Earthquakes | ICSE Class 9 Geography Notes

This note covers the meaning and causes of earthquakes, their points of origin, earthquake waves, recording and measurement, destructive and constructive effects, world earthquake zones, the Bhuj and Himalayan examples, and ways of reducing earthquake damage.

What is an earthquake, and where does it begin?

An earthquake is shaking of the Earth caused by a sudden release of energy. The released energy travels as seismic waves, meaning waves produced by an earthquake. These waves spread away from the place where the energy is released.

The crust is the outermost solid part of the Earth. Beneath it lies the mantle, the layer between the crust and the core, the central part of the Earth. The crust and the uppermost part of the mantle together form the lithosphere, the Earth's rigid outer layer.

The lithosphere is divided into tectonic plates, large, rigid slabs that carry continents and ocean floors. A plate boundary is the edge where plates meet. Their movement changes the Earth's surface. Forces operating inside the Earth are called endogenic forces. Earthquakes are an example of the sudden movements associated with these forces.

How do focus and epicentre differ?

The focus, also called the hypocentre, is the point inside the Earth where earthquake energy is released. The epicentre is the point on the Earth's surface directly above the focus. It is the surface point nearest to the focus.

Greatest damage is usually closest to the epicentre, and the strength of shaking decreases away from the centre. Keep the word “usually”: the location of the epicentre identifies a surface position; it does not itself describe every damaged building.

The block diagram labels the focus below the surface, the epicentre above it, and a plate boundary. Curved lines spread from the focus, while arrows show movement on opposite sides of the boundary.

See Fig. 3.3 in your NCERT textbook

In a labelled sketch, place the focus within the block and the epicentre at the surface above it. Show waves spreading from the focus. This keeps the source of the energy distinct from the point used to locate its position at the surface.

What causes earthquakes?

The most common earthquakes are tectonic earthquakes, caused by rocks sliding along a fault. A fault is a sharp break in crustal rocks along which movement occurs. The surface along which the rocks move is the fault plane.

Rocks on opposite sides of a fault tend to move relative to one another. Pressure from overlying rock layers presses them together. Friction, the resistance between surfaces in contact, can lock the rocks even while forces continue to act on them.

How does locked rock produce sudden shaking?

1. Forces act on rock blocks along a fault, tending to move them in different directions.
2. Friction holds the blocks together while the forces continue to act.
3. The rocks become deformed, meaning that their shape changes under the applied forces.
4. When the tendency to move overcomes friction, the blocks slide abruptly past one another.
5. Energy is released at the focus and travels outwards as seismic waves, producing shaking.

This sequence connects slow movement with a sudden earthquake. The plates do not have to move rapidly all the time for an earthquake to occur. The crucial change is the abrupt release of accumulated energy when the locked rocks move.

What other types of earthquake occur?

These causes should not be confused with the effects of an earthquake. Sliding at a fault explains the origin of a tectonic earthquake. A building falling down describes a consequence of the shaking. Keeping cause and effect separate makes the whole process easier to explain.

Earthquakes of tectonic origin have a large area of influence and have proved highly destructive. Earthquakes associated with volcanic activity, rock falls, downward ground movement in mining areas and reservoirs have a more limited area of influence and scale of damage.

How do earthquake waves travel?

Earthquake waves fall into two main groups. Body waves travel through the interior of the Earth after energy is released at the focus. Surface waves travel along the surface and are generated when body waves interact with surface rocks.

How do primary and secondary waves compare?

P-waves means primary waves. They move faster and are the first body waves to arrive. S-waves means secondary waves. They arrive after P-waves. The letters P and S identify these two wave types; they are not units of measurement.

Parallel vibration runs along the same direction as the wave travels. Perpendicular vibration acts at right angles to that direction. A crest is a raised part of a wave, and a trough is its lowered part.

Surface waves are the last to report on a seismograph, an instrument that records earthquake waves. They are considered to be the most damaging waves. Their movement displaces rocks and can cause structures to collapse.

