Earthquakes ICSE Class 9: Causes, Measurement, Effects and Zones

What Earthquakes is about

Weightage: CISCE publishes no chapter weightage and no question-paper layout for ICSE Class 9 Geography. Class IX is examined by your school: one 80-mark paper of 2 hours plus 20 marks of internal assessment (a record file and one project). Earthquakes is topic 2(e), Structure of the Earth, of the 2028 syllabus.Source: CISCE ICSE 2028 Geography syllabus, Class IX

Key concepts

Meaning: focus, epicentre and seismic waves

An earthquake is a sudden vibration or shaking of the earth's surface caused by the abrupt release of energy within the crust or upper mantle.

Focus (hypocentre): the point inside the earth where the rocks first break and the earthquake originates.
Epicentre: the point on the earth's surface directly above the focus. Shaking is usually strongest and damage greatest near the epicentre, decreasing outwards.

Seismic waves carry the energy outwards:

• P-waves (primary): fastest; push-pull (longitudinal) waves; travel through solids, liquids and gases; arrive first.
• S-waves (secondary): slower; side-to-side (transverse) waves; travel only through solids; arrive second.
• Surface waves (L-waves): travel along the surface; slowest but cause most of the damage.

Diagram checklist: draw a block of crust with a fault; mark the focus on the fault below the surface, the epicentre directly above it, concentric wave fronts spreading from the focus, and label 'P', 'S' and 'surface waves'.

Worked example

(2 marks) Distinguish between the focus and the epicentre of an earthquake.

Try it first, then show the solutionSolution

1. Focus: the point within the earth where the earthquake originates - 1 mark.
2. Epicentre: the point on the surface vertically above the focus, where shaking is usually strongest - 1 mark.

Answer: The focus is the point inside the earth where the rocks break and the earthquake begins. The epicentre is the point on the earth's surface directly above the focus; it usually experiences the strongest shaking.

Causes of earthquakes

Natural causes:
1. Plate movements (tectonic earthquakes), the main cause. Stress builds up where plates collide (convergent, e.g. the Himalayas), move apart (divergent, mid-oceanic ridges) or slide past each other (transform, e.g. the San Andreas Fault in California). When the rocks snap or slip, energy is released.
2. Faulting: sudden movement of rocks along a fracture (fault) in the crust; plate movement is the usual cause of the stress.
3. Volcanic eruptions: movement of magma and explosive eruptions shake the ground (e.g. before and during eruptions in Hawaii and Italy).
4. Collapse of underground caves and large landslides cause small, local tremors.

Human (induced) causes:

• Large reservoirs behind dams: the weight of water and water seeping into faults can trigger quakes. The Koyna earthquake in Maharashtra (1967) is widely linked to the Koyna reservoir.
• Mining and quarrying with explosives; collapse of mines.
• Underground nuclear tests.
• Pumping fluids deep into the ground (for oil, gas or waste disposal).

Worked example

(3 marks) Explain how plate movements cause earthquakes.

Try it first, then show the solutionSolution

1. The crust is broken into plates that move slowly - 1 mark.
2. Where they collide, separate or slide past one another, stress builds up in the rocks - 1 mark.
3. When stress exceeds the rocks' strength they slip suddenly along a fault, releasing energy as seismic waves - 1 mark.

Answer: The crust is divided into plates that move slowly. Along their boundaries, where plates collide, pull apart or slide past each other, stress accumulates in the rocks. When it exceeds their strength the rocks slip suddenly along a fault, and the stored energy spreads out as seismic waves, causing an earthquake.

Measurement: seismograph, magnitude and intensity

Seismograph (seismometer): an instrument that detects and records earthquake waves. The record, a zigzag line on paper or screen, is a seismogram. Bigger zigzags mean stronger ground movement. Seismology is the study of earthquakes.

Magnitude (energy released) - the Richter scale:

• Devised by Charles F. Richter in 1935.
• Open-ended, usually quoted from about 0 to 9 and above; based on the amplitude of waves on a seismogram.
• Logarithmic: each increase of 1 means 10 times greater ground movement (amplitude) and about 31.6 (roughly 32) times more energy released. Two steps mean 100 times the amplitude and about 1,000 times the energy.
• Today seismologists use the closely related moment magnitude scale for large earthquakes, but news reports still speak of 'magnitude on the Richter scale'.

