Earthquakes and Tsunamis

What an earthquake is

An earthquake is the sudden shaking of the ground caused when stress built up in rocks is released along a fault, a fracture where blocks of crust move against each other. The point inside the Earth where the rupture starts is the focus (hypocentre); the point on the surface directly above it is the epicentre. Energy spreads out as seismic waves – P-waves, S-waves and surface waves – recorded by a seismograph. GS Paper 1 lists earthquakes and tsunamis under "important geophysical phenomena", and GS Paper 3 covers them again under disaster management, so you need both the science and the Indian context.

Causes of earthquakes

• Tectonic – by far the most common and destructive; caused by plate movements at boundaries. The elastic rebound theory (H. F. Reid, after the 1906 San Francisco quake) explains how rocks bend under stress and then snap back.
• Volcanic – linked to magma movement.
• Collapse – in mining areas or caves.
• Induced (reservoir-induced) seismicity – the weight of water in large dams can trigger quakes; the Koyna earthquake of 1967 in Maharashtra is the classic Indian case.
• Explosions – such as underground nuclear tests.

Earthquakes are classed by depth of focus: shallow (0–70 km), intermediate (70–300 km) and deep (300–700 km). Shallow quakes are usually the most damaging.

Measuring earthquakes

Where earthquakes happen

About 80 per cent of the world's earthquakes occur in the Circum-Pacific belt, the "Ring of Fire", along the subduction zones around the Pacific – Japan, Indonesia, the Philippines, Chile, Alaska and California. About 15 per cent occur in the Alpine–Himalayan (Mediterranean–Asiatic) belt, from the Alps through Turkey, Iran and Afghanistan to the Himalaya and Myanmar. The rest occur along mid-oceanic ridges and in plate interiors. The Turkey–Syria earthquake of February 2023 (Mw 7.8) and the Myanmar earthquake of March 2025 (Mw 7.7, on the Sagaing fault) are recent examples in the Alpine–Himalayan belt.

Earthquakes in India

India is highly prone because the Indian plate is pushing into the Eurasian plate at about 5 cm a year. About 59 per cent of India's land is vulnerable to moderate to severe shaking.

The Bureau of Indian Standards (IS 1893) map divides India into four seismic zones (II to V):

• Zone V (very high): the whole north-east, Jammu and Kashmir and Ladakh, Himachal Pradesh, Uttarakhand, the Kachchh region of Gujarat, north Bihar and the Andaman and Nicobar Islands.
• Zone IV (high): Delhi, parts of the Indo-Gangetic plain, Sikkim and the rest of the Himalayan foothills.
• Zone III (moderate) and Zone II (low): most of the peninsula, although intraplate quakes such as Latur (1993) show that even "stable" regions are not safe.

In 2025, BIS released a revised seismic zonation map that adds a new highest zone (Zone VI) covering the entire Himalayan arc, so check the latest version before the exam.

Major Indian earthquakes: Assam (1950, Mw ~8.6, one of the largest ever recorded on land), Koyna (1967), Uttarkashi (1991), Latur (1993), Chamoli (1999), Bhuj (26 January 2001, Mw 7.7, about 20,000 deaths), Kashmir (2005), Sikkim (2011) and the Nepal earthquake of 2015 (Mw 7.8), which also affected Bihar and Uttar Pradesh. Seismologists warn of a possible "central seismic gap" in the Himalaya between the 1905 Kangra and 1934 Bihar–Nepal ruptures, where a great quake is overdue.

Tsunamis

A tsunami (Japanese for "harbour wave") is a series of long ocean waves generated by a sudden vertical displacement of the sea floor – mostly by undersea earthquakes at subduction zones, but also by submarine landslides, volcanic eruptions (such as Hunga Tonga, 2022) and meteorite impacts.

How it behaves: in the deep ocean, tsunami waves may be less than a metre high but travel at 700–900 km per hour, with wavelengths of hundreds of kilometres. As they reach shallow coastal water, they slow down and pile up – a process called shoaling – to heights of 10–30 metres. Often the first sign is the sea drawing back from the shore, exposing the sea bed.

The 2004 Indian Ocean tsunami: on 26 December 2004, a Mw 9.1 earthquake off Sumatra, where the Indian plate subducts under the Burma microplate, triggered waves that killed about 2.3 lakh people in 14 countries. In India, about 10,000+ people died, mainly in Tamil Nadu (Nagapattinam was worst hit) and the Andaman and Nicobar Islands; Indira Point lost land to subsidence.

Response: India set up the Indian Tsunami Early Warning Centre (ITEWC) at INCOIS, Hyderabad in 2007. It uses seismic stations, bottom-pressure recorders (tsunameters) and tide gauges, and can issue alerts within about 10 minutes. It is a regional tsunami service provider for the Indian Ocean under UNESCO's IOC. Coastal communities are being certified under UNESCO's "Tsunami Ready" programme; two villages in Odisha were the first in the Indian Ocean region to get it, in 2020.

Reducing the risk

Earthquakes cannot be predicted reliably, so the focus is on preparedness and mitigation:

• Building codes: IS 1893 and IS 13920 for earthquake-resistant design; base isolation, shear walls, ductile detailing; retrofitting of schools and hospitals.
• Land-use planning: avoid building on loose fill, steep slopes and soils prone to liquefaction (when water-saturated sand behaves like a liquid during shaking).
• Early warning: IIT Roorkee's Uttarakhand earthquake early warning system sends alerts seconds before strong shaking arrives.
• Institutions: the NDMA guidelines, the NDRF, and the National Centre for Seismology in the Ministry of Earth Sciences.
• Coastal defences against tsunamis: mangroves and coastal shelterbelts (which reduced damage at Pichavaram in 2004), CRZ rules and evacuation drills.
• Awareness: "drop, cover and hold on", mock drills, community training.

Why the Himalaya keeps shaking

As the Indian plate pushes under Asia, the strain is taken up along a set of great thrust faults that run the length of the range: the Main Central Thrust (MCT) at the base of the Greater Himalaya, the Main Boundary Thrust (MBT) at the base of the Lesser Himalaya, and the Main Frontal Thrust (MFT) at the foot of the Shiwaliks. These faults lock up for centuries and then rupture in great earthquakes. Rapid, unplanned construction on steep slopes, weak hillside buildings, and the loose soils of river valleys multiply the damage. Earthquakes in the hills also trigger landslides, block rivers and create temporary lakes that can burst, so they cascade into floods – as the land subsidence at Joshimath (2023) and the 2021 Chamoli flood reminded planners. The Delhi–NCR region is at risk too, because of its dense population, many old buildings and deep alluvial soils that amplify shaking.

Mains practice question

Why is the Himalayan region so prone to earthquakes? Discuss the measures India has taken for earthquake and tsunami preparedness since 2001. (250 words)

Answer framework

• Intro: plate collision, seismic zones, past quakes.
• Causes: convergent boundary, thrust faults (MCT, MBT), central seismic gap.
• Measures: building codes, NDMA, NDRF, ITEWC (2007), early warning, Tsunami Ready, mangroves.
• Gaps: enforcement of codes, unplanned hill construction.
• Conclusion: resilience over prediction.