The recorded trace labels P-waves first, S-waves next and surface waves last. Arrows identify arrival times. A vertical marker labels amplitude, the size of the vibration measured from its resting position.

See Fig. 3.1 in your NCERT textbook

Why do these waves help us study the Earth?

Different waves behave differently as they cross materials inside the Earth. Their speed and direction change. Reflection means a wave rebounds, while refraction means it changes direction as it passes into different material. These changes can be studied from recorded wave patterns.

The inability of S-waves to travel through liquids is particularly useful in investigating the Earth's interior. Wave records provide indirect evidence about layers that cannot be reached and examined directly. An earthquake therefore supplies information as well as causing shaking.

How are earthquakes recorded and measured?

A seismograph records waves reaching its location. Its trace shows the arrival and pattern of vibrations. Recording is different from assigning a number to an earthquake: the instrument supplies a record, while a scale expresses a particular aspect of the event.

Magnitude relates to the energy released during an earthquake. The Richter scale is used to express earthquake magnitude. Intensity describes the observed effects of shaking, including visible damage. The Mercalli scale expresses intensity and is named after Mercalli, an Italian scientist who studied earthquakes.

What is the difference between magnitude and intensity?

A statement about fallen objects or damaged buildings describes an effect. A statement giving a Richter magnitude describes the earthquake's size in relation to energy release. Do not replace one with the other merely because both are expressed using numbers.

What do the magnitude examples indicate?

The expressions “over 5.0” and “6.0 or higher” have different meanings. The first excludes 5.0 itself; the second includes 6.0. Also retain “can cause” when describing damage from falling objects, rather than turning a possibility into a fixed outcome.

The values in this table are descriptive reference points. They are not casualty figures, wave speeds or measurements of the duration of shaking. Read the value together with its description so that the meaning of the measurement remains clear.

Note: A seismograph is an instrument, the Richter scale expresses magnitude, and the Mercalli scale expresses intensity. The instrument and the two scales answer different questions about an earthquake.

What destructive effects can earthquakes produce?

An earthquake is a natural hazard, a natural event with the potential to harm people or property. When damage and disruption become widespread and severe, the event becomes a disaster. Shaking affects the ground, buildings, water and the services on which people depend.

How are land and structures affected?

Ground displacement is a change in the position of the ground. Earthquakes can also open fissures, or cracks. Buildings may crack, slide, overturn or collapse. Falling objects and damaged structures threaten people even where the ground itself remains recognisable.

Landslides are rapid downslope movements of rock and debris, loose broken material. Earthquakes can trigger them, obstructing roads or river channels. Such obstructions can hold back river water, and rivers sometimes change their course, causing flooding and other damage in affected areas.

Fires are another possible consequence. Damage to transport, communication, settlements and industries can continue to affect daily life after shaking ends. Homelessness adds to the difficulties of affected people, while the destruction of services makes rescue and relief harder.

How can an earthquake produce a tsunami?

A tsunami is a series of sea waves generated by sudden displacement of water, including displacement associated with an earthquake beneath the sea. An earthquake-related tsunami requires an epicentre beneath oceanic waters and a sufficiently high magnitude; sea-floor displacement is the link to the water above.

1. An earthquake occurs beneath the sea and displaces part of the sea floor.
2. The displacement moves the water above the affected area.
3. Waves travel away from the disturbed region through the ocean.
4. On reaching shallow coastal water, the waves can become high and flood coastal land.

The earthquake and tsunami are distinct phenomena: one is ground shaking and the other is moving seawater. A tsunami is therefore an associated water hazard. An account of earthquake damage should distinguish shaking damage from damage caused by the sea waves.

What constructive effects can accompany earthquakes?

Constructive effects are changes that create land features or may provide useful conditions. The term describes a particular result of Earth movements. It does not mean that the destruction of homes, services and lives becomes beneficial.

How can landforms and water storage change?

Earth movements associated with an earthquake can raise or lower land. Uplift means upward movement of part of the Earth's surface, while subsidence means downward movement. Uplift can expose new land, including parts of a coast that previously lay below water.

