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!

The marvellous mechanics of the subduction zone

I'm trying out calligraphy with Inkscape. You can add mass, tremor and other neato stuff to mask your normal horrible strokes! I'm still improving, but already I'm better than Brittney in an MTV concert!

Lots of earthquakes lately in subduction zones. I get some questions (none, here, unfortunately), mainly: Why all these earthquakes now? -- Why not? is the correct answer. :) Also, why do these earthquakes have to be so much bigger to get the same shaking destruction as other types of earthquakes. (I made that up myself!)

To answer that question, we have to look at the mechanics, which is slightly different than the other articles I have written. First, I poo-poo the notion that the oceanic crust is pushed down. All oceanic crust eventually gets cold, old, and tired (like me!). It eventually sinks under its own weight, like elephants going to their grave site. Even New York City will (one day) have its own subduction zone!

As show in my great figure, this slab tends to fall at an angle, and rub up against the island arc complex. Mechanically, this slab is really goopy, full of water and sea-shit (see my Carbon Cycle articles), and has different properties than regular earthquake rock. Once in a while, the descending slab gets hung up on the upper rock, and slippery bathtub feet engage (from Fault Mechanics).

All the normal mechanisms for earthquakes now apply, but the dynamic friction drop may (or may not!) be greater because of all the goop. The important thing is that this is an 'upside down' thrust fault, with the moving element being the slab. As such, the rupture, and the high velocity pulse goes downward.

This reduces the impact of the shaking, but things are not so happy for the inhabitants of the ocean shore. The combination of the rapid slip downward, and the attendant push upward on upper crust, makes for some dramatic ocean wave mechanics. It's like suddenly getting up in your bathtub! If we are lucky (like this last one), most of the oceanic energy gets beamed down to the penguins, who can take care of themselves.

As in my beautiful drawing, the oceanic crust eventually gets a nice warm, peaceful death, and the water-carbon gets pooped out at the top, to cause some other problems for the locals....

Of course, the island arc actually gets pushed by all this, and on the other side we get the juicy high-speed thrust earthquakes that cause so much trouble, for their size (as in the Japanese nuclear earthquake).