Geological disasters

Geological disasters

Geological disasters are caused by the transition of rock mass from stable equilibrium to the unstable state. This transition is accompanied by release of large amounts of energy. The disasters of this type are classified as:

1. Earthquake
2. Landslides
3. Tsunami
4. Mining

Earthquake

Earthquakes are one of the most dangerous and instantaneously destructive natural hazards. It is not possible to forecast earthquakes with regard to time and place of occurrence or magnitude. A large portion of our country is vulnerable to earthquake activity of varying magnitudes. The nature and impact of earthquake disaster is dependent on:

• its magnitude
• geological and soil conditions
• location of fault
• construction of major structures and 
• prevailing construction practices in the particular areas.

India has a very long history of earthquakes. A few major earthquakes are listed below:

1. Rann of Kutch (1819) magnitude 8.0
2. Assam (1897) magnitude 8.7
3. Kangra (H.P.) (1905) magnitude 8.0
4. Bihar-Nepal border (1934) magnitude 8.4
5. Andaman Islands (1941) magnitude 8.0 and
6. Assam (1950) magnitude 8.6.

In the recent past, India experienced major earthquakes at 

1. Uttarkashi (1991)
2. Latur (1993) and Jabalpur (1997)
3. Chamoli (1999) 
4. Bhuj (2001).

In the earthquake that occurred at Uttarkashi:

• The affected area had a known history of earthquake occurrence and widespread damage took place due to this earthquake.
• The non-engineered buildings found throughout the rural areas and the old stone buildings in the towns suffered severe damage.
• The engineered buildings faced the earth quake in a very safe manner and suffered no damage, except for minor cracks in the buildings
• There was large scale damage to infrastructural facilities in the earthquake affected area.
• The damage to roads was due to rockfall, landslides and' rock-slides along the road side slopes causing heavy damage
• Services like communication network and power supply system were affected very badly
• The buildings that house health and education facilities suffered damaged the joints in the piped supply lines failed due to ground shaking, resulting in disruption of water supply
• A large number of bridges were located to cross the rivers and deep river valleys throughout the hilly area. 
• The Gawana bridge on the road to Gangotri suffered 'slight to medium damages.

In the earthquake that occurred at Latur:

This earthquake struck the Marathwada region of Maharashtra state on 30th September 1993. The impact of this earthquake was felt in the adjoining states of Andhra Pradesh and Karnataka

also.

• Widespread damage took place due to this earthquake.
• Due to the earthquake most of the houses were destroyed' causing death of people in large numbers as also a wide spread damage to installations and properties
• The entire region had a traditional system of dwelling unit construction which involved heavy stone walls, and a massive roof over the wooden timber sub-structure. 
• During the earthquake most of the houses were destroyed' causing death of people in large numbers along with widespread damage to installations and properties
• Army services were pressed into action for the rescue operation. This involved clearing rubble, rescuing the injured, removal and cremation of dead bodies.
• Provisions were made for temporary relief shelters to the survivors of the earthquake. These
• shelters were made up ofG.1. (galvanized iron) sheeted roofs over the bamboo
• or wooden frames. About 30,000 families were provided the temporary
• shelters in the two worst affected districts of Latur and Osmanabad,

Landslides

• Landslides are simply defined as the mass movement of rock, debris or earth down a slope and have come to include a broad range of motions whereby falling, sliding and flowing under the influence of gravity dislodges earth material. 
• They often take place in conjunction with earthquakes, floods and volcanoes. At times, prolonged rainfall causing heavy block the flow or river for quite some time. 
• The formation of river blocks can cause havoc to the settlements downstream on it's bursting. In the hilly terrain of India including the Himalayas, landslides have been a major and widely spread natural disaster the often strike life and property and occupy a position of major concern.

The two regions most vulnerable to landslides are 

• the Himalayas and 
• the Western Ghats. 

The Himalayas mountain belt comprise of tectonically unstable younger geological formations subjected to severe seismic activity. The Western Ghats and Nilgiris are geologically stable but have uplifted plateau margins influenced by neo- tectonic activity. Compared to Western Ghats region, the slides in the Himalayas region are huge and massive and in most cases the overburden along with the underlying lithology is displaced during sliding particularly due to the seismic factor.

Incidences of landslides in India

Himalayas                                                                       - High to very high

North-eastern Hills                                                         - High

Western Ghats and the Nilgiris                                       - Moderate to high

Vindhayachal                                                                  - Low

Tsunami

• The term ‘Tsunami’ has been coined from the Japanese term Tsu meaning ‘harbour’ and nami meaning ‘waves’. Tsunamis are waves generated by earthquakes, volcanic eruptions, or underwater landslides and can reach 15m or more in height devastating coastal communities.
• Tsunamis caused by nearby earthquakes may reach the coast within minutes. When the waves enter shallow water, they may rise to several feet or, in rare cases, tens of feet, striking the coast with devastating force. The Tsunami danger period can continue for many hours after a major earthquake.
• Tsunamis are nearly always created by movement of the sea floor associated with earthquakes which occur beneath the sea floor or near the ocean. Tsunamis may also be generated by very large earthquakes far away in other areas of the Ocean. Waves caused by these travel at hundreds of kilometres per hour, reaching the coast several hours after the earthquake.
• Tsunamis can be very large. In coastal areas their height can be as great as 10m or more (30m in extreme cases), and they can move inland several hundred meters. All low-lying coastal areas can be struck by tsunamis.
• A tsunami consists of a series of waves. Often the first wave may not be the largest. The danger from subsequent tsunami waves can last for several hours after the arrival of the first wave.
• Sometimes a tsunami causes the water near the shore to recede, exposing the ocean floor. This is nature’s Tsunami warning.
• The force of some tsunamis is enormous. Large rocks weighing several tons along with boats and other debris can be moved inland several meters by tsunami wave activity. Homes and other buildings are destroyed. All floating material and water move with great force and causing mortality or injuries to people.
• Tsunamis can occur at any time of day or night.
• Tsunamis can travel up rivers and streams that lead to the ocean thereby polluting them.
• Tsunamis may also be generated by very large earthquakes far away in other areas of the Ocean. Waves caused by these travel at hundreds of kilometres per hour, reaching the coast several hours after the earthquake. Unlike ordinary tides, which are short, frequent and surface level, tsunami, are barely noticeable in their deep-sea formation stage. At this point despite a wavelength up to 100 km, they are shallow in depth and move at hundreds of kilometre per hour.

Mining