Friday, September 14, 2007

Tsunami dynamics

That series of earthquakes is absolutely fascinating to us 'movers and shakers'. They seem to be rupturing around a large patch that is known to be seismic gap, and has always been expected to rupture in the next 30 years. The articles on this page, basically say that the city of Padang is waiting to die.

The only way that the outer edges of a large zone could be active, is if the zone has begun to move, but it requires a larger critical displacment than the edge zones (see my fault mechanics). It still might take 30 years, or it could happen tomorrow!

We're talking about another M9 here. As explained, the shaking will be average, but the tsunami will be horrendous!

This leads me into tsunami dynamics for people like me (dummies!). I've read a lot about tsunami wave modelling, and the physics of the thing, but I think all the scientists have missed the point! They keep calling this thing a wave, when it's nothing like a wave at all! (at least to the public). This allows everybody to think of this monster as a nicey-nicey surface wave which can be surfed on! (Hey, Dudes!)

I have attempted a drawing showing the real scale of things.

We basically have water, which might be 1000 m deep, spread over a length of thousands of kilometres! My thin blue line should be thinner (try drawing that!), and plastered right onto the crust.

The difference is like walking with a full cup of coffee from Timmy's. You see the little waves, but you can handle it. Now, try filling a cookie tray with water (thought experiment only!), and walking with it. You can always make a few steps, you see the little shimmering waves on the surface, but all of a sudden the water starts to slide, and sloosh! -- bye-bye water!

This is a water slosh that encompasses the full depth of the water. Now, on the figure, the arrow shows where we suddenly pull down on the crust. This is our big subduction earthquake. The water sloshes into the hole, and this starts to propagate over to Padang and Africa (negative slosh). It doesn't take long for the water to slam together, and send out a positive slosh. These slosh pulses involve the whole depth of the ocean, and travel quite slowly, as one would expect.

In one way, the people may be luckier with a big subduction earthquake, because the pull-down causes a leading negative slosh. Thus, when confronted with the ocean disappearing, it behooves one to run like hell to higher ground! Apparently, this might be too late for Padang, they have to start running as soon as they feel the shaking, because the run-up could be kilometres!

Not everybody gets whacked the same. As we can see in this figure, boats out on the cookie sheet don't feel much. To get the full impact, you need a coast geometry that acts like a giant bulldozer, and can scoop up the water from a great depth. A nice gentle slope is the most effective. As we can see, this is no wave, but a giant wall of water slooshing up over your house!

So, forget all those scientists and their talk of 'long period waves'. This is a giant landslide (waterslide?) of water coming right at you!


Chris said...

"Another M9".

You know, if you say that quickly, it goes past before your blood can run cold much at all. Or maybe my pschye is warped by going through a M6.7 with ongoing aftershocks when I was a kid...

Harold Asmis said...

If it is a Northridge or Kobe-type high-speed thrust, it hits you much worse than the distant big ones. These big subduction quakes are not so much known for shaking destruction, as for the tsunamis. Still, a stone house doesn't buy you much...

Chris said...

So if I understand you right, a lot of the displacement would be vertical and get transmitted into the water column? A shallow 6.3 (not 6.7, I misremembered) was quite enough for me, shaking-wise.