Effects of earthquakes and earthquake mitigation

Detail

Why earthquakes become disasters

• Earthquake: sudden shaking of the ground. It happens when energy stored in rocks is released along a fault (a crack in the crust where blocks move past each other).
• An earthquake becomes a calamity (a large disaster) only where many people live. The same shaking in an empty desert does little harm.
• It destroys houses, roads, bridges, railway lines and telephone links.
• People lose in seconds what they built over generations.

• Broken roads and communication links slow down rescue and relief.

• Scale of risk in India:

• About 59% of India's land area could face moderate to severe earthquakes. This means shaking of MSK intensity VII and above (MSK: Medvedev–Sponheuer–Karnik scale, which measures shaking by its effects on people and buildings) [2].
• Between 1990 and 2006, 6 major earthquakes in India killed more than 23,000 people [2].

• About 12 crore buildings stand in seismic Zones III, IV and V. Most of them are not earthquake-resistant [2].

• Great Himalayan earthquakes: In about 50 years, the Himalayan region had four earthquakes of magnitude 8 or more [2]:

• Shillong 1897 (M 8.7)
• Kangra 1905 (M 8.0)
• Bihar–Nepal 1934 (M 8.3)

• Assam–Tibet 1950 (M 8.6)

• "Safe" areas are not fully safe. The Koyna earthquake (1967, Maharashtra) and the Latur earthquake (1993, Maharashtra) hit the stable Peninsular plateau. Both led to revisions of the seismic zoning map [2].

• Economic reach: The Bhuj earthquake of 26 January 2001 damaged homes, government buildings, infrastructure and industry in more than 7,900 villages in 21 districts of Gujarat. Its effects also reached savings, investment and the stock market [2].
• NDMA's key message: "earthquakes don't kill, buildings do." Most deaths come from buildings that collapse.

Effects of earthquakes (NCERT Class 11, Table 6.2)

Each effect can set off chain effects: one damage leads to the next.

Where	Primary effects	Possible chain effects
On ground	Fissures, settlements, landslides, liquefaction, earth pressure	Flooding from fissures; lakes formed by landslide dams; rivers changing course; tilted or sunken buildings
On structures	Cracking, sliding, overturning, buckling, collapse	Fires, broken water and gas lines, people trapped, roads and links cut
On water	Waves, hydrodynamic pressure, tsunami	Coastal flooding; stress on dams and tanks

Terms in the table:

• Fissure: a long, open crack in the ground.
• Settlement: the ground sinks unevenly, and buildings sink with it.
• Earth pressure: extra sideways push of soil against walls and foundations during shaking.
• Buckling: a column or beam bends sideways under load, then fails.
• Overturning: a tall, narrow building tips over as a whole.
• Hydrodynamic pressure: extra push of moving water against dams, tank walls and piers while the ground shakes.
• Tsunami: a series of very long sea waves. They form when an undersea earthquake suddenly lifts or drops the sea floor.

Chain effects on ground and rivers

• Fissures → flooding
• Shaking opens cracks in the ground.
• Underground water and volatile materials (gases and fluids that escape easily) gush out through the cracks.

• The water floods nearby land.

• Landslides → dammed rivers → floods

• Shaking loosens slopes, and the loose material falls into a valley.
• The debris blocks the river, and a lake forms behind it (a landslide dam).

• The dam later bursts, or the river finds a new path. Both can cause floods downstream.

• Examples:

• 1950 Assam earthquake (M 8.6): landslides blocked and changed the courses of tributaries of the Brahmaputra in Arunachal Pradesh and Assam.
• 2015 Nepal (Gorkha) earthquake: landslides dammed several Himalayan streams and created lakes that could burst.

Soil liquefaction

• Soil liquefaction: strong shaking makes loose sand that is soaked with water lose its strength. For a short time, the sand behaves like a liquid.
• Process (cause → process → result):
• Cause: loose sand or silt with little clay, filled with groundwater, is shaken hard.
• Process: shaking breaks down the grain structure, and the small spaces between grains collapse. Water pressure in the pores rises, so the grains stop pressing on one another, and the soil loses its strength [5].

