Earthquakes and tsunamis

The first principle

The plates of the Earth's crust move a few centimetres a year, but where they meet they lock against each other by friction. The rocks on either side bend and store strain energy like a stretched spring, and when the stress exceeds the strength of the fault, the rocks slip suddenly and spring back. This is the elastic rebound theory of H F Reid, who studied the San Francisco earthquake of 1906. The energy released travels as seismic waves that shake the ground, and if the slip lifts the sea floor, it lifts the water above it too, which is how most tsunamis begin.

The core

Causes of earthquakes

• Tectonic. Most earthquakes occur along plate boundaries, at subduction zones, collision zones such as the Himalaya, and transform faults such as the San Andreas.
• Volcanic. Magma forcing its way up cracks rock and causes swarms of small earthquakes.
• Collapse. The roofs of mines and caves can collapse and cause small local shocks.
• Reservoir induced. The weight of water in a large reservoir and water seeping into faults can trigger slip on faults already close to failure; the Koyna earthquake of 1967 in Maharashtra, of magnitude about 6.3, is the classic case.

Focus, epicentre and waves

• The focus, or hypocentre, is the point inside the Earth where the rupture begins; the epicentre is the point on the surface directly above it. Shallow earthquakes, with a focus less than 70 km deep, do the most damage.
• Body waves travel through the Earth. Primary (P) waves push and pull the rock in the direction they travel and pass through solids, liquids and gases; secondary (S) waves shake the rock across their path and pass only through solids, which is how we know the outer core is liquid.
• Surface waves travel along the surface and arrive last; Love waves shake the ground side to side and Rayleigh waves roll it like an ocean wave, and they cause most of the damage.

Magnitude and intensity

• Magnitude measures the energy released at the source. The moment magnitude scale, now standard, is calculated from the area of the fault, the amount of slip and the rigidity of rock; each whole number step means about 32 times more energy, so a magnitude 9 releases about a thousand times the energy of a magnitude 7.
• Intensity measures the effect at a place, on the Modified Mercalli scale from I (not felt) to XII (total destruction). One earthquake has one magnitude but many intensities, which fall with distance and rise on soft soil.

Where earthquakes happen

• About 80 per cent of the world's earthquake energy is released in the Circum Pacific belt, about 15 per cent in the Alpine Himalayan belt that runs from the Mediterranean through Iran and the Himalaya to Indonesia, and the rest along mid ocean ridges and within plates.

Dimensions

The geophysical characteristics of the Circum Pacific zone

The Circum Pacific belt, or Ring of Fire, is a horseshoe about 40,000 km long, from New Zealand through Indonesia, the Philippines, Japan and the Aleutians, and down the western Americas to Chile.

• Subduction zones. Along most of its length, dense oceanic plates such as the Pacific, Nazca and Philippine plates sink beneath other plates.
• Deep ocean trenches. Where the plate bends down, trenches form, including the Mariana Trench, at about 11,000 m the deepest point on Earth, and the Peru Chile, Japan and Tonga trenches.
• Volcanic arcs. At a depth of about 100 km, water released from the sinking plate lowers the melting point of the mantle above it, so magma rises and forms island arcs such as Japan and the Aleutians and continental arcs such as the Andes and the Cascades.
• Earthquakes. The belt has about 90 per cent of the world's earthquakes. Earthquake foci deepen away from the trench along the sinking slab, down to about 700 km, forming the Wadati Benioff zone.
• Megathrust earthquakes. The largest earthquakes happen on the locked interface between the plates: Chile in 1960 (magnitude 9.5, the largest ever recorded), Alaska in 1964 (9.2) and Tohoku in Japan in 2011 (9.0).
• Volcanoes and tsunamis. About three quarters of the world's active volcanoes lie on the belt, and because megathrusts lift the sea floor, the Pacific produces most of the world's tsunamis.
• Transform segments. Not all of it is subduction; the San Andreas fault in California is a transform boundary.

Earthquakes in India

India is pushing north into Eurasia at about 4 to 5 cm a year, which builds stress along the Himalaya and the north east, while the Andaman and Nicobar Islands lie over a subduction zone. Peninsular India is more stable, but Latur (1993) and Koyna showed that it is not immune.

In practice

Seismic zoning

The Bureau of Indian Standards (BIS) code IS 1893 (Part 1): 2016 divides India into Zones II to V, with Zone V the highest. Zone V covers the north east, parts of Jammu and Kashmir and Ladakh, Himachal Pradesh, Uttarakhand, north Bihar, the Rann of Kutch and the Andaman and Nicobar Islands; Delhi lies in Zone IV. Buildings must be designed for the shaking expected in their zone.

In late 2025 the BIS issued a revised code, IS 1893: 2025, which added a new Zone VI covering the whole Himalayan arc, but it withdrew the revision in March 2026 after concerns over building costs and too little consultation, so the 2016 map still applies.

