Selina Concise Solutions for ICSE Class 9 Geography Chapter 8 Earthquakes

Exercises

I. Fill in the blanks below

1. Most earthquakes occur on account of the strain in the earth's crust.
2. P (primary or push waves) are the first earthquake waves to be recorded on a seismograph of an earthquake.
3. The last of the earthquake waves to arrive on the surface of the earth are L (surface or long) waves.
4. The giant sea waves caused by earthquakes in the oceans are called Tsunamis.
5. About $70\text{ per cent}$ of the earthquakes occur in the Circum- Pacific Mountain Belt.
These sentences describe the basic facts about how earthquakes start and the different types of vibrations they send through the ground. It is important to know that different waves travel at different speeds and that certain parts of the world are much more likely to have earthquakes than others.
Teacher's Tip: Remember "P" for "Primary" and "L" for "Last" to keep the wave order in your head!
Exam Tip: Be careful with the spelling of "Tsunamis" and "Circum-Pacific," as these are high-frequency words in geography exams.

II. Short Answer Questions

Question 1: What is called an earthquake ?
Answer: An earthquake is defined as a tremor below the surface of the earth which causes shaking of the earth.
An earthquake is basically a sudden vibration or movement in the Earth's crust. These shakes can be very small or large enough to change the shape of the land.
Teacher's Tip: Think of an earthquake like a giant "earth-shiver" caused by energy escaping from underground.
Exam Tip: Use the word "tremor" in your definition to sound more scientific and score higher marks.

Question 2: State two natural causes of an earthquake.
Answer: Natural causes are the movement of tectonic plates and volcanic activities.
Tectonic plates are giant pieces of the Earth's shell that are always moving and occasionally bumping into each other. Volcanic eruptions also cause shakes because of the massive explosion of gas and liquid rock bursting through the crust.
Teacher's Tip: Imagine the Earth's surface as a giant jigsaw puzzle where the pieces are always trying to move.
Exam Tip: If asked for causes, always list "Plate Tectonics" as your first point, as it is the primary cause of most quakes.

Question 3: Name one man-made cause of an earthquake.
Answer: Construction of large scale dams, thrust of rivers along fault lines, dumping of chemical wastes also cause earthquakes due to imbalance in isostatic balance of the earth's landforms, e.g. Koyna dam caused an earthquake in Maharashtra.
When humans build huge structures like dams, the weight of the water can put too much pressure on the rocks below. This extra weight can cause the ground to crack and slip, leading to an earthquake in a place that used to be stable.
Teacher's Tip: Think of "isostatic balance" like a see-saw; if we put too much weight on one side (the dam), the other side reacts!
Exam Tip: Always mention the "Koyna dam" example when discussing man-made earthquakes to prove your answer with a real-world fact.

Question 4: What are known as seismic focus and epiceptre with respect to an earthquake ?
Answer: The point of origin of earthquake waves is called seismic focus and the centre vertically above the seismic focus nearest to the earth's crust is called epicentre.
The focus is the exact "starting point" hidden deep underground where the rocks first break. The epicentre is the spot directly above it on the surface where people first feel the shaking.
Teacher's Tip: The "Focus" is in the earth, and the "Epicentre" is on the surface - remember "E" for "Earth's surface"!
Exam Tip: Make sure to mention that the epicentre is "vertically above" the focus to get the technical definition right.

Question 5: What is known as Richter scale ? State its uses.
Answer: Ritcher scale is used to measure the intensity of earthquake through a scale, which is motivated by a needle attached to this instrument The zigzag wavelength of the earthquake comes on the screen and calculated from $1$ to $9$ scale measured in $centimetres$.
The Richter scale is like a giant ruler for measuring the strength of an earthquake's vibrations. Each number on the scale represents a much stronger shake than the one before it.
Teacher's Tip: Remember that a magnitude $6$ quake is $10$ times stronger than a magnitude $5$!
Exam Tip: Mention the range "$1$ to $9$" when explaining how scientists categorize the earthquake's power.

