reply to post by blackthorne
Yes, as you say, the last mega-quake on the Cascadia fault zone at the junction between the Juan de Fuca and Nth American plates produced a tsunami
that was recorded in Japan the next day. To be precise, it shows that this quake occurred on Jan 26, 1700.
The effects on the local native people in the BC/Washington region were horrendous. Whole communities were washed away, areas of land rose or fell,
and according to oral histories, the shaking lasted many hours.
For some more data, maps and animated simulations of this event, go to this page
If you’d like more data just send me a u2u and I’ll post some references.
reply to post by winotka
Hi winotka and thank you for the thread.
The Chile quake occurred due to subduction of the Nazca plate under the Sth American plate, so it’s quite similar to what happened in the
mega-thrust quake of 1700 on the Juan de Fuca/Nth American plate subduction zone on the Cascadia fault.
You’re right that we may not get much warning. Just as in Chile, the off-coast Oregon/Washington/BC region gets smaller quakes quite often, but
rarely much over a mag of mid-5. The last seriously big one was an 8.1 on 22 August 1949, on Queen Charlotte Island, BC (just north of Vancouver
Island). Luckily it was a sparsely populated area and relatively speaking, it didn’t do a great deal of harm.
Problem is, even this big quake 60 years ago has probably not done a lot to alleviate the next really big one. There’ve been mag 8 quakes on the
Cascadia fault before but they didn’t stop the mag 9’s. (More on this aspect below.)
Someone commented that small quakes can help relieve the pressure and prevent bigger quakes. Yes, it would
help -- if
the total amount
of energy released by the smaller quakes used up a significant amount of the energy that would otherwise
be released by a single big one. Well,
while that sounds good and actually even seems pretty logical, it’s not really the case in reality, because quakes are measured on a logarithmic
scale, not a linear scale.
Don’t want to bore members to tears, so in simple terms it just means that if you hear of a quake that’s a magnitude 5.0, for example, then
another that’s a magnitude 6.0, it means that the 6.0 released 32 times the energy of the 5.0, and had about ten times the amount of shaking. The
same applies if they are not exactly whole numbers. A 7.2 is about 32x32* times as powerful as a 5.2, and an 8.2 is 32x32x32 times more powerful, and
so on up and down the scale.
[* For the technically minded, the number 32 is an approximation derived from the formula M2 = 10^3/2 .M1
, where M2 is the next whole number on
the logarithmic scale above M1. In the formula, M1’s energy is taken as a base value of “10”, which is then treated with the fractional exponent
3/2 to derive the value of M2. So, we solve 10^3 (giving us 1000) and then solve its square root, which gives approx 31.6.]
We’ve probably all heard or read recent news reports that compared the Chile quake to the Haiti quake, in terms of energy release. The Chile quake
was many hundreds of times more powerful, so even if there’d been a mag 7 in Chile shortly before their latest big one, it wouldn’t have made even
a one percent difference to its effects.
Here’s an interesting statistic to really
put this into perspective: scientists calculated that the mag 9.5 Chile quake in 1960 released
about 25% as much energy as all the other earthquakes of the 20th century combined
And nearly all that energy was released in just a few
So, getting back to the Pacific Northwest: even if we had a couple of hundred mag 5-range quakes along the Cascadia fault zone in a shorter time (like
a few months or weeks, even), it would not make any noticeable difference to a subsequent mega-thrust mag 9 quake. In fact, although that part of the
PNW typically gets plenty of micro-quakes and small quakes every year (mag 1’s, 2’s and 3’s), it only averages a dozen or so over mag 5. Two
dozen at the outside. So their total energy release will have almost no effect whatsoever on a future mega-thrust event.
Imagine this: a mag 6 is equal to 32 mag five’s, and it takes 32 mag 6 quakes to equal one mag 7, and 32 of them
to add up to the energy of a
single mag 8. And it takes about 32 mag 8’s to equal the energy of just one
mag 9. So, to have enough relatively harmless offshore mag 5’s
to even reduce a mag 9 down to a still highly-dangerous mag 8, we’d need a total energy release equivalent to 32 mag 8’s -- and in a fairly short
space of time. And that means we’d need about 32x32x32x32 mag 5’s. Around one million mag 5 quakes, in other words. (A few thousand either way
doesn’t really matter.) That’s just to reduce one mag 9 quake down to a mag 8. And a mag 8 is still big enough to do a heck of a lot of damage,
especially to coastal areas around Vancouver and down to Seattle.
This “million-mag-five-quake” thing just doesn’t happen. As far as we know, it never has. And the reason is simple: these mega-thrust quakes are
so called because when they thrust, the results are mega. The quake of Jan 26, 1700 involved a rupture and thrust along about 1,000 km (600 miles) of
coastline. The chunk of real estate involved was about 150 km (90 miles) wide. The average movement -- thrust in one direction -- was about 15 metres.
That’s nearly 50 feet
-- and most likely, the majority of that movement occurred in just a few minutes.
If the plates move over or against each other fairly smoothly and gradually, all relatively good. But these mega-thrust quakes happen where the plates
let go gradually. That’s what happened in Sumatra off Banda Aceh, and it’s what happened in Chile. They occurred because a huge
chunk of a plate got stuck, and the pressure on it built and built over many years. Then, when the stuck region can take no more, it lets go like an
avalanche. The longer it takes before it lets go, the bigger the thrust and the more powerful the quake. Not even a few thousand smaller quakes will
make much difference to that power. And as far as we know, a big chunk of the Juan de Fuca plate is stuck.