Earthquakes: Causes, Richter & Mercalli Scales, Belts & Seismic Zones (UPSC)
Why the Earth Shakes
🎯 Exam priority: Important. Earthquakes recur in Prelims (distribution, measurement) and Mains (the tsunami mechanism in 2025, the Circum-Pacific zone in 2020). Master why and where the Earth shakes, how we measure it, and India's seismic risk.
In a few violent seconds, an earthquake can level a city. Yet it is simply the Earth releasing, in one jolt, strain that has built up for centuries along a crack in the crust. This note explains the cause, the scales we measure it on, where quakes cluster, and why so much of India lives on shaky ground.
Why the Earth shakes
An earthquake is the shaking of the ground caused by a sudden release of energy in the crust. The energy is released along a fault — a fracture where rock blocks move in opposite directions. Friction locks the blocks while stress builds; when the stress finally overcomes the friction, the rocks snap past each other and the stored energy radiates outward as seismic waves. (This is the elastic rebound idea.)
The point inside the Earth where the rupture begins is the focus (hypocentre); the point on the surface directly above it is the epicentre — the first and usually worst-hit place. The energy travels as P-, S- and surface waves (their behaviour, and how they reveal the Earth's layers, is covered in the Interior of the Earth note); a seismograph records them.
Focus depth decides the damage. Earthquakes are shallow-focus (0–70 km), intermediate (70–300 km) or deep-focus (300–700 km, along subducting slabs). Shallow-focus quakes are the most destructive because the energy reaches the surface with least loss. A big quake is usually preceded by small foreshocks and followed by many aftershocks; a long-quiet stretch of an active fault is a "seismic gap" that is overdue — much of the central Himalaya is one such gap.
Types of Earthquakes
Most earthquakes are tectonic, but several other kinds exist — a favourite "odd one out" in Prelims.
Types of earthquakes
Tectonic — by far the most common; caused by the sliding of rocks along a fault. These are the big, damaging quakes.
Volcanic — a special class of tectonic quake, confined to areas of active volcanoes.
Collapse — minor tremors from the collapse of underground mine roofs in areas of intense mining.
Explosion — ground shaking from the detonation of chemical or nuclear devices.
Reservoir-induced — quakes triggered by the weight and water pressure of large dam reservoirs (e.g. linked to the 1967 Koyna quake in Maharashtra).
Measuring Earthquakes — Richter vs Mercalli
Two different scales describe an earthquake — one measures its raw energy, the other the damage it does. Confusing them is a classic trap.
Measuring earthquakes — Richter vs Mercalli
Richter scale | Mercalli scale | |
Measures | Magnitude — the energy released | Intensity — the visible damage caused |
Expressed as | Numbers 0–10 (logarithmic) | Numbers I–XII (1–12) |
Based on | Instrument (seismograph) readings | Observed effects on people, buildings, land |
Named after | Charles Richter | Giuseppe Mercalli (Italian seismologist) |
The Richter scale is logarithmic: each whole number up means about 10× the ground shaking and ~32× the energy — so a magnitude 7 is vastly bigger than a 6. Quakes of magnitude 8+ are rare (about once or twice a year worldwide), while tiny tremors happen every minute.
For very large quakes, scientists today prefer the Moment Magnitude scale (Mw), which more accurately captures the total energy of giant events than the older Richter scale.
Where Earthquakes Occur — the Global Belts
Earthquakes are not scattered at random — they trace the edges of the tectonic plates almost exactly. There are three great belts.
Where earthquakes occur — the global belts
The Circum-Pacific Belt (the "Ring of Fire") rims the Pacific Ocean and accounts for the great majority of the world's earthquakes (~68%). It follows the subduction zones around the Pacific, where quakes can be very deep.
The Mid-Continental / Alpine-Himalayan Belt runs from the Mediterranean through the Alps, Iran and the Himalayas (~21%) — the collision belt where India pushes into Asia.
The Mid-Atlantic Ridge Belt follows the divergent ocean ridge; here quakes are frequent but shallow and milder.
