West Coast USA: Pay Attention, Cascadia May Be Ready to Rupture

Originally posted by JohnVidale
As you note, Mb is a strange measure, which originated from discriminating nuclear tests from earthquakes. Even a big nuke would let out its energy in the first few seconds (or much less), and score a big Mb but moderate MS, while a big earthquake would take much longer, and have a littler Mb but a huge MS.

Which is exactly why I brought up the point. Mb is notoriously inaccurate. And I will now refer that back to a previous poster plotting depth charts off Oregon, for further note of comprehension of circumstance, and the reliability of estimates characterizing quakes there.

But then my wife, in her thesis with Hiroo Kanamori, invented the Mb-hat (not sure how to put the little hat over the b) scale, which measured the peak short-period P amplitude anywhere in the P wave train. It still saturates, but continues to grow with magnitude even up to the biggest quakes.

So to what extent, if any, is this new calculation actually being used and referenced in the modern networks? If there is a network using and displaying this magnitude type, could you please point us to an example?

MS is mostly obsolete - MW covers the same range for regional networks, and is more universal.

True, but are quakes off Oregon well enough constrained to be able to reliably calculate Mw in the first place? Especially without OBS data. Mw relies on calculation of the seismic moment, and that is particularly difficult there, if I am adding up all the pieces right.

But in a general sense, you guys are watching cascadia constantly, John. So in your opinion, do you feel like there is any reason whatsoever to be concerned with what appears to be a slight increase in the frequency of earthquakes off the coast in this last period of a month or two? Or is this completely normal, and has not raised eyebrows one iota in professional circles? In other words, is that a faulty premise, and thus, I really have no basis for the creation of this thread?

Because so far, I don't see you arguing that....yet. Just curious. Give us a sense of what the pros are saying about these quakes behind the scenes. Berkeley itself seems to be concerned, as this article would show:
seismo.berkeley.edu...

reply to post by TrueAmerican


good questions, I've no time at the moment, will try to get back here later tonight.

John, I am really happy to have you aboard You may also want to visit the Quake Watch 2012 Thread it is hosted by Puterman another quake watcher. Muzzy from NZ is also a major slugger there and would have much interest in your thinking on the different values and such. Again, Welcome Aboard and don't forget to check your supplies and expiration dates in your BO Bag

reply to post by TrueAmerican

So to what extent, if any, is this new calculation actually being used and referenced in the modern networks? If there is a network using and displaying this magnitude type, could you please point us to an example?

It was only used in some research papers. Frankly, I don't keep track of which networks are using which magnitude scales except for a few. They are all generally equivalent, when used appropriately, and the exact magnitude of any earthquake doesn't matter at the level of +/- 0.5 magnitude units. NEIC uses a bewildering variety, partly because they collect data from many thousands of seismometers around the world, and collate the results from dozens of networks.

True, but are quakes off Oregon well enough constrained to be able to reliably calculate Mw in the first place? Especially without OBS data. Mw relies on calculation of the seismic moment, and that is particularly difficult there, if I am adding up all the pieces right.

It depends on the size. The further offshore the quake, the more limited are our capacities. An M5 anywhere in the world will get a good MW from NEIC and the Lamont-Doherty guys. An M4, maybe not. An M3, we can barely do an MW in the middle of the PNSN - it just doesn't generate enough 10-20s-period energy to rise above the noise.

But in a general sense, you guys are watching cascadia constantly, John. So in your opinion, do you feel like there is any reason whatsoever to be concerned with what appears to be a slight increase in the frequency of earthquakes off the coast in this last period of a month or two? Or is this completely normal, and has not raised eyebrows one iota in professional circles? In other words, is that a faulty premise, and thus, I really have no basis for the creation of this thread?

It's not black and white. Activity is a bit higher than usual, so we wonder a little more, and peek at the spectrograms and triggers a little more often. In a year or two, we'll check the GPS charts more frequently. But we're usually obsessed about something - in fact it has been quiet on our turf recently. Since Paul and I arrived in 2006, the biggest events in OR & WA have been M4.5, and nothing has come out of any of the volcanoes.

It's a bit like reading tea leaves. No pattern of activity is guaranteed to warn us of coming events, and not many unsettling events really raise the odds of an impending earthquake to a dangerous level. We just need to be ready to be surprised.

AnMarie - thanks, I added the Quake Watch to my subscriptions. My desktop is already cluttered with CISN display and quake alert to tell me what just happened on the West Coast and around the world, but it will be interesting to track what people think of the activity. I won't be watching constantly, though.

reply to post by PuterMan


I'll add an opinion from time to time, and it may not be as wild as some here.

Originally posted by JohnVidale
reply to post by PuterMan

 

I'll add an opinion from time to time, and it may not be as wild as some here.

Oh, go on. I bet you get tempted when you have been here for a while, simply because you will quickly come to realise that your views would be taken as gospel. I would save it for when you are having a bad day, just to cheer yourself up!

Ok, so it's like that, is it?

Fine. I just got an inside tip on some new activity happening somewhere of interest. And this time I am keeping my mouth shut.

Go find it boys, somewhere on the planet.

By the way, off topic posts are strongly discouraged at this forum. And moderated. Those last posts contributed absolutely NOTHING to this topic, and are merely just to make fun of me.



Keep talking.

reply to post by JohnVidale


reply to post by Flavian


It's good to see some folks can maintain a sense of humor and not approach every topic with a sense of regal self-importance unbecoming an internet conspiracy theory forum.

reply to post by TrueAmerican


Off topic, but I apprecite what you do TrueAmerican, keep it up, you have come close in the past, with your personal research.

