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Earthquakes 101

Earthquakes can cause the ground to shake and crack apart. Earthquakes can be very powerful, and if they occur in or near areas where people live, they can make buildings collapse, bridges sway, and roads buckle.
An Alaskan earthquake caused the ground to crack apart. © USGS
But not all earthquakes are powerful enough to cause damage. In fact, earthquakes are happening all the time, on land and in the ocean. Most are so small that people don't even feel them.
An earthquake is the sudden movement of the Earth's crust. Earthquakes occur along fault lines, cracks in the Earth's crust where tectonic plates meet. They occur where plates are subducting, spreading, slipping, or colliding. As the plates grind together, they get stuck and pressure builds up. Finally, the pressure between the plates is so great that they break loose. Depending on how much pressure has built up, the ground may tremble slightly or shake forcefully.
This highway in California collapsed during an earthquake. © USGS
Scientists describe the intensity of an earthquake using the Richter Scale. It measures earthquakes on a scale of 1 to 10. People barely feel a magnitude 3 earthquake, and windows might rattle at magnitude 4. A magnitude 6 earthquake is considered major, causing houses to move and chimneys to fall. The largest earthquake on record had a magnitude of 9.5.

Why do Earthquakes Happen?

In 1906, the seismologist Henry Reid developed the “elastic rebound theory” to explain earthquakes. When rocks begin to press against each other, they initially bend, like a spring, to accommodate the opposing forces. Eventually, when the rocks reach a point where they cannot bend further, they break. The bent rocks snap back, or rebound, to their original shape. The break is the fault itself, and the shock waves emanating from the rebound are the earthquake. The shock waves vibrate through the Earth, making it “ring” like a bell.

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  • blobby green style avatar for user jendaj
    You say: Scientists describe the intensity of an earthquake using the Richter Scale.
    That is not true. The "Intensity" of an earthquake is what is felt, not the measure of the size, and uses the Modified Mercalli Scale (http://earthquake.usgs.gov/learn/topics/mercalli.php). This is an important distinction. The Richter scale is only good for earthquakes that are close and under magnitude 7. The newer and more accurate scale is the Moment Magnitude Scale described here: https://www.youtube.com/watch?v=HL3KGK5eqaw
    (11 votes)
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  • old spice man green style avatar for user Blake longdon
    what if earthquakes never had a role in natural phenomenons?
    (6 votes)
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    • starky ultimate style avatar for user Diana
      Earthquakes don't happen by themselves. They appear after rocks and tectonic plates meet. That happens because of the movement of mantle under crust.
      Earthquakes wouldn't happen if there weren't thermal reactions happening in mantel and in Earth core(cuz that's the reason why mantle is "half-solid" and able to move) but then there probably wouldn't be humanity. Climate and shape of Earth would be totaly different. So as ecosystem if there would be any at all. It's not like earthquakes are good but they're the result of thermal reactions happening under our feet that play big role in shaping climate and ecosystem the way we know and need for living.
      (4 votes)
  • piceratops seed style avatar for user abigail.thiel
    What does the focus of the earthquake have to do with the seismic waves and the scale point it deserves?
    (5 votes)
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  • marcimus red style avatar for user Aleziah
    What causes the ground to push together?
    (2 votes)
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    • duskpin ultimate style avatar for user Betty :)
      Tectonic plates are large slabs of rock that lie underneath us, named after the geographical location in which they are found. They make up what is called the lithosphere (oceanic crust and continental crust), which slides around on top of the asthenosphere. The asthenosphere is the top level of mantle, semi-liquid magma which lets the lithosphere move around. In the asthenosphere, convection currents (imagine a pot boiling on a stove, the water that is at the bottom is heated by the fire, moves upwards, it then is pushed back down because it cools a bit, and the next batch of heated molecules would push it away.) push the crust along. Earthquakes form when these tectonic plates rub against each other and cause friction and energy to build up at the point of contact. When one plate slips under another, it causes a subduction zone! These zones are a result of one plate being denser than the other, causing it to sink below another upon contact. Earthquakes are more common at subduction zones than any other kind of tectonic plate boundary. I hope that helps you!
      (6 votes)
  • leafers seedling style avatar for user bethzy
    so im guessing by the way earthquakes are rated 1 through 10 which one will cause the most damage that is noticeable enough? like if a 3*3 is about not really feeling it, why does it escalate so quickly when its a 4*3 or a 4*5
    (4 votes)
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    • old spice man green style avatar for user Sarah Wanger
      While the 1 to 10 part is true, it's not quite so simple as a 4 being a little bit stronger than a 3. The scale is exponential. That means, the difference in numbers on the scale corresponds to the exponent on 10 that the strength is multiplied by. For example: 4 is actually 10 times stronger than a 3 (since 10 to the first power is 10) and a 5 is actually 100 times stronger than a 3 (since 10 to the second power is 100).
      (2 votes)
  • blobby green style avatar for user Jillian Zhu
    So basically, the rock strains itself until it snaps and the aftermath is that everything around it can feel it's pressure? Do people see the fault lines of earthquakes afterwards? And since there were 4 earthquakes within a week on the fault line of Alabama and Florida, I know that different earthquakes can happen in the same general area. But does this instance mean that the plate tectonics move at different rates? How do scientists detect those rates and prepare for the future? Do they even know the exact location of the different plate tectonics? Are they tracking them down? If so, how so?
    (3 votes)
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  • male robot hal style avatar for user Emmanuel Danison
    If earthquakes have to do with tectonic plate movement, can they be used to detect volcanoes
    (2 votes)
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  • blobby green style avatar for user Carbon Dioxide
    if earthquakes have plates that can detect volcanoes and volcanoes detect tsunamis
    so if you have a earthquake in japan than wow! they better get ready for a tsunami and a volcano

    but that will rarely happen
    (2 votes)
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  • blobby green style avatar for user bronzebull2005
    How long does it take for earthquakes to form?
    (2 votes)
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  • duskpin seed style avatar for user Grace Schlaud
    Why do people not feel all earth quakes? Why are there some small Earthquakes that no one feels?
    (2 votes)
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