GeographyICSE Class 9 · Earthquakes

Earthquakes: Causes, Measurement and Effects (ICSE Class 9 Geography)

Updated 17 October 2026 · Draft, not yet fact-checked

An earthquake is a sudden shaking of the earth's crust caused by the release of energy at the focus, with the epicentre directly above it on the surface. Most are caused by the movement of tectonic plates; waves are recorded by a seismograph and the magnitude by the Richter scale. Earthquakes occur mainly in the Circum-Pacific belt and the Mediterranean-Himalayan belt.

Understand Earthquakes: Causes, Measurement and Effects

1. Meaning. An earthquake is a sudden trembling of the earth. The point inside the earth where it starts is the *focus*; the point on the surface directly above it is the *epicentre*, where the shaking is strongest.

2. Seismic waves.

*P (primary) waves*: fastest, travel through solids and liquids.
*S (secondary) waves*: slower, travel only through solids.
*L (surface) waves*: travel along the surface and cause most damage.

3. Causes.

*Tectonic*: movement of plates along faults, building stress that is suddenly released (most earthquakes).
*Volcanic*: eruptions and moving magma.
*Man-made*: reservoirs behind large dams, mining, underground nuclear tests.

4. Measurement. A *seismograph* records the waves as a seismogram. *Magnitude* (energy released) is measured on the *Richter scale*, a logarithmic scale: each step is about ten times larger in recorded wave amplitude. *Intensity* (effects) is measured on the *Mercalli scale*.

5. Effects. Destructive: collapse of buildings, fires, landslides, tsunamis, floods from broken dams, loss of life. Constructive: new lakes and landforms, fault scarps, new springs, helpful to geologists and an understanding of the interior.

6. Earthquake zones. The *Circum-Pacific belt* (Ring of Fire), the *Mediterranean-Himalayan belt* and zones along mid-ocean ridges. In India, the Himalayan region and the north-east are highly active; the Indo-Gangetic plain moderate; the peninsula less so.

Earthquakes: Causes, Measurement and Effects: key facts

Parts: Focus: inside | Epicentre: on surface above focus
Waves: P: fastest, through solid and liquid
S: only solid
L: surface, most damage
Measurement: $S e i s m o g r a p h ∣ R i c h t er : ma g ni t u d e, l o g a r i t hmi c ∣ M er c a l l i : in t e n s i t y$
Zones: Circum-Pacific belt
Mediterranean-Himalayan belt
Himalayan region in India

Earthquakes: Causes, Measurement and Effects 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.

Hammer under a table

Suppose a hammer strikes the underside of a wooden table. The student places a hand on the top of the table and feels the strongest tremor directly above the spot of impact, with weaker tremors farther away.

Match it to an earthquake.

Try it first, then show the solutionSolution

The point of impact under the table is the focus, where the energy is released.
The spot on the top surface directly above it is the epicentre.
The shaking weakens with distance from the epicentre.

Answer: The strongest tremor is felt at the epicentre, directly above the focus.

Memory hook: Focus below, epicentre above.

Tricks to remember Earthquakes: Causes, Measurement and Effects

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

P first
P is Primary, arrives first.
Use it when: Waves.
Richter is Rise x10
Each Richter step is about ten times in amplitude.
Use it when: Scale.

Earthquakes: Causes, Measurement and Effects: solved examples

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

Example 1: Define · 2 marks · Short answerBoard level

Differentiate between focus and epicentre. [2]

Try it first, then show the solutionSolution

Focus is the point within the crust where the earthquake starts.
Epicentre is the point on the surface directly above it.

Answer: Focus is the origin underground; epicentre is the point above it on the surface.

Example 2: Waves · 2 marks · Short answerBoard level

Give two differences between P waves and S waves. [2]

Try it first, then show the solutionSolution

Speed: P is faster, S is slower.
Medium: P through solid and liquid; S through solids only.

Answer: P waves are faster and travel through liquids too; S waves are slower and only in solids.

Example 3: Richter scale · 3 marks · Short answerHarder board level

Two quakes are recorded at the same station, with magnitudes 6.5 and 4.5. Compare the sizes of their recorded waves and say what the scale tells about damage. [3]

Try it first, then show the solutionSolution

Difference = 6.5 - 4.5 = 2 steps.
Each step is 10 times in amplitude.
10 x 10 = 100 times; a higher magnitude suggests more energy, though damage also depends on depth, distance and construction.

Answer: The larger quake shows waves 100 times larger in amplitude.

