Tsunami

Tsunami

• A Tsunami is a series of large, long-wavelength ocean waves generated by the sudden displacement of a massive volume of water, usually due to an undersea earthquake, volcanic eruption, submarine landslide, or meteorite impact.
• Unlike ordinary sea waves generated by wind, tsunamis are generated by geological disturbances.
• The word “Tsunami” is derived from the Japanese words:
  
  • Tsu = Harbour
  • Nami = Wave.  Hence, Tsunami literally means “Harbour Wave.”

Characteristics of Tsunami

Wave Length

• Extremely long, ranging from 100 km to over 500 km.

Wave Height

• In the open ocean, wave height is generally less than 1 metre.
• Near the coast, wave height may increase to 10–30 metres or even more.

Wave Speed

Depends upon ocean depth.

• • Deep Ocean: 500–900 km/hr
  • Near Coast: Speed decreases significantly.

Wave Frequency

• Successive waves may arrive 5 to 60 minutes apart.
• The first wave is not always the largest.

Water Movement

• A tsunami involves the movement of the entire water column, from the ocean surface to the seabed.

Causes of Tsunami

Underwater Earthquakes

• The most common cause of tsunamis.
• Usually generated by earthquakes of Magnitude 7.5 or above.
• Occur mainly along Subduction Zones.

Example

• 2004 Indian Ocean Tsunami
• 2011 Japan Tsunami

Volcanic Eruptions

• Violent volcanic eruptions beneath or near the sea can displace enormous quantities of water.

Examples

• Krakatoa (1883)
• Hunga Tonga–Hunga Ha’apai (2022)

Submarine Landslides

• Sudden collapse of underwater slopes can generate local tsunamis.

Meteorite Impact

• Rare but capable of producing extremely large tsunamis by displacing vast quantities of ocean water.

Glacier Collapse

• Rapid collapse of glaciers into oceans or fjords can generate localized tsunamis.

Conditions Necessary for Tsunami Generation

A tsunami is most likely when:

• An earthquake occurs beneath the ocean.
• Magnitude is generally greater than 7.5 Mw.
• The movement is vertical rather than horizontal.
• Large quantities of water are displaced.
• Earthquake occurs at shallow depth.

Mechanism of Tsunami Formation

Stage 1: Earthquake

• Sudden rupture occurs beneath the ocean floor.

Stage 2: Seafloor Displacement

• The ocean floor is uplifted or depressed.

Stage 3: Water Displacement

• Huge masses of seawater are displaced vertically.

Stage 4: Wave Generation

• Energy spreads outward in all directions as tsunami waves.

Stage 5: Ocean Propagation

• Waves travel rapidly across deep oceans with very little loss of energy.

Stage 6: Coastal Amplification

• As water becomes shallow:
  
  • Speed decreases.
  • Wavelength shortens.
  • Wave height increases dramatically.

Stage 7: Inundation

• Massive waves flood coastal regions causing widespread destruction.

Types of Tsunami

Local Tsunami

• Originates close to the affected coastline.
• Reaches shore within minutes.
• Leaves very little warning time.

Regional Tsunami

• Travels several hundred kilometres.
• May take 1–3 hours to reach affected areas.

Distant (Tele-Tsunami)

• Travels across entire ocean basins.
• May require several hours before reaching distant coastlines.

Parts of a Tsunami Wave

Crest

• Highest point of the wave.

Trough

• Lowest point of the wave.

Wave Height

• Vertical distance between crest and trough.

Wavelength

• Horizontal distance between successive crests.

Run-up Height

• Maximum vertical height reached by tsunami water above mean sea level.

Inundation Distance

• Maximum inland distance travelled by tsunami waters.

Difference Between Tsunami and Ordinary Sea Waves

Tsunami-Prone Regions of the World

Major tsunami-prone regions include:

• Pacific Ring of Fire
• Japan
• Indonesia
• Philippines
• Chile
• Alaska
• New Zealand
• Western Coast of North America

Tsunami Risk in India

India is vulnerable due to its long coastline.

Highly vulnerable areas include:

• Andaman & Nicobar Islands
• Eastern Coast
• Tamil Nadu
• Andhra Pradesh
• Odisha
• Kerala
• Lakshadweep
• Gujarat Coast (moderate risk)

Indian Ocean Tsunami (2004)

Date

• • 26 December 2004

Cause

• • Magnitude 9.1–9.3 Megathrust Earthquake near Sumatra, Indonesia.

Effects

• • More than 2.3 lakh deaths across 14 countries.
  • India, Indonesia, Sri Lanka, Thailand, Maldives and Somalia were severely affected.

India

Affected regions:

• • Tamil Nadu
  • Andaman & Nicobar Islands
  • Andhra Pradesh
  • Kerala
  • Puducherry

2011 Japan Tsunami

Date

• • 11 March 2011

Cause

• • Magnitude 9.1 Earthquake off the coast of Tohoku.

Effects

• • Severe destruction.
  • Triggered the Fukushima Daiichi Nuclear Disaster.
  • More than 18,000 people died or went missing.

Tsunami Warning System

Purpose

To provide early warning before tsunami waves reach the coast.

Components

• Seismographs
• Ocean Bottom Pressure Sensors
• DART Buoys
• Tide Gauges
• Satellite Communication
• Numerical Models

Indian Tsunami Warning Centre

India established the Indian Tsunami Early Warning Centre (ITEWC) at:

• INCOIS
• Hyderabad

Functions:

• Earthquake detection.
• Tsunami forecasting.
• Warning dissemination.
• Monitoring Indian Ocean region.

Warning Signs of a Tsunami

Natural warning signs include:

• Strong coastal earthquake.
• Sudden withdrawal of sea water.
• Loud roaring sound from the sea.
• Unusual rise or fall of sea level.

Effects of Tsunami

Primary Effects

• Coastal flooding.
• Destruction of buildings.
• Loss of human life.
• Damage to ports.
• Infrastructure collapse.

Secondary Effects

• Soil salinisation.
• Water contamination.
• Disease outbreaks.
• Displacement of people.
• Economic losses.
• Ecosystem destruction.

Environmental Impacts

• Coastal erosion.
• Destruction of mangroves.
• Coral reef damage.
• Habitat loss.
• Saltwater intrusion into groundwater.
• Loss of biodiversity.

Economic Impacts

• Damage to fisheries.
• Tourism losses.
• Agricultural land degradation.
• Port destruction.
• Huge reconstruction costs.

Tsunami Preparedness

Before Tsunami

• Hazard mapping.
• Early warning systems.
• Public awareness.
• Coastal zoning.
• Evacuation planning.

During Tsunami

• Move immediately to higher ground.
• Stay away from beaches.
• Follow official evacuation routes.
• Do not return after the first wave.

After Tsunami

• Wait for official clearance.
• Avoid floodwater.
• Assist injured people.
• Ensure safe drinking water.
• Inspect buildings before entry.

Tsunami Mitigation Measures

Structural Measures

• Tsunami shelters.
• Sea walls.
• Elevated buildings.
• Coastal embankments.
• Mangrove plantation.

Non-Structural Measures

• Early warning systems.
• Disaster education.
• Community drills.
• Coastal Regulation Zone (CRZ) management.
• Land-use planning.

Role of Mangroves and Coral Reefs

Mangroves

• Reduce wave energy.
• Minimise coastal erosion.
• Protect coastal settlements.

Coral Reefs

• Act as natural offshore barriers.
• Dissipate wave energy before reaching the coast.

Important Organisations

Major Tsunamis in History