Movement of Ocean Water — UPSC Geography Notes

1. What exactly are Tides?

Imagine a glass of water. If you tilt it, water rises on one side and falls on the other. On Earth, gravitational forces 'tilt' the ocean water.

High Tide (Jwar): When the water level rises to its highest point and covers more of the beach.

Low Tide (Bhata): When the water recedes to its lowest point, exposing the seabed.

Tidal Range: The vertical difference in height between the High Tide and the Low Tide.

2. The Science: Why do they happen?

Two main forces work together to create tides:

A. Gravitational Pull (The Magnet):

The Moon is the main driver. Even though the Sun is bigger, the Moon is much closer, so its pull on oceans is 2.17 times stronger than the Sun's.

The Moon pulls the water on the side of Earth facing it, creating a Bulge (High Tide).

B. Centrifugal Force (The Spin):

Earth is spinning. Just like when you spin a bucket of water and the water pushes outwards, Earth's spin creates an outward force.

This force creates a second bulge on the opposite side of the Earth, away from the Moon. This ensures we have two high tides a day.

3. The Timing: Why not exactly 24 hours?

You might think tides happen at the same time every day, but they don't.

The Tidal Day: A tidal cycle takes 24 hours and 52 minutes.

Why the delay? While Earth rotates, the Moon also moves forward in its orbit. By the time Earth spins back to the same spot (24 hours later), the Moon has moved a bit further ahead. Earth takes an extra 52 minutes to catch up to the Moon's new position.

4. Types of Tides (Based on Sun-Moon Alignment)

The height of the tide changes depending on where the Sun, Moon, and Earth are standing.

A. Spring Tides (The Strongest Tides)

When: Full Moon (Purnima) and New Moon (Amavasya).

Alignment: The Sun, Moon, and Earth are in a Straight Line (called Syzygy).

Effect: The Sun and Moon pull together in the same direction. This creates the Highest High Tides and Lowest Low Tides (Maximum Tidal Range).

B. Neap Tides (The Weakest Tides)

When: 1st and 3rd Quarter of the Moon (Half Moon).

Alignment: The Sun and Moon are at Right Angles (90°) to each other.

Effect: The Sun's pull cancels out some of the Moon's pull (like a tug-of-war from sides). This creates Low High Tides and High Low Tides (Minimum Tidal Range).

5. Why are Tides Important?

Navigation: High tides raise water levels, helping big ships enter shallow harbors (e.g., Kandla Port and Diamond Harbour in India).

Fishing: Fish come closer to the shore during high tide to feed, helping traditional fishermen catch them easily.

Desilting: Tides rush in and out, naturally clearing mud and silt from river mouths (Estuaries), keeping them open for boats.

Energy: The strong force of moving tidal water can be used to generate electricity (Tidal Energy).

1. Semidiurnal Tide (The Regular Pattern)

What is it? 'Semi' means half. This pattern repeats every half-day.

Pattern: 2 High Tides + 2 Low Tides every 24 hours.

Height: The two high tides are roughly the same height.

Where: This is the most common type. Found along the Atlantic Coast (USA, Europe) and Indian Coast.

2. Diurnal Tide (The Daily Pattern)

What is it? 'Diurnal' means daily. This pattern happens only once a day.

Pattern: 1 High Tide + 1 Low Tide every 24 hours.

Where: Rare. Found in the Gulf of Mexico and parts of Southeast Asia.

3. Mixed Tide (The Uneven Pattern)

What is it? A mix of the two patterns above.

Pattern: 2 High Tides + 2 Low Tides, BUT they are of unequal heights.

Visual: Imagine having breakfast and dinner, but breakfast is a huge feast (High High Tide) and dinner is just a snack (Low High Tide).

Where: Pacific Coast of North America and many Pacific islands.

1. Spring Tides (The Strongest)

Don't get confused: 'Spring' has nothing to do with the season. It comes from the word 'springing forth' (jumping up).

When: Twice a month (New Moon & Full Moon).

Alignment: Sun, Moon, and Earth are in a straight line (called Syzygy).

Mechanism: The Sun helps the Moon. It's like two people pulling a rope in the same direction. ($1 + 1 = 2$).

Result: The water bulges significantly. High tides are very high; low tides are very low (Maximum Range).

2. Neap Tides (The Weakest)

When: Twice a month (1st and 3rd Quarter Moon - Half Moon).

Alignment: Sun and Moon make a 90° angle with Earth.

Mechanism: The Sun pulls water one way, the Moon pulls another way. They cancel each other out partially.

