A disaster is a result of natural or man-made causes that leads to sudden disruption of normal life, causing severe damage to life and property to an extent that available social and economic protection mechanisms are inadequate to cope.

Catostrophic events majorly caused of :

• Volcanoes
•  Earthquakes
•  Tsunami

Volcanoes:

1. Volcanism and causes of volcanism aren’t random. The areas of high volcanic activity are of ocean – ocean convergence and ocean continent convergence.
2. One of the regions is the Pacific ring of fire located in the pacific ocean and consists of border areas of China, Philippines, Japan, rocky mountains, Mexico , Andes mountains, Indonesia, new Zealand, Tanzania.
3. The second reason for volcanoes is the ocean – ocean divergence which creates peaceful basaltic eruptions as mid ocean ridges are formed.
4. The third reason for volcanic eruptions is breaking up of plates into blocks. The andesitic eruptions are seen in this type.

Distribution of Earthquakes and Volcanoes across the World

Volcanism along the Pacific Ring of Fire

• Circum-Pacific region popularly termed the ‘Pacific Ring of Fire’, has the greatest concentration of active volcanoes. Volcanic belt and earthquake belt closely overlap along the ‘Pacific Ring of Fire’.
  • Regions with active volcanism along ‘Pacific Ring of Fire’
  • The Aleutian Islands into Kamchatka, Japan,
  • the Philippines, and Indonesia (Java and Sumatra in particular),
  • Pacific islands of Solomon, New Hebrides, Tonga and North Island, New Zealand.
  • Andes to Central America (particularly Guatemala, Costa Rica and Nicaragua), Mexico and right up to Alaska.

Converging plate margins and Mid-oceanic ridges

• Most known volcanic activity and the earthquakes occur along converging plate margins and mid-oceanic ridges.
• It is said that nearly 70 per cent of earthquakes occur in the Circum-Pacific belt.
• Another 20 per cent of earthquakes take place in the Mediterranean-Himalayan belt including Asia Minor, the Himalayas and parts of north-west China.
• The belts of highest concentration are Aleutian-Kurile islands arc, Melanesia and New Zealand-Tonga belt.

Earthquake

• An earthquake is the shaking of the Earth when two blocks of the earth suddenly slip past one another.
• The surface where they slip is called the fault or fault plane.
• The location below the earth’s surface where the earthquake starts is called the hypocenter, and the location directly above it on the surface of the earth is called the epicenter.

Types of Earthquakes

• Tectonic: A tectonic earthquake is one that occurs when the earth’s crust breaks due to geological forces on rocks and adjoining plates that cause physical and chemical changes.
• Volcanic: A volcanic earthquake is any earthquake that results from tectonic forces which occur in conjunction with volcanic activity.
• Collapse: A collapse earthquake are small earthquakes in underground caverns and mines that are caused by seismic waves produced from the explosion of rock on the surface.
• Explosion: An explosion earthquake is an earthquake that is the result of the detonation of a nuclear and/or chemical device.
• Reservoir induced Earthquakes: These occur in the areas of huge reservoirs like dams.

Seismic Zones of India

• Seismic zones in Indian subcontinent is divided into four seismic zones (II, III, IV, and V) based on
• scientific inputs relating to seismicity,
• earthquakes occurred in the past and
• tectonic setup of the region.
• The Bureau of Indian Standards is the official agency for publishing the seismic hazard maps and codes.
• It has brought out versions of seismic zoning map: a six zone map in 1962, a seven zone map in 1966, and a five zone map 1970/1984.

Seismic Active Zones

• Seismic Zone II
• Seismic Zone III
• Seismic Zone IV
• Seismic Zone V

Tsunami

1. Underwater earthquakes and volcanic eruptions are the primary causes of tsunamis.
2. Meteorite fall has also caused tsunamis. Earthquakes and volcanic eruptions are the most frequent causes of tsunamis. 

Formation of Tsunamis

• Undersea Earthquakes
• Landslides
• Volcanic Eruptions
• Meteorites and Asteroids

Characteristics/Features of a Tsunami

• The wavelength of a deep ocean tsunami can be up to 200 kilometres
• Tsunamis are fast in deep waters and slow in shallow waters
• Small tsunamis can be destructive because they may carry debris
• Tsunamis come in multiple waves
• The time difference between one wave and another can be hours
• The later waves are usually more destructive
• When a tsunami reaches the shore, there is a temporary rise in sea levels, which is called a “run-up”
• About 80% of the tsunami waves happen in the Pacific Ocean
• Tsunamis can travel from one end of the pacific ocean to the other in just one day.

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