Tsunami

Tsunami: Large sea waves caused by sudden displacement of ocean water due to Earthquakes and volcanic eruptions. that cause the sea floor to move abruptly resulting in the form of high vertical waves called Tsunamis (harbour waves) or seismic sea waves.

Formation of Tsunamis:

Seismic waves may cause only one instantaneous vertical wave due to the movement of the sea floor.

Subsequently, a series of after-waves are created in water that oscillates between high crest and low troughs in order to restore the water level.

Propagation of Tsunami:

The speed of the waves in the deep ocean is faster in the shallow water than in the ocean and the Impact of Tsunamis is less over the ocean and more near the coast: –

• Deep waters: it is difficult to detect a tsunami in the deeper parts of the sea as over deep water, the tsunami has a very long wavelength and limited wave height. It raises the ship only by a metre or two and each crest or trough takes several minutes. However, Tsunamis travel at a very high speed here at over 750km/h.
• In Shallow water, its velocity gets reduced due to greater friction but the period between each wave (i.e. its frequency) remains unchanged. The speed of the wave can be even below 100km/h here. The height of the wave can be up to 15m or more, which causes large-scale destruction along the shores.

Indian Ocean Tsunami of 2004

The Indian Ocean Tsunami (December 26, 2004) was triggered by a 9.1 magnitude undersea earthquake off the coast of Sumatra, Indonesia. The massive seismic activity displaced ocean water, generating waves over 30 meters high, which travelled at speeds exceeding 800 km/h. The tsunami took approximately 2 to 3 hours to reach India’s eastern coast, striking Tamil Nadu, Andhra Pradesh, Kerala, and the Andaman & Nicobar Islands. India lacked a proper tsunami warning system, leading to an unprepared response. Coastal states suffered massive destruction, with over 12,400 deaths and thousands displaced. Infrastructure, fisheries, and livelihoods were severely impacted. In the aftermath, India established the Indian Tsunami Early Warning System (ITEWS) in 2007, improved disaster preparedness, and strengthened coastal resilience measures to mitigate future risks.

Causes of Tsunami:

Tsunami is caused by the shaking of the seabed. This can happen in the following ways:

1. Earthquakes
2. Underwater Landslides
3. Volcanic Eruption:
4. Extra-terrestrial Collision:

Let’s consider each of the causes in detail.

Earthquakes:

Earthquakes can be generated by movements along fault zones associated with plate boundaries. The region where two plates come in contact is a plate boundary, and the way in which one plate moves relative to another determines the type of boundary:

• Subduction of plates causes Strongest earthquakes;
• Seafloor Spreading, where two plates move away from each other;
• Transform boundaries where two plates slide horizontally past each other.

All earthquakes do not cause tsunamis. There are four conditions necessary for an earthquake to cause a tsunami:

1. The earthquake must occur beneath the ocean or cause the material to slide into the ocean.
2. The earthquake must be strong, with at least a magnitude 6.5 on the Richter Scale.
3. The earthquake must rupture the Earth’s surface and it must occur at shallow depth – less than 70km below the surface of the Earth.
4. The earthquake must cause vertical movement of the seafloor (up to several meters).

Underwater Landslides:

Underwater landslides can also result in tsunamis when the material loosened by the landslide moves violently, pushing the water in front of it. A landslide that occurs along the coast can force large amounts of water into the sea, disturbing the water and generating a tsunami.

Volcanic Eruption:

Although relatively infrequent, violent volcanic eruptions represent also impulsive disturbances, which can displace a great volume of water and generate extremely destructive tsunami waves in the immediate source area. According to this mechanism, waves may be generated by the sudden displacement of water caused by a volcanic explosion, by a volcano slope failure, or more likely by a phreatomagmatic explosion and collapse/engulfment of the volcanic magmatic chambers.

Krakatoa Volcano and Tsunami

One of the largest and most destructive tsunamis ever recorded was generated on August 26, 1883, after the explosion and collapse of the volcano of Krakatoa (Krakatau), in Indonesia. This explosion generated waves that reached 135 feet, and destroyed coastal towns and villages along the Sunda Strait in both the islands of Java and Sumatra, killing 36,417 people.

Extra-terrestrial Collision:

Tsunamis caused by extra-terrestrial collisions (i.e. asteroids, meteors) are an extremely rare occurrence. Although no meteor/asteroid-induced tsunami has been recorded in recent history, scientists realize that if these celestial bodies strike the ocean, a large volume of water would undoubtedly be displaced to cause a tsunami.

