How are earthquakes caused?
We know that tremors are produced by the disturbances taking place deep down in the uppermost layer of the Earth known as the crust.
This exterior portion of the Earth is not continuous, and is broken into many pieces known as plates. These plates move constantly. They can slide past one another, or move into each other because of the collisions taking place between them.
When this happens, disturbances are produced in the earth’s crust, which lead to an earthquake on the surface of the Earth. However, this is not the only reason which can give rise to an earthquake.
Earthquakes can also be caused by the following:
Meteors falling on the Earth’s surface
Underground explosions. For example, underground nuclear tests.
Collapsing structures such as mines etc.
Earthquakes are usually caused when rock underground suddenly breaks along a fault. 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 earth is basically a tectonic plate. this plate keeps moving (almost 1mm in 10 yrs). when 2 plates touch each other a wave is caused. this place where the plates touch is the focus(focus can also be called as the starting point of the earthquake). the place above the focus is the epicentre. as the waves reach further its speed or energy decreases.this is an earthquake.an earthquake is measured on a richter scale .