Intensity (effects at a place) - the Modified Mercalli scale:

• Grades I to XII (Roman numerals) based on what people feel and the damage seen: I = not felt, XII = total destruction.
• One earthquake has one magnitude but different intensities at different places (highest near the epicentre).

Worked example

(2 marks) An earthquake of magnitude 7 is compared with one of magnitude 5. How many times greater is its ground movement, and roughly how many times more energy does it release?

Try it first, then show the solutionSolution

1. Difference = 2 steps; amplitude ratio = 10 × 10 = 100 - 1 mark.
2. Energy ratio ≈ 31.6 × 31.6 ≈ 1,000 - 1 mark.

Answer: The magnitude 7 earthquake has 100 times the ground movement and releases about 1,000 times as much energy as the magnitude 5 earthquake.

Destructive effects of earthquakes

• Collapse of buildings, bridges and dams: the main cause of death. The Bhuj (Gujarat) earthquake of 26 January 2001 destroyed thousands of buildings and killed about 20,000 people.
• Landslides and avalanches in hilly areas block roads and rivers (common in Himalayan earthquakes, such as Nepal in 2015).
• Fires from broken gas pipes and electric lines; after the San Francisco earthquake of 1906 fires caused much of the damage.
• Tsunamis: undersea earthquakes displace the sea floor and send giant waves across the ocean. The Indian Ocean tsunami of 26 December 2004, set off by an earthquake off Sumatra (Indonesia), struck Indonesia, Sri Lanka, India (Tamil Nadu and the Andaman and Nicobar Islands) and Thailand. The 2011 earthquake and tsunami in Japan also damaged the Fukushima nuclear power plant.
• Liquefaction: shaking turns waterlogged soil into a quicksand-like mass, so buildings tilt or sink.
• Floods from broken dams and blocked rivers; changes in river courses.
• Disruption of services: power, water, transport and communication fail; disease can spread in relief camps.
• Economic loss and long-term trauma.

Worked example

(2 marks) What is a tsunami? How is it related to an earthquake?

Try it first, then show the solutionSolution

1. Tsunami: a series of very long, high sea waves - 1 mark.
2. Caused when an undersea earthquake suddenly displaces the sea floor and the water above it, e.g. 2004 Indian Ocean - 1 mark.

Answer: A tsunami is a series of huge sea waves. It is set off when an undersea earthquake suddenly lifts or drops the sea floor, displacing the water above it; the 2004 Sumatra earthquake caused the Indian Ocean tsunami.

Constructive effects of earthquakes

Though mainly destructive, earthquakes have some useful results:

• New landforms: coasts and sea floor may be raised or lowered; new islands, lakes and valleys may form.
• Springs: shaking can open new paths for groundwater, so new springs may appear (and old ones dry up).
• Minerals and resources: faulting can bring mineral deposits nearer the surface.
• Fertile land: sea-bed areas raised above the sea can add new land for use.
• Knowledge of the earth's interior: P- and S-waves change speed and direction in different layers, and S-waves cannot pass through liquids. Studying them told scientists that the outer core is liquid.
• Better preparedness: each earthquake improves building codes and warning systems.

Worked example

(2 marks) How have earthquake waves helped us learn about the earth's interior?

Try it first, then show the solutionSolution

1. Seismic waves change speed and direction as they pass through layers of different materials - 1 mark.
2. S-waves do not pass through the outer core, showing it is liquid - 1 mark.

Answer: Seismographs around the world record how seismic waves bend and change speed in different layers. Since S-waves cannot travel through liquids and do not pass through the outer core, scientists concluded that the outer core is liquid.

Earthquake zones of the world and of India

Earthquakes, like volcanoes, occur mainly along plate boundaries.

1. Circum-Pacific belt (Ring of Fire): around the Pacific margins - Chile, Peru, Mexico, California (USA), Alaska, Japan, the Philippines, Indonesia, New Zealand. It has the largest share of the world's earthquakes, including most of the strongest. The largest earthquake recorded struck Chile in 1960.

2. Mid-continental (Alpine-Himalayan) belt: along the young fold mountains from the Alps and the Mediterranean (Italy, Greece, Turkey) through Iran and Afghanistan to the Himalayas and Myanmar. The Turkey-Syria earthquake of 2023 and the Nepal earthquake of 2015 lie in this belt.