Example 4: Dams and quakes · 3 marks · Short answerHarder board level

Explain how a large reservoir can lead to small earthquakes. [3]

Try it first, then show the solutionSolution

The weight of water adds stress on rocks.
Water seeps into cracks and reduces friction.
Faults slip, causing tremors.

Answer: Added load and seeped water make faults slip.

Example 5: Case: coastal shake · 6 marks · Case-basedHOTS / competency

Case. A strong undersea earthquake occurs near a coast on the Pacific rim (for example, Japan or Chile). (i) Which belt is it likely to belong to? [1] (ii) Name the sea wave that may follow. [1] (iii) Why may the epicentre be on the sea floor? [2] (iv) Suggest two preparations for coastal people. [2]

Try it first, then show the solutionSolution

(i) Circum-Pacific belt, since the coast is on the Pacific rim.
(ii) Tsunami.
(iii) The focus lies below the sea floor.
(iv) Early warning systems and evacuation plans.

Answer: (i) Circum-Pacific belt (ii) Tsunami (iii) Focus under the sea bed (iv) Warning systems and evacuation routes.

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

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

Mistake: Writing that S waves pass through liquids
Wave types are swapped.
Do this: S waves are blocked by liquids.
Usually costs: 1 mark.
Mistake: Confusing focus and epicentre
Positions reversed.
Do this: Focus below, epicentre on the surface.
Usually costs: 1 mark.
Mistake: Treating Richter scale as linear
Magnitude 7 treated as 2 more than 5.
Do this: Each unit is ten times in amplitude.
Usually costs: 2 marks.

Exam tips

Draw a diagram of focus, epicentre and waves.
State the wave type with its medium.
Give examples of zones with names.
Use logarithmic reasoning for scale questions.

Practice questions on Earthquakes: Causes, Measurement and Effects

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 wave causes the most damage?
(a) P wave
(b) S wave
(c) L wave
(d) None
Hint
Surface.
Answer
(c) L wave.
Q2 · 1 mark · Very short answerBoard level
Name the instrument that records earthquake waves. [1]
Hint
Seismo.
Answer
Seismograph.
Q3 · 2 marks · Very short answerBoard level
State two causes of earthquakes other than plate movement. [2]
Hint
Volcano/man.
Answer
Volcanic eruptions and man-made causes such as mining or dams.
Q4 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Earthquakes are common along plate boundaries.
Reason (R): Plates press and slip against one another there.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Boundaries.
Answer
(a).
Q5 · 3 marks · Short answerHarder board level
Distinguish between magnitude and intensity. [3]
Hint
Energy/effects.
Answer
Magnitude measures the energy released (Richter scale); intensity measures the effects at a place (Mercalli scale).
Q6 · 3 marks · Short answerHarder board level
Why is the Himalayan region prone to earthquakes? [3]
Hint
Plates.
Answer
The Indian plate pushes against the Eurasian plate, building stress along faults.
Q7 · 2 marks · Very short answerHarder board level
Give two constructive effects of earthquakes. [2]
Hint
Landforms.
Answer
New lakes or landforms and release of groundwater (springs).
Q8 · 3 marks · Short answerHarder board level
Why do buildings on loose soil suffer more damage than those on rock? [3]
Hint
Shaking.
Answer
Loose soil amplifies shaking and can liquefy, so foundations fail.
Q9 · 4 marks · Case-basedHOTS / competency
Case. In a school, students are taught a drill of covering the head under a table and then moving to an open area. Explain two reasons for each step. [4]
Hint
Safety.
Answer
Under a table, falling objects are blocked and the body is protected; open areas are away from collapsing buildings and glass.
Q10 · 4 marks · Case-basedHOTS / competency
Case. A seismograph station records P waves at 10:00:00 and S waves at 10:02:00. (i) Which arrives first and why? [2] (ii) What does the time gap show? [2]
Hint
Distance.
Answer
(i) P waves, being faster. (ii) The gap grows with the distance from the epicentre, so it helps to locate the epicentre.

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 magnitude?Show answer
Richter scale.
2.Surface wave?Show answer
L wave.
3.Instrument?Show answer
Seismograph.
4.Point above the focus?Show answer
Epicentre.
5.Most earthquakes occur in?Show answer
Circum-Pacific belt.

Earthquakes: Causes, Measurement and Effects: frequently asked questions

Can earthquakes be predicted?

Not exactly; zones can be mapped.

What is a tsunami?

A large sea wave caused by an undersea earthquake.

What is an aftershock?

A smaller tremor following the main shock.