• Result: buildings tilt or sink. Sand boils appear. These are spouts of water and sand pushed up from the pressurised layer below, also called sand blows [5].

• Where it happens:

• India: Bhuj and the Rann of Kachchh, Gujarat (2001). The low, salty, waterlogged flats of the Rann (about 23–24°N) are ideal ground for liquefaction.

• World: Niigata, Japan (1964). Several four-storey buildings of the Kawagishi-cho apartment complex tipped as much as 60°. This is the classic example of soil failing under buildings [5].

• Risk zones: river deltas, floodplains, reclaimed coastal land and filled-up wetlands. Examples are the Indo-Gangetic plains, the Brahmaputra valley and coastal cities.

Earthquake hazard mitigation — NCERT's four steps

Mitigation means steps taken before a disaster to reduce the loss it causes.

1. Monitoring and fast information - Seismological centres watch earthquakes and send out information quickly. - GPS (Global Positioning System) stations track the slow movement of tectonic plates.

2. Vulnerability map and awareness - A national vulnerability map (a map showing which areas face the greatest danger) is prepared. - The public learns about the risk and how to reduce it.

3. Planning and design - House types and building designs are changed to suit the risk. - High-rise buildings, large industries and big cities are discouraged in highly risky zones.

4. Mandatory earthquake-resistant design - Earthquake-resistant design is made compulsory. - Light materials are used, because a lighter building faces smaller shaking forces and kills fewer people if it falls.

Seismic zoning — current status

• Seismic Zoning Map: a map that divides India by expected strength of shaking. It is part of the BIS code IS 1893 (Part 1) (BIS: Bureau of Indian Standards).
• The 2016 map has four zones: II, III, IV and V. Zone V is the most dangerous. High-risk areas are Zones III, IV and V [2].
• Revised map: it placed the entire Himalayan arc in a new Zone VI, the highest category. Only a small part of north-eastern Ladakh was left in Zone V [3].
• This revised zonation was withdrawn in March 2026. IS 1893 (Part 1): 2016 remains the current standard (verify current) [3].

Building codes and guidelines (Beyond NCERT)

Code or document	What it covers
IS 1893 (Part 1)	Criteria for earthquake-resistant design; seismic zone map [2][3]
IS 4326	Earthquake-resistant construction of buildings [2]
IS 13920	Ductile detailing of reinforced concrete (RCC) structures [2]
IS 13935	Seismic strengthening (retrofitting) of existing structures [2]
National Building Code 2016	National model code that brings these standards into building rules

• Ductile detailing: placing and tying steel bars in concrete so that a building bends and absorbs energy instead of snapping suddenly.
• NDMA Guidelines on Management of Earthquakes (April 2007) rest on six pillars [2]: 1. Earthquake-resistant construction of new structures 2. Selective seismic strengthening and retrofitting of existing priority and lifeline structures 3. Regulation and enforcement 4. Awareness and preparedness 5. Capacity development (education, training, research and development, documentation) 6. Emergency response

Retrofitting of lifeline buildings

• Retrofitting: strengthening an existing building so it can survive shaking.
• Lifeline structures: buildings and services needed during and after a disaster, such as hospitals, schools, water works, power plants, bridges and dams.
• Why selective: It is not practical or affordable to retrofit all buildings. NDMA therefore recommends a structural safety audit (a technical check of strength) and retrofitting of chosen lifeline and priority buildings [2].
• First focus: government and public buildings, including primary schools, primary health centres, panchayat offices and post offices in Zones III, IV and V [2].
• Priority cities: metropolitan cities and big towns in Zones III to V. North-eastern state capitals are included because their risk is high, even though some have small populations [2].
• Non-structural parts: finishes and contents also need care, because they make up over two-thirds of a building's cost [2].

Traditional earthquake-safe houses

These houses use local materials. They are light and flexible, and their timber frames hold the walls together.

