The subsurface location (below the surface to a depth of 700 kilometres) where the earthquake originated is called the earthquake focus. The earthquake epicentre is the point on Earth’s surface that lies directly above the focus (where the shocks felt). The vibrations generated during earthquakes are called seismic waves and is caused by the sudden collapse of rock within the earth or an explosion (Fig. 6).
Earthquake is a shaking of the ground produced by the release of energy. It can occur anywhere (land and ocean), but strong earthquakes are most likely to happen near the plate boundaries.
There are 3 types of seismic waves:
- P Waves or Primary Waves (1 to 14 kilometres/second): These are first waves generated during the earthquake and fastest among the other waves to reach earth crust. It can pass through solid, liquid and gases.
- S waves or Secondary Waves (1 to 8 kilometres/second): They are the waves which travel after the primary waves and can pass only through solids. The particle movement in the S waves is perpendicular to the wave direction
- Surface Waves or L-Waves (2 to 6 kilometres/second): The Surface waves travel near the ground’s surface and velocity is very slow. These are the most destructive waves.
Seismographs are instruments used to record the motion of the ground during an earthquake. A seismogram is the recording of the ground shaking at the specific location of the instrument. The Richter scale is the most common method of measuring earthquakes.
Fig. 6 – Earthquake: Focus, Epicentre and Seismic waves
Generally, earthquakes occur mainly in the margins of tectonic plates caused by the sudden release of massive amount of energy within some finite regions from the interior of the Earth. Following this, there is another type of earthquake termed as volcanic earthquake which is caused by volcanic activity. Human induced earthquakes are caused by the activities of humans like dumping of fluid waste into deep wells, testing of nuclear bomb under the ground, mining, and water storage in huge dam.
Causes of Earthquakes
Effects of Earthquakes
The effects of earthquakes can be classified as primary effects and secondary effects.
The primary effects are:
- Ground motion - can damage and sometimes completely destroy buildings, and
- Surface rupture - faults break the ground surface, buildings can be split, roads disrupted.
The secondary effects are:
- Fires - ground movement displaces stoves, breaks gas lines, and loosens electrical wires, thereby starting fires,
- Landslides - earthquake vibrations may cause soil to slip and cliffs to collapse in the steep slope regions,
- Liquefaction - is a phenomenon that turns solid ground into a liquid-like state.
- Tsunami - is a seismic sea wave initiated by sudden movement of the seafloor caused by an earthquake, volcanic eruption, or underwater landslide.
Distribution of Earthquakes
Earthquake Belts of the World
Earth’s major earthquake belts are the margins of tectonic plates.
- The most important earthquake belt is Circum-Pacific Belt. This Belt is associated with volcanic activity and popularly known as the “Pacific Ring of Fire”. This Belt accounts for about 68 per cent of all earthquakes.
- The second belt termed as “Alpine Belt” includes “The Alps” in Europe and the “Great Himalayas” in Asia accounts for 31%.
- Apart from the two belts it may also occur in some other regions mostly along oceanic ridges (ridges of Arctic, Atlantic and Western part of Indian Ocean), and the rift valleys of East Africa. (Fig. 7).
Fig. 7 – Earthquake Regions of the World
Earthquake Zones of India
The earthquake (seismic) zones of Indian subcontinent are divided into four zones with respect to its severity (Fig. 8).
Fig. 8 – Earthquake Regions of India
- Zone V: Andaman & Nicobar Island, all of North-Eastern India, Parts of North-Western Bihar, Eastern Section of Uttranchal, Himachal Pradesh, the surrounding region of Srinagar in Jammu & Kashmir and the Rann of Kutchh in Gujarat.
- Zone-IV: Includes Jammu & Kashmir exclusive of Srinagar region, parts of Himachal Pradesh, Union Territory of Delhi, Sikkim, northern parts of Uttar Pradesh, Bihar and West Bengal, parts of Gujarat and small portions of Maharashtra near the west coast and Rajasthan.
- Zone-III: Consists of Kerala, Goa, Lakshadweep islands, Uttar Pradesh excluding northern part, Gujarat, West Bengal, parts of Punjab, Rajasthan, Madhya Pradesh, Bihar, Jharkhand, Chhattisgarh, Maharashtra, Orissa, Andhra Pradesh, Tamil Nadu and Karnataka.
- Zone-II: covers other than the above said parts of India.