Question 6: How are earthquakes useful ?
Answer: Earthquakes cause energy release to put the earth in good shape; several landforms are uplifted to build large plains e.g. Sagami Bay uplifted $200\text{ m}$, Landslides cause the formation of lakes in Himalayan region.
While they are scary, earthquakes help the planet "stretch" and release energy that builds up inside. They can actually create new land features like mountains or beautiful lakes that didn't exist before.
Teacher's Tip: Think of earthquakes as the Earth's way of "renovating" its landscape.
Exam Tip: Use the term "constructive effects" when talking about the useful side of earthquakes.

Question 7: Give any two destructive effects of earthquakes ?
Answer: Destructive effects are collapse of structures, submergence of coastal parts e.g. Dwarka submergence, these change the course of rivers, danger to human life and motivation of tsunami waves causing drastic calamities.
Large earthquakes can knock down buildings and bridges, which is very dangerous for people living nearby. They can also force parts of the coast to sink under the ocean, destroying ancient cities or farmland.
Teacher's Tip: Remember "Collapse" and "Submergence" as the two big "C" and "S" of destruction.
Exam Tip: Mentioning the "submergence of Dwarka" is an excellent way to add historical detail to your answer.

Question 8: What is tsunami ? How is it caused ?
Answer: Tsunami is a large furious destructive sea waves caused by the force of an earthquake along the moving tectonic plates, volcanic activity landslides and meteorite impact.
A tsunami is a series of massive waves that travel across the ocean at very high speeds. They are usually triggered when the sea floor moves suddenly during an earthquake, pushing the water up like a giant plunger.
Teacher's Tip: A tsunami isn't just one wave, but a whole "train" of waves coming one after another.
Exam Tip: Do not call them "tidal waves," as they have nothing to do with the moon or tides.

Question 9: How do Japanese predict earthquakes ?
Answer: Japanese use the methods of measuring changes in sea level and variations in Earth's magnetic field to predict earthquakes.
Because Japan has many earthquakes, their scientists watch for tiny signs like the water rising or the Earth's magnetic pull changing slightly. These small clues can sometimes warn people before a big shake happens.
Teacher's Tip: Japan is one of the most prepared countries in the world because they live on the "Ring of Fire"!
Exam Tip: Focus on "sea level" and "magnetic field" as the two specific indicators mentioned in the text.

Question 10: Name the two belts where most of the earthquakes occur.
Answer: Two belts are the Circum-Pacific Mountain belt and the midworld mountain belt of high fold mountains like the Alps, Himalayas etc.
Most of the world's earthquakes happen in these two specific lines because that is where the Earth's crust is the weakest. One goes around the edge of the Pacific Ocean, and the other travels through the middle of the continents.
Teacher's Tip: "Circum" means around - so it's the belt *around* the Pacific!
Exam Tip: Be sure to name the "Himalayas" as an example of the midworld mountain belt.

III. Match the following

Column A
1. Tsunami
2. Seismograph
3. S-waves
4. Richter scale
5. Epicentre

Column B
(a) The waves that make the inhabitants feel the ground motion.
(b) The instrument for measuring the intensity of an earthquake.
(c) Harbour waves generated by oceanic earthquakes.
(d) The point on the earth surface directly above the seismic focus.
(e) An instrument for recording the movement of earthquake waves.
Answer:
1. Tsunami - (c) Harbour waves generated by oceanic earthquakes.
2. Seismograph - (e) An instrument for recording the movement of earthquake waves.
3. S-waves - (a) The waves that make the inhabitants feel the ground motion.
4. Richter scale - (b) The instrument for measuring the intensity of an earthquake.
5. Epicentre - (d) The point on the earth surface directly above the seismic focus.
This exercise helps connect the scientific names of tools and events with their actual meanings. For example, knowing that a seismograph is the machine and the Richter scale is the measurement is very important.
Teacher's Tip: Think of the Seismograph as the "machine" and the Richter scale as the "math" behind it.
Exam Tip: In matching questions, double-check that you haven't used the same letter twice!