Earthquake-triggered landslides often obstruct rivers and channels, forming reservoirs behind the obstruction. Where water collects in a hollow or behind a blockage, a new water body may form. This is a landform-creating effect, even though the blockage may also threaten nearby settlements.

Cracks can alter the routes followed by underground water. In some places this may allow water to emerge as a new spring, a place where underground water flows naturally onto the surface. This is a possible local result, not an effect of every earthquake.

Why must constructive effects be described carefully?

The same change can have different consequences. Water collected behind a blocked channel creates a reservoir, but the obstruction may also cause flooding. A raised coastal area adds exposed land, while the movement producing it may damage existing settlements.

Keep the physical change and its possible usefulness separate. First identify what happened to the land or water. Then explain why that change can be classed as constructive. Avoid assuming that a newly formed water body is automatically safe or useful for settlement.

Earthquake waves also help scientists investigate the Earth's internal structure. This is a benefit of studying the event, rather than a newly created landform. Distinguishing this scientific value from physical constructive effects keeps the two ideas clear.

Where are the major earthquake zones of the world?

Earthquakes are concentrated in particular belts, elongated zones of activity. They are not spread evenly across the globe. Important concentrations follow the rim of the Pacific Ocean, the Alpine-Himalayan system and the mid-oceanic ridges, which are underwater mountain chains.

What are the principal belts?

Circum-Pacific means around the Pacific. This belt follows the rim rather than covering the entire ocean uniformly. The Pacific rim is also called the Ring of Fire because active volcanoes occur there. Earthquakes and volcanoes show a broadly similar world distribution.

The world map uses shading for deep earthquake zones and small dots for shallow earthquake centres. The shaded belt follows the Pacific rim and the Alpine-Himalayan region. Separate symbols identify volcanic eruptions and hot spots, localised centres of volcanic activity.

See Fig. 4.2 in your NCERT textbook

How do the oceanic belts continue?

A line of earthquake activity runs through the central Atlantic, almost parallel to its coastlines, and continues into the Indian Ocean. South of the Indian subcontinent it divides. One branch extends into East Africa, and another meets the line from Myanmar to New Guinea.

In general, earthquake foci along mid-oceanic ridges are shallow, whereas those along the Alpine-Himalayan belt and the Pacific rim are deep-seated. “In general” describes a broad geographical pattern. It should not be turned into an absolute claim about every earthquake in either setting.

On an outline world map, trace the central Atlantic ridge belt and its continuation into the Indian Ocean. Mark the branch towards East Africa and the connection towards the Myanmar to New Guinea belt. Label the oceans and use a key for the earthquake belt.

A map key explains the symbols used on a map. Read it before interpreting dots and shaded areas. A dot marking an earthquake centre and a symbol marking a volcano do not represent the same feature, even where their distributions overlap.

What does the Bhuj case study show about earthquake damage?

Case study: Bhuj, 26 January 2001

A massive earthquake measuring 6.9 on the Richter scale struck Bhuj town in Gujarat on 26 January 2001. The event illustrates how shaking can be followed by fires, failure of essential services, loss of shelter and difficulties in delivering relief.

The school figures are people feared to have lost their lives, not confirmed final totals. The reported collapse of school buildings shows why the strength of buildings matters. The figures also show why a report's qualifications must remain attached to its numbers.

Hundreds of fires started when charcoal cookers overturned. Phone lines, water pipelines and power transmission lines were knocked out. These failures affected the connections and services that people needed after the earthquake as well as during the immediate emergency.

Three days after the earthquake, concern arose that food, blankets and medical supplies were not reaching everyone. Relief means immediate assistance to affected people. Here it involved basic supplies needed by survivors, and the reports show that organising assistance did not guarantee that it reached every person.

This example links several effects without treating them as identical. Ground shaking was the initiating event; school collapses and overturned cookers were physical consequences; fires and disrupted services added further difficulties; shortages in relief distribution affected the response.

