GeographyISC Class 11 · Earth's Interior, Earthquakes and Volcanoes

Earthquakes: Causes, Distribution and Measurement (ISC Class 11 Geography)

Updated 17 October 2026 · Fact-checked

Earthquakes are caused mainly by tectonic movement along faults, and cluster along the circum-Pacific belt, the Alpine-Himalayan belt and mid-ocean ridges. Magnitude (Richter) measures energy and is logarithmic: each unit means 10 times the amplitude and about 32 times the energy. Intensity (Mercalli) measures effects at a place.

Understand Earthquakes: Causes, Distribution and Measurement

Terms. The focus is where energy is released; the epicentre lies directly above it on the surface.

Causes. Tectonic (plates moving on faults, building up stress that is released suddenly; the majority); volcanic; collapse of mines or caverns; explosions; reservoir-induced shocks (Koyna, Maharashtra, 1967).

Distribution. The circum-Pacific belt (Ring of Fire) has most earthquakes; the Alpine-Himalayan belt runs from the Mediterranean through Iran to the Himalayas and Indonesia; mid-ocean ridges such as the Mid-Atlantic Ridge also shake.

Effects. Ground shaking and cracks, landslides and avalanches, liquefaction, building collapse, fires, dam failures and flooding, and tsunamis when the epicentre is under the sea (Indian Ocean, 2004).

Lines. Isoseismal lines join places of equal intensity and form rough rings around the epicentre; homoseismal (coseismal) lines join places where waves arrive at the same time.

Measurement. Seismographs record waves. Magnitude (Richter, 1935) measures energy and is logarithmic: amplitude ratio = 10^(M1 - M2) and energy ratio = 10^(1.5 x (M1 - M2)), about 31.6 per whole unit. Intensity (Modified Mercalli, I to XII) records observed effects and varies from place to place.

Nepal, 25 April 2015. Magnitude 7.8 in central Nepal, epicentre in Gorkha district north-west of Kathmandu, followed by an aftershock of 7.3 on 12 May. Cause: the Indian Plate pushing under the Eurasian Plate along the Himalayan thrust fault. Effects: nearly 9,000 deaths, hundreds of thousands of houses damaged, collapse of monuments such as the Dharahara tower, landslides and an avalanche at Everest base camp. Lessons: earthquake-resistant codes, retrofitting, community preparation.

Earthquakes: Causes, Distribution and Measurement: key facts

Amplitude ratio: $1 0^{(} M 1 - M 2) ∣ E a c h u ni t i s 10 t im es$
Energy ratio: $1 0^{(} 1.5 x (M 1 - M 2)) ∣ A b o u t 31.6 p er u ni t$
Belts: Circum-Pacific
Alpine-Himalayan
Mid-ocean ridges
Scales: $R i c h t er : ma g ni t u d e, e n er g y ∣ M er c a l l i I - X I I : in t e n s i t y, e f f ec t s$

How to answer Earthquakes: Causes, Distribution and Measurement questions

Expect causes plus distribution (4 marks), scale distinctions and ratio sums (3 to 4 marks) and case-study questions.

1For causes, give tectonic first then two others with examples.
2For distribution, name the three belts.
3For ratios, subtract magnitudes first, then apply 10^ or 10^(1.5 x).
4For the case study, give date, magnitude, cause, effects and lessons.

Earthquakes: Causes, Distribution and Measurement in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Energy of two quakes

Suppose one invented quake measures 6.5 and another 5.0.

How many times larger is the amplitude and the energy of the first?

Try it first, then show the solutionSolution

Difference = 1.5.
Amplitude ratio = 10^1.5, about 31.6.
Energy ratio = 10^(1.5 x 1.5) = 10^2.25, about 178.

Answer: Amplitude about 31.6 times, energy about 178 times.

Memory hook: Subtract, then power ten.

Damage differs

Suppose two towns equally far apart lie on hard rock and on loose wet sand, and both feel the same quake.

Which scale shows the different damage?

Try it first, then show the solutionSolution

Intensity differs because ground and buildings differ.
The Mercalli scale records effects at a place.

Answer: Mercalli intensity.

Memory hook: Magnitude is one; intensity varies by place.

Tricks to remember Earthquakes: Causes, Distribution and Measurement

Short memory aids. Say each one aloud once, then use it on a question the same day.

Focus below, Epi above
Epicentre = Earth surface; Focus = Fault depth.
Use it when: Terms.
Ten and thirty
Amplitude x10 per unit; energy x32 per unit.
Use it when: Richter jump.

Earthquakes: Causes, Distribution and Measurement: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Scale identification · 1 mark · MCQBoard level

The Mercalli scale measures:
(a) energy released
(b) magnitude only
(c) observed intensity of effects
(d) wave speed

Try it first, then show the solutionSolution

Intensity.

Answer: (c) Observed intensity.

Example 2: Two terms · 3 marks · Short answerBoard level

Define focus and epicentre and name one example of a reservoir-induced earthquake mentioned in the chapter.