IV. Long Answer Questions

Question 1: Describe how earthquakes are caused on the surface of the earth.
Answer: When the earth's crust is unable to accommodate itself to the strain due to isostatic imbalance, so sudden release of energy results in violent shock or an earthquake. The earthquakes are caused by severe tectonic movement of plates, volcanic eruption forces and isostatic imbalance often taking place due to diastrophism creating ups and downs in landform construction.
Earthquakes happen because the Earth's crust is under a lot of stress, like a bent stick that is about to snap. When the pressure becomes too much, the rocks break or slip, and all that trapped energy explodes outward as a shockwave.
Teacher's Tip: Use the word "diastrophism" to describe the slow bending and breaking of the Earth's crust.
Exam Tip: List three main causes: plate movement, volcanic eruption, and isostatic imbalance to provide a complete answer.

Question 2: Explain the three types of earthquake waves. What is the difference between seismic focus and epicentre ?
Answer: Three types of earthquake waves are :
1. P-primary waves,
2. S-Secondary (or Shear waves) and
3. L-Long waves (or Surface waves).
Seismic focus.is located in the interior of earth's crust from where the earthquake waves originate and the epicentre is the nearest point vertically below the region of earthquake effect.
Earthquakes send out different kinds of vibrations: P-waves are the fastest and arrive first, S-waves move side-to-side, and L-waves travel along the surface and cause the most damage. The focus is the underground starting point, while the epicentre is the surface spot right above it.
Teacher's Tip: "P" for "Push" (they arrive first), "S" for "Shake" (side-to-side), and "L" for "Land" (surface).
Exam Tip: Study the wave diagram carefully, as you might be asked to label the P, S, and L waves in an exam.

P Q: What is the Tsunami ? How ist it produced ? What are its effects ?
Answer: Tsunami is very destructive sea wave rising high upto several metres and jump furiously along the coastal areas like the hood of a snake. The name 'tsunami' is from the Japanese words : 'Tsu' and 'nami' meaning 'harbour' and 'wave' respectively. So tsunamis are harbour waves.
It is produced mainly by undersea earthquake along the gaps of tectonic plates, volcanic thrusts, great landslides and meteorite impact. They can cause a lot of destruction on the shore. They are sometimes mistakenly called “tidal waves,” but tsunamis have nothing to do with the tides.
Tsunamis are giant walls of water that can wash away entire towns near the ocean. They are caused by huge disturbances under the water, like an earthquake on the sea floor that pushes the ocean upward.
Teacher's Tip: Imagine dropping a big rock into a bathtub; the big splash that travels to the edge is like a tsunami!
Exam Tip: Be sure to clarify that tsunamis are "harbour waves" and are NOT caused by tides.

Question 3: Compare and contrast the constructive effects and the - destructive effects of an earthquake.
Answer: Constructive effects :
1. Earthquakes help the earth release its stored up energy. The majority of the earthquake occur around. The plate margins. This energy release helps to keep the earth in good shape.
2. On account of both vertical and lateral displacement of the earth's crust, earth quakes may raise or lower parts of earth especially near the seashore. Sagami Bay of Japan was uplifted $200\text{ m}$ in $1923$.
3. Landslides triggered by earthquakes cause formation of lakes as has happened at many places in the Himalayan region.
Destructive effects :
1. Human beings have settlements in active earthquake zones. Most often the houses and other structures collapse causing great loss of property.
2. The forces of uplift and subsidence also cause submergence of coastal parts. For example: Dwarka in Gujarat which now lies submerged under the sea.
3. Earthquakes have changed the course of rivers in the past. They have thus rendered many areas unsuitable for irrigation and agriculture. They cause danger to human life.
Earthquakes are a mix of "creating" and "destroying." They build new mountains and lakes, but they also knock down our homes and can even sink whole pieces of land under the sea.
Teacher's Tip: Think of it as a "Natural Reset" button that is very messy.
Exam Tip: Use specific examples like "Sagami Bay" or "Dwarka" to make your comparison much stronger.

