Earthquakes: The Science of Shaking Earth

There is simple science behind the quake that shook Japan recently and those that have occurred throughout Earth’s history. We consider here, only those earthquakes that are caused by forces arising from beneath the earth’s surface, commonly called tectonic forces.

Our earth’s crust is broken up into many fragments, all floating on semi-solid magma in the mantle of the earth. These fragments, called tectonic plates, are thus capable of moving, albeit very slowly and just a bit each year. Stress can build up along the edges between the plates if one cannot move against the other. Earthquakes are caused by tectonic plates suddenly sliding on each other at edges where they meet. The earth shakes as huge amounts of energy are released into the surrounding medium.

tectonic plates

There are two types of waves by which energy is transferred. One is called the primary or P wave, the other, secondary or S wave. They are also fundamentally different. The P wave travels much faster than the S wave. The P wave travels by compressing the ground at certain points and stretching it out at some other points, just like sound waves in air. This mode is called the longitudinal mode. The S wave travels by undulating the ground in the direction perpendicular to the propagation, like light waves. This mode is called the transverse mode.

types of waves
Longitudinal and Transverse Waves


Locating the epicenter: How do scientists know where the center of the earthquake (the epicenter) is? Since the P wave travels faster than the S wave, it arrives at a certain place before the S wave. Places nearer the epicenter will experience the two in quick succession and far off places will experience a greater delay between the two waves. Using delay data from different earthquake monitoring centers, one can triangulate the position of the epicenter and even track how far below the earth’s surface it lies.

Measuring an earthquake: What does a “magnitude 7 earthquake on the Richter Scale” mean? The Richter scale magnitude measures how much the earth shakes. It does so by considering how much the needle of the seismograph (figure below) oscillates.


The Richter scale is a logarithmic scale. This means that a 5 magnitude earthquake is 10 times as powerful (causes 10 times more shaking on the seismograph) than a 4 magnitude quake. A 6 magnitude one is 10 times as powerful as a 5 magnitude one and 100 times as powerful as a 4 magnitude one.

Earthquakes are often followed by smaller ones called aftershocks, (and sometimes, preceded by ‘foreshocks’).   This happens because the earth restores itself and takes some time to do so.

Prediction of earthquakes is impossible with the current technology and will likely remain so for a very long time. As far as data suggests, earthquakes are completely random and no occurrence pattern has ever been observed. As to whether animals can really sense’ earthquakes before they happen is a matter of speculation.

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Debjyoti Bardhan

Is a science geek, currently pursuing some sort of a degree (called a PhD) in Physics at TIFR, Mumbai. An enthusiastic but useless amateur photographer, his most favourite activity is simply lazing around. He is interested in all things interesting and scientific.