Earthquake, its types, effect and Ways to measure it Upsc notes

An earthquake in simple words is the shaking of the earth. It is a natural event and it is caused due to release of energy that generates seismic waves that travel in all directions.

Why does the earth shake?

The shakes are due to the release of energy, that occurs along the fault. The fault is a sharp break in the crustal rocks. These rocks along the fault tend to move in opposite directions. The overlying rocks strata press them and the friction locks them together.

But over time, the tendency to move part over this friction. As a result, these blocks get deformed and slide past each other abruptly. Due to this movement, there is a release of energy and energy waves that travel in all directions.


The point where the energy is released is called the focus of an earthquake or hypocentre.


The energy waves travel in different directions and reach the surface and the point on the surface, nearest to the focus, is called the epicenter. It is the first one to experience the waves and it is a point directly above the focus.

Types of Earthquakes

  • Tectonic Earthquakes
  • Collapse Earthquakes
  • Explosion Earthquakes
  • Reservoir Induced Earthquakes

The most common ones are the tectonic earthquakes which are generated due to the sliding of rocks along a fault plane.

A special class of tectonic earthquakes such as volcanic earthquakes, only volcanic zones.

In the areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors called collapse earthquakes.

Ground shaking may also occur due to the explosion of chemical or nuclear devices called explosion earthquakes.

The earthquakes that occur in the areas of large reservoirs are called reservoir-induced earthquakes.

Measuring Earthquakes

The earthquake events are scaled either according to the magnitude or intensity of the shock. The magnitude scale is known as the Richter scale.

The magnitude relates to the energy released during the quake. The magnitude is expressed in numbers, 0-10.

The intensity scale is named after Mercalli, an Italian seismologist.

The intensity scale takes into account the visible damage caused by the event. The range of intensity scale is from 1-12.


Ground Shaking, Differential ground settlement, Land and mudslides, Soil liquefaction, Ground lurching, Avalanches, Ground displacement, Floods from dam and levee failures, Fires, Structural collapse, Falling objects, Tsunami, etc.

The first six listed above have some bearings upon landforms.

The effect of the tsunami would occur only if the epicenter of the tremor is below oceanic waters and the magnitude is sufficiently high.

Tsunamis are waves generated by tremors and not an earthquake in themselves.

Though the actual quake activity lasts for a few seconds, its effects are devastating provided the magnitude of the quake is more than 5 on the Richter scale.

The earthquake is a natural hazard. If a tremor of high magnitude takes place, it can cause heavy damage to the life and property of people.

However, not all parts of the globe necessarily experience major shocks

Note that the quakes of high magnitude, i.e. 8+ are quite rare; they occur once in 1-2 years whereas those of ‘tiny’ types occur almost every minute.

* * All the Notes in this blog, are referred from Tamil Nadu State Board Books and Samacheer Kalvi Books. Kindly check with the original Tamil Nadu state board books and Ncert Books.