3. Mid-Atlantic Ridge belt: along the ridge down the middle of the Atlantic, including Iceland; mostly mild shocks.

4. Other zones: the East African Rift Valley and some old faults within plates.

India: the Himalayan region and the north-east are the most earthquake-prone (Assam 1950; Uttarkashi 1991); Kutch in Gujarat (Bhuj 2001) and the Andaman and Nicobar Islands are also highly prone. The peninsular plateau is more stable, although quakes such as Latur (Maharashtra, 1993) do occur. India's seismic zone map, drawn up by the Bureau of Indian Standards, grades regions by hazard: the higher the zone number, the greater the risk, and the Himalayan belt, the north-east, Kutch and the Andaman and Nicobar Islands fall in the highest zones.

Worked example

(3 marks) Name the three main earthquake belts of the world and give one country in each.

Try it first, then show the solutionSolution

1. Circum-Pacific belt: Japan or Chile - 1 mark.
2. Mid-continental (Alpine-Himalayan) belt: Turkey, Iran or Nepal - 1 mark.
3. Mid-Atlantic Ridge belt: Iceland - 1 mark.

Answer: (1) Circum-Pacific belt, e.g. Japan. (2) Mid-continental or Alpine-Himalayan belt, e.g. Turkey or Nepal. (3) Mid-Atlantic Ridge belt, e.g. Iceland.

Earthquakes in real life

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

Topics in this chapter

• Earthquakes: Causes, Measurement and Effects

Earthquakes: key facts

Focus and epicentre

$Focus=origininsidetheearth;epicentre=pointonthesurfacedirectlyabove$

Damage usually greatest at the epicentre.

Seismic waves

1. 1.P (fastest, solids and liquids)
2. 2.S (solids only)
3. 3.surface waves (slowest, most damaging)

Order of arrival at a seismograph.

Richter scale

$+1magnitude=10\times amplitude\approx 31.6\times energy;+2=100\times amplitude\approx 1,000\times energy$

Logarithmic; measures magnitude (energy).

Mercalli scale

Intensity I (not felt) to XII (total destruction)

Based on observed effects; varies with place.

Zones

Circum-Pacific; Mid-continental (Alpine-Himalayan); Mid-Atlantic Ridge

India: Himalayas, north-east, Kutch, Andaman and Nicobar most prone.

How the school exam asks Earthquakes

The question patterns to expect, and the method that works for each.

Tricks to remember Earthquakes

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

• P before S

  Primary arrives first (fast); Secondary second (Solids only); Surface waves last but Smash the most.

  Use it when: Order and nature of seismic waves.

• Richter = Rich in energy; Mercalli = Mercy on buildings

  Richter measures energy at the source; Mercalli grades the damage you see.

  Use it when: Magnitude versus intensity.

• Ten-fold rule

  Each step up the Richter ladder multiplies the shaking by 10.

  Use it when: Comparison numericals.

Earthquakes: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section B style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 6-mark question split into 2 + 2 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Earthquakes

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

• ✗Mistake: Confusing focus and epicentre

  Both are 'where the earthquake happens'.

  Wrong working

  The epicentre is the point inside the earth where the earthquake starts.

  Right working

  The focus is inside the earth; the epicentre is on the surface directly above it.

  ✓Do this: Epi- means 'upon': the epicentre is upon the surface.

  Usually costs: 1 mark.

• ✗Mistake: Treating the Richter scale as linear

  Numbers look like an ordinary scale.

  Wrong working

  Magnitude 6 is twice as strong as magnitude 3.

  Right working

  Three steps: 1,000 times the amplitude and about 32,000 times the energy.

  ✓Do this: Multiply by 10 for every step in amplitude.

  Usually costs: 1 to 2 marks.

• ✗Mistake: Saying the Richter scale measures damage

  Mixing magnitude and intensity.

  ✓Do this: Damage = intensity (Mercalli); energy = magnitude (Richter).

  Usually costs: 1 mark.

• ✗Mistake: Effects listed without examples

  Lists learnt as bare points.

  ✓Do this: Pair each effect with a real event: Bhuj 2001, 2004 tsunami, San Francisco 1906 fire.

  Usually costs: 1 mark per point.

Practice questions on Earthquakes

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

1. Q1 · 1 mark · MCQExam level

   The point on the earth's surface directly above the focus is the:
   (a) hypocentre
   (b) epicentre
   (c) fault
   (d) crater

   Hint

   On the surface.

