Earthquakes, Tsunamis and Volcanoes

Prerequisites: Interior of the Earth and Plate Tectonics

The Hook: Two Earthquakes in One Day

The Core Concept

Earthquakes

An earthquake is the shaking of the ground caused by a sudden release of energy, usually when rocks under stress along a fault slip. The energy travels outward as seismic waves.

By depth of focus: shallow (0–70 km; most destructive), intermediate (70–300 km), deep (300–700 km, found only in subduction zones).

Types by cause:

• Tectonic — slip on faults at or within plates; the most common and most powerful.
• Volcanic — linked to magma movement.
• Collapse — roof collapse of mines or caverns.
• Explosion — nuclear or chemical blasts.
• Reservoir-induced — water loading of large dams; the Koyna earthquake in Maharashtra (December 1967) is the standard Indian example.

Global distribution:

Effects: ground shaking, surface rupture, landslides and avalanches, liquefaction (water-saturated sand behaves like a liquid — buildings sink or tilt), tsunamis, fires, and dam or embankment failures. Most deaths are caused by collapsing buildings, which is why building codes matter more than prediction.

India’s seismic zones

India’s seismic hazard is mapped by the Bureau of Indian Standards in its earthquake-resistant design code (IS 1893). The long-used map divides the country into four zones (II to V), from low to very high hazard; Zone V includes the North-East, parts of Jammu and Kashmir, Himachal Pradesh and Uttarakhand, the Rann of Kachchh, north Bihar and the Andaman and Nicobar Islands. BIS revises the map from time to time, so always check the current edition.

Tsunamis

A tsunami (Japanese for “harbour wave”) is a series of very long ocean waves caused by a sudden vertical displacement of a large body of water.

Causes: undersea earthquakes at subduction zones (the most common — usually magnitude above about 7 with vertical sea-floor movement), submarine landslides, volcanic eruptions and, very rarely, meteorite impacts.

The 2004 Indian Ocean tsunami: on 26 December 2004, a magnitude 9.1 earthquake off northern Sumatra, where the Indian plate subducts beneath the Burma microplate along the Sunda Trench, generated a tsunami that struck coasts across the Indian Ocean. In India, Tamil Nadu (especially Nagapattinam), Kerala, Andhra Pradesh and the Andaman and Nicobar Islands were badly affected.

Response: India set up the Indian Tsunami Early Warning Centre (ITEWC) at INCOIS, Hyderabad (under the Ministry of Earth Sciences), operational since 2007. It uses seismic stations, bottom-pressure recorders and tide gauges, and acts as one of the regional tsunami service providers for the Indian Ocean. A year after the tsunami, Parliament enacted the Disaster Management Act, 2005, which set up the National Disaster Management Authority framework.

Volcanoes

A volcano is an opening in the crust through which magma (molten rock below the surface), gases and fragments erupt. Once on the surface, magma is called lava. Eruptions also release pyroclastic material (ash, cinders, volcanic bombs) and gases (mainly water vapour, carbon dioxide and sulphur dioxide).

By activity: active (erupting or recently erupted — Barren Island in the Andaman Sea is India’s only confirmed active volcano), dormant (not erupting now but may erupt — Narcondam, also in the Andaman Sea), and extinct (no eruption expected).

Intrusive landforms (magma cools inside the crust and is later exposed by erosion):

Distribution: most active volcanoes lie on the Pacific Ring of Fire; others lie along mid-ocean ridges (Iceland), continental rifts (East African Rift — Kilimanjaro, Nyiragongo), the Mediterranean (Etna, Vesuvius, Stromboli — the “Lighthouse of the Mediterranean”), and hotspots in the middle of plates (Hawaii).

Effects: lava flows, pyroclastic flows and lahars (volcanic mudflows) destroy settlements; ash disrupts aviation (Iceland’s Eyjafjallajökull eruption of April 2010 closed much of European airspace); large eruptions inject sulphate aerosols into the stratosphere and cool global climate for a year or two (Mount Pinatubo, Philippines, 1991). Benefits include fertile soils, geothermal energy (Iceland) and minerals.

graph TD
    A[Plate boundaries] --> B[Divergent: shallow quakes, fissure volcanoes]
    A --> C[Convergent - subduction: deep quakes, explosive volcanoes, tsunamis]
    A --> D[Convergent - collision: large quakes, few volcanoes]
    A --> E[Transform: shallow damaging quakes, no volcanoes]
    F[Hotspots within plates] --> G[Shield volcanoes - Hawaii]

Memory Anchors

Magnitude vs intensity: “Magnitude Measures the Monster (energy); Intensity is the Impact at Individual places.”

Richter steps: “10 shake, 32 energy” — one step up = 10× amplitude, about 32× energy.

Tsunami behaviour: “Fast and flat at sea, slow and tall at shore.”

Volcano shape follows lava: runny basalt → broad shield; sticky andesite → steep cone and explosions.

Intrusives: “Sill Sleeps (horizontal); Dyke Dives (vertical); Laccolith Lifts (dome); Lopolith Ladles (saucer).”

For Prelims

For Mains

• Dimensions: cause (plate tectonics, faults, magma); distribution (Ring of Fire, Alpine–Himalayan belt, ridges, hotspots); impacts (human, economic, environmental, climatic); vulnerability (building stock, population density, preparedness); mitigation (codes, zoning, early warning, community preparedness).
• Keywords: seismic gap, liquefaction, retrofitting, base isolation, earthquake-resistant design, tsunami early warning, shoaling, pyroclastic flow, lahar, volcanic winter.
• Value-addition: “earthquakes don’t kill people, buildings do” — the Bhuj (2001) and Türkiye–Syria (2023) disasters were worsened by non-compliant construction; the 2004 tsunami was followed by India’s tsunami warning centre and the Disaster Management Act, 2005; mangroves and coastal regulation reduce tsunami and storm-surge damage.

Common Traps

In the News

• February 2023 — Earthquakes in Türkiye and Syria; India’s Operation Dost.
• January 2022 — The Hunga Tonga–Hunga Ha’apai submarine volcano in Tonga erupted explosively, sending a pressure wave around the world and generating tsunamis across the Pacific.

Practice

Prelims

Mains

Quick Revision

Key Takeaways

Related Topics

• Tropical Cyclones
• Geophysical Phenomena — quick revision
• Geophysical Phenomena — practice questions
• Interior of the Earth and Plate Tectonics
• Disaster Management