Scientists have calculated that if a moderately large asteroid, 5-6 km in diameter, should strike the middle of a large ocean basin such as the Atlantic Ocean, it would produce a tsunami that would travel all the way to the Appalachian Mountains in the upper two-thirds of the United States.

On both sides of the Atlantic, coastal cities would be washed out by such a tsunami. An asteroid 5-6 kilometres in diameter impacting between the Hawaiian Islands and the West Coast of North America, would produce a tsunami that would wash out the coastal cities on the West coasts of Canada, the U.S., and Mexico and would cover most of the inhabited coastal areas of the Hawaiian Islands.

Areas Susceptible to Tsunamis:

Tsunamis are frequently observed along the Pacific Ring of Fire, particularly along the coast of Alaska, Japan, the Philippines, and other islands of Southeast Asia, Indonesia, Malaysia, Myanmar, Sri Lanka, and India etc.

Several factors intensify the impact of a tsunami, making it more destructive:

• Earthquake Magnitude and Depth: Higher magnitude and shallow-depth earthquakes generate more powerful tsunami waves.
• Seafloor Displacement: Large vertical shifts in the ocean floor create bigger and faster waves.
• Coastal Topography: Narrow bays, inlets, and low-lying coastal areas amplify wave height and impact.
• Underwater Landslides and Volcanic Eruptions: These events displace large volumes of water, intensifying tsunami strength.
• Wave Speed and Energy: Tsunamis travel up to 800 km/h, gaining energy in deep waters and rising in height near shorelines.

Impact of Tsunami

On Coast: tsunami waves release enormous energy stored in them and water flows turbulently onto the land destroying port cities and towns, structures, buildings and other settlements – coastal areas are densely populated all over the world.

The 2004 Indian Ocean Tsunami had a devastating impact on India’s coastal regions, particularly in the Andaman & Nicobar Islands, Tamil Nadu, Kerala, Andhra Pradesh, and Pondicherry. The destruction was widespread due to the high wave intensity and lack of preparedness.

• Loss of Life: Over 12,400 people died, with thousands missing and injured, mainly in Tamil Nadu and the Andaman & Nicobar Islands.
• Displacement: Around 650,000 people were displaced, leading to humanitarian crises in coastal regions.
• Infrastructure Damage: Roads, bridges, ports, and coastal buildings were destroyed, disrupting communication and transport.
• Livelihood Impact: Fishermen and coastal communities lost boats, nets, and fishing grounds, severely affecting their income.
• Environmental Damage: Seawater intrusion salinized agricultural land, making it unfit for farming. Coastal ecosystems, including mangroves and coral reefs, were also damaged.
• Economic Losses: The overall economic loss was estimated at ₹11,000 crores ($2.5 billion), affecting multiple sectors.
• Psychosocial Impact: Trauma and mental health issues emerged due to the loss of families, homes, and livelihoods.

The tsunami underscored the need for early warning systems, coastal preparedness, and better disaster management strategies in India.

Steps taken to Reduce the risk of Tsunami

India has volunteered to join the International Tsunami Warning System after the December 2004 tsunami disaster. Therefore, several systems have been engineered to reduce its risk.

Indian Tsunami Early Warning System (ITEWS)

Indian Tsunami Early Warning System (ITEWS) was established in 2007 and is based at & operated by INCOIS, Hyderabad.

1. It is an integrated effort of different organizations including the DOS), DST, the CSIR, Survey of India (SOI) and the National Institute of Ocean Technology (NIOT).
2. It comprises a real-time network of seismic stations, tide gauges and a 24X7 operational tsunami warning centre to detect tsunamigenic earthquakes, monitor tsunamis and provide timely advisories to vulnerable communities.

Intergovernmental Oceanographic Commission (IOC)

Intergovernmental Oceanographic Commission (IOC) of UNESCO (also known as UNESCO-IOC) has approved the recognition of two communities of Odisha viz., Venkatraipur and Noliasahi as Tsunami Ready Communities. It promotes preparedness and participation of the public, community leaders, and national and local emergency management agencies.

1. With this recognition, India has become the first country in the Indian Ocean Region to achieve the honour from the UNESCO-IOC.
2. Odisha is the first state in India to have such recognised communities.

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