.

Prolly a 3.0 or so...maybe less

reply to post by Rockbeard

Thank you again for the excellent, first-hand information. Good to hear that pretty well everyone there can get up to high ground very quickly. I mean, considering this happened in the middle of the night (and in very cold conditions), you all did great.

I think the tsunami tragedies of Banda Aceh in 2004 and Japan in 2011 have made many people much more aware of how essential it is to head for high ground as soon as possible, and stay there as long as necessary. Now that we all have seen the videos of mutiple tsunamis sweeping in and simply wiping out almost everything in their path, we've learned that with these events, only a fool will stick around and try and "ride it out".

You all did the right thing -- the smart thing. Yes, this time there was no big tsunami, but the blessing is that you've now had a "trial run" and even though you may never face the real thing, you know that as a community you can work together and survive.

And it's also good for the kids, because they see how all you adults were prepared. You all knew what to do and even though this happened at the worst possible time -- in the dead of a winter's night -- you were ready to act and in just minutes you got them to safety. And once they were safe on high ground, they saw how other members of the community opened their homes to them and offered them shelter, friendship and reassuring words. Those are very valuable lessons for them and ones they'll carry for life -- and most likely will pass on to their own children one day.

With that kind of attitude, I think that no matter what might happen in the future, you'll all get through it okay as a community. Hats off to you all.

Mike

reply to post by Olivine and also to post by TrueAmerican


Yup, saw that one. Came in at about 50 micr/sec (100 micron range) at Wrangell Island. So, as TA says, likely a low mag 3 range. (If that.)

We need something just a mag or so bigger, so they'll post it up and give us a good benchmark... Okay, we can extrapolate as the mag scales are logarithmic, but it'd be a lot easier to have an actual local event to work from.

And in case anyone's wondering, a mid-range mag 5 is only 1/100th the strength of the 7.5 (and 1/1000th of the energy release). So I'm not wanting anything nasty, just something small(ish) for better comparison.

EDIT: TA, do you recall the size of the traces for Japan's big one? Was it up in the cm/sec range on the nearer seismos?

reply to post by JustMike


Yeah, that 9.1 clipped all the nearest stations, and I remember that the first good read came from station YSS, or maybe ASAJ, far on the north island. And those stations registered in the neighborhood of 9 mm/s.

In this case, the 7.5 clipped out station CRAG, cause I pulled the data on that one, and probably clipped SIT and WRAK too, dunno, didn't check on them.

These big quakes are a bitch for the closest seismometers, which is why they have been moving more towards co-located accelerometers, which can handle much bigger quakes. But those cost mucho mulah.

Ok, we're zeroing in:

That 15:32 event was a 4.0, and it showed 150 microns/s peak amplitude on my rig at CRAG in GEE, and about 100k counts on the beast.

earthquake.usgs.gov...

reply to post by Olivine


What worries me about the location of the main shock here is that I have read some scientists feel that if Cascadia were to unzip and let loose its fury, that it would likely start at one end and move towards the other. And this quake is getting pretty far up there towards the north end of the zone.

reply to post by TrueAmerican


I am watching with baited breath, I can't sleep!

Years ago when we did the Experiment in earthquake prediction thread I caught on to something. Every time Alaska has one Japan has one just days after. Some how they effect each other. I would keep your eyes glued to Japan, since this was a big one they may have the mother of all recorded quakes there after the 9.0 and how that area is still shaking.

reply to post by TrueAmerican

That mag 4.0 is very useful to know. It's stronger than I expected but not to worry, at least we have a good guide to estimate by.

And about the Japan event: yes, I recall you mentioned way back when that the nearer seismos basically went off the scale. It's one reason I'm running GEE on three stations, progressively further away from the AK main one. If the nearer one maxes out (praying it won't as that could be very bad!), at least the others will give some guide.

Originally posted by Olivine
[snippy snip]
Ack! I forgot this portion of text from the summary linked above:

The January 5th, 2013 earthquake is related to that Haida Gwaii earthquake three months previously, and is an expression of deformation along the same plate boundary system.

So today's earthquake is associated with the Haida Gwaii event 3 months ago. But we haven't seen similarly sized activity south of here (read Cascadia Subduction Zone). Should we infer that stress is even higher in the northern CSZ, the stress regimes between the Queen Charlotte Fault and CSZ are unrelated, or that these EQ sequences to the north have lessened stress on the CSZ?

What is your thinking quake-watchers of ATS?
I'm leaning toward this movement bringing a large event on the CSZ one step closer.

