DAV Class 8 Science Chapter 18 Question Answers – Earthquakes
An earthquake can strike without warning, and in a few seconds it can bring down buildings and change a landscape. Chapter 18 of the DAV Class 8 Science book explains why the ground shakes, how we measure that shaking, and how we can protect ourselves. This post gives easy answers to every question in the chapter, with a short reason behind each one, so you can understand the idea and write it in your own words.
The chapter in one minute
The Earth’s outermost layer, the crust, is not one solid piece. It is broken into huge slabs called tectonic plates, and these plates move very slowly, only a few millimetres a year. Sometimes the edges of two plates get stuck against each other because of friction. When the pressure builds up enough to overcome the friction, the plates suddenly slip and a large amount of energy is released as waves called seismic waves. These waves shake the ground, and that shaking is an earthquake.
The spot deep underground where the earthquake begins is called the focus. The point on the surface directly above it is the epicentre. The strength of an earthquake is measured on the Richter scale, and the instrument that records it is a seismometer. A smaller quake before the main one is a foreshock. If the quake happens under the sea, it can create giant waves called tsunamis.
A. Fill in the blanks
1. A foreshock is a smaller earthquake that happens in the same place before the main quake.
2. The huge sea waves caused by underwater earthquakes are called tsunamis.
3. An earthquake is caused by a disturbance deep inside the crust of the Earth.
4. The waves linked with earthquake tremors are called seismic waves.
5. Geologists call the point where an earthquake begins its focus.
6. The destructive energy of an earthquake is measured on the Richter scale.
B. True or false
| Statement | Answer | Why |
|---|---|---|
| 1. It is now possible to predict earthquakes exactly. | False | Scientists can tell which regions are at risk, but they cannot give the exact time and place. |
| 2. Earthquakes under the ocean floor can create huge waves. | True | These waves are called tsunamis. |
| 3. When plates slide past each other, the released energy creates seismic waves. | True | This sudden release of stored energy is what shakes the ground. |
| 4. The Richter scale is a simple linear scale. | False | It is a scale where each step up means a much bigger quake, not just one more unit. |
| 5. An earthquake of magnitude 2.5 cannot be recorded on a seismometer. | False | A seismometer is sensitive enough to record even weak quakes like this. |
| 6. In highly seismic areas, houses of mud or timber are better than heavy construction. | True | Light buildings cause far less harm if they fall. |
C. Choose the correct option
1. A small quake that may come before the main earthquake is called a foreshock.
2. A natural event that is not triggered by an earthquake is a flood. Tsunamis, landslides and fires can all follow an earthquake, but a flood caused by heavy rain has nothing to do with one.
3. Earthquakes happen when stored-up energy is suddenly released in the Earth’s crust.
4. The layers of the Earth, from the outside to the centre, are: A is the crust, B is the mantle, C is the outer core and D is the inner core. Check the labels in your figure against this order.
5. The epicentre of an earthquake is on the surface of the Earth.
6. If two earthquakes measure 7 and 5 on the Richter scale, earthquake A is 100 times stronger than earthquake B. Each step on the scale means a ten times stronger quake, and a difference of two steps means ten times ten, which is 100.
D. Short answer questions
1. What is an earthquake?
An earthquake is a sudden shaking or movement of the Earth’s crust that lasts for a short time.
2. State any three damaging effects of an earthquake.
Earthquakes can destroy buildings, bridges and dams. They can cause a large loss of human life. They can also set off floods and landslides.
3. Explain how shock waves are produced.
The plates of the Earth’s crust slide past each other, and their edges can get stuck because of friction. The stress keeps building up until it becomes stronger than the friction. Then the plates suddenly slip, and the stored energy is released in the form of waves. These are called seismic or shock waves.
4. Which is deep underground, the focus or the epicentre?
The focus. The epicentre is the point on the surface directly above it.
5. Name the scale used to measure the strength of an earthquake. What is the likely magnitude of (a) a mild earthquake and (b) a highly destructive one?
The Richter scale is used. (a) A mild earthquake measures about 2.5 to 4.5. (b) A highly destructive earthquake measures 8.0 or more.
6. An earthquake measures 3 on the Richter scale. (a) Will a seismometer record it? (b) Will it cause much damage?
(a) Yes, a seismometer will record it. (b) No, it is a very mild earthquake that causes little or no damage, and many people may not even feel it.
E. Long answer questions
1. Explain briefly the cause of an earthquake.
Earthquakes are caused by disturbances deep inside the crust, the outermost layer of the Earth. This layer is cracked into several large pieces called tectonic plates. The plates keep moving very slowly, a few millimetres every year. As they push against, pull away from or slide past each other, stress builds up at their edges. When this stress is suddenly released, the ground shakes, and this is an earthquake.
2. Where are earthquakes most likely to occur?
More than 80 per cent of earthquakes occur around the edges of the Pacific Ocean. This region is known as the Ring of Fire. It is the most active zone in the world for both earthquakes and volcanoes, because the Pacific plate is pushed beneath the plates around it.
3. Mr Sarthak’s company must build a housing complex in a seismic zone. What precautions should it take?
Buildings in a seismic zone should be designed to survive strong tremors. The design should be simple and well balanced, because simple structures are safer in a quake. The company should use light materials and strong foundations. Fires often follow an earthquake, so every building, especially tall ones, should have working firefighting equipment.
4. What precautions should one take during an earthquake (a) inside the house and (b) outdoors?
(a) Inside the house:
- Take shelter under a strong table and stay there until the shaking stops.
- Keep away from tall or heavy objects that could fall on you.
- If you are in bed, stay there and protect your head with a pillow.
- Stay inside until the shaking stops and it is safe to go out.
(b) Outdoors:
- Move to an open area away from buildings, trees, bridges and electric wires.
- If you are in a moving vehicle, drive slowly to a clear spot and stay inside until the tremors stop.
5. What main precautions can reduce the effects of an earthquake?
After an earthquake, help the people who are injured. Use the telephone only for emergencies so that the lines remain free for rescue work. If you live near the coast, watch out for a possible tsunami. Always keep disaster supplies at hand, such as a torch, a first-aid box and drinking water.
Mistakes to avoid in the exam
Students often mix up the focus and the epicentre. Remember: the focus is the source, deep underground, and the epicentre is the point on the surface directly above it.
Another common slip is treating the Richter scale like a ruler. Each step is about ten times stronger than the one before, so a magnitude 7 quake is far stronger than a magnitude 5 quake.
Do not write that earthquakes can be predicted exactly. Scientists can only tell which areas are at high risk.
Finally, in the safety question, mention both indoor and outdoor precautions, and give at least three points for each part.
Frequently asked questions
What is the difference between a foreshock and an aftershock?
A foreshock is a smaller quake that comes before the main earthquake. An aftershock is a smaller quake that follows it in the same area.
What instrument records earthquakes?
A seismometer records the seismic waves and shows how strong the earthquake was.
What is a tsunami?
A tsunami is a series of giant sea waves, usually caused by an earthquake under the ocean floor.
What is the safest thing to do indoors during an earthquake?
Take cover under a strong table or desk, protect your head, stay away from windows and heavy objects, and stay inside until the shaking stops.
Written for students following the DAV Science book for Class 8. Please cross-check answers with your own textbook and teacher.