Figure 1: Hypocentre and Epicentre

1.1. Types of Earthquakes

1. Tectonic Earthquakes: These are generated due to sliding of rocks along a fault plane. This movement causes imbalance in the crustal rocks which results in earthquakes of varying magnitude, depending upon the nature of dislocation in the rock strata.

2. Volcanic Earthquakes: Volcanic activity is considered to be one of the main causes of earthquakes. In fact, volcanic activity and seismic events are so intimately related to each other that they become cause and effect for each other. Each volcanic eruption is followed by an earthquake and many of the severe earthquakes can cause volcanic eruptions.

The explosive violent gases during the process of volcanic activity try to escape upward and hence they push the crustal surface from below with great force. This leads to severe tremors of high magnitude, which depend upon the intensity of volcanic eruptions.

3. Collapse Earthquakes: In areas of intense mining activity, sometimes the roofs of underground mines collapse causing minor tremors.

4. Explosion Earthquakes: Ground shaking may also occur due to the explosion of chemical or nuclear devices.

5. The earthquakes that occur in the areas of large reservoirs are referred to as reservoir induced earthquakes.

Above may also be referred as various causes of earthquakes with one and two being the natural causes of earthquakes while three, four and five represent anthropogenic or man- made causes of earthquakes.

1.2. Seismic Waves

The waves generated by an earthquake are called the 'seismic waves' or ‘earthquake waves’. These are recorded by an instrument called the seismograph or the seismometer. For further understanding of earthquake waves, refer to the portion of the notes on ‘Interior of Earth’.