Question 4: Is it possible to predict an earthquake ? Give reasons to support your answer.
Answer: No till now it is not possible to predict an earthquake. Earthquake prediction in the past was left to astrologers and mystics. Today it is a respectable scientific pursuit. The Russians were the first to discover P and S seismic waves in $1960\text{s}$ on the basis of which predictions were made. These waves still are the only means available to seismologists to predict earthquakes.
The theory of Plate Tectonics offers another means of prediction on scientific lines. Japanese use the methods of measuring changes in sea level and variations in Earth's magnetic field to predict earthquakes. However, no method is still foolproof as has been proved by recent earthquakes in Japan, India and the USA.
Even with all our fancy machines, we still can't tell exactly when an earthquake will happen. We can see signs like "P-waves" or changes in the ocean, but the Earth is very unpredictable.
Teacher's Tip: Predicting a quake is like predicting exactly when a popcorn kernel will pop—you know it *will* happen, but not the exact second!
Exam Tip: Mention that no method is "foolproof" to show you understand the limitations of modern science.

Question 5: How are earthquakes distributed over the globe ?
Answer: The earthquakes are common along the weak tectonic belts affected by continuous tremors due to the movement of plates one upon another. Main belts are Circum-Pacific Mountain belt and Mid-World Mountain belt of fold mountains which are still in the process of upthrust forces increasing their height.
Earthquakes don't just happen anywhere; they follow the lines where the Earth's giant plates are pushing against each other. This is why places like the Himalayas or the countries around the Pacific Ocean have so many shakes.
Teacher's Tip: Map out the "Ring of Fire" to see where most of the world's quakes are concentrated.
Exam Tip: The "Circum-Pacific Belt" is the most important location to remember for this question.

Question 6: How is the intensity of earthquakes measured ? What instruments are used ?
Answer: Ritcher scale and Mercalli scale are used to measure the intensity of earthquakes. Ritcher scale measures through a graphical scale in zigzag graph from $1$ to $9$ and Mercalli scale observes it from $1$ to $12$ points scale.
Scientists use two different scales to understand a quake: the Richter scale measures the energy released (the "power"), while the Mercalli scale measures the damage people actually see and feel.
Teacher's Tip: Think of Richter as measuring the "Engine Power" and Mercalli as measuring the "Crashed Car."
Exam Tip: Remember the numbers: $1$ to $9$ for Richter and $1$ to $12$ for Mercalli.

Practice Questions (Solved)

