Maarg Manthan · Topic 3.1

Earthquakes: Types, Causes, Effects and Frequency

Geography › Geomorphic Processes and Landform Evolution · Topic 3.1

An earthquake is the sudden shaking of the ground caused by the release of energy in the Earth's crust or upper mantle, mostly along faults. Stress builds slowly in rocks as plates push or slide past each other, and when the rock breaks, the stored energy is released as seismic waves from the focus. Earthquakes are classed by depth as shallow, intermediate and deep, and by cause as tectonic, volcanic, collapse and induced. Millions of them occur each year, but only a small number are strong enough to cause damage.

7 min read · UPSC Prelims · Geography · Geomorphic Processes and Landform Evolution

At a Glance

Origin point Focus, with the epicentre above it
Main cause Sudden slip along faults at plate boundaries
Scales Magnitude (Richter, moment) and intensity (Mercalli)
Common depth Shallow (0 to 70 km) causes the most damage
Frequency About one great (M8+) earthquake a year
Exam link UPSC Prelims, Geography: Geomorphic Processes and Landform Evolution

Where Does It Fit?

This is the first topic of the macro on geomorphic processes, and it begins with the sudden, endogenic process of earthquakes. It uses the seismic waves and plate boundaries described in earlier macros, and leads to the topics on locating epicentres, volcanoes and tsunamis. Previous topic: The Rock Cycle. Next topic: Shadow Zone and Locating an Epicentre.

What Is an Earthquake and How Does It Occur?

In short: an earthquake is the shaking caused when rocks that have been slowly strained suddenly slip along a fault.

The Earth’s plates keep moving, and the rocks along their edges and along faults get stuck by friction while the strain builds up. According to the elastic rebound theory, proposed by Harry Reid after the 1906 San Francisco earthquake, the rocks bend like a stretched spring until the stress exceeds the strength of the rock, and then they break and snap back, releasing the stored energy as seismic waves. The point at which the rupture begins is the focus or hypocentre, and the point on the surface directly above it is the epicentre, where shaking is usually strongest. The waves, described in Seismic Waves: Body Waves and Surface Waves, spread out and are recorded by seismographs as a seismogram.

What Causes Earthquakes?

Tectonic causes: by far the most important; movement of plates at convergent, divergent and transform boundaries produces over 90 per cent of earthquakes.
Volcanic causes: the movement of magma and explosive eruptions produce earthquakes, generally of small magnitude.
Human-induced causes: loading by large reservoirs behind dams (reservoir-induced seismicity, as at Koyna in Maharashtra, where a magnitude 6.3 earthquake occurred in 1967), mining, injection of fluids into deep wells including hydraulic fracturing, and underground nuclear tests.
Other causes: landslides, collapse of caves and mines, and glacial rebound.

How Are Earthquakes Classified by Depth?

Class	Focal depth	Features
Shallow	0 to 70 km	Most damaging; the majority of earthquakes
Intermediate	70 to 300 km	Found at subduction zones
Deep	300 to 700 km	Only at subduction zones, along the Wadati-Benioff zone; less damage at the surface

Small tremors that come before the main shock are foreshocks, and the many smaller shocks that follow it as the crust adjusts are aftershocks, which can continue for weeks and may be dangerous for damaged buildings. Earthquakes that originate deep occur mostly under the island arcs of the Pacific, in places described in Plate Boundaries: Divergent, Convergent, Transform.

What Are the Scales of Earthquakes?

Two different things are measured. Magnitude is a measure of the energy released at the source, independent of the place. Intensity is a measure of the effects at a particular place, so it varies with distance from the epicentre, the local ground and the buildings.

Scale	Measures	Features
Richter scale	Magnitude	Devised by Charles Richter in 1935; logarithmic, so each whole number means ten times greater ground motion and about 31.6 times more energy; open-ended but rarely exceeds 9
Moment magnitude scale (Mw)	Magnitude	Based on the area of the fault slip and the rock rigidity; used by modern seismologists for large earthquakes because the Richter scale saturates
Modified Mercalli scale	Intensity	Twelve levels from I (not felt) to XII (total destruction), based on observed effects

The largest earthquake recorded was the 1960 Valdivia earthquake in Chile, of magnitude 9.5, followed by the 1964 Alaska earthquake, of 9.2, and the 2004 Sumatra earthquake, of about 9.1, and the 2011 Tohoku earthquake in Japan, of about 9.1. In July 2025 an earthquake of magnitude 8.8 struck off the Kamchatka Peninsula of Russia and set off tsunami warnings across the Pacific, ranking among the strongest ever recorded. Earthquakes are monitored in India by the National Centre for Seismology under the Ministry of Earth Sciences. As an analytical point, magnitude alone does not predict damage, since a moderate shallow earthquake near a city of weak buildings can kill more people than a larger deep one far offshore.

Can Earthquakes Be Predicted?

Earthquakes cannot yet be predicted accurately in time and place. Scientists can map faults, estimate long-term probabilities and identify seismic gaps along faults where a large earthquake is overdue. Earthquake early warning systems, which detect fast P waves and warn of the slower, more damaging S and surface waves, can give seconds to tens of seconds of notice in some countries. The most effective protection is therefore preparation: earthquake-resistant construction, land-use control and public drills. The distribution of earthquakes is in Distribution of Volcanoes: Pacific Ring of Fire, Mediterranean Volcanism.

How Often Do Earthquakes Occur?

The Earth is never still. Instruments record millions of tremors a year, but only a few are large. The figures below are approximate yearly averages, based on global records.

