Originally posted by Olivine
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
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
So, it takes around 177 mag 7.5 quakes to release the energy of one
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?
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
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.
edit on 5/1/13 by JustMike because: (no reason given)