reply to post by amarenell
You've asked some pretty good questions and as TrueAmerican is undoubtedly a bit snowed under at the moment I hope he won't mind if I offer something
by way of answers. This will be a bit long but some explanation is needed.
The Mercalli scale isn't used much these days, and in fact neither is the Richter scale. Most of the larger quakes are reported by USGS and similar
agencies on the MMS -- the Moment Magnitude Scale -- and are denoted as "Mw". (The small "w" refers to "mechanical work".) So for example, a quake
reported as 5.1 Mw means it's 5.1 on that scale. The annoying thing is that some mainstream media still say "on the Richter scale", even though that
scale has not been widely used for some years. It's not a major worry though, because much of the time, the two scales give very similar values.
Without going into lots of boring details, MMS gives the magnitude of a quake based upon the amount of energy it released and also indicates its size.
How "big" a quake is -- its size -- refers to the amount of shaking it creates, and how strong it is refers to the energy it releases.
Like the Richter scale, it's also logarithmic. This means that for example, a quake of magnitude 6.0 is ten times bigger in terms of the amount of
shaking than a magnitude 5.0. In terms of the energy released, it's 31.622 times stronger. But a magnitude 9.0 is 10,000 times bigger than a mag 5.0
and releases 1,000,000 times more energy. Yes, one million times more.
Oh, here's a great calculator on the USGS website that shows what I mean. Just put in any two numbers for different quake magnitudes and it will show
you their difference in size and strength: USGS Earthquake Calculator
Now, when TrueAmerican was referring to giant tsunamis as he did -- and cataclysmic earthquakes -- he was simply presenting the ultimate worst-case
scenario. He was not saying that this is what is going to happen, he was only saying what is accepted as theoretically possible. So, it wasn't
fear-mongering, just presenting a scenario.
The biggest (and strongest) earthquake ever recorded was a magnitude 9.5. That is to say, it was more than three times bigger than the Japan quake
(and over 5.5 times stronger in energy released).
But the experts agree that just because we have not recorded an earthquake bigger or stronger than a 9.5, it doesn't mean that they are not possible.
In theory, a magnitude 10.0 quake is possible, but it would require a huge, probably megathrust event. And yes, there are subduction zones in the
world where such an event could occur. The western side of South America is one region, off Tahiti is another, and the entire Cascadia Subduction zone
off the US Pacific Northwest is another as well -- if
the entire zone ruptured in a single event. The same applies to the zone off Japan and on
down further along the Phillipine plate.
The fact is, there are several regions in the world that could have a massive megathrust event. Would a magnitude 10.0 quake produce a wave a mile
high? It's impossible to know for sure, but the energy released would be more than 30 times greater than that released by a magnitude 9. It's the
energy released that allows the waves to be created. More energy can mean a faster and more powerful wave, and when it reaches shallow water, the
surge inland would be much greater. How high the wave is when it hits the shore just depends on the terrain, but very high waves are possible.
While such an awful scenario is very unlikely, it's far better to consider it than to make the mistake of thinking that the possible won't happen.
This is one reason why the Japanese are having such serious problems with some of their nuclear reactors. They were designed to withstand quakes in
the lower magnitude 8 range, because the designers' thinking was: "We've never had a bigger quake, so we don't need to design for one."
They should have at the very least designed those power stations to survive the biggest megathrust quake ever recorded. To me, that simply makes
sense. Japan has that huge subduction zone on its doorstep. They've had megathrust quakes before. Terrible ones. To simply hope that a bigger one
won't happen -- well, we all can see what a bad idea that was.
After all, now they not only have all the worries of dealing with damaged nuclear reactors, they also have to think about the fact that they have
several others that could be damaged if they get another huge quake. And as the "Tokai" event has not yet happened, they have very good reason to be
worried. And that's not fear-mongering either. That's telling the plain and simple truth. Ask the Japanese: they were surprised by this event on
Friday because the one they have long expected was for the Tokai region.
And now, they are still waiting for that one.
edit on 16/3/11 by JustMike because: Typos and linky