Question 1: (a) What is an Earthquake ? (b) How it is caused ? (c) Describe the world's distribution of earthquakes. (d) What are the advantages and disadvantages of Earth quakes. OR What are the effects of Earthquakes ? (e) Name the major earthquakes of India from 1991 to 1997.
Answer: (a) The sudden mild or violent shaking of a part of the earth is called an earthquake. An earthquake is generally accompanied by a rumbling and tremors.
(b) Causes The chief known causes of earthquakes are believed to be as follows :
1. When a dormant volcano erupts into activity or an active volcano discharges matter with greater violence the surrounding areas feel tremors or earthquakes.
2. When the interior of the earth cools and contracts the outer crust in some places cracks and faults are caused. This movement causes earthquake.
3. Some time water percolates so deep down into the earth that it turns into steam on account of the internal heat. This steam expands and tries to force its way out. This pressure causes an earthquake.
(c) The earthquakes are distributed along two major belts namely circum Pacific earthquake belt (Ring of fire) and the Mid-World, Mountain Earthquake belt along the great fold mountain zone.
(d) Earthquakes brings about changes on the surface of the earth which are very harmful to man.
Destructive Effects :
1. Many towns are destroyed and there is considerable loss of life and property.
2. They cause floods by uplifting of land in the' course of river.
3. Some places are submerged under the sea.
4. Big cracks and fissures are formed on the surface of the river and they interrupt communications.
5. Some times rivers disappear or change their courses or get flooded. Railway lines are twisted.
6. They cause great tidal waves, which may bring havoc to the coastal areas.
Constructive Effects :
1. Precious metals and minerals come up to the surface for the use of man.
2. New lands for habitation above the surface of the sea are formed.
3. Some times new springs and water falls are formed which help in irrigation.
4. Violent earthquakes have led to the formation of hills and mountains.
5. Bays and gulfs are formed along the coastal land to provide new ports and harbours.
(e) Major earthquakes of India from 1991 to 1997 : 1. Latur -1991, 2. Uttarkashi - 1993
This is a giant summary of the whole chapter. Earthquakes are caused by volcanoes, cooling rocks, or steam pressure, and while they can destroy towns and twist railway lines, they also bring gold to the surface and create new land for us to live on.
Teacher's Tip: This "solved" section is your best study guide for a big test!
Exam Tip: Be ready to name at least one Indian earthquake (Latur or Uttarkashi) for any question about local quakes.

Question 2: Name the important Earthquake-belts of the world. OR Indicate two major belts of Earthquakes.
Answer: 1. Circum - Pacific Earthquake Belt :- i.e. round the Pacific Ocean. This belt goes along with the coast of America and East coast of Asia.
2. Mid World Mountain Earthquake Belt :- This belt runs through the middle of Asia from East and goes beyond the Mediterranean sea as far as the West Indies. This belt, however, is not so active as Pacific Belt.
Think of these belts as the most "exciting" places on Earth where the crust is always moving. The Pacific belt is like a giant horseshoe of activity, while the Mid World belt travels through huge mountain ranges like the Himalayas.
Teacher's Tip: If you live near a "Fold Mountain," you are likely in an earthquake belt!
Exam Tip: Note that the Pacific Belt is the "most active" one; this is a common comparison point.

Question 3: Name the instrument used to record Earthquakes.
Answer: The instrument used to record Earthquakes' is called Seismograph.
A seismograph is a very sensitive machine with a pen that draws lines on a rolling piece of paper. When the ground shakes, the pen makes big "zig-zags" to show how strong the shake is.
Teacher's Tip: Think of it as the Earth's "heartbeat monitor."
Exam Tip: Don't confuse "Seismograph" (the machine) with "Seismology" (the study of quakes).

Question 4: Name some Earthquakes which caused great damage.
Answer: Some Earthquakes which caused serious damage to property and life are :
1. Lisbon Earthquake of 1755
2. South Carolina Earthquake of August 1886
3. The Japan Earthquake of Oct. 28, 1891
4. Kanga (Himachal pradesh) Earthquake of April 1905
5. Tokyo (Japan) Sep. 1923
6. Quetta 1935
7. Bihar 1934
8. California Earthquake of April, 1966
9. Turkey Sep. 1975
10.Iran 1968
11.Peru 1970
12.Tangshan (China) Earthquake of July, 1976
13.Broach Earthquake of 1970 affected part of Gujrat.
14.Earthquake of 1988, had its epicentre near Darbhanga and affected parts of North Bihar, Nepal and Bangladesh.
15.The Bhuj earthquake of 2001 devastated many cities of Gujarat.
This list shows that earthquakes happen all over the world, from Japan to the USA to India. Some of these, like the Bhuj earthquake in 2001, are remembered because they were extremely powerful and changed many people's lives.
Teacher's Tip: Notice how many of these happened in Japan and India - they are both on major belts!
Exam Tip: Memorize at least three of these dates and locations for a "History of Earthquakes" question.

