Geography [3 Mark Question Answer]

Earthquake Waves:- Earthquake waves travel in all directions from the Focus. There are three types of earthquake waves :

1. Longitudinal Waves: These are known as Primary Waves ‘P’
2. Transverse Waves: These are known as Secondary Waves ‘S’.
3. Surface Waves: These are called Long Waves or ‘L’ Waves.

Koyna Dam is located in Maharashtra on the Deccan plateau. This part was considered a stable block free from earthquakes. But the Koyna earthquake in 1986 was a big surprise. In the reservoir of the dam water gathered was more than its capacity. It disturbed the local isostatic balance. It caused an earthquake and caused cracks in the rocks. It may be called a man-made earthquake.

Causes The chief known causes of earthquakes are believed to be as follows :

1. When a dormant volcano erupts into activity or an active volcano discharges matter with greater violence the surrounding areas feel tremors or earthquakes.
2. When the interior of the earth cools and contracts the outer crust in some places cracks and faults are caused. This movement causes an earthquake.
3. Some time water percolates so deep down into the earth that it turns into steam on account of the internal heat. This steam expands and tries to force its way out. This pressure causes an earthquake.

The earthquakes are common along the weak tectonic belts affected by continuous tremors due to the movement of plates one upon another. Main belts are Circum-Pacific Mountain belt and Mid-World Mountain belt of fold mountains which are still in the process of upthrust forces increasing their height.

When the earth’s crust is unable to accommodate itself to the strain due to isostatic imbalance, so sudden release of energy results in violent shock or an earthquake. The earthquakes are caused by the severe tectonic movement of plates, volcanic eruption forces and isostatic imbalance often taking place due to diastrophism creating ups and downs in landform construction.