Your body can let out sudden bursts.  So can the earth!  The ground we walk on might seem quiet, but then, without warning BOOM!  SHAKE!  The earth's crust is made up of giant pieces of rock that are always on the move.  When the plates rub up against each other, they can get stuck. This traps energy that can escape all at once and cause a lot of problems for us on the surface.  Even though the plates are only sliding a few inches, they let out a lot of energy because of their size.  Think about the difference between getting hit by a slow tennis ball or a truck.  Even if the truck is going slowly, it will still push you a lot more than the ball.  When the earth's plates slip, they can make buildings break and rip apart the very ground we walk on.  In order for us and our buildings to stay in one piece, we have to know an earthquake's every move -- and that is no small job.

The thought that the ground beneath our feet can start shaking like Jell-O is a scary one.  So let's talk about how we know when earthquakes might come and, more importantly, how big they will be.  That way we can decide if we can stand where we are, or need to grab onto something sturdy.  A seismometer is a tool that lets us measure the earth's shaking and helps us find the size and starting place of an earthquake.  They can only tell us these things after it's already happened.  The plates move and slide against each other and that's where the energy comes out.  We place these tools close to these places where they meet so they can tell us two things:
1) The size of the earthquake.  Was there a lot of movement or just a little? 
2) Where it began. 
If we read at least three measurements, they can point us to where the shaking began.

Hey, did you feel something?

Now it's time to go to the heart of the shaking!  We talked about the place where the shaking starts, which is actually under the ground.  Those earthquakes will send waves around the whole earth.  The place that shakes us is different.  It's several miles up.  The place where an earthquake starts to shake the land is called the epicenter.  When you hear about earthquakes on the news, they will say epicenter, because it is a lot easier to say it happened in a city instead of 40 miles below that city.  Even though it starts shaking deep under the ground, the shaking we care about happens closer to the crust and is a lot more dangerous to us. 

Epicenter, like the bullseye of a target.

What is an earthquake?  Hit the top of some water and watch what happens.  You made waves, right?  That is what's happening with earthquakes, only the energy that comes from the middle of the earth is so strong it can make rocks and roads move like waves in water.  The fastest waves can even move through the inside of the earth and are called body waves.  These fast waves push and pull the ground together and apart like a Slinky.  It's easy to remember because it's a wave that pushes and pulls and starts with a "p."  P-waves come first in an earthquake and they push and pull the earth forward and back.  P-waves can move through water, which means they can be felt everywhere on the earth.  If they happen in the ocean, the shaking can still make it to land.  P-waves only make way for the real shaking . . .

S-waves, the second wave of an earthquake, roll the earth and can only move through land.  They move things up and down, making waves that look like what you would see rolling across the ocean or if you gave a quick flap to a bed sheet.  S-waves will stop the second they hit liquid.  This includes the magma in the center of the earth.  They also move about 1.7 miles slower than p-waves.  These are much easier to see coming because scientists can read the size and speed of the last wave and figure out when it will come and how heavy it will be.

Earthquakes can be caused by many things.  Volcanoes, rocks from space, even stuff we do, like digging into the ground.  However, most of them come from our earth's moving plates.  People who live near the cracks must learn everything they can about earthquakes: how they move, how they start, how to make your house safe, everything.  The ground turning to Jell-O beneath our feet may seem bad, but with some learning, it does not have to be the end of the world.


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National Geographic. "Earthquakes--Seismic Destruction." Nat Geo, 2010. <http://environment.nationalgeographic.com/environment/natural-disasters/earthquake-profile/>

Earthquake Country. "Protect Yourself During an Earthquake . . . Drop, Cover, and Hold On!"  Earthquake Country, 2012. <http://www.earthquakecountry.info/dropcoverholdon/>

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