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

It's tending to look at fact such as there being tremor as we speak off Vancouver Island (see link)

www.pnsn.org...

but that probably facts the odds hardly at all.

reply to post by JohnVidale


Interesting. So you guys really are looking at that slow slip situation to gauge potential. Perhaps then we all need to take that more seriously too. So keep those "slow slip of the day" awards coming, z!

And looks like today it's all the way up in Vancouver Island?

reply to post by TrueAmerican


Tremor may well exert such small stresses that it doesn't matter, but we don't know for such how close it is to the locked asperities on the megathrust.

It happens from time to time pretty much continuously from Cape Mendocino to the tip of Vancouver Island.

reply to post by JohnVidale


Might I possibly suggest that we get away from the term "tremor" when we are talking about slow slip? That is extremely confusing to those that aren't in this as deep. ETS gets the point across, but slow slip is probably the best term- and I think I may have read that on your blog.

"Tremor" is starting to mean too many things.

We have volcanic tremor, harmonic tremor, people call earthquakes tremors sometimes, and now you are going to start calling slow slip tremor too?



Umm...

reply to post by TrueAmerican


Quite right. It is essentially slow slip, which we can study because the accompanying tremor gives away its location.

reply to post by JohnVidale


So you guys using mostly broadband to detect it? Or what? And further, how are you distinguishing slow slip tremor from microseisms? I take it you must have the frequency ranges pinned down on those, at the very least?

reply to post by TrueAmerican


No, it best comes out of the noise only at 3-8 Hz. When it is strong or we have an array, it can be seen 1-20 Hz.

reply to post by JohnVidale


Oh ok. So you can see it on broadbands and shortbands too. I see. That explains then one way in which you can differentiate the two (without giving away some crucial details here, that is.)

Was wondering, cause it seems you guys have a LOT of shortbands in your network.

reply to post by TrueAmerican


Maybe I wasn't clear. We think the slow slip has power at all frequencies from that of the entire episode, a period of several weeks, up past where it fades into the noise at 20 Hz. But we can only see it above the noise at periods longer than a day on GPS, periods of an hour to a couple of days on strain meters, and above 1Hz on seismometers.

So slow slip and tremor the same process, just spotted in two different passbands with two different instruments.

reply to post by JohnVidale


Very interesting, at least to me. So it appears you are in a constant battle with very low amplitude tremor and noise thresholds, and also dealing with higher frequencies getting severely attenuated because of the low amplitude and depth of the tremor. And thus the need for passbands. Yuk. Nasty!

reply to post by TrueAmerican

So back to John. Everybody wants to talk to John.

Not necessarily...like he said, he's just the new kid....

Anyway, I have something for you;

SOSUS Hydrophone Derived Earthquake Epicenters

It seems as though they estimate location and magnitude using an array of hydrophones.

The Ocean Acoustic/Marine Geophysics Project of the VENTS Program began continuous monitoring of north Pacific Ocean seismicity in August 1991 using the U.S. Navy's SOund Surveillance System (SOSUS) In May, 1996, monitoring was extended to the eastern equatorial Pacific using moored autonomous hydrophones, then to the North Atlantic in 1999, Antarctic in 2005, Indian Ocean in 2006, and Lau Basin (west Pacific) in 2009. The hydroacoustic method allows detection of low-magnitude seismicity and volcanic activity and provides more accurate source locations than land seismic networks.

NE Pacific Sesimicity VENTS Program

reply to post by jadedANDcynical


As I think TA is driving at, we'd love realtime data from a network of OBS, and deformation meters like pressure sensors (which tell water depth) and tilt meters, as well. It would reveal the earthquakes with more sensitivity and locate them better. Maybe it would even show signs of anomalous and/or accelerating deformation before dangerous earthquakes.

It is simply very expensive. Right now, there are only a couple of realtime sensor up off Canada. The Cascadia Initiative deployment TA mentions only returns data once a year, when a ship cruise by.

reply to post by JohnVidale


Does that mean then that in the current financial climate there is little scope for more sensors / kit?

Or is it more that even when finance is ok, there isn't much will for spending money in these areas? (important distinction, at least in my mind).

reply to post by Flavian


I think the former, although earthquake hazard mitigation is so driven by what's happened recently that, in the absence of more quakes like New Zealand and Japan, momentum could be lost, and nothing happen for a long time.

reply to post by JohnVidale


Thank you for answering. That is actually rather reassuring as, to be honest, i suspected the latter! So really, we want more Japan / New Zealands? (if they could happen without casualties).

Or the interest of something like the Bill Gates Foundation! (not everyone's cup of tea but frankly i suspect funding would be welcome whatever the source).

An interesting scenario for the known rupture on the Seattle fault in 900AD.

crosscut.com...:-echoes-of-Japan/

reply to post by Flavian


Gee thanks

Unfortunately you need casualties to get the politicians off their arses.
We have been having Mag 6s and 7's here in NZ since the year dot but there has never been such a rush as now, after Christchurch, to upgrade the Building Codes to new standards to withstand big earth quakes since the Napier quake of 1930, again when there were casualties.
There was a bit of a shuffle around in 1944 when a couple of 6's hit Masterton city and quite a few buildings collapsed, but it never gained momentum, as there was a World War on and the Govt didn't want to distract the citizens from the task at hand.
Here in Wellington the Council has given notice to hundreds of old unreinforced masonry building owners to upgrade and make their buildings safe, but with the economy stagnant at the moment the timing couldn't be worse. Since Christchurch some Wellington schools have had whole blocks closed down following inspection due to the risk of collapse in a medium to strong earthquake.


reply to post by JohnVidale

in that article is the sentence;

Seattle does not require old buildings to be retrofitted to match new building codes.

Surely they have some standard to aspire to?, like at least 30-50% of the 2011 code or something similarly pro rata rated.
They do have a 2011 Building Code don't they

Otherwise in the event of a strong to very strong quake, say a 7, some building owners and the (is it County?) Authorities could be facing Manslaughter charges because they did nothing to make their buildings safe.

At the end of the day, and a few Architects I know have told me this, the earthquake resistance design of the building is not so much to make the building survive the big quake but "stay up long enough for people to get out safely"

Seems like the tremor along the continental margin (at pnsn.org) is continuing in the Olympics Mts. region in 2012. I wonder if there has been associated slip measured with it?

reply to post by JohnVidale

So John, you think that slow slip and tremor are just bandwidth partitions of the same process? I would think that the data suggest now that slow slip is the fundamental process and well documented world wide. It seems that the 'tremor' is a secondary feature generated by the fluid pressure responses to the small motions at the plate interface. Or is that not the accepted view now?

reply to post by geotrician


You're quite right. There are some indications (New Zealand, Mexico) that sometimes tremor shows up without slow slip, or slow slip shows up without tremor, but most places, and the best places, the two are coincident, and slow slip would be the bigger motion and process involving more energy.

I still call it tremor by habit because that is what seismologists can most easily and most precisely measure.