Result: The difference between high and low tide is very small.

3. Apogee and Perigee (Distance Matters)

The Moon's orbit is oval, not a perfect circle. Distance affects the pull.

Perigee (Near): Moon is closest to Earth. Tides are much higher. (Memory Tip: 'P' for Pass/Near).

Apogee (Far): Moon is farthest. Tides are lower. (Memory Tip: 'A' for Away).

1. What exactly is a Tidal Bore?

Usually, rivers flow into the sea. A Tidal Bore is when the sea fights back and wins.

The Phenomenon: The incoming tide is so strong and fast that it pushes a vertical wall of water upstream into the river.

Visual: It looks like a surf wave traveling inland, sometimes for kilometers.

Sound: It makes a loud roaring sound (like a train approaching) because of the water crashing against the riverbed.

2. The "Funnel Effect": How does it form?

Not every river has a bore. Three specific conditions act like a funnel:

1. Narrow Mouth: The river mouth must be narrow. When the massive ocean tide enters, it gets squeezed, forcing the water level to rise instantly.

2. Shallow Depth: The river must be shallow. Friction with the riverbed slows down the bottom water, but the top water keeps moving fast, causing it to 'tumble' over and form a wave.

3. Extreme Tides: The difference between low and high tide must be very large (usually over 6 meters).

3. Why are Tides Important? (Significance)

Tides act like the heartbeat of the coast, regulating life and trade.

A. Navigation (Helping Ships)

The Problem: Big ships need deep water. Many river ports (like Kolkata or London) get shallow due to silt.

The Solution: Ships wait for High Tide. The tide acts like a water elevator, lifting the ship so it can safely enter the harbor.

B. Fishing Industry

Feeding Time: During high tide, ocean water rushes into creeks and inlets, bringing nutrients and small fish. Big fish follow them.

Catch: Fishermen can catch more fish near the shore during high tide without venturing into the dangerous deep sea.

C. Natural Cleaning (Desilting)

Rivers naturally deposit mud (silt) at their mouths, which can block them. Tides rush in and out with great force, scrubbing the river mouth clean and keeping it open.

D. Tides & Ecology

Mangroves: Forests like the Sundarbans survive because tides bring in salty ocean water and wash away plant waste daily.

Intertidal Zone: Creatures like crabs, oysters, and snails live in the zone that is wet during high tide and dry during low tide. They depend on this cycle for food.

1. What are Ocean Currents?

Think of them as 'Rivers in the Sea'. They flow in a specific direction over long distances.

Surface Currents: Driven by Wind (Top 10% of ocean water).

Deep Currents: Driven by Density (Saltier/Colder water sinks). This is called Thermohaline Circulation.

2. Types of Currents (Temperature Based)

This is the most important classification for understanding climate.

A. Warm Currents (The Heaters)

Analogy: Like a hot water pipe running under the floor.

Movement: They flow from the Equator (Hot) towards the Poles (Cold).

Example: The Gulf Stream carries warm Caribbean water to Europe. This keeps European ports ice-free in winter.

Effect: They cause rainfall. Warm air holds moisture.

B. Cold Currents (The Coolers)

Analogy: Like an air conditioner blowing cold air.

Movement: They flow from the Poles (Cold) towards the Equator (Hot).

Example: The Peru (Humboldt) Current brings cold Antarctic water to South America.

Effect: They cause deserts. Cold air cannot hold moisture, so it doesn't rain over nearby land (e.g., Atacama Desert near Peru Current).

3. Forces: What makes them move?

Primary Forces (Start the movement):

1. Solar Energy (Heating): Water near the Equator heats up and expands. This makes the water level physically higher (about 8 cm) than at the Poles. Gravity pulls this water down the slope towards the poles.

2. Wind (Friction): Trade winds and Westerlies blow constantly across the ocean surface, dragging the water along with them.

3. Coriolis Force (The Spin): Because Earth spins, currents don't flow straight. They bend to the Right in the Northern Hemisphere and Left in the Southern Hemisphere.

Secondary Forces (Modify the path):

1. Landmasses: When a current hits a continent (like South America), it has to split and turn.

2. Salinity & Density: Saltier or colder water is heavier. It sinks down, pulling surface water in to replace it. This starts the deep ocean currents.

4. Gyres: The Giant Whirlpools

Because currents bend due to the Earth's spin (Coriolis effect) and hit continents, they form huge circular loops called Gyres.

There are 5 main Gyres in the world. They trap debris, forming things like the 'Great Pacific Garbage Patch'.

