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Earthquakes and Tsunamis

I frequently get questions about earthquakes that causes devastation, so I thought we would experiment a bit to help you understand more about them. This is a topic that many people, and the news media especially, frequently misunderstand.

You will need:

  • a wooden pencil
  • a large, zipperlock, plastic bag
  • water
  • a large plate

Many people want to know why some earthquakes produce a tsunami, when most earthquakes do not. To understand that, we need to know about faults. Before you start listing all my faults, such as forgetting to include parts of the experiment, I am talking about geologic faults. Imagine the rocks in the Earth as a wooden pencil. If you hold the ends of the pencil and push upwards on the center with your thumbs, the pencil will bend slightly and then suddenly break. When pressure causes the same thing to happen in the Earth's crust, the break is called a fault.

There are different kinds of faults, depending on the direction of the pressure. Hold your hands in front of you, side by side, with the palms up. Imagine them as the rocks on either side of the fault. If you move one of your hands away from you, you can simulate a strike slip fault. This is a very common type of fault. A good example is the San Andreas Fault in California. Because the sides of the fault move horizontally, this type of fault often results in fences and roads that are broken, with one side shifted several feet to the right or left of the other.

While strike slip faults are common, there are other ways that the rocks can move. If the pressure of the sides is towards each other, then one side is forced upwards, causing a thrust fault. If the pressure is away from each other, then one side can move downwards, forming a gravity or normal fault.

That kind of fault has a big impact on the formation of a tsunami. Fill a large zipperlock bag half-full of water. Seal it well, and work over the sink, just in case. Hold your hands in front of you as you did before, with the bag of water laying on your palms. Watch what happens to the water when you slide one hand away from you, simulating a strike slip fault. You should not see much movement of the water. Next, move one hand upwards quickly to simulate a thrust fault. You get a lot more movement as the water rushes from the lifted hand to the lower hand. You get the same sort of rush if you lower one hand, as in a gravity fault.

Experiment with this for a while, and you should get some understanding about why different kinds of faults carry different risks for the production of a tsunami.

Have a wonder filled week.