Earthquakes
ICSE Class 9 · Geography · Chapter 8
What this chapter covers
- Meaning of an earthquake
- Seismic waves
- Causes of earthquakes
- Measurement of earthquakes
- Destructive effects of earthquakes
- Constructive effects of earthquakes
- Earthquake zones of the world
Key points
- An earthquake is a sudden shaking of the earth's surface caused by the release of energy stored in rocks; the energy travels as seismic waves.
- The focus (hypocentre) is where the rocks break inside the earth; the epicentre is the point on the surface directly above it, where shaking is usually strongest.
- Shallow-focus earthquakes (0 to 70 km) are the commonest and usually the most destructive.
- P-waves are fastest and pass through solids, liquids and gases; S-waves are slower and pass only through solids; surface (L) waves are slowest and do the most damage.
- Most earthquakes are tectonic: stress builds up where plates meet until rocks break suddenly along a fault.
- Other causes are volcanic activity, collapse of caves and landslides, and human activities such as filling reservoirs (Koyna, 1967), mining and nuclear tests.
- A seismograph records ground vibrations; its record is a seismogram.
- Magnitude (energy at the focus) is measured on the Richter scale, where each step of 1 means waves 10 times larger and about 32 times more energy.
- Intensity (shaking and damage at a place) is measured on the Modified Mercalli scale from I to XII.
- Destructive effects: collapse of buildings, loss of life, damage to transport and services, fires, landslides, tsunamis, floods, liquefaction and economic loss.
- Constructive effects: new landforms (Allah Bund, raised coasts), lakes (Reelfoot Lake) and springs, exposure of minerals, knowledge of the earth's interior and lessons for safer building.
- The main earthquake belts are the Circum-Pacific Belt (Ring of Fire, the most active), the Mid-Continental or Alpine-Himalayan Belt, and the Mid-Atlantic Ridge Belt.
- In India the Himalayas, the North-East, Kutch and the Andaman and Nicobar Islands face the highest risk.
Questions answered
Define an earthquake.
An earthquake is a sudden shaking or trembling of the earth's surface caused by the release of energy stored in the rocks of the crust or upper mantle, which travels outwards as waves.
Distinguish between the focus and the epicentre of an earthquake.
The focus (hypocentre) is the point inside the earth where the rocks first break or slip and the energy is released. The epicentre is the point on the earth's surface vertically above the focus.
Why is the damage usually greatest near the epicentre?
The epicentre is the point on the surface closest to the focus, so the seismic waves reach it first and after travelling the shortest distance. They have lost the least energy there, so the shaking is strongest.
Why is a shallow-focus earthquake usually more destructive than a deep-focus earthquake of the same magnitude?
Waves from a shallow focus travel only a short distance to the surface and lose little energy on the way, so the surface shakes more strongly.
Name the three kinds of seismic waves in the order in which they reach a seismograph. Which kind does the most damage?
Primary (P) waves, secondary (S) waves and surface (L) waves. Surface waves do the most damage because they make the ground roll and sway with the largest movements.
Give a reason why S-waves are not recorded on the side of the earth opposite an earthquake.
S-waves can travel only through solids. The outer core is liquid, so it stops them, leaving a region on the far side of the earth where they do not arrive.
Explain how the movement of plates causes earthquakes.
The edges of neighbouring plates lock together by friction while the plates keep moving, so stress builds up in the rocks. When the stress exceeds the strength of the rocks, they break or slip suddenly along a fault and spring back, releasing the stored energy as seismic waves.
Name two human activities that can cause earthquakes, with an Indian example of one.
Filling large reservoirs behind high dams, and deep mining (others: underground nuclear tests, pumping fluids into or out of the ground). The Koyna earthquake of 1967 in Maharashtra is linked to the Koyna reservoir.
Past board exam questions
ICSE 2019 sample paper · 2 marks
Define the following terms: (i) Epicentre (ii) Focus
Show answer
(i) Epicentre: the point on the Earth's surface vertically above the focus, where the earthquake waves reach first and the shaking is the strongest.
(ii) Focus: the point inside the Earth where an earthquake originates, i.e. where the energy is first released (also called the hypocentre).