1. What is a Gyre?

A 'Gyre' is simply a large system of circulating ocean currents. The North Atlantic Gyre spins clockwise due to the Earth's rotation (Coriolis effect) and wind patterns.

2. The Engine: Equatorial Currents

Start Point: Near the Equator, the sun heats the water intensely.

Movement: The Trade Winds blow from east to west, pushing this warm water towards the Americas (North and South Equatorial Currents).

The Split: When this water hits Brazil, it splits. The northern part enters the Caribbean Sea, getting even warmer.

3. The River in the Ocean: Gulf Stream

As warm water piles up in the Gulf of Mexico, it is forced out through the narrow Florida Strait.

This creates the Gulf Stream, a powerful, fast-moving current (like a river flowing inside the ocean).

It hugs the US East Coast, carrying heat energy northward.

4. The Heat Pump: North Atlantic Drift

As the Gulf Stream moves east towards Europe, it widens and slows down, becoming the North Atlantic Drift.

Impact: It releases heat into the atmosphere. This is why London (Europe) creates mild winters, while places at the same latitude in Canada are freezing cold.

5. The Return Journey: Canary Current

After reaching Europe and Africa, the water cools down.

It turns southward along Northwest Africa as the Canary Current.

Desert Maker: This cold water cools the air above it, preventing cloud formation. This lack of rain is a major reason why the Sahara Desert exists nearby.

6. Cold Intruders from the North

Labrador Current: Flows south from the Arctic along Canada. Where this cold current meets the warm Gulf Stream (Grand Banks), it creates dense fog and one of the world's richest fishing grounds.

Irminger Current: Another cold branch near Iceland.

7. The Quiet Center: Sargasso Sea

In the middle of this spinning loop lies the Sargasso Sea.

It is a calm region with no coastlines, known for its distinct seaweed (Sargassum) and still waters.

1. South Atlantic: The Anti-Clockwise Loop

Unlike the North Atlantic, the South Atlantic Gyre spins Anti-Clockwise because the Earth's rotation (Coriolis force) deflects wind and water to the left in the Southern Hemisphere.

2. The Journey of the Water (South Atlantic)

The Warm Start (South Equatorial Current): Driven by Trade Winds, warm water flows west along the Equator towards Brazil. Think of this as the start of the heating cycle.

The Warm Slide (Brazilian Current): Upon hitting Brazil, the water turns south. This warm water acts like a heater for the Brazilian coast, bringing humidity and rain.

The Cold Connector (South Atlantic Current): As it goes further south, it hits the 'West Wind Drift' (freezing Antarctic water), turns east, and cools down significantly.

The Cold Return (Benguela Current): This cold water flows up along the west coast of Africa to complete the circle.

Key Fact: Cold water does not evaporate easily. Because the Benguela current is so cold, it creates very little rain. This is why the dry Namib Desert exists right next to the ocean.

3. Pacific Ocean: The Giant System

The Pacific is so wide that its currents form two separate huge loops. It covers nearly one-third of the Earth's surface, making its circulation vital for the whole planet's weather.

4. North Pacific (Clockwise Gyre)

Kuroshio Current (The 'Black Tide'): This is the Pacific's version of the Gulf Stream. It is a warm, fast current flowing north past Taiwan and Japan, bringing heat to the region. It is dark blue in color, hence the name.

North Pacific Drift: The water moves east towards North America. It acts as a barrier separating polar water from tropical water.

California Current: A cold current flowing south along the US West Coast. This cool water is why beaches in California are often foggy and chilly, and why the nearby land is dry (contributing to deserts like Mojave).

Oyashio Current: A cold current coming from the freezing Bering Sea. It meets the warm Kuroshio near Japan. Tip: Where warm and cold water meet, it creates a nutrient explosion, making it one of the world's best fishing grounds.

5. South Pacific (Anti-Clockwise Gyre)

East Australian Current: Made famous by the movie 'Finding Nemo', this warm current flows south along Australia's east coast, bringing warm water to places like Sydney.

Peru (Humboldt) Current: A massive cold current flowing north along South America (Chile/Peru). It is one of the most important currents for marine life.

The 'Upwelling' Effect: As this current moves, it pulls deep, nutrient-rich cold water to the surface. Fish love these nutrients, making the coast of Peru a global fishing hub.

6. The Balancer: Equatorial Counter Current

Because strong winds pile up so much water in the western Pacific (near Indonesia), the sea level is actually higher there. Gravity pulls some of this water back towards the east. This 'backflow' runs right along the Equator.

