Something For Everyone . . .

We hear about a tsunami in the ocean.  But how does a wave move underwater at 500 miles per hour, clear across the Pacific, then suddenly pop up higher than a building?  Here’s how that works.

In a chemistry class, our instructor said he would show us how to pound a nail into a board using a glass bottle.

We all laughed.  We were anxious to watch him smash the bottle.

He tricked us.  He filled the bottle with water, screwed on a cap, and inverted it to make sure there were no air bubbles (air can be compressed).  He explained that liquids cannot be compressed, so he couldn’t just press in a rubber stopper without exploding the bottle.  Since water cannot be compressed, the glass could not change shape even when used to pound on a nail.  He proceeded to pound the nail into the wood with a glass bottle.

Later, he pointed out that liquids can be compressed just a tiny, tiny bit.  That’s very good for us.

If an undersea earthquake causes two pieces of seabed to shear sideways, there’s no compression of water, no tsunami.  But when an undersea earthquake causes a huge area of seabed to jut upwards suddenly, the water above that upheaval can only be compressed a tiny bit.  The rest is an enormous volume of water that’s jerked violently into motion, dispersed into the surrounding water, and sent in every direction.

If water were not just a tiny bit compressible, the other end of that column would instantly appear across the ocean, devastating the land without warning.

Instead, being just slightly compressible, it moves across the ocean at about 500 m.p.h.

We can’t see the wave in the ocean because it’s underwater.  It passes under a ship without incident, though some areas have sensors to warn us.  The original water doesn’t move to a different continent, it just pushes the water in front of it, so the “movement” energy is transferred.  One molecule bumps the next quickly, but not instantly.

When the column of water suddenly hits the shallows in front of the shore, it gets pushed up into the air, the same as any other wave.

So, the speed of “compressibility” of water in real life is about 500 m.p.h.  That’s about 9 miles in a minute, about 800 feet per second, the speed of a commercial jetliner, or 10 times as fast as the family car.  If you’re 1,000 miles away, you’ve got about 2 hours to prepare.

The amount of water that ends up on land compares to the area of seabed lifted, and how high it lifts.

A compressed water volume is a little slower than the speed of sound in air (~ 700 m.p.h.).  Basically, they’re both the same phenomenon.  A wave in the air is faster because air is not as dense as water, so less air gets moved.  It’s quicker, but doesn’t go as far.

Charlie Gosh

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