The plate-boundary logic: quakes at mid-oceanic ridges are shallow, while those along subduction zones are deep-focus (the descending slab generates the inclined Wadati–Benioff zone of deep quakes). Match the belt to the boundary type and the pattern makes sense.
Earthquake Zones of India
India sits on the leading edge of a colliding plate, so a large part of the country faces real seismic risk — mapped by the Bureau of Indian Standards into seismic zones.
Earthquake zones of India
India is divided into four seismic zones, II to V (Zone V is the most dangerous; the old Zone I has been merged into Zone II). The risk rises sharply towards the north and north-east.
Zone V (very high risk): the entire Himalayan belt, the North-East, the Kachchh (Kutch) region of Gujarat, the Andaman & Nicobar Islands and parts of north Bihar. Zone IV (high): the Indo-Gangetic plains, Delhi, J&K. Zone III (moderate) and Zone II (low) cover most of the Peninsula.
Why the north is so risky: the ongoing India–Eurasia collision loads the Himalayan front with strain. Major Indian quakes include Bhuj/Gujarat (2001, Mw 7.7, ~20,000 dead), Latur (1993), Uttarkashi (1991), Kashmir (2005) and the Nepal–Gorkha (2015) quake felt across north India.
Effects, the Tsunami, and Mitigation
The shaking is only the start. Earthquakes trigger a chain of secondary hazards — and under the sea, the deadliest of all: the tsunami.
Effects, the tsunami, and mitigation
Hazards: direct ground shaking and displacement; soil liquefaction (saturated soil behaves like liquid and buildings sink); landslides and avalanches; fires (from ruptured gas/electric lines); floods from dam or levee failure; and structural collapse.
Tsunami — the ocean hazard. A shallow-focus undersea earthquake at a subduction zone can jerk the sea floor vertically, displacing a huge column of water. The waves race across the deep ocean at 500–800 km/h, barely noticeable at sea, then rear up into towering walls of water at the coast. (A tsunami is generated by the tremor — it is not the earthquake itself.)
The 2004 Indian Ocean tsunami — triggered by the Sumatra–Andaman earthquake (Mw ~9.1) on 26 December 2004 — killed over 2.2 lakh people across the region, including more than 10,000 in India (Tamil Nadu, Andhra, Andaman & Nicobar). India's east coast is more tsunami-prone than the west. It led India to set up the Indian Tsunami Early Warning Centre (INCOIS, Hyderabad, 2007).
Mitigation: enforce seismic zoning and earthquake-resistant building codes (BIS codes, base isolation, ductile detailing), retrofit old structures, run early-warning systems and public drills, and regulate construction on weak/landfill soils. In India this is coordinated by the National Disaster Management Authority (NDMA), with the National Centre for Seismology (NCS) monitoring quakes — the disaster-cycle of prevention, preparedness, response and recovery is the standard Mains framework.
Why earthquakes (unlike cyclones) cannot be predicted: there is no reliable way to forecast the exact time, place and size of a quake, so the only defence is preparedness and resilient construction — "earthquakes don't kill people, badly-built buildings do." That single line is the heart of any earthquake answer.
Why this matters for UPSC
Prelims tests distribution and measurement (Ring of Fire, Richter vs Mercalli, deep vs shallow quakes). Mains links earthquakes to plate boundaries, the Circum-Pacific zone, tsunamis and disaster management — always connect the hazard to its tectonic cause and to mitigation.
Further Reading
Standard NCERT-level geography textbooks and reference books, plus a good atlas (any UPSC reading list).
Test Yourself: Practice Questions & PYQs
Test yourself on the causes, types, measurement, distribution and hazards of earthquakes. Attempt each question before revealing the answer.
Practice Questions
Q1. The point on the Earth's surface directly above the focus of an earthquake is called the:
(a) seismic zone
(b) epicentre
(c) fault line
(d) hypocentre
Show answer
Answer: (b) — The focus (hypocentre) is where the rupture begins inside the Earth; the epicentre is the point on the surface directly above it and is usually the worst affected.
