The earth is made up of a number of layers, including the core, that radiate outwards at different depths and sizes. The surface of the earth is the layer called the crust, and the crust is divided into many segments called plates. These plates are constantly in motion, though they move so slowly that any significant change in the plates will only be noticed after thousands of years. Every now and then the movement of one plate over another builds up energy, and it is released at the point where the plates meet, called the fault line. This release of energy is the basis for an earthquake. Energy can also be released in other ways, such as when a volcano erupts with enough force. Man-made explosions can also release enough energy to cause an earthquake if they are strong enough. Whatever the catalyst is for a major release of energy in or against the earth's crust, it is what triggers earthquakes to occur.
Seismic Waves Are Produced
The point at which the mass amount of energy needed to start an earthquake originates is known as a focal point. These are usually found at shallow depths from the earth's surface and go hand in hand with the point on the surface right above it, called the epicentre. From this point, seismic waves emanate in all directions. Seismic waves travel at various speeds depending on the material they pass through. They continue to move through the crust, though they lose strength the farther they travel from the focal point. Eventually, the energy that created them is expunged and the seismic waves disappear.
The Earthquake Occurs
As seismic waves travel through the Earth's crust, the effects of an earthquake we are familiar with occur. These seismic waves carry with them an enormous amount of energy, so whatever they travel through will be affected by that. This results in the quivering and shaking of the earth during an earthquake. Depending on the strength of the seismic waves, the earthquake they cause will vary in strength and duration. Mild earthquakes may not cause very much damage at all, while the more powerful ones can crumble buildings and cause the earth to open up. While the earthquake is occurring, the seismic waves also case the ground to make a noise or ring like a tuning fork. The combination of these things are the earthquakes that are common to people living on or near fault lines, and it is these earthquakes that have the ability to cause a great deal of damage.
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