1. South Indian Ocean (The Steady Loop)

The southern part acts like a normal ocean because it is open and wide. The currents here form a permanent Anti-Clockwise loop (gyre), just like in the South Atlantic and Pacific.

South Equatorial Current (The Engine): Driven by the Trade Winds, a massive stream of warm water flows from Australia towards Africa (East to West) along the Equator. This piles up warm water near Africa.

Agulhas / Mozambique Current (The Warm River): When this water hits the African coast and Madagascar, it is forced south. It flows down the east coast of Africa as a narrow, fast, and warm current. It brings humidity and storms to the region.

West Wind Drift (The Cold Connector): As the current reaches the bottom of Africa, it meets the freezing waters of the Antarctic. It turns east, driven by the strong westerly winds.

West Australian Current (The Cooler): To finish the loop, cold water flows north along the west coast of Australia. This cold water cools the air, preventing rain, which is why Western Australia has large deserts.

2. North Indian Ocean (The Shifting System)

This is the only ocean in the world where currents reverse direction twice a year. This happens because of the Monsoon (seasonal winds). The land mass of Asia heats up and cools down much faster than the ocean, acting like a giant pump.

A. Summer: The South-West Monsoon (June–Oct)

The Heating Effect: In summer, the Asian landmass gets very hot. Hot air rises, creating a vacuum (Low Pressure). This pulls wind from the ocean towards the land.

Current Flow: The wind pushes the water in a Clockwise direction.

Somali Current (The Exception): Usually, currents on the east side of continents are cold. However, in summer, the wind pushes water so hard that the Somali Current becomes a strong, Warm current flowing North along East Africa.

B. Winter: The North-East Monsoon (Dec–Mar)

The Cooling Effect: In winter, the Asian landmass gets very cold. The air creates high pressure and pushes wind away from the land towards the ocean.

Current Flow: The wind pushes the water in an Anti-Clockwise direction.

Reversal: The currents that were flowing towards India in summer now turn around and flow away towards Africa.

3. Why is this Significant?

The Monsoon Engine: These currents are essential for the Indian Monsoon. The warm water moving towards India in summer supplies the moisture needed for the rains that feed billions of people.

Agulhas Leakage: The Agulhas current is so fast that some of its warm water 'leaks' around the tip of Africa into the Atlantic Ocean. This transfer of heat is crucial for balancing the global climate.

Historical Trade: Ancient sailors (like Arab traders) used these predictable current changes to sail easily between Africa and India—riding the wind one way in winter and returning in summer.

1. The Earth's Thermostat (Heat Balance)

The Equator is very hot, and the Poles are freezing. If currents didn't move, the Equator would get hotter and the Poles colder.

How it works: Warm currents take excess heat from the Equator to the Poles. Cold currents bring cooling water back to the Equator. This keeps the Earth habitable.

2. The 'Buffet' for Fish (Fishing Grounds)

The best places in the world to catch fish are where a Warm Current meets a Cold Current.

Why? Cold water brings food (nutrients) from the deep. Warm water provides a comfortable temperature. This creates an explosion of plankton (fish food).

Example: The Grand Banks near Canada is a world-famous fishing zone because the cold Labrador Current meets the warm Gulf Stream there.

3. Upwelling: Bringing Food to the Surface

Sometimes, wind pushes surface water away, allowing deep, cold, nutrient-rich water to rise up to replace it. This process is called Upwelling.

Impact: This deep water is full of 'fertilizer' for the ocean. This is why the coast of Peru (Humboldt Current) has so many fish.

4. Influencing the Weather (Rain vs. Deserts)

Warm Currents = Rain: Warm water evaporates easily, creating clouds and rain (e.g., Brazil, Japan, UK).

Cold Currents = Deserts: Cold water chills the air, preventing clouds from forming. This creates dry conditions.

Fact: Most big deserts (Sahara, Atacama, Namib) are located on the West Coast of continents where cold currents flow.

5. Navigation and Trade

Currents are like 'moving walkways' for ships. Historically, sailors used them to travel faster and save fuel. If a ship travels with the current, it moves much faster.

6. Marine Highways

Animals like turtles, whales, and eels use currents to migrate thousands of miles without using much energy. They ride the currents like a highway.

1. What is the 'Normal' Condition? (Walker Circulation)

To understand El Niño, you first need to know what happens normally in the Pacific Ocean.

Wind Flow: Strong Trade Winds blow from East (Americas) to West (Australia). Imagine a giant fan blowing across the ocean.

