ICSE Class 6 Geography Chapter 8 Earthquakes

Chapter Eight

Earthquakes

Syllabus

Earthquakes: Meaning, causes and measurement.

Effects: destructive and constructive. Earthquake zones of the World.

An earthquake is defined as a tremor below the surface of the earth which causes shaking of the crust. These devastating tremors are caused by the stresses that break the earth's crust.

Causes of Earthquakes

Earthquakes are caused by the strain in the earth's crust which in turn is a result of a number of factors. When the earth's crust is unable to accommodate itself to the strain, it results in a sudden release of tremendous energy in the form of a sudden violent shock.

A series of shocks that result from sudden earth movement, e.g. along a fault, are known as earthquake (seismic) waves. The waves spread out from the seismic focus of the earthquake and can cause widespread destruction. In areas of the world that lie along the edges of lithospheric plates, for example, down the western edge of the Pacific Ocean, earthquakes are common. The severity of earthquakes is measured on the Richter scale. The main factors that cause earthquakes are the following:

Volcanoes

Volcanic eruptions are one of the main causes of earthquakes. During volcanic eruptions, hot gases are pushed upward and they push violently the earth's crust surface upward causing earthquakes. Volcanic activities and seismic occurrences (earthquakes) are interrelated: an earthquake follows a volcanic eruption and a volcanic eruption follows an earthquake. The violent eruption of Krakatoa volcano between the islands of Sumatra and Java caused severe earthquake around a vast area.

Plate Tectonics

According to the plate tectonic theory, the crust of the earth is made of a series of moveable plates. These plates move and slide over each other. Most earthquakes occur on account of plate movements. When two plates slip past each other or collide against each other, their edges produce faults along the lines of weakness.

The tectonic earthquakes are most frequent. These earthquakes are immensely powerful. They occur in high risk regions of the Pacific Ring of South East Asia (Philippines and Indonesia) and northern India, along plate boundaries.

In India, a severe earthquake occurred at Bhuj in Gujarat in 2001 due to lowering of the Indian plate below the Asiatic plate.

Folding and Faulting

Internal horizontal and vertical movements due to compressional and tensional forces cause folding and faulting in the earth's crust. This causes displacement of rocks in the crust. Such an imbalance causes earthquakes. For example, the San Andreas Fault of California (USA) gave rise to a major earthquake that measured 8.3 on the Richter scale and broke the earth's surface along a 402 km length.

Man-made Causes

Human activity like construction of huge dams, nuclear explosions, blasting of rocks, mining, etc., near fault lines are responsible for earthquakes. Koyna earthquake in Maharashtra was caused by construction of the Koyna dam in a seismically active region. Chemical wastes dumped into the ground and underground explosions in the USA caused several earthquakes in Colorado in the 1960s.

Anatomy of an Earthquake

The tension caused by the movement of the earth's plates builds up until it is released in the form of powerful vibrations. These vibrations travel in three types of waves: P-Primary waves, S-Secondary or Shear waves and L-Long waves (or Surface waves).

P (primary or push) waves

P waves are the first earthquake waves to be recorded on a seismogram of an earthquake. They are compressional, i.e., the particles vibrate in the direction of movement of the wave, similar to a sound wave. They can pass through solids, liquids and gases.

S (secondary or shear) waves

S waves are recorded after the P waves. They are distortional waves in which the particles vibrate at right angles to the direction of movement of the wave. S-waves travel through the earth's interior but cannot be transmitted by liquids.

L (surface or long) waves

L waves travel along the surface of the earth and are recorded after the P and S waves. Two types of L waves are identified: Love waves and Rayleigh waves.

The P-waves and S-waves travel through the interior of the earth. The surface waves (or L-waves) are the last to arrive. The S-Waves are slower and do not pass through liquids. It is these waves that make the inhabitants of the earth feel the ground motion. The point of origin of these waves (called Seismic waves) is called Seismic focus. The point on the earth's surface directly above the seismic focus is called epicentre. Most of the earthquakes have their seismic focus between 8 to 35 km in depth.

Measurements

Seismograph, the Richter Scale and the Mercalli scale are used for measuring the direction and intensity of earthquakes.

Seismograph

The direction of movement of waves and their passage at a particular point is recorded by an instrument called Seismograph. It has a pen attached to it. The pen vibrates with the earthquake waves, recording their movements on a travelling strip of paper. It also calculates the difference in the arrival of P and S waves. This interval is then used to calculate the distance between the recording station and the epicentre.

Richter Scale and the Mercalli Scale

Both these instruments measure the intensity (or the amount of energy released during an earthquake). The Richter scale measures the power of an earthquake on a scale of 1 to 9. It measures the absolute intensity with mathematical precision. The Mercalli scale grades the earthquakes on the basis of observed effects on a 12-point scale. The Richter scale is most commonly used. Each point upward from 1 represents 10-fold increase in the amplitude of shock waves. Thus an earthquake wave of magnitude 6 will be 10 times stronger than magnitude 5.

The Assam earthquake in India of 1897 was of 8.7 magnitude. The Gujarat earthquake in 2001 had the magnitude of only 7.7 on the Richter Scale yet it caused the greatest damage. In Nepal, an earthquake measuring 7.8 on the Richter scale occurred on April 25, 2015, which killed over 8,000 people and injured more than 21,000. It made many homeless, with entire villages flattened across many districts of Nepal. It also destroyed centuries-old buildings including UNESCO World Heritage sites in the Kathmandu Valley. Continued

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