Monday, March 14, 2011

Apple #512: How Earthquakes and Tsunamis Happen

So with all the recent articles and conversations about the enormous earthquake that Japan suffered at the end of last week, there's been lots of talk about plate tectonics.  Plates of the earth's crust shifting and bumping into each other.  People are using words like subduction and convergent plates and all sorts of stuff like that.  Even though I studied earthquakes in school like everybody else, I'm having a hard time visualizing the complete process by which an earthquake occurs.  So, like a good Apple Lady, I looked it up.

Because it's a process, static diagrams weren't very helpful to me.  I wanted animations.  Drawings in action, with arrows and helpful identifiers to show me what's happening, to show me how the stuff under the crust can build up, or how an earthquake can send shock waves through the ground.  So I went looking for lots of videos.

I found some helpful stuff.  I'm going to give them to you from basic and general to more specific, zeroing in on what happened in Japan, first to create an earthquake, and then to create the tsunami.

Plate Tectonics Basics

Basically, the earth's crust is a series of floating flat islands of rock, or plates. The plates are sliding around on top of molten rock. As the plates bang into each other or drift apart, the shape of the landscape changes. Mountains might form, or volcanoes could form and erupt, or earthquakes could happen.

In this animation, the process ends with the creation of a volcano:

Types of Plate Movements and Boundaries

As the hunks of the earth's crust are floating around, they bump into each other or interact with each other in different ways. Sometimes they float away from each other (diverge), sometimes they bump into each other (converge), and sometimes they rub against each other (transform).

This presentation shows how the plates can interact with each other to create different results. Specifically, it describes the various types of boundaries and what happens to the crust in each case:

This soundless animation focuses specifically on what happens as the crust on one plate is forced beneath another.  This process is called subduction.  As shown here, the crust that gets pushed down into the hot magma gets melted and eventually bubbles back to the surface in the form of volcanoes:

The voice-over for this one is in French, but the images are still helpful, I think. This animation shows plate movement at the bottom of the ocean. First it shows what happens when plates diverge, and then it shows convergent plates where one is subducted beneath another and volcanoes and mountains form as a result -- all at the bottom of the ocean:

Earthquakes in the Ocean Creating Tsunamis

The process by which volcanoes are formed is very similar to the process by which earthquakes are created. The difference is, instead of one plate happily subducting another below it to be melted and turned into a volcano, the plate getting butted against pushes back or rebounds, and that causes an earthquake.

This animation shows an oceanic plate pressing against a continental plate until an earthquake results. There's no sound, but explanations are included in text along with the animation:

This is an animation of the earthquake and subsequent tsunami that happened in Sumatra in 2004. It shows one oceanic plate pressing against a continental plate. The passage of time is indicated in the right hand corner. The pressure builds up until finally the continental plate pushes back, so to speak, creating an earthquake.  By the way, the speed with which the "island pops up" is deceptively fast.  In real life, that would take many years.

But this is the closest depiction I could find of what happened in the earthquake in Japan:


Compared to the creation of an earthquake, the process by which tsunamis occur is relatively simple. Also, since they are in effect reactions to earthquakes in the ocean, they happen much more quickly.  But because so much water is involved, they can be far more destructive than the earthquakes that created them.

Quick & dirty overview of how a tsunami is formed:

I can't embed this next video, but I can give you a link.  This meteorologist's animation isn't great, but his explanation is helpful and it's a more complete explanation than the animation immediately above.

This guy with a British accent explains pretty much the same thing as the American meteorologist.  His images are rather static and though they're kind of hard to read, they're actually more descriptive than the meteorologist's. This guy also goes on to talk about where tsunamis tend to be more common:

This animation generated by NOAA shows the path(s) of the tsunami following the earthquake in Japan.  Suddenly the Pacific Ocean seems much smaller:

Here's another graphic from NOAA, this one static.  It shows the intensity of the tsunami that started at its originating earthquake off the shore of Japan.

The black triangles, I think, are tide gauges. The black lines are "computed tsunami arrival times."
(Graphic by NOAA, sourced from

Pretty intense.  All of it courtesy of the seething hot mass that is the mantle beneath our feet.

If you want to know more about how tsunamis work, read this brief article by Jeremy Bernstein, a physicist and regular contributor to the New York Review of Books.  It's descriptive without being too technical, informed without being stuffy.  Good stuff.
(Thanks, regular reader Jason, for the link.)

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