Now, that's the million-dollar question. The main problem is megathrust events are so rare it's very hard to characterize what sort of seismic activity could indicate that a major shock is imminent.

Take the Cascadia Subduction Zone (CSZ) for example. The last megathrust event there was almost 313 years ago, so because there's no information for what may have happened in the days, months or even years prior to that event on Jan 26, 1700, we are in the dark until the next one happens. We can't even use the events in Japan 2011, Asia 2004 (Banda Aceh), Chile 1960 or Alaska 1964 as guides, because they are different regions with their own seismic histories.

For example, the great quake in Chile was preceded by some major foreshocks in the days prior, and Japan had a mag 7 range event just 3 days before its megaquake. But Banda Aceh didn't, and neither did Alaska as far as I know.

I think it's also worth bearing in mind that even if a region gets a couple of mid-7 range events, the energy they release is miniscule compared to a magnitude 9 quake. So we should never fall for the argument that "at least these quakes are releasing energy, so the big one won't be so bad".

It just ain't so and here's the math: a magnitude 9.0 is 31.622 times bigger than a mag 7.5 (in shaking), and it's 177.827 times stronger (in energy release). (Anyone can check this with the USGS "How Much Bigger" Quake Calculator.)

So, it takes around 177 mag 7.5 quakes to release the energy of one mag 9.0!

Worldwide, we get one mag 7-range quake about every three weeks or so, on average. We are not going to see more than 100 of them in one small region in any subduction zone within a short enough time to have any significant effect on the energy building up there.

As for the smaller ones, like the mag sixes and fives? Well, it would take over 31,000 mag 6.0 quakes to release the energy of one mag 9.0 -- and we only average one mag 6-range event every 3 days. Worldwide. So even if the CSZ were to have a couple of hundred mag 6 quakes within (say) a two-week period (extremely unlikely!) what difference would that make when it takes 31,000 of them to release the energy of a single mag 9?

Practically zilch.

And there is another factor that is often overlooked in the "little quakes help to reduce the big ones" argument. Namely, even if a whole bunch of minor quakes do reduce the pent-up energy by a tiny fraction, the energy is still building up, and will continue to do so until a really big quake lets loose.

In summary: there's no way to tell if the seismic activity in or near the CSZ indicates a megathrust event is about to happen or not. We just don't have historic data before 1700 for smaller quakes there. Maybe these mag 7 range events have weakened things a little, but they haven't made any serious impact on the stress energy that's been building up over the past few centuries. Ergo, we can't tell if they are the straws that will break the camel's back.

All we know is that going on the history of the past major events (from the sediments laid down in the PNW by their tsunamis), we are "in the window" for another megathrust there. Geologically, it's "imminent" -- meaning that while it could happen today, it might not occur for another few centuries.

So as Olivine already knows all of the above (so I wrote it for others who might not ), all I can say is: mate, I have no idea! I wish I did. I only hope that we don't find out the answer the hard way.

Mike

reply to post by TrueAmerican

If it were to unzip along the entire length then it will be a very bad day. That's one very long zone, so it could have intense shaking for up ten minutes and it could peak out in the low to mid 9 range.

As for the potential tsunamis, the worry there is that there could be several. While that's pretty typical, some could overlap and cumulate into larger ones.

I suspect this might have happened with some previous ones. Hard to imagine how else they could get a 30 metre high tsunami in places otherwise -- even allowing for topographical variations.

Mate, I hope you'll be a very old man (at least) before that happens. In which case I'd likely have to be the oldest man on Earth. But if it'll hold off for a few more decades and just twitch a little with 7s from time to time to keep people alert and practised with their drills, that would be help.

reply to post by TrueAmerican

Thanks for the input, TA. Watching the progression of the aftershocks, my guess is that if today's events are going to push a secondary fault into action, it will probably be farther to the NW of today's mainshock.

What I find unnerving is how quiet the CSZ proper, has been. (Technically, today's event is considered to have occurred on the Queen Charlotte fault.)
Besides the Mag 6.5 in September 2011, and the Mag 7.0 intraplate offshore northern California in 2005, it has been uneventful the last decade. (please don't mistake this for me wishing for a devastating quake--just an observation)

This map from the Iris EQ browser shows magnitude 6+
quakes since 01/01/2002. Notice the "quiet" inside the oval, where the CSZ is locked.

I have a suspicion that today's earthquake is going to be severely downgraded--possibly below 7...just a hunch.