A direct definition of an earthquake is, the vibrations felt due to the physical breaking of rocks under the Earth’s surface from stress.
Knowing the structure of what makes up the planet we live on can intensify our understanding of what causes these natural, sometimes catastrophic disasters. Beginning from the inside out, the Earth is made up of 4 main layers: the inner and outer core, the mantle and the crust. The crust, often referred to as the ‘thin outermost skin of Earth’ is broken up into many pieces that cover the surface of the planet which are called Tectonic Plates. These plates are constantly moving and sliding around, (at approximately 1-15cm/year) occasionally bumping against each other on their edges or plate boundaries(fault lines). This interaction can cause the plates to interlock, inhibiting their greater movement. The plates continue to try and overcome their hold on each other meanwhile building pressure from underneath to a point where the two plates finally break apart in a violent manner, releasing all the pent up energy and causing an earthquake.
The point where the energy is first released deep down within the Earth is known as the focus point, the point directly above this on the surface of the Earth is called the epicentre. The energy from the break radiates out from this point in the form of seismic waves which shake the ground and can be the cause of significant damage and destruction depending on the relative energy of the break. Seismographs are tools used to record and measure the intensity and position of earthquakes. They provide a seismogram which is a series of wavelengths that provide seismologists with the information needed to determine where and how large the earthquake was.
Image from: http://www.mlms.loganschools.org/~mlowe/LoweHome/SciberText/GraphicPNG/EarthQuake%20Diagram.png
After an earthquake occurs, the amplitude of the seismic waves is measured and converted to a magnitude which gives the extent of the energy released (Richter Scale), this value can then be rated using the Modified Mercalli intensity scale. The Richter Scale evaluates earthquakes on a logarithmic scale of 0-10 depending on the intensity and therefore the amount of damage caused.
Image from: http://wiki.ubc.ca/images/e/ea/Richterscale.png
Earthquakes Among us 