Why is the Himalayan region earthquake-prone?

Case study: Energy release along the Himalayan arch

The Indian plate moves towards the north and north-east, where its movement is obstructed by the Eurasian plate, the plate to its north. Their interaction provides a regional example of how plate movement can lead to earthquakes.

The plates become locked, and energy accumulates. Stress is force per unit area acting within rock and tending to deform it. As stress builds, the locking can break down. Sudden energy release then produces earthquakes along the Himalayan arch, the curved Himalayan mountain belt.

1. The Indian plate moves towards the north and north-east.
2. The Eurasian plate obstructs this movement, and the interacting plates become locked.
3. Energy accumulates and stress builds at the locked region.
4. The lock breaks, energy is released suddenly, and an earthquake occurs.

Vulnerable areas include Jammu and Kashmir, Ladakh, Himachal Pradesh, Uttarakhand, Sikkim, the Darjeeling subdivision of West Bengal and the north-eastern states. These examples connect a broad world earthquake belt with particular regions of India.

The Himalayan case explains the cause of repeated earthquake activity in a region. The Bhuj case illustrates the consequences of one event. Together they show why a study of earthquakes needs both a physical explanation and an account of effects on people.

Earthquakes are not confined to the Himalayan region. Gujarat and Maharashtra have also experienced severe earthquakes. A broad concentration along a mountain belt therefore does not justify treating every area outside that belt as free of earthquake hazard.

How can earthquake damage be reduced?

It is not possible to prevent an earthquake from occurring. Preparedness means making arrangements before a disaster, while mitigation means reducing its harmful effects. Both are important because damage to transport and communication can make timely relief difficult.

What can be done before an earthquake?

Vulnerability means susceptibility to harm. Earthquake-resistant design means designing a structure to resist earthquake forces.

• Monitor activity: earthquake monitoring centres record activity and help information reach people in vulnerable areas quickly.
• Map vulnerability: identify areas likely to suffer damage and share the information with people living there.
• Improve construction: use earthquake-resistant designs and appropriate light materials in vulnerable areas.
• Spread awareness: explain earthquake hazards and ways of reducing their effects before an emergency occurs.

A vulnerability map identifies differences in exposure to possible damage. It supports preparation and planning; it does not state the exact time when the next earthquake will occur.

Changes in house types and building design can help reduce losses. Construction planning is therefore part of earthquake mitigation, alongside information, monitoring and public awareness.

What is needed during and after a disaster?

During a disaster, rescue and relief involve moving affected people to safer places, arranging shelters and supplying water, food, clothing and medical assistance. These needs are linked: shelter alone does not meet the need for drinking water or treatment of injuries.

After the emergency, rehabilitation means helping affected people restore their lives and livelihoods. Recovery also includes rebuilding the capacity to cope with future disasters. The response therefore extends beyond the first delivery of supplies to longer-term restoration.

The Bhuj reports show why this preparation matters. When communication and power connections fail, affected people still need assistance. Planning for damaged services is part of reducing the disruption that follows the earthquake itself.

Glossary

• Earthquake — Shaking of the Earth caused by a sudden release of energy that produces seismic waves.
• Lithosphere — The rigid outer layer consisting of the crust and the uppermost part of the mantle.
• Tectonic plate — A large, rigid slab of the lithosphere carrying continental land or part of the ocean floor.
• Fault — A sharp break in crustal rocks along which rock blocks can move relative to one another.
• Focus — The point inside the Earth where earthquake energy is released, also known as the hypocentre.
• Epicentre — The point on the Earth's surface directly above and nearest to the earthquake's focus.
• Seismic waves — Waves produced by the release of energy during an earthquake and transmitted through or along the Earth.
• Body waves — Earthquake waves generated at the focus that travel through the Earth's interior in different directions.
• Surface waves — Waves generated through interaction with surface rocks that travel along the Earth's surface.
• Seismograph — An instrument that records earthquake waves arriving at the location where it is installed.
• Magnitude — A measure of earthquake size related to the energy released during the earthquake event.
• Intensity — A description of the observed effects of earthquake shaking, including visible damage at an affected place.
• Tsunami — Sea waves generated by sudden water displacement, including displacement associated with an earthquake beneath the sea.
• Mitigation — Measures intended to reduce the harmful effects of a hazard on people, property and daily life.
• Preparedness — Arrangements made before a disaster to improve the ability of people and services to respond.