Magnitude	Class	Average number per year	Effects
8 and above	Great	About 1	Total destruction near the centre; felt over thousands of km
7 to 7.9	Major	About 15	Serious damage over large areas
6 to 6.9	Strong	About 130	Damage in populated areas
5 to 5.9	Moderate	About 1,300	Damage to weak buildings
4 to 4.9	Light	About 13,000	Noticeable shaking, little damage
Below 4	Minor	Hundreds of thousands	Mostly not felt

Because the scale is logarithmic, small earthquakes are very much more frequent than large ones. Over 80 per cent of the energy released comes from the largest events along subduction zones, whose distribution is covered in Distribution of Earthquakes and Earthquake Observatories.

What Are the Direct Effects of an Earthquake?

In short: earthquakes shake, break and shift the ground, and most deaths come from collapsing buildings.

The effects of earthquakes fall into direct effects, caused by the movement of the ground itself, and secondary effects, set off by the shaking.

Ground shaking: the main cause of damage; vibrations from body and surface waves shake buildings, bridges and pipelines, and may bring down poorly built structures.
Surface rupture and ground deformation: the fault may break the surface, forming scarps and cracks, and the ground may be lifted, dropped or shifted horizontally by metres, as in the 2023 Turkey earthquakes in which the ground moved several metres along the fault.
Change of land and rivers: permanent uplift or subsidence of coasts, and changed river courses; the 1950 Assam earthquake altered the courses of rivers in the Brahmaputra basin.

What Are the Secondary Effects?

Effect	Description
Liquefaction	Water-saturated loose sand and silt behaves like a liquid when shaken; buildings tilt and sink; occurred at Niigata in 1964 and Bhuj in 2001
Landslides and avalanches	Shaking triggers slides on steep slopes, blocking rivers and forming landslide lakes; frequent in the Himalaya
Tsunamis	Undersea earthquakes displace water and create waves that travel across oceans, as in Tsunami: Mechanism, Properties and Effects
Fires	Broken gas pipes and electric lines start fires, as in San Francisco 1906 and Tokyo 1923
Floods	Damaged dams, landslide dams and river changes cause flooding
Disease and disruption	Water supply and sanitation fail; hospitals and roads are damaged

The sudden shift of the ground also affects the Earth as a whole: very large earthquakes, such as the 2004 and 2011 events, are estimated to have slightly shortened the length of the day by a few microseconds.

Why Is This Topic Important?

Concepts Focus, epicentre and scales
Hazards Disaster management link
Current affairs Frequent earthquakes in news

What Came Next?

The next topic explains how seismologists locate an epicentre and why shadow zones exist. Next topic: Shadow Zone and Locating an Epicentre.

Key People and Terms

Focus Origin of the rupture
Epicentre Surface point above the focus
Magnitude Energy released
Intensity Observed effects
Foreshock Tremor before the main shock
Aftershock Tremor after the main shock

Exam Corner

Points to Remember

Richter devised the scale in 1935.
One magnitude step is about 31.6 times more energy.
Mercalli has 12 levels.
Valdivia 1960 is the largest recorded, magnitude 9.5.
Koyna is an example of reservoir-induced seismicity.

Do Not Confuse With

Magnitude and intensity Source energy versus local effects
Richter and Mercalli Instrumental versus observed scale
Focus and epicentre Underground versus surface point

Memory Hook

Strain stores, fault slips; focus below, epicentre above; Richter measures, Mercalli feels.

Mains Angle

Questions ask about causes and measurement. These points add depth.

Causes Tectonic, volcanic, human
Scales Magnitude versus intensity
Preparedness Prediction is not possible

A Question You May Face

An original practice question, not a past paper question.

“Explain the causes of earthquakes and distinguish between the Richter and Mercalli scales.”

How to Answer

Cause Elastic rebound.
Types Tectonic and induced.
Scales Magnitude and intensity.
Response Preparedness.

GS Relevance

Prelims: types and causes, focus and epicentre, magnitude and intensity scales, frequency, effects. GS Paper 1: earthquakes; GS Paper 3: disaster management.

Frequently Asked Questions

What is the elastic rebound theory?

The elastic rebound theory, proposed by Harry Reid after the 1906 San Francisco earthquake, says that rocks along a fault slowly bend and store strain energy, and when the stress exceeds their strength they snap back, releasing the energy as an earthquake.

What is the difference between focus and epicentre?

The focus is the point underground where the rock breaks and the earthquake begins. The epicentre is the point on the surface of the Earth directly above the focus, and it is usually where the shaking is strongest and the damage greatest.

What is the Richter scale?

The Richter scale, devised by Charles Richter in 1935, measures the magnitude of an earthquake on a logarithmic scale. Each whole number increase means ten times larger ground motion and about 31.6 times more energy released. Modern work often uses the moment magnitude scale.

What is the difference between magnitude and intensity?

Magnitude measures the total energy released by an earthquake at its source, and has one value for each earthquake. Intensity measures the effects at a place, on the Mercalli scale from I to XII, and varies with distance, ground type and building quality.

What is liquefaction?

Liquefaction is the process in which water-saturated loose sand or silt loses its strength and behaves like a liquid when shaken by an earthquake. Buildings on it can tilt or sink, and it caused major damage in Niigata in 1964 and in Bhuj in 2001.

PYQ Practice — Statement Analysis

1 The Richter scale measures the intensity of an earthquake.

False

It measures magnitude; Mercalli measures intensity.

2 A magnitude 7 earthquake releases more energy than a magnitude 6 earthquake.

True

About 31.6 times more.

3 The epicentre of an earthquake lies directly above the focus.