Rocks breaking underground along a fault in the Earth's crust is how earthquakes are caused. This releases a large burst of energy which shakes the ground we stand on. This "energy" or "waves" are called seismic waves.
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Earthquakes are caused by sudden tectonic movements in the earth's crust. The main cause is that when the tectonic plates, one slides over the other, they cause the collision of the orogeny (formation of mountains), earthquakes.