   Answer

   (b) epicentre.

2. Q2 · 1 mark · MCQExam level

   Which seismic waves cause the most damage?
   (a) P-waves
   (b) S-waves
   (c) Surface waves
   (d) All equally

   Hint

   Travel along the surface.

   Answer

   (c) Surface waves.

3. Q3 · 2 marks · Very short answerExam level

   What is the Modified Mercalli scale? What is its range?

   Hint

   Intensity.

   Answer

   A scale that measures the intensity of an earthquake at a place from its observed effects on people and buildings. It ranges from I (not felt) to XII (total destruction).

4. Q4 · 2 marks · Very short answerHarder exam level

   An earthquake of magnitude 4 releases a certain amount of energy. Roughly how much more energy is released by one of magnitude 6?

   Hint

   About 32 per step.

   Answer

   Two steps: about 31.6 × 31.6 ≈ 1,000 times as much energy (and 100 times the ground movement).

5. Q5 · 2 marks · Very short answerHarder exam level

   Why are fires a major danger after an earthquake in a city?

   Hint

   Pipes, wires, water.

   Answer

   Shaking breaks gas pipes and electric lines, starting fires; at the same time broken water mains and blocked roads make it hard for fire engines to put them out, as in San Francisco in 1906.

6. Q6 · 2 marks · Very short answerHarder exam level

   What is liquefaction? Why is it dangerous?

   Hint

   Waterlogged soil.

   Answer

   Liquefaction is when strong shaking makes loose, waterlogged soil lose its strength and behave like a liquid. Buildings standing on it tilt, sink or topple even if they are well built.

7. Q7 · 3 marks · Short answerExam level

   Give three human activities that can cause earthquakes.

   Hint

   Dams, mines, tests.

   Answer

   (1) Filling large reservoirs behind dams (Koyna). (2) Mining and blasting, and collapse of mines. (3) Underground nuclear tests; also injecting fluids deep underground.

8. Q8 · 2 marks · Very short answerHarder exam level

   Explain two constructive effects of earthquakes.

   Hint

   Landforms; knowledge.

   Answer

   (1) They create new landforms, raising parts of the sea floor or coast and forming lakes and valleys; new springs may appear. (2) Seismic waves reveal the structure of the earth's interior, such as the liquid outer core.

9. Q9 · 3 marks · Short answerHOTS / competency

   Japan suffers frequent strong earthquakes but its death tolls are often lower than in poorer countries hit by weaker ones. Suggest three reasons.

   Hint

   Buildings, warnings, drills.

   Answer

   (1) Strict earthquake-resistant building codes, with flexible frames and base isolation. (2) Early-warning systems that stop trains and alert people seconds before strong shaking. (3) Regular public drills and well-equipped rescue services; poorer countries often lack these.

10. Q10 · 3 marks · Short answerHOTS / competency

    Why do the Andaman and Nicobar Islands face both earthquakes and tsunamis?

    Hint

    Plate boundary under the sea.

    Answer

    The islands lie close to a plate boundary in the sea, where the Indian plate sinks beneath another plate. Movement along it causes strong earthquakes, and because these occur under the sea they displace the water and can set off tsunamis, as in December 2004.

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. 1.Point of origin of an earthquake inside the earth?Show answer

   Focus (hypocentre).

2. 2.Which waves arrive first at a seismograph?Show answer

   P-waves (primary).

3. 3.Who devised the magnitude scale of 1935?Show answer

   Charles F. Richter.

4. 4.Range of the Modified Mercalli scale?Show answer

   I to XII.

5. 5.Earthquake belt with most of the world's earthquakes?Show answer

   The circum-Pacific belt.

6. 6.Indian earthquake of 26 January 2001?Show answer

   Bhuj (Kutch), Gujarat.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Earthquake: sudden shaking from energy released when rocks slip along a fault.
• ☐Focus inside the earth; epicentre on the surface above it.
• ☐Waves: P (first), S (solids only), surface (last, most damaging).
• ☐Richter: magnitude, logarithmic (+1 = 10 × amplitude, ≈ 32 × energy); Mercalli: intensity I-XII.
• ☐Destructive: building collapse, landslides, fires, tsunamis, liquefaction; constructive: landforms, springs, knowledge of the interior.
• ☐Belts: circum-Pacific, Alpine-Himalayan, Mid-Atlantic Ridge.

Earthquakes: frequently asked questions