CBSE Class 8 Science • Chapter 18 • Detailed Master Notes
Chapter Overview
Unlike weather which can be predicted, earthquakes strike without warning. This chapter explores what causes the ground to suddenly shake, how we measure earthquakes, and the safety measures we must take during one.
Definition: An earthquake is a sudden shaking or trembling of the earth which lasts for a very short time. It is caused by a disturbance deep inside the earth's crust.
Earthquakes occur all the time all over the earth. They are not even noticed. Major earthquakes are less frequent. They can cause immense damage to buildings, bridges, dams and people. There can be a great loss to life and property.
Earthquakes can cause floods, landslides and tsunamis. A major tsunami occurred in the Indian Ocean on 26 December 2004.
The outermost layer of the earth is not in one piece. It is fragmented. Each fragment is called a plate (tectonic plate). These plates are in continual motion.
When plates brush past one another, or a plate goes under another due to collision, they cause disturbance in the earth's crust. It is this disturbance that shows up as an earthquake on the surface of the earth.
Although we know for sure what causes an earthquake, it continues to be impossible to predict when and where the next earthquake will happen.
AI Image Prompt: A cross-section diagram of the Earth showing the crust, mantle, outercore, and innercore. Magnified view shows two tectonic plates colliding and creating a fault line where an earthquake originates (the focus). Clean, scientific style.
Since earthquakes are caused by the movement of plates, the boundaries of the plates are the weak zones where earthquakes are more likely to occur. The weak zones are also known as seismic or fault zones.
In India, the areas most threatened are Kashmir, Western and Central Himalayas, the whole of North-East, Rann of Kutch, Rajasthan and the Indo-Gangetic Plane. Some areas of South India also fall in the danger zone.
The tremor produces waves on the surface of the earth. These are called seismic waves. The waves are recorded by an instrument called the seismograph.
Richter Scale: The power of an earthquake is expressed in terms of a magnitude on a scale called the Richter scale. Really destructive earthquakes have magnitudes higher than 7 on the Richter scale.
The Richter scale is not linear. This means that an earthquake of magnitude 6 does not have one and a half times the destructive energy of an earthquake of magnitude 4. In fact, an increase of 2 in magnitude means 1000 times more destructive energy.
Q1. Why are earthquakes more common in specific geographic areas?
Ans: Earthquakes originate from the friction and movement of the earth's tectonic plates. Geographic areas located at the boundaries of these plates are weak zones (seismic zones) and experience earthquakes more frequently.
We know that earthquakes cannot be predicted. Hence, it is important that we take necessary precautions to protect ourselves.
If you are at home:
If you are outdoors: