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 How tsunamis work
작성자
 이** (jean)
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Description: 

The immense swell of a tsunami can grow up to 100 feet, hitting speeds 

over 500 mph -- a treacherous combination for anyone or anything in its 

path. Alex Gendler details the causes of these towering terrors and explains

how scientists are seeking to reduce their destruction in the future.

 

Transcription:

In 479 BC, when Persian soldiers besieged the Greek city of Potidaea, the tide

retreated much farther than usual, leaving a convenient invasion route. 

But this wasn't a stroke of luck. Before they had crossed halfway, 

the water returned in a wave higher than anyone had ever seen, drowning the

attackers. The Potiidaeans believed they had been saved by the wrath of 

Poseidon. But what really saved them was likely the same phenomenon that 

has destroyed countless others: a tsunami. Although tsunamis are commonly

known as tidal waves, they're actually unrelated to the tidal activity caused by 

the gravitational forces of the Sun and Moon. In many ways, tsunamis are just 

larger versions of regular waves. They have a trough and a crest, and consist

not of moving water, but the movement of energy through water. The difference

is in where this energy comes from. For normal ocean waves, it comes from 

wind. Because this only affects the surface, the waves are limited in size and 

speed. But tsunamis are caused by energy originating underwater, from a 

volcanic eruption, a submarine landslide, or most commonly, an earthquake 

on the ocean floor caused when the tectonic plates of the Earth's surface 

slip, releasing a massive amount of energy into the water. This energy travels

up to the surface, displacing water and raising it above the normal sea level, 

but gravity pulls it back down, which makes the energy ripple outwards 

horizontally.Thus, the tsunami is born, moving at over 500 miles per hour. 

When it's far from shore, a tsunami can be barely detectable since it moves 

through the entire depth of the water. But when it reaches shallow water, 

something called wave shoaling occurs. Because there is less water to move

through, this still massive amount of energy is compressed. The wave's 

speed slows down,while its height rises to as much as 100 feet. 

The word tsunami, Japanese for "harbor wave," comes from the fact that

it only seems to appear near the coast.If the trough of a tsunami reaches 

shore first, the water will withdraw farther than normal before the wave hits, 

which can be misleadingly dangerous. A tsunami will not only drown people

near the coast, but level buildings and trees for a mile inland or more,

especially in low-lying areas. As if that weren't enough, the water then 

retreats, dragging with it the newly created debris, and anything,or anyone,

unfortunate enough to be caught in its path. The 2004 Indian Ocean

tsunami was one of the deadliest natural disasters in history, killing over 

200,000 people throughout South Asia. So how can we protect ourselves 

against this destructive force of nature? People in some areas have attempted

to stop tsunamis with sea walls, flood gates, and channels to divert the water.

But these are not always effective. In 2011, a tsunami surpassed the flood wall

protecting Japan's Fukushima Power Plant, causing a nuclear disaster in addition 

to claiming over 18,000 lives. Many scientists and policy makers are instead 

focusing on early detection, monitoring underwater pressure and seismic activity,

and establishing global communication networks for quickly distributing alerts.

When nature is too powerful to stop, the safest course is to get out of its way. 

 

Questions:

1. What's a tsunami? 

2. What causes a tsunami? 

3. Talk about the deadliest tsunami occurred in 2004.

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2020-11-11 오후 12:37:10
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