How Earthquakes Happen

In the past two months the world has encountered a series of earthquakes of high magnitudes. With the devastating 7.0 earthquake hitting Haiti in January which took away more than 200,000 precious lives, the world saw three other earthquakes following in close succession. The 7.0 magnitude earthquake near Japan, the massive 8.8 earthquake near Chile on Saturday, and a 6.4 near Taiwan after that, all make us shudder. The Chile quake was so massive that NASA says it slightly tilted the earth's axis and shortened the day. Wow! What an impact! Moreover, the destruction caused by these quakes are simply mind-boggling! The oodles of destruction with respect to lives and material destruction leaves the land hit by one, completely shattered. Geographical surveys state that several million earthquakes shake the earth every year. The only reason why these four earthquakes have been in the news so much is because the quakes hit highly populated regions of the world. As we see the destruction, death, pain and sorrow associated with earthquakes, it often makes us wonder why and how earthquakes happen?

How Do Earthquakes Happen

Earth's Tectonic Plates
The earth can be divided into three main layers: the outermost crust, middle mantle and innermost core (inner and outer core). Now the crust and the upper mantle of the earth form a strong layer called the lithosphere. This strong lithosphere is further broken down into plates called tectonic plates. Thus, the earth's crust is not actually one thick blanket, instead it's something like a puzzle, with pieces coming together to form one picture. The size of these puzzle pieces or plates can vary from a few hundred to thousands of kilometers across while the thickness may vary from less than 15 km to 200 km. When these plates underline the oceans, they are termed as oceanic plates, whereas when they underline the continents, they are referred to as continental plates.

Movement of Plates
As the mantle within the earth's crust moves in the deeper part of the earth, the plates begin to move as well. They begin to pull away from each other, bump into one another or slide past one another. There are three main tectonic movements: extensional, transform, and compressional which conduce to three different types of earthquakes. These movements of plates is quite slow and to help you understand better, the movement is usually the same speed as how our fingernails grow. It's when these plates collide against each other that rock layers are forced upwards in the form of mountains. When these plates move away from one another, lava from the mantle fills the gap and forms new crust. These plates also converge over a period of thousand years by slowly positioning them one beneath the other.

Science Behind Earthquakes

Earthquakes occur at the edges of the plates, and more commonly at the edges of large plates forming the earth's crust. The plate boundaries mostly lie beneath the oceans and cannot be seen, however, oceanic plate boundaries can be mapped accurately from outer space, via measurements from GEOSAT satellites. Earthquakes happen when these plates move under, slide past one another or collide into one another.

The surface where these plates slip are termed as fault planes or faults and the plate boundaries are made up of several such faults or slipping zones. Since the edges of these plates are rough, the sliding process is not very smooth. As the plates slide past one another, parts of the plate get stuck, while the rest continues moving. So we have one portion of the plate clinging on to opposing portion of the other plate, while the rest of the plate is gradually moving forwards. Potential energy keeps building up in the locked or stuck portion of the plates. When the unstuck part of the plate has moved far enough, the force exerted by it pulls the stuck regions apart. This ripping of the stuck part of opposing plates results in release of oodles of potential energy that was stored for the sliding movement.

Here's when the earthquake seismic waves come into picture. The energy radiated outwards from the ripping action comes out in the form of seismic waves. These waves resemble the ripples formed in a pond. As the ripples of seismic waves move through the earth, they shake our earth and we experience the quaking of the earth. There are two kinds of waves that radiate through the earth during an earthquake, one called the Rayleigh waves which move with a rolling, up and down motion, while the other are love waves which cause the ground to twist from side to side. These two waves are responsible for all the quaking, cracking of the earth's crust and crumbling of buildings, etc. Depending on the amount of energy released in the form of seismic waves, the magnitude of the earthquake will vary. They are also triggered by volcanic eruptions wherein molten magma under the earth's crust exert enormous pressure, releasing it through openings in the plates.

There is a lot of research being conducted by scientists in the field of earthquake forecasting, so as to minimize the amount of devastation caused by the earth's quaking. Intimating people before an earthquake and evacuating the area would help save millions of lives. However, despite all the decades of efforts scientists have not been able to predict earthquakes and it's quite unlikely they will be able to predict the occurrence of this natural disaster. So the best thing to do as of today is to update yourself about earthquake management.