Try it first, then show the solutionSolution

Focus: point inside the earth where energy is released.
Epicentre: point on the surface directly above it.
Reservoir-induced: Koyna, Maharashtra, 1967.

Answer: Focus, epicentre, and Koyna 1967.

Example 3: Ratio sum · 4 marks · Long answerHarder board level

Compare an earthquake of magnitude 7 with one of magnitude 4: find the amplitude ratio and the energy ratio.

Try it first, then show the solutionSolution

Difference = 7 - 4 = 3.
Amplitude ratio = 10^3 = 1,000.
Energy ratio = 10^(1.5 x 3) = 10^4.5, about 31,600.

Answer: Amplitude 1,000 times; energy about 31,600 times.

Example 4: Nepal aftershock · 4 marks · Long answerHarder board level

The 2015 main shock was 7.8 and the main aftershock 7.3. Find the amplitude ratio and the energy ratio of the main shock to the aftershock.

Try it first, then show the solutionSolution

Difference = 7.8 - 7.3 = 0.5.
Amplitude ratio = 10^0.5, about 3.16.
Energy ratio = 10^(1.5 x 0.5) = 10^0.75, about 5.62.

Answer: Amplitude about 3.2 times; energy about 5.6 times.

Example 5: Case analysis · 5 marks · Case-basedHOTS / competency

Case: Two villages in the same hills suffer different damage after one quake: poorly built brick houses collapse in one, while newer code-built houses survive in the other. (a) Identify which scale shows this. (b) Explain two effects of earthquakes in hills. (c) Suggest two lessons.

Try it first, then show the solutionSolution

(a) The Modified Mercalli scale, since intensity depends on local conditions.
(b) Landslides and avalanches; building collapse.
(c) Enforce earthquake-resistant codes and retrofit old structures.

Answer: (a) Mercalli (b) landslides and collapse (c) codes and retrofitting.

Where students lose marks in Earthquakes: Causes, Distribution and Measurement

The slips examiners see most often, and what to write instead.

Mistake: Treating each Richter unit as double
The scale is read as linear.
Wrong working
Magnitude 7 is twice magnitude 6
Right working
Magnitude 7 has 10 times the amplitude of 6
Do this: Each unit is ten times in amplitude and about 32 times in energy.
Usually costs: 2 marks.
Mistake: Confusing magnitude and intensity
Richter and Mercalli swapped.
Do this: Richter: energy; Mercalli: effects at a place.
Usually costs: 1 mark.
Mistake: Epicentre placed underground
Focus and epicentre swapped.
Do this: The focus is underground; the epicentre is on the surface.
Usually costs: 1 mark.

Exam tips

Name all three earthquake belts.
Subtract magnitudes before applying powers of ten.
For Nepal write date, magnitude, cause, effects and lessons.
Do not state casualty figures beyond those in your notes.

Practice questions on Earthquakes: Causes, Distribution and Measurement

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which belt has the most earthquakes?
(a) Alpine-Himalayan
(b) Circum-Pacific
(c) Mid-Atlantic
(d) Equatorial
Hint
Ring of Fire.
Answer
(b) Circum-Pacific.
Q2 · 3 marks · Short answerHarder board level
How many times more energy does a magnitude 8 earthquake release than one of magnitude 6?
Hint
10^(1.5 x 2).
Answer
Difference = 2; energy ratio = 10^(1.5 x 2) = 10^3 = 1,000 times.
Q3 · 3 marks · Short answerHarder board level
Distinguish between isoseismal and homoseismal lines.
Hint
Equal intensity versus equal time.
Answer
Isoseismal lines join places with the same intensity of shaking; homoseismal lines join places where shock waves arrive at the same time.
Q4 · 4 marks · Long answerHarder board level
Explain the tectonic cause of the 2015 Nepal earthquake.
Hint
Indian plate under Eurasian plate.
Answer
Nepal lies where the Indian Plate pushes northward under the Eurasian Plate. Stress built up on the great thrust fault under the Himalayas and was released suddenly, causing the magnitude 7.8 shock.
Q5 · 4 marks · Case-basedHOTS / competency
Case: A magnitude 6 quake at sea is followed by a wave on the coast. (a) Name the hazard. (b) Give one example from the chapter. (c) Explain why intensity could differ between two coastal towns.
Hint
Epicentre under the sea.
Answer
(a) A tsunami. (b) Indian Ocean, 2004. (c) Distance, ground type and building quality differ, and intensity records effects at each place.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Scale of intensity?Show answer
Modified Mercalli, I to XII.
2.Year of the Richter scale?Show answer
1935.
3.Nepal main shock magnitude?Show answer
7.8.
4.Date of the Nepal earthquake?Show answer
25 April 2015.
5.Which lines join equal intensity?Show answer
Isoseismal lines.

Earthquakes: Causes, Distribution and Measurement: frequently asked questions

Why are most quakes tectonic?

Moving plates build stress on faults that is released suddenly.

Does a higher magnitude mean more damage everywhere?

Not everywhere; intensity varies with distance, ground and buildings.

Are modern scales different?

Scientists often use the moment magnitude scale, which follows the same idea.