Q2. Which one of the following is NOT a recognised type of earthquake?
(a) Reservoir-induced earthquake
(b) Volcanic earthquake
(c) Tectonic earthquake
(d) Sedimentary earthquake
Show answer
Answer: (d) — There is no 'sedimentary' earthquake. The recognised types are tectonic, volcanic, collapse, explosion and reservoir-induced.
Q3. The Richter scale and the Mercalli scale, respectively, measure an earthquake's:
(a) frequency and magnitude
(b) magnitude (energy) and intensity (damage)
(c) intensity and magnitude
(d) depth and duration
Show answer
Answer: (b) — The Richter scale measures magnitude — the energy released, from instrument readings. The Mercalli scale measures intensity — the visible damage, on a I–XII scale.
Q4. The Richter scale is logarithmic, which means a magnitude 7 earthquake releases roughly how much more energy than a magnitude 6?
(a) exactly 2 times
(b) about 10 times
(c) about 32 times
(d) about 100 times
Show answer
Answer: (c) — Each whole number on the Richter scale means about 10 times more ground shaking and roughly 32 times more energy — so a 7 is about 32 times more energetic than a 6.
Q5. The 'Ring of Fire', which records the majority of the world's earthquakes, surrounds the:
(a) Pacific Ocean
(b) Atlantic Ocean
(c) Indian Ocean
(d) Arctic Ocean
Show answer
Answer: (a) — The Circum-Pacific Belt or Ring of Fire rims the Pacific Ocean along its subduction zones and accounts for about 68% of earthquakes (and most active volcanoes).
Q6. Deep-focus earthquakes are most characteristic of:
(a) stable continental shields
(b) transform faults
(c) subduction (convergent) zones
(d) mid-oceanic ridges
Show answer
Answer: (c) — The descending slab at a subduction zone generates deep-focus quakes along the inclined Wadati–Benioff zone. Mid-oceanic ridge quakes, by contrast, are shallow.
Q7. Which of these regions of India falls in the highest earthquake risk category, Seismic Zone V?
(a) Coastal Tamil Nadu
(b) The Kachchh (Kutch) region and the North-East
(c) The Thar Desert core
(d) The Deccan Plateau interior
Show answer
Answer: (b) — Zone V (very high risk) covers the Himalayas, the North-East, the Kachchh region, the Andaman & Nicobar Islands and parts of north Bihar — the most seismically active parts of India.
Q8. A tsunami is most likely to be generated by:
(a) a deep-focus earthquake far inland
(b) a reservoir-induced tremor
(c) a shallow-focus undersea earthquake that displaces the sea floor
(d) a collapse earthquake in a mine
Show answer
Answer: (c) — A tsunami needs vertical displacement of the sea floor — typically by a high-magnitude, shallow-focus undersea earthquake at a subduction zone, as in the 2004 Indian Ocean event.
Mains Practice Questions
Use these to frame full-length answers. You don't have to answer one exactly — they show the angles UPSC tests, so let them guide which points you cover.
What are tsunamis? How and where are they formed, and what are their consequences? (UPSC Mains 2025)
Discuss the geophysical characteristics of the Circum-Pacific Zone. (UPSC Mains 2020)
Why are the fold-mountain belts also the major earthquake and volcanic belts of the world? (UPSC Mains 2014)
Distinguish between the Richter and Mercalli scales of measuring earthquakes.
Why is a large part of northern India highly vulnerable to earthquakes? Suggest mitigation measures.
UPSC Mains — Previous Years’ Questions on this topic (last 10 years)
These are the actual GS Mains questions UPSC has asked on this theme — real proof of how, and when, it is tested. Try to write a full answer to each.
2025: What are Tsunamis? How and where are they formed? What are their consequences?
2020: Discuss the geophysical characteristics of the Circum-Pacific Zone.
2014: Why are the world's fold-mountain systems located along the margins of continents? Bring out the association between the global distribution of fold mountains and the earthquakes and volcanoes.