The Piling Up: These winds push warm surface water towards Australia and Indonesia. This creates a large pool of warm water there.

The Rain: Warm water evaporates and creates clouds. So, Australia and Indonesia usually get plenty of rain and have lush forests.

The Cold Coast: Near Peru (South America), the wind pushes the surface water away. To fill the gap, cold, nutrient-rich water from the deep ocean rises to the top. This is called Upwelling. Fish love this cold water.

2. What happens during El Niño? (The Disruption)

Every 2 to 7 years, this system gets disrupted.

Weak Winds: The Trade Winds get tired and weaken (or sometimes even reverse direction).

The Slosh Back: Since the wind isn't pushing hard enough to keep the water in the West, the warm water flows back towards South America (East).

The Result: Now, the East Pacific (Peru) is warm, and the West Pacific (Australia) is cooler than usual. The system has flipped.

3. The Consequences (The Weather Flip)

Because the warm water moved, the rain moves too.

Peru/Ecuador (Floods): They get the warm water and the rain clouds. Deserts can turn into lakes, causing massive floods. (This is bad for fishing because the cold nutrient water stops rising).

Australia/Indonesia (Droughts): They lose the warm water and clouds. This leads to severe droughts and often causes massive forest fires.

India (Weak Monsoon): The global wind system is connected. El Niño often suppresses the monsoon winds that bring rain to India, leading to less rainfall and potential drought.

4. What is the 'Southern Oscillation'?

While El Niño is about the Ocean (temperature), Southern Oscillation is about the Atmosphere (pressure).

The Seesaw: It is a 'seesaw' of air pressure. When pressure is high over the Pacific, it is low over the Indian Ocean, and vice versa.

Connection (ENSO): When the Ocean gets warm (El Niño), the Pressure patterns flip (Southern Oscillation). Because they happen together, scientists call the whole phenomenon ENSO.

1. Pacific Region (The Center of Action)

The Shift: Normally, trade winds push warm water to Australia. During El Niño, winds weaken, and warm water sloshes back to the Central & East Pacific.

South America (Peru/Ecuador): The arrival of warm water creates massive rain clouds. Result: Heavy Floods in usually dry areas.

Australia & Indonesia: The warm water (and rain) leaves this region. Result: Severe Droughts, forest fires, and water shortages.

2. Marine Ecosystems (The Food Chain Crash)

The Fertilizer Problem: Normally, cold water rises up near Peru carrying nutrients (fish food). This is called Upwelling.

The Collapse: El Niño's warm water acts like a lid, blocking this cold water. Without nutrients, Phytoplankton (tiny plants) die. Without plants, small fish (Anchovies) die or migrate. This crashes the fishing industry.

3. India & Monsoon (The Teleconnection)

The Link: The global wind system is connected. When it rains heavily in the Pacific (Chile/Peru), the air circulation suppresses the winds over the Indian Ocean.

Impact: This leads to a Weak Monsoon in India. Farmers suffer because crops like Rice (Paddy) need plenty of water.

4. Cyclones & Storms (The Trade-off)

Pacific Ocean: Since the water is warmer, there is more energy for storms. Result: More Cyclones/Typhoons.

Atlantic Ocean: El Niño creates strong winds in the upper atmosphere (wind shear) that rip storms apart. Result: Fewer Hurricanes in the US/Atlantic.

5. North America (Distant Effects)

Warmer North: Canada and Northern US experience milder, warmer winters (less snow).

Wetter South: The Southern US gets more rain and storms than usual.

1. Definition & Nature (The Anti-El Niño)

Meaning: 'La Niña' means 'The Little Girl' in Spanish.

What happens? It is an overdrive of the normal system. The central and eastern Pacific Ocean gets colder than usual.

Cycle: It is part of the ENSO cycle. If El Niño is the 'Warm Phase', La Niña is the 'Cold Phase'.

2. Causes & Mechanism (The Super-Charged Winds)

Strong Winds: The Trade Winds (which blow from Americas to Asia) become extremely strong.

The Push: These strong winds push warm surface water all the way to Asia and Australia.

The Cold Rise: Because so much water is pushed away from South America (Peru/Ecuador), huge amounts of deep, cold water rise to the surface (Intense Upwelling).

3. Impact on India & Asia (The Rain Booster)

Low Pressure: Because all the warm water is piled up near Asia, it creates massive rain clouds.

Monsoon: For India, La Niña is usually Good News for agriculture. It strengthens the monsoon winds, bringing plenty of rain.