REMINDER

This is the topic:

West Coast USA: Pay Attention, Cascadia May Be Ready to Rupture

Please stick to the topic. Off topic remarks can derail and even destroy good threads.
--Off Topic, One Liners and General Back Scratching Posts--

Thanks for your cooperation.

At least a small lake in Cascadia may be ready to rupture, and in fact is ripping already:

pnsn.org...

What I think TA may be expressing is a certain amount of incredulity over the apparent lack of interest in the potential dangers that the CSZ representa by those who hold the purse strings. Having recently been very involved with a small, local bit of politics, I can scarcely imagine the sheer intertia must be staggering.

I am curious as to how much attention and efforts are beig put in to sifting for possible seismic precursors of a large quake in that area. Here I will reference some posts I've made in various threads regarding this topic:

A post which references SOSUS

Are these some the OBS you've referenced TA?

The capabilities of INSAR, the M8 (and variants) algorithm, and GESS among others interest me a bit also; some of which are referenced in this thread

Sorry TA if this goes a bit off topic, but I couldn't see how else to respond to JohnVidale

reply to post by JohnVidale


Hi there John.
I'm interested to read what you have to say about magnitude scales, as it is the most common reason that makes me keep coming bacck onto ATS (Quake Watch) to point out the differing magnitudes (and types) that the Networks are giving for particular events that someone has posted up.

The Russians still use MS, although they also always give mb as well.

Both Puterman and I have tended towards looking at the energy released by a quake rather than the magnitude, as it gives a better idea as to the possible destruction caused.
The formula I use is;
TTNT (tonnes of TNT energy released) = (10^(4.8+(magnitude*1.5))/4184)/10^6
but of course you need the magnitude to make that work to start with

hense I am surprised to read that "exact magnitude of any earthquake doesn't matter at the level of +/- 0.5 magnitude units."

I think it matters a great deal

For example using that formula

M3 = 0.48TTNT
M3.5 = 2.68TTNT

M5.0 = 476.88TTNT
M5.5 = 2,681.69TTNT

M6.3 = 15,080.24TTNT
M6.8 = 84,802.44TTNT


M7.0 = 476,879.14TTNT
M7.5 = 2,681,688.47TTNT

take the 6.3 and the 6.8, +/- 0.5 units of magnitude difference, 15k or 85k of TNT,? a huge difference IMO.

even the 3.0 and 3.5 comparison, if it went off under your house either half a ton of TNT or 2.7 TTNT would probably destroy it, but if it was 1km away the damage would tend to favour your house surviving the half ton, where 2.7 tonnes would likely knock it off its foundations

I enjoyed reading the seismo blog its interesting to see what the people behind the data are thinking.


Any chance you could put us out of our misery and give the formula used by USGS to convert from mb to Mw?
If there is one [

reply to post by muzzy


I was being a bit glib in minimizing the difference in magnitude scales, but basically, any earthquake has a source amplitude spectra, and one gets the magnitude by reducing its complexity to a single number by averaging estimates from a selection of stations. Different estimates emphasize different frequencies. Energy emphasizes the high frequencies, moment the low frequencies, and the rest something in between.

As you say, there are meaningful differences, but also enough inconsistencies and artifacts from poor data to frustrate observers. In fact, the topic is endless. Paul, who wrote that PNSN blog entry, has much more patience with it than do I.

reply to post by JohnVidale


Thanks for the quick reply
Yes it has taken me a few years to grasp the complexity of establishing the magnitudes accurately.
I note that Me was used for a period in the 1980's by NEIC, but you rarely see it now except perhaps in the occasional Tensor Solution report on USGS.
Is this because Me is too unrelaible? or too hard to calculate?

reply to post by TrueAmerican


I'm sorry, I know you are very knowledgeable in all this .....but...Your title to this thread (6 weeks ago) is:

West Coast USA: Pay Attention, Cascadia May Be Ready to Rupture

Then your opening line is:

"it probably isn't...but it might"

So you think it probably "isn't" going to rupture......yet you post a sensationalist headline?

Why not title your thread

West Coast USA: Pay Attention, Cascadia probably isn't going to Rupture ?

I'm not knocking your knowledge, it just seems like a little fear mongering that's all.

No offence intended.

reply to post by muzzy


I think people weren't using it so much, so they don't post it for simplicity. As I recall, it is quantity George Choy likes to calculate.

Now I see it is still there - here for an earthquake 4 days ago

earthquake.usgs.gov...

check under "scientific and technical" and "energy and broadband solution" under that.

Originally posted by jadedANDcynical
A post which references SOSUS

Are these some the OBS you've referenced TA?

I was talking more specifically about these in this project:

cascadia.uoregon.edu...

Read around there, and you'll get an idea.

But I am all for any OBS project that can help directly contribute to qualifying data in Cascadia. Because as you can see from information presented in this thread, help is needed for quakes far out at sea there.

So back to John. Everybody wants to talk to John.
What size of quake and what epicenter location, would seriously get you worried about an impending rupture in Cascadia?

Just so I'll know when not to start a thread in the future...

reply to post by TrueAmerican


I'm just a new, probably occasional, visitor. Now I'm counting the seconds until my wife finishes her work and chases me home for dinner.

My concern would gradually rise with increasing magnitude of seismicity and/or slow slip activity. A good rule of thumb is any earthquake has a 1 in 10 chance of being followed by a bigger quake within a week. I know you're concerned about the longer term, but as far as I'm concerned, the odds I described in the link below for the next 50 years apply today.

pnsn.org...

1 in 300 a year, now that we're 310 years out from the last one. I haven't seen activity in the last 5 years, since I've been up here that more than temporarily would raise those odds. The Eureka M5.5 and the M6 or so event off Vancouver Island might have bumped up the odds for a few days or weeks, but not to any alarming level.