Sunday, January 09, 2005

Tsunamis and Polar Ice

So on The Carnegie Vandertramp in post Prick Me, Jenn Onofrio said:

Theo? If you're out there: do you know if the recent tsunami/earthquake had any great impact on the position of the polar ice caps in the Arctic Circle?

To which I responded:

First, I want to point out that tsunami is a Japanese word literally meaning "habor wave." It is meant to represent a sea wave that is not caused by periodic tidal forces (i.e., gravitational interaction with the moon). It is meant to represent a sea wave that really only exists AT coastlines. If it weren't for harbors, we wouldn't notice them. Somewhere something was lost in the translation and we ignorant English speakers started to call it a "tidal wave." Now, in most English phrases, "tidal" would modify "wave," implying that this is a slow periodic movement. That's what "tidal" means. However, we make a special exception for "tidal wave" where, basically, "wave" modifies "tidal." This is a wave that sits ON TOP OF the normal tidal ebbing and flowing of the sea. So, personally, I'd be happier if everyone called this a "harbor wave." Of course, it would take a lot for everyone to do to make me truly happy...

Now, that being said, you're really asking about the earthquake. So let's talk a little bit about earthquakes, or at least the particular kind that the area around Sumatra.

You see, there are a number of different types of earthquakes.

For example, the earthquakes you hear about in California are caused by two plates sliding by each other, just like two cars side-swiping each other. This type of earthquake will never result in a volcano (despite what Hollywood movies tell you), only earthquakes.

However, the earthquake that occurred near Sumatra is due to a very different type of interaction. It is what is known as a "subduction zone." This particular subduction zone is what fueled the great 1883 explosion of Krackatoa. When continential plates interact this way, a soft plate collides head-on with a hard continential plate and the soft plate is forced downward beneath the hard plate, taking with it lots of water. The composition of the material that is driven deeper into the earth actually sets up conditions for massive volcano eruptions later.

In the case of this recent 600-mile-long "subduction event," six miles below the earth, the soft plate dived down into the planet in a matter of minutes. The entire island of Sumatra was moved 100 feet to the southwest. These movements are what drove the wall of water toward India at ~500 miles per hour. Note that this movement of water was only a few meters high away from the coastline. Boats on the water would have hardly noticed it (keep in mind that energy is what moved hundreds of miles an hour toward India, not water; the water just translated the energy but did not drift much with the wave). As the wave moved closer to land, the sea became more shallow. As the sea became more shallow, the drag on the bottom of the sea started to increase and slow the movement of the wave down there. This is what caused the top of the wave to surge forward ahead of the bottom of the wave. This is what caused the sudden crest of the wave. In order to dissipate all of this energy, the water then had to receed and return a number of times, damping with each new time. Also note that the BACKSIDE of islands in its path often were hit the hardest because of the dynamics of the wave's movements.

Now, going back to the earthquake... That soft plate was pushed deep into the earth. This changed the earth's rotational inertia. Imagine a skater spiraling around with her arms spread far apart. As she brings those arms closer together, it becomes easier to spin her since there is less mass to be moved around the large circle etched out by her extended arms. The SAME THING happened with the earth. In order to conserve its angular momentum, the EARTH'S ROTATION HAS INCREASED. A day on earth is now about one second shorter. This may not seem that significant, but keep in mind that scientists (including those who study polar ice) need to know the length of a day so precisely that they have invented a "leap second" (like a "leap day") that they apply at regular intervals to help keep our clock in sync. Those calculations thus need to be updated.

And so all this alone changes the environment slightly. To a bacteria, for example, one second is a long time.

But I still haven't really answered your question. As you probably heard, the earthquake completely changed the underwater toplogy around Sumatra (I'm guessing the Sunder Straight is probably a lot different than it used to be). Heck, Sumatra actually MOVED, so of course the underwater topology changed.

However, there really was no DIRECT effect on the position of polar ice. However, as mentioned before, this changes the DYNAMICS of the earth's spin.

Let's go back to the skater example. Her arms are extended as she spins slowly. She moves her arms inward. Her angular velocity increases. What happens to her pony tail? Since its mass hasn't changed, in order to maintain circular rotation, its radius of rotation changes. In other words, it moves farther out.

So as the earth spins faster, there is more stress on the polar ice caps to move toward the equator. This probably is a neglible change, but it is a difference.

Also keep in mind that since the days are shorter, the temperature profile of the planet changes. On average, every point on the planet still receives the same amount of sunlight. However, how that average is delivered changes. Picture someone turning on and off a light switch. Over ten minutes, the light is on half the time. It could be on for five minutes and off for the other five, but it also could be on for 30 seconds every minute. This would cause a very different effect in a room. Imagine that we're not only turning on a light, but also turning on a heater. How does it affect things by speeding the "carrier signal" of that energy up?

So there are those issues. But that's not what all of us science types are really concerned with. The big issue involves global warming. We know that a "tiny" little earthquake like this one can cause a change in the earth's rotation and stability. Imagine what would happen if the polar ice caps melted?

In other words, let's go back to our spinning skater. Let's say she's pretty strong and holds a bowling ball at full arm extension in one hand and nothing in the other. Now, if she's a light girl, this bowling ball is going to cause her to wobble like a boomerang. In other words, her center of rotation is going to be closer to the bowling ball than herself. Her body is going to want to rotate around the bowling ball, and it will wobble and destabilize her desired spin because of it. Now, if she is a REALLY REALLY heavy girl, this bowling ball isn't going to make a dent. She'll spin around her own center with no trouble.

The earth is a lot like this skater. The polar ice caps give the earth a bunch of stability. It is unclear how the dynamics of the earth would change without so much mass being concentrated near our axis of rotation.

So does the tsunami affect the polar ice caps? Probably not in really important short and medium term ways. What the tsunami does do is point out how a mass movement can change the dynamics of our spin (even in the short term). Movement of ice can do the same thing.

(NOTE that we currently do not have a stable rotation; we wobble like a top. If you know how to look at the stars and point out constellations, you'll notice that the REAL Zodiac is far different than the one that is published in the paper. Just 2000 years ago our earth was at a different angle with respect to the plane of the planets' rotation. 2000 years isn't a long time. Hopefully the top is settling down, but with too much mass movement, it could destabilize. The top could "fall"...


Anonymous said...

Wow! Very interesting read. I'm fascinated by tsunamis and earthquakes. Mostly because I love living in the littoral regions... Question comparing the spin of the ice skater to the spin of the earth. Is gravity in terms of the ice skater on earth different than gravity in terms of the earth in the solar system? I really don't know!

Ted said...

The analogy comparing the spinning skater to the earth didn't really involve gravity in any consequential way. That is, in the example given in the post, the ice skater's "arms" play the same role as the gravity of the earth. Gravity holds the "bowling ball" close to the body. After that, the dynamics are the same. If gravity is able to pull mass closer to its center, a spinning object will have a "more stable" rotation. Likewise, the closer the skater can hold the "bowling ball" to her body, the more stable her rotation.

The important thing is the spinning -- not the gravity. The effects come from angular momentum (and its conservation); they are not anything special related to gravity.

THAT BEING SAID, certainly the geometry of the earth changes the "amount" of gravity. It's as if the position of the skater's bowling ball actually changed the "stretchiness" of her arms. However, the earth's diameter is very large, and we're talking about *relatively* small changes in the shape of a SMALL part of the earth (it's surface). Those small changes should not change the gravity holding the planet together. They just might change how it spins (i.e., speed and "stability").

I hope that answers your question.