Design	Region	Key feature
Dhajji-dewari	Jammu & Kashmir	Timber frame filled with brick or stone [2]
Taq	Kashmir	Masonry walls with timber bands laid flat at floor levels
Koti banal	Uttarakhand (Yamuna valley)	Stone and wood walls in layers; heavy base, lighter top
Bhonga	Kachchh, Gujarat	Round mud hut with a cone-shaped roof [2]
Brick-nogged wood frame	Himachal Pradesh	Wood frame with brick filling [2]
Ekra / Assam-type house	Assam	Light bamboo and reed walls [2]

• Problem: these houses are being replaced by RCC buildings that often lack earthquake-resistant features and ignore building codes [2].

Monitoring and early warning

• National Center for Seismology (NCS), under the Ministry of Earth Sciences:
• It is India's nodal agency for watching earthquakes.

• It runs the national seismic network and the earthquake catalogue (the official record of every recorded earthquake).

• Earthquake Early Warning (EEW):

• Sensors detect the first, weaker wave (P-wave).
• An alert is sent out before the stronger, damaging waves (S-waves and surface waves) arrive.

• This gives people a few seconds to take cover.

• BhuDEV ("Bhukamp Disaster Early Vigilante"):

• It is an earthquake early warning mobile app made by IIT Roorkee and the Government of Uttarakhand (verify current).
• It uses IIT Roorkee's seismic sensors to send real-time alerts to phones.
• It has an SOS button that shares the user's location with trusted contacts and with authorities [4].

Prelims Hooks

• Table 6.2 groups: ground → fissures, settlements, landslides, liquefaction, earth pressure; structures → cracking, sliding, overturning, buckling, collapse; water → waves, hydrodynamic pressure, tsunami.
• Liquefaction pairs: Bhuj and Rann of Kachchh (2001) in India; Niigata, Japan (1964), where buildings tipped up to 60° [5]. Sign on the ground: sand boils.
• Order of great Himalayan earthquakes: Shillong 1897 (M 8.7) → Kangra 1905 (M 8.0) → Bihar–Nepal 1934 (M 8.3) → Assam–Tibet 1950 (M 8.6) [2].
• About 59% of India's land is prone to moderate to severe earthquakes (MSK VII and above) [2].
• Code–subject pairs: IS 1893 → design criteria and zone map; IS 4326 → earthquake-resistant construction; IS 13920 → ductile detailing; IS 13935 → retrofitting [2].
• Trap: the Himalaya-wide Zone VI map was withdrawn in March 2026. The 2016 four-zone map (II–V) is still current [3].
• House–region pairs: dhajji-dewari → Kashmir; koti banal → Uttarakhand; bhonga → Kachchh; ekra → Assam [2].
• Koyna 1967 and Latur 1993 showed that the Peninsular plateau is not free of earthquakes [2].
• BhuDEV is an early warning app made by IIT Roorkee and the Uttarakhand Government [4].

Mains Points

• Buildings, not shaking, cause most deaths.
• There are about 12 crore non-engineered buildings in Zones III to V [2].
• Traditional houses are being replaced by RCC buildings that do not follow the codes [2].

• Response: make codes binding through municipal by-laws, train masons, and use the NBC 2016.

• Chain effects make the damage bigger.

• Landslide dams and changed river courses (Assam 1950, Nepal 2015) cause floods.
• Liquefaction (Bhuj 2001) makes buildings tilt and sink.
• Cut transport and phone links delay relief.

• Planning needs: multi-hazard maps, microzonation of cities on soft soil, and strong, backed-up communication systems.

• Mitigation is about tight budgets and choosing priorities.

• Selective retrofitting of lifeline buildings such as schools and hospitals [2].
• Better monitoring and early warning (NCS, BhuDEV) [4].
• Zoning that discourages high-rises, large industries and big cities in risky zones.
• These steps support the Sendai Framework's priority of investing in disaster risk reduction (GS-III).