Common errors and misconceptions

• Misconception: The focus and epicentre are the same point. Correct: The focus lies inside the Earth; the epicentre lies on the surface directly above it.
• Misconception: A seismograph and the Richter scale are the same thing. Correct: The seismograph is a recording instrument, while the Richter scale expresses magnitude.
• Misconception: Magnitude means the visible damage at a particular place. Correct: Magnitude relates to energy release; intensity describes observed effects, including visible damage.
• Misconception: S-waves travel through liquids as well as solids. Correct: S-waves pass only through solids, while P-waves can pass through solids, liquids and gases.
• Misconception: A tsunami is another name for an earthquake. Correct: A tsunami consists of sea waves; an earthquake is shaking of the Earth.
• Misconception: Constructive effects make an earthquake beneficial overall. Correct: A new landform or water body may form alongside severe destruction and danger to people.
• Misconception: Only Himalayan regions of India experience severe earthquakes. Correct: Severe earthquakes have also occurred in Gujarat and Maharashtra, outside the Himalayan region.
• Misconception: People feared dead in an early report are confirmed final deaths. Correct: “Feared” expresses uncertainty and must remain attached to the reported figures.

Exam-style questions with model answers

Q1. Distinguish the focus from the epicentre by stating the location and meaning of each. [2 marks]

1. The focus is the point inside the Earth where energy is released during an earthquake.
2. The epicentre is the point on the Earth's surface directly above and nearest to the focus.

Q2. State what a seismograph records and what the Richter scale expresses. [2 marks]

1. A seismograph records earthquake waves arriving at the place where the instrument is located.
2. The Richter scale expresses earthquake magnitude, which relates to the energy released during the event.

Q3. Explain in three stages how movement along a fault produces an earthquake. [3 marks]

1. Rock blocks on opposite sides of a fault tend to move, but friction can lock them together as overlying rocks press upon them.
2. Continued forces deform the rocks until their tendency to move overcomes the friction that has been holding the blocks together.
3. The blocks then slide abruptly, releasing energy at the focus. This energy travels in different directions as seismic waves and produces shaking.

Q4. Compare P-waves, meaning primary waves, and S-waves, meaning secondary waves, using arrival order and the materials through which each travels. Give four points. [4 marks]

1. P-waves move faster than S-waves and are the first of these two types of body wave to arrive.
2. S-waves arrive after P-waves, so their appearance on the record follows a time lag.
3. P-waves can travel through solid, liquid and gaseous materials, making their range of transmission broader.
4. S-waves travel only through solid materials; they cannot pass through liquid regions within the Earth.

Q5. The Bhuj earthquake struck on 26 January 2001 with Richter magnitude 6.9. Reports described collapsed school buildings, overturned charcoal cookers starting fires, damaged phone lines, water pipelines and power transmission lines, and concern three days later that food, blankets and medical supplies were not reaching everyone. Identify five distinct consequences using these details. [5 marks]

1. The collapse of school buildings illustrates structural damage. Places used for education were physically destroyed or damaged by the earthquake, creating danger for their occupants.
2. Overturned charcoal cookers started fires. This shows how shaking can produce an additional destructive effect through objects and equipment disturbed inside the affected settlement.
3. Damaged phone lines disrupted communication. Communication failure was therefore part of the earthquake's consequences, alongside damage to buildings and other physical structures.
4. Damage to water pipelines and power transmission lines disrupted essential services. The affected population faced difficulties extending beyond the immediate movement of the ground.
5. Concern about supplies three days later shows difficulty delivering relief to everyone. Food, blankets and medical supplies were needed after the initial shaking had ended.

