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.,. On shaky ground
The attempt to understand, measure and predict earthquakes is by no means a modern fascination.
Ancient wisdom thought earthquakes were the result of underground winds, while others blamed them
fire spirits living deep underground. It was not until the mid-1800s that Robert Mallet, an Irish engineer,
concluded that earthquakes were caused by the movement of plates beneath the earth's surface.
It is now scientifically accepted that earthquakes are the result of underground volcanic forces pushing
just beneath the surface, building up until a sudden release of pressure causes a movement of the
tectonic plates which cover the earth. These movements are known as shock waves and can be classified
into two different categories: primary and secondary.
Primary waves, also called compression waves, travel upwards through the earth and through the
earth's crust creating the epicentre of a volcano. They are the most powerful waves and the first to
register on a seismograph. Secondary waves travel along the earth's crust moving considerably slower
than compression waves as they spread the shock-wave energy from the epicentre outwards. There are
three types of earthquake: tectonic, volcanic or artificial. Tectonic earthquakes are caused by movements
of the earth's plates far below the surface which make up the crust of the earth. As the plates move over
a bed of molten lava, the friction they cause can result in massive shock waves, and, as a result, tectonic
earthquakes are the most powerful and destructive. Volcanic earthquakes are generally much smaller and
less intense and often signal the creation of a volcano. Finally, there are artificial earthquakes caused by
underground atomic testing or the building of new reservoirs, although these rarely cause much damage.
Contrary to what is portrayed in many movies, the main reason for injury or death from an earthquake
is being struck by falling objects. With an earthquake's ability to collapse buildings, bridges and any other
artificial constructions, people in heavily built-up areas such as Tokyo are particularly vulnerable; but
there are other effects. Earthquakes can trigger a number of secondary natural disasters such as
flooding, fires or landslides, and the effects are just as dangerous.
Answer the questions below. Choose NO MORE THREE WORDS from the passage for each answer.
1 Who concluded that earthquakes are the result of the movements of tectonic plates?
2 How many types of shock waves are there?
3 Which shock waves are stronger?
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