Download Answer the following questions. 1. What are Earthquakes

GEOGRAPHY
EARTHQUAKES
STD. 8
Answer the following questions.
1. What are Earthquakes?
Earthquakes are tremors or vibrations on the Earth’s surface produced by internal forces or
man-made stresses. They result from sudden movements of the Earth’s crust along lines of
weakness.
2. State the geographical definition of an earthquake?
A shock or series of shocks or tremors due to a sudden movement of crustal rocks
generated at a point known as the hypocenter or seismic focus, within the crust or mantle.
3. Where is the intensity of tremors maximum?
The intensity of the tremors is maximum near the epicenter.
4. When is the effect of an earthquake at its greatest?
Its at its greatest if the seismic focus / hypocenter lies near the surface of the earth.
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5. Define seismology .How is an earthquake recorded?
Seismology is the science of earthquakes. The earthquake waves generated by an
earthquake are
recorded by an instrument called a seismograph or seismometer. The
passage of earthquake waves is recorded by a graph indicating both the intensity of the waves
and its time of passage.
6a. How is the intensity of earthquake waves measured?
The intensity of earthquake waves is measured by the Richter scale from 0 to 9.
6b. Whom is it named after?
It is named after the American seismologist Charles Francis Richter in 1935.
7. State the causes of Earthquakes?
The causes of earthquakes are: Folding and faulting.
Plate Tectonics.
Volcanic eruptions and volcanic earthquakes.
8. Define a Fault.
A fault consists of a fracture in a rock where in a great deal of displacement has taken
place.
9. Where do faults occur? State any one example.
Faults occur in rocks of all ages and types. If the fault is located in a zone where one plate
moves against another, the likelihood of destruction is tremendous.
egs. San Andreas fault in California.
10a. Explain how plate tectonics is a cause for an occurrence of an earthquake?
Plate tectonics is a theory of global dynamics in which a series of separate plates in the
lithosphere move or slide over each other in response to convection in the upper mantle.
10b. Why does such a phenomenon occur?
As the ocean flow is subjected to geological activity in the form of volcanic activity ,
mountain building , crustal faulting etc .Earthquakes of varied intensity occur along the
boundaries of these tectonic plates.
11. Why is the Pacific Ring of Fire ridden with active volcanoes and tectonic
earthquakes?
The Pacific Ring of Fire is located around plate boundaries of moving plates in areas
where major oceanic plates are undergoing subduction ( subduction is the process that takes
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place at convergent boundaries by which one tectonic plate moves under another tectonic
plate, sinking into the Earth’s mantle)
12. How are volcanic eruptions/ earthquakes caused? Give examples.
These are usually caused by gas explosions .The eruption and explosion of both volcanoes
and earthquakes maybe simultaneous or separated by brief intervals.
Egs : The violent explosion of Krakatau volcano ( near Java) Its impact was experienced as
far as Capehorn .
13a. Name the kind of shockwaves sent out from the focus of an earthquake.
13b.State each of it distinct properties.
Primary Waves[P]
Can pass through gases, liquids and solids.
They travel fastest and its velocity increases as it passes through the mantle.
‘ P’ waves intensity drops in the outer core and rises in the inner core due to pressure.
They are called compressional waves .
Secondary waves [S]
Travel only through solids and can’t penetrate through dense, molten matter.
‘S’ waves travel along curved paths where they meet concentric layers of increasing
density which turn the waves towards the surface.
Long Waves [L]
These are slow waves which travel which travel along the Earth’s surface.
Their movement being vertical or horizontal, vibrating horizontally at right angles or like
sea waves.
14. Where do the seismic zones exist in India?
They exist in Peninsular India, North-East India, Himalayan region and Gujarat .
15. State the consequences of earthquakes .
Loss of human life
Loss of human property – buildings, railways, roads.
Collapse of buildings.
Landslides – can change course of rivers. Fire – break out due to damage to gas lines, short
circuit, and flammable material being thrown, flash floods and tsunamis.
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16. Where do earthquakes occur?
Earthquakes occur in unstable portions of the earth’s crust. These are regions which are
subject to earth movements of various kinds.
17. Name the two belts in which the Earthquakes are frequent.
The Circum-Pacific Mountain belt. It includes mountain ranges along the Pacific Ocean in
S.America, N.America and Asia.
The Mid-World Mountain belt covers the Alpine Himalayan belt in Europe and Asia.
18. State the constructive effects of earthquakes.
a) The changing of coastlines leads to the formation of bays, gulfs etc.
b) Earthquakes may give rise to geysers and hot springs which have medicinal properties.
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