Q6. Describe the three principal world earthquake belts, the general difference in earthquake-focus depth between ridge and mountain/rim belts, and why the Pacific rim is called the Ring of Fire. [5 marks]

1. The Circum-Pacific belt follows the rim of the Pacific Ocean. It forms a major concentration of earthquake activity around the ocean's margins.
2. The Alpine-Himalayan belt follows the Alpine-Himalayan mountain system. It is another major elongated concentration of earthquake activity across the mountain region.
3. The mid-oceanic ridge belt follows underwater mountain chains, including the central Atlantic ridge system and its continuation into the Indian Ocean.
4. In general, earthquake foci at mid-oceanic ridges are shallow, whereas those along the Alpine-Himalayan belt and Pacific rim are deep-seated.
5. The Pacific rim is called the Ring of Fire because active volcanoes occur there. The name refers to volcanic activity in the same broad region.

Q7. A landslide triggered by an earthquake blocks a river, and water collects behind it. Explain the constructive landform change and the possible destructive consequence in two points. [2 marks]

1. Water collecting behind the obstruction forms a new reservoir or water body, a constructive landform change.
2. The blocked river may cause flooding, so the same physical change can also damage the surrounding area.

Q8. Explain four measures that can reduce earthquake losses before an event: monitoring, vulnerability mapping, construction design and public awareness. [4 marks]

1. Earthquake monitoring centres record activity and help information reach people in vulnerable areas quickly, supporting an organised response to the hazard.
2. Vulnerability maps identify areas susceptible to damage, allowing risk information to be shared with the people who live in those areas.
3. Earthquake-resistant building designs and appropriate light construction materials help reduce the damage suffered by buildings in vulnerable regions.
4. Public awareness prepares people to understand earthquake hazards and ways of reducing their effects before an emergency disrupts daily life.

Key takeaways

• An earthquake releases energy at a focus inside the Earth; the epicentre lies directly above that point on the surface.
• Tectonic earthquakes occur when rock blocks locked by friction move abruptly along a fault and release accumulated energy.
• P-waves travel through solids, liquids and gases; S-waves travel only through solids, helping scientists investigate the Earth's interior.
• A seismograph records earthquake waves, while magnitude relates to energy release and intensity describes observed effects such as visible damage.
• Earthquakes can damage ground, buildings, services and transport; associated fires, landslides, floods and tsunamis can add further destruction.
• Constructive effects include possible creation of land or water features, but these changes can accompany severe hazards and losses.
• Major earthquake belts follow the Pacific rim, the Alpine-Himalayan system and mid-oceanic ridges, giving earthquakes an uneven world distribution.
• Earthquake preparedness combines monitoring, vulnerability mapping, public awareness and suitable construction with plans for relief and longer-term recovery.

Test yourself

What is another name for the focus, and where is it?

The focus is also called the hypocentre. It lies inside the Earth at the point where earthquake energy is released.

Why can slow rock movement result in sudden shaking?

Friction can lock moving rock blocks while energy accumulates. When the blocks overcome friction and slide abruptly, energy is released suddenly.

Which earthquake waves are considered the most damaging?

Surface waves are considered the most damaging. They travel along the surface, displace rocks and can cause structures to collapse.

How is earthquake intensity different from magnitude?

Intensity describes observed effects and visible damage, whereas magnitude relates to the energy released during the earthquake.

What qualification belongs with the location of greatest earthquake damage?

Greatest damage is usually closest to the epicentre. The word “usually” preserves the qualification rather than claiming an absolute rule.

Why can a river blockage have both constructive and destructive effects?

Water collecting behind the blockage can form a new reservoir, while the obstruction may also cause flooding and damage.

What distinguishes the mid-oceanic ridge earthquake belt?

It follows underwater mountain chains, and earthquake foci in these ridge areas are generally at shallow depths.

Why should preparation include transport and communication failures?

Earthquakes can destroy these connections, making timely relief difficult. Affected people still need shelter, water, food and medical assistance.