Risk: Sometimes, it brings too much rain, causing Floods in India, Bangladesh, and Indonesia.

4. Impact on the Americas (Drought & Cold)

South America (Peru/Chile): The water is very cold. This means no rain clouds form, leading to Droughts. However, the cold water brings nutrients, so Fishing is excellent.

North America: The Jet Stream (wind current) shifts. This brings very cold winters to Canada and Northern US, while the Southern US stays dry and warm.

5. Cyclones (The Atlantic Danger)

Atlantic Ocean: Unlike El Niño (which stops hurricanes), La Niña makes the atmosphere unstable in the Atlantic. This leads to More Hurricanes hitting the USA and Caribbean.

Pacific Ocean: Cyclone activity decreases here because the water is too cold to fuel the storms.

1. What is the IOD? (The Bathtub Analogy)

Think of the Indian Ocean as a giant bathtub filled with water. The IOD is the difference in temperature between the two ends of this tub.

The Seesaw: The warm water sloshes back and forth between the West (Arabian Sea/Africa) and the East (Bay of Bengal/Indonesia). This movement is called the 'Dipole' because it has two poles (ends).

2. Positive IOD (The 'Good' Phase for India)

In this phase, the Western side (near Africa and India) becomes warmer than usual.

How it works: Warm water acts like a steam engine. It evaporates quickly to form massive clouds. The winds then push these clouds over India.

Result: This acts as a 'Turbo Boost' for the Indian Monsoon, bringing Heavy Rainfall to India and East Africa. It is called 'Positive' because it is generally positive for India's rain.

3. Negative IOD (The 'Bad' Phase for India)

In this phase, the warm water shifts to the Eastern side (near Indonesia and Australia).

How it works: Since the heat source (warm water) has moved to Indonesia, the rain clouds form there instead. India is left with cooler water and drier air.

Result: This suppresses the Indian Monsoon, often causing Droughts in India, while causing heavy floods in Indonesia and Australia.

4. Neutral IOD

This is the most common phase. The temperatures on both sides are roughly the same, like a balanced seesaw. During this time, the IOD does not really disturb the normal weather patterns.

5. The Connection with El Niño (The Savior)

We know that El Niño is usually a villain for the Indian Monsoon (causing drought).

The Shield: However, if a Positive IOD happens at the same time as El Niño, it can act as a shield.

Real Life Example: Even if El Niño is trying to dry out India, a strong Positive IOD can generate enough rain on its own to save the monsoon season. It acts like a backup generator for rainfall.

1. How Tsunamis Form

Earthquakes: Vertical movement of the seafloor at subduction zones (megathrust earthquakes) lifts the water column; usually magnitude 7.5 or above and shallow focus.

Submarine landslides and volcanic eruptions (e.g. Krakatoa, 1883; Hunga Tonga, 2022).

Meteorite impacts (very rare).

2. Characteristics

In the deep ocean, a tsunami travels at 500–800 km/h with a height of less than a metre and a wavelength of hundreds of kilometres, so ships may not notice it.

Near the coast, the wave slows and its height grows dramatically (shoaling).

The sea often recedes before the wave arrives—a natural warning sign.

Unlike wind waves, a tsunami moves the entire water column.

3. The 2004 Indian Ocean Tsunami

Triggered by a magnitude ~9.1 earthquake off Sumatra–Andaman on 26 December 2004.

Worst-hit in India: Tamil Nadu (Nagapattinam), Andaman and Nicobar Islands, Kerala, Andhra Pradesh and Puducherry.

Mangroves (e.g. Pichavaram) and coastal vegetation reduced damage in some places.

4. Risk to India

Two main source zones: the Andaman–Sumatra subduction zone (east) and the Makran subduction zone off Pakistan–Iran (west).

5. Early Warning and Preparedness

ITEWC at INCOIS (2007): Seismic network, tide gauges and ocean-bottom pressure sensors; issues warnings within minutes; serves as a Regional Tsunami Service Provider for Indian Ocean countries under UNESCO-IOC.

UNESCO 'Tsunami Ready': Venkatraipur and Noliasahi villages in Odisha were recognised in 2020.

World Tsunami Awareness Day: 5 November (designated by the UN in 2015).

Coastal zone regulation, mangrove and shelterbelt plantation, evacuation routes and drills.

6. Other Geophysical Hazards

Earthquakes (most of India lies in moderate to high seismic zones), volcanoes (India's only active volcano is Barren Island in the Andamans), and landslides.