What causes earthquakes?

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Earthquakes occur because of the movements of the tectonic plates. Tectonic plates are large, flat pieces of stone that form the Earth's crust. These tectonic plates are actually moving in slow motion. Since they are moving, the edges of the tectonic plates will rub against each other or one of them may slide over another, When the sides are stuck together and the rest of the plate continues moving, the rocks will start to distort. As the motion keeps going, the strain will build up until the plate cannot hold up the bending. The result is that the rocks will break and the two sides of the tectonic plates will tremble. The shaking of the moving tectonic plates causes the earthquake.
The movement of continental plates against each other, and the adjustment of fracture zones.
The processes of "plate tectonics" are what cause an earthquake.

An answer for young students:

Earthquakes are shakes and jiggles in the huge pieces of the Earth that form the outside of the big ball of our planet. We and the dirt and trees and other living things live on top of these big pieces of our Earth. These pieces are called plates. They "float" around on the Earth's hot crust very very slowly, so slowly you can't see them move. But as they creep along, they bump into each other sometimes. They can even get in a "traffic jam" and push real hard against each other.

When they bump and push together, the pressure builds up until something has to give and move from all the force. All at once when pieces find a way to get unjammed, the pressure that has made them squish together really tight lets go and a sudden jerk can happen as one plate gets on top of another or as they slide past each other. This jerking causes the scrunches and wrinkles of the plates that were jamming into each other to suddenly let go and "Boing!" Shakes and jiggles go rushing out of the tight spot as the plates move over or under or around each other. These jerks move in waves all across the huge plates to relieve the tension. These released forces are very powerful and they are what make the ground move, wiggle and vibrate so that we can feel it from very far away.

A basic answer for adults:

The surface of the earth is made up of great tectonic plates of rock. These plates are all moving slowly towards, or away from, each other. Sometimes when they push together they slip and there is a jolt, and that is what we feel as an earthquake.

The release of pressure caused by the grinding together of the tectonic plates creates the earthquake. Imagine the Earth's outer layer as being cracked into many pieces. These pieces, called tectonic plates "float" on the melted core of the earth. Because of this, they shift around ever so slightly and as they shift, they can grind against each other. This builds up pressure until the two plates slip, releasing their energy in what we feel as an earthquake.

If you slowly but firmly rub your hands together you can feel how they seem to stick for a moment before finally sliding. It's the same thing with the tectonic plates.

More Information:

Movement of the Earth's crust along the plate margins causes up-heaving and instability in land or ocean masses. Plate Tectonics is what this movement of the Earth's crust is called. The Earth's plates are constantly moving and can push together in places. Sometimes they get into a "traffic jam". As plates move and interact with each other, stress (pressure) builds up over time. When the stress increases to a certain point, it overcomes the tendency of the plates to press together and creates a sudden shift in the plate positions, resulting in what are called seismic waves. These waves are what we feel as an earthquake as they move out and away from the epicenter of the plate shift.

There are a few other occurrences that can cause earthquakes, including human activities (described below), but the majority are caused by plate tectonics.

A more detailed and technical answer:

The accumulation of stress from the plate motions described above cause the rocks that make up the crust to deform elastically (this is very similar to what happens when you squash or stretch a spring). The stress causes a form of energy to be stored in the rocks of the crust which is technically described as "elastic potential energy". When this stress gets too intense, it exceeds the strength of the rocks in the crust and causes a brittle failure. Brittle failures are failures where fractures propagate through the material.

This sudden brittle failure causes all of the elastic potential energy to be released at one time in the form of seismic waves (just as a spring or elastic band that is being over-stretched can suddenly snap). These seismic waves cause the tremors that people feel on the surface and are what can cause damage to buildings and other structures. The ground shakes, and cracks in the earth begin to form. During a sufficiently powerful earthquake, poorly designed structures can collapse and cause a lot of damage and deaths.

Other and Human causes of earthquakes:

Earthquakes of differing magnitudes (also known as seismic activity) may also be triggered by volcanic eruptions, the detonation of explosives or nuclear weapons, landslides and the impact of large meteorites or asteroids on the Earth.

Additional Information and Terminology:

Earthquakes are recorded with a seismometer. Seismologists can use the data to estimate the strength (more correctly termed the magnitude) of an earthquake; the distance to the center of the earthquake; and, if data from enough seismometer stations are available, the location of the earthquake's epicenter. [An earthquake's point of initial rupture is called its focus or hypocenter. The term epicenter refers to the point at ground level directly above the hypocenter.]

