Maarg Manthan · Topic 4.9

Earthquakes – Causes, Focus and Richter Scale

Geography › Mountains, Volcanoes, Earthquakes and Tsunami · Topic 4.9

An earthquake is the sudden shaking of the ground caused by the release of stored strain energy along a fault, as described by the elastic rebound theory. The release point underground is the focus and the point above it is the epicentre. Magnitude, the energy released, is measured on the Richter and moment magnitude scales, and intensity, the effect at a place, on the Mercalli scale.

5 min read · UPSC Prelims · Geography · Mountains, Volcanoes, Earthquakes and Tsunami

At a Glance

Cause Release of strain along faults, mostly at plate boundaries
Focus Point of origin underground
Epicentre Surface point above the focus
Magnitude Richter or moment magnitude scale
Intensity Modified Mercalli scale, I to XII
Exam link UPSC Prelims, Geography: Mountains, Volcanoes, Earthquakes and Tsunami

Where Does It Fit?

This is the ninth topic of the category on mountains, volcanoes, earthquakes and tsunami. The previous topic dealt with the effects of volcanoes, and this one begins the study of earthquakes. The next topic covers the effects of earthquakes. Previous topic: Effects of Volcanoes. Next topic: Effects of Earthquakes.

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 and Their Types, 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 Ocean-Ocean Convergence and Island Arcs.

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 and Earthquakes – Ring of Fire.

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 describes the effects of earthquakes. Next topic: Effects of Earthquakes.

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: causes, focus, Richter and Mercalli scales, reservoir-induced seismicity. 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.

Can human activities cause earthquakes?

Yes. Reservoir-induced seismicity occurs when the weight of water behind a large dam loads the crust, as in the 1967 Koyna earthquake. Mining, deep fluid injection, hydraulic fracturing and underground nuclear tests can also cause earthquakes, usually of modest size.

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.