Tsunamis: what they are and how they form

• A tsunami is a series of long ocean waves caused by a sudden displacement of a large volume of water. Most are caused by undersea megathrust earthquakes that lift or drop the sea floor; others by submarine and coastal landslides, as at Lituya Bay in Alaska in 1958; by volcanic eruptions and collapses, such as Krakatoa in 1883 and Hunga Tonga Hunga Ha'apai in January 2022; and, rarely, by meteorite impact.
• They form mostly where subduction zones lie under the sea: the Pacific Ring of Fire, the Sunda arc of Indonesia and the Andaman and Nicobar Islands, the Makran coast of Pakistan and Iran, and the eastern Mediterranean.
• Speed and shoaling. A tsunami's wavelength is hundreds of kilometres, far greater than the ocean depth, so it travels as a shallow water wave whose speed equals the square root of gravity times depth. In 4,000 m of water it moves at about 700 km/h, with a height under a metre that ships do not notice. As it enters shallow water near the coast its front slows while the back is still fast, so the energy is squeezed into a shorter, taller wave that may reach 10 to 30 m. Often the sea first withdraws from the shore, which is a natural warning.

Consequences, with examples

• 2004 Indian Ocean. A magnitude 9.1 earthquake off Sumatra on 26 December killed about 230,000 people in 14 countries, including over 10,000 in India, mostly in Tamil Nadu and the Andaman and Nicobar Islands. There was no warning system in the Indian Ocean then.
• 2011 Tohoku. Waves up to about 40 m in places overtopped sea walls, killed about 18,000 people, and flooded the Fukushima Daiichi nuclear plant, causing meltdowns and a large evacuation.
• Other consequences. Coasts are eroded and reshaped, saltwater ruins farmland and wells, mangroves and coral reefs are damaged, fishing boats and harbours are destroyed, and land can subside, as parts of the Nicobar Islands did in 2004.

Institutions and laws

• The Indian Tsunami Early Warning Centre at the Indian National Centre for Ocean Information Services (INCOIS), Hyderabad, under the Ministry of Earth Sciences, became operational in 2007; it uses seismometers, bottom pressure recorders and tide gauges and can issue warnings within about ten minutes, and it serves as a regional provider for the Indian Ocean.
• The National Centre for Seismology (Ministry of Earth Sciences) runs India's national seismic network.
• The Disaster Management Act, 2005 (Ministry of Home Affairs), passed after Bhuj and the 2004 tsunami, set up the National Disaster Management Authority (NDMA).

Issues and way forward

• Weak building practice. Most deaths in earthquakes come from collapsing buildings, because codes are not enforced; NDMA's earthquake guidelines (2007) call for code compliance, licensed engineers and retrofitting of schools and hospitals.
• Unplanned growth in high zones. Hill towns such as Joshimath and cities like Delhi pile weight on weak soil; microzonation of cities, as mapped by the National Centre for Seismology, should guide land use.
• Last mile warnings. A warning in ten minutes helps only if it reaches people; tsunami ready communities under the UNESCO Intergovernmental Oceanographic Commission, two of which were recognised in Odisha in 2020, show the way.
• Natural buffers. Mangroves and coastal forests reduced damage in some villages in 2004, so coastal regulation should protect them.

Common confusions

• Magnitude and intensity. Magnitude is energy at the source; intensity is shaking at a place.
• Focus and epicentre. The focus is underground; the epicentre is on the surface above it.
• Tsunami and tidal wave. Tsunamis have nothing to do with tides; they are caused by displacement of water.
• Tsunami and storm surge. A storm surge is a rise in sea level pushed by cyclone winds and low pressure; a tsunami comes from a sudden movement of the sea floor or a landslide.

For the exam

• Keyword bank: elastic rebound, focus, epicentre, P and S waves, shadow zone, moment magnitude, Modified Mercalli, Ring of Fire, subduction, trench, Wadati Benioff zone, megathrust, reservoir induced seismicity, IS 1893, wave shoaling, INCOIS.
• Previous year questions: Practise the previous year questions on this topic →
• Use in an answer:
  • Introduction (data): Nearly three fifths of India's land lies in moderate to very high seismic zones, and about 90 per cent of the world's earthquakes occur along the Circum Pacific belt.
  • Introduction (judgement, quote or report): The 2004 Indian Ocean tsunami killed about 230,000 people largely because no warning system existed, which led India to set up its tsunami warning centre at INCOIS in 2007.
  • Conclusion: Earthquakes cannot be predicted, but their deaths can be prevented, because it is buildings, not tremors, that kill.
  • Conclusion: A tsunami gives minutes to hours of warning, so detection, communication and community drills together decide whether it becomes a disaster.