Question 5: Earthquakes occur in the Mid-Atlantic belt.
Answer: Earthquakes occur in the Mid-Atlantic belt because here the sea floor spreading is the main cause for earthquakes.
In the middle of the Atlantic Ocean, the ground is actually tearing apart and creating new land. This "stretching" of the ocean floor causes lots of earthquakes deep under the water.
Teacher's Tip: This belt is mostly hidden because it is at the bottom of the ocean!
Exam Tip: Use the term "sea floor spreading" to explain quakes in the Atlantic region.

Question 6: How do earthquakes affect landscape ?
Answer: Earthquakes cause rise and fall of landmasses, create fault scarps, offset streams and other land features and cause landslides. Earthquakes waves also destroy houses and man-made structures, buckle and twist the railway lines electric and telegraph lines etc.
An earthquake can literally rewrite the map by moving rivers or creating new cliffs called "fault scarps." It also tears up our human-made things, like bending train tracks into wavy lines and breaking power poles.
Teacher's Tip: Think of it like a giant hand "re-shuffling" the ground and buildings.
Exam Tip: Mention "fault scarps" and "offset streams" to show you understand how nature changes after a quake.

Question 7: What was the cause of Koyna earthquake in India ?
Answer: Koyna Dam is located in Maharashtra on the Deccan plateau. This part was considered a stable block free from earthquakes. But Koyna earthquake in 1986 was a big surprise. In the reservoir of the dam water gathered was more than its capacity. It disturbed the local isostatic balance. It caused an earthquake and caused cracks in the rocks. It may be called a man-made earthquake.
The Koyna earthquake was special because it was caused by humans building a dam that was too heavy for the local rocks to handle. The weight of all that extra water "unbalanced" the ground and triggered a surprise shake in a place where nobody expected it.
Teacher's Tip: This is the most famous example of a "Reservoir Induced Earthquake."
Exam Tip: Be sure to use the phrase "man-made earthquake" when describing the Koyna event.

Question 8: Describe the main type of Earthquake Waves.
Answer: Earthquake Waves :- Earthquake waves travels in all directions from the Focus. There are three types of earthquake waves :
1. Longitudinal Waves : These are known as Primary Waves 'P'
2. Transverse Waves : These are known as Secondary Waves 'S'.
3. Surface Waves : These are called Long Waves or 'L' Waves.
When an earthquake hits, it sends out "energy ripples" in every direction. The P-waves are the fastest and go straight, the S-waves move side-to-side, and the L-waves are the big ones that roll along the surface where we feel them most.
Teacher's Tip: Think of P-waves as "Pushing" and S-waves as "Shaking"!
Exam Tip: Always associate L-waves with "surface" and "maximum damage."

Question 9: Name the causes of earthquakes.
Answer: 1. Volcanic eruptions
2. Faulting
3. Elasticity of rocks
4. Local causes
5. Epeiro, Genetic events.
Earthquakes have many triggers, from exploding volcanoes to rocks snapping back into place like a rubber band. "Faulting" is simply the ground breaking and slipping along a crack.
Teacher's Tip: "Faulting" is the most common cause of big earthquakes.
Exam Tip: Listing five causes will help you ensure you get the maximum points on a short-answer question.

Question 10: Why are earthquakes related to volcanoes ?
Answer: There is a close relationship between an earthquake and a volcano. Earthquakes and volcanoes occur in small belts, i.e. Mid world belt and Circum pacific belt. Their distribution shows a similar pattern. Volcanic eruption lead to earthquakes. Volcanic eruptions are the local cause of earthquakes.
Volcanoes and earthquakes are like "partners in crime" because they both happen when the Earth's crust is weak and moving. A volcano's explosion is so powerful that it naturally shakes the ground for miles around, causing a local earthquake.
Teacher's Tip: If you see a volcano erupting on the news, you can almost bet there was a small earthquake right before it!
Exam Tip: Mention the "Circum-Pacific Belt" as a place where both are frequently seen together.