The spot underground where the rock breaks is called the focus of the earthquake. The place right above the focus (on top of the ground) is called the epicenter of the earthquake.

The Earth's tectonic plates are constantly moving very slowly (at a rate of cm/year).

Convergent, divergent and transform plate boundaries:

There are convergent, divergent and transform plate boundaries. Some plates are moving away from each other (divergent boundaries), some are moving towards each other (convergent boundaries) and some are sliding past each other (transform boundaries).

In a convergent plate boundary, two plates will approach each other. The denser plate will then subduct. Friction is produced and the ground shakes, depending on the strength of the quake, causing an earthquake. Shock waves are radiated around the area, creating tremors on land.

In a divergent plate boundary, two plates will move away from each other, causing the ground to sink, forming rift valleys. This will also cause earthquakes due to the sinking of the land.

In transform plate boundaries two plates move side by side in opposite directions. This will produce lots of friction when the two plates come into contact. The ground will vibrate and the tremors can be felt, however, not felt as strong as those in convergent, as they definitely will not exceed 8.5 on the Richter Scale. At transform boundaries, the fault zone can become locked together due to friction, which again causes stress to build up as the plates attempt to continue moving.

Earthquakes can cause tsunami:

If earthquakes occur in or very near large bodies of water, a tsunami can occur. A tsunami is a huge wave of water caused by the sudden upheaval or subduction of the sea floor or the bottom of a large lake (see the related questions below for more details about a tsunami). It will originate at the epicenter of the earthquake and move at high speeds deep in the water. At first it cannot be clearly seen since it moves deeply under water. When it gets nearer to the coastal area, it will start to form great tall waves as it moves into the more shallow waters. The wave may rise up to 7-10 stories high! It will then move ashore and cause the coastal areas to be flooded. (See more about tsunami in the related questions below.)

Interesting Note:

The crack lines where this happens are called fault lines. Wherever there are fault lines, you normally find more earthquakes.

There are four tectonic plates near Japan: the Eurasian Plate, the Pacific Plate, the North American Plate and the Philippine Plate. The March 2011 earthquake involved the Pacific Plate diving under the North American Plate. This move of one plate below another is called subduction.

There are also big fault lines under Los Angeles. There are two main tectonic plates that meet under California. New Zealand also has large fault lines in the area.

That's a reason why there are so many massive earthquakes and tsunami in those areas.
The Earth's crust is made up of several large areas called tectonic plates. These move in relation to each other, some moving around, over, or under the other plates. Most earthquakes occur when the plates have been held in place, allowing stress to build up along the boundary where they are in contact (which is called a fault). When they finally do move, the sudden shift in position creates a displacement and shock wave that can affect the surface of the Earth above them. This series of movements is called an earthquake.

More rarely, quakes can be created or triggered by surface events that create similar motion in the crust : volcanic eruptions, landslides, floods, building collapses, or large man-made explosions.
Earthquakes are usually caused when rock underground suddenly moves along a fault (a fracture accompanied by displacement of one side against the other). This sudden release of energy causes the seismic waves that make the ground shake. When two blocks of rock or two plates are rubbing against each other, they stick a little. They don't just slide smoothly; the rocks catch on each other. The rocks are still pushing against each other, but not moving. After a while, the rocks break because of all the pressure that's built up. When the rocks break, the earthquake occurs. During the earthquake and afterward, the plates or blocks of rock start moving, and they continue to move until they get stuck again. The spot underground where the rock breaks is called the focus of the earthquake. The place right above the focus (on top of the ground) is called the epicenter of the earthquake.
The tectonics plates push,pull,or scrape agiainst each other,stress builds up along faults near the plates edges.
Earthquakes are caused by sudden motions in zones of the Earth's crust called tectonic plates. The plates touch one another and might be pushing against one another, pulling apart or twisting beside. Friction of the jagged earth/rock will keep the plate locked against the other until sufficient pressure builds to overcome the jaggedness. Sudden motion of the plate in a jagged zonewill cause the area to shake. Certain places where the plates touch will be very prone to earthquakes such as the area near Santiago, Chilie where 3 plates border one another.
Earthquakes occur because of movements in the earth's crust. When built up pressure of two or more plates moving against each other is suddenly released, the ground can shake violently.

Because stress forces have exceeded the strength of rock. :)

plates of the earth cross over each other
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