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March 11, 2011.

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It's 2:46 on a Friday afternoon
in northeastern Japan.

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Seventy kilometers off the coast, deep
beneath the seafloor, something gives way.

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For about 6 minutes, the ground shakes.

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And along the coast, people have roughly
half an hour before the sea arrives.

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This is the story of the strongest
earthquake ever measured in Japan.

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Why it happened.

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Why the warnings were 1,000 years old.

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And what Japan changed afterward.

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To understand that day,
you have to start underground.

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Japan sits right where four
of Earth's tectonic plates meet.

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The Pacific plate.

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The Philippine Sea plate.

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The North American plate.

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And the Eurasian plate.

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Each one is a slab of rock
up to 100 kilometers thick,

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slowly drifting across the planet.

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And they never stop moving.

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The Pacific plate is creeping toward
Japan at about 8 centimeters a year.

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That's roughly twice as fast
as your fingernails grow.

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Off the northeast coast, the Pacific plate
meets northern Japan at the Japan Trench,

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a gash in the seafloor
about 8 kilometers deep.

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Here's what's happening down there.

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The Pacific plate bends,
and dives underneath Japan,

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sinking slowly into the hot mantle below.

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But it doesn't slide smoothly.

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Where the plates touch,
they lock together.

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So year after year, Japan's edge
gets dragged down and squeezed,

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like a spring being wound
tighter and tighter.

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Eventually, the strain wins.

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The fault slips in seconds.

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The squeezed plate springs back,
and the seafloor lurches upward,

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lifting the whole ocean above it.

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That's why Japan gets about one in five
of the world's strong earthquakes.

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And it's why the country has
more than 100 active volcanoes.

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Japan has written records of earthquakes
going back more than 1,000 years.

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One of them describes
a night in the year 869.

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A great earthquake struck near
what's now the city of Sendai.

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And the sea rushed far
inland, across the plain.

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About 1,000 people drowned.

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Starting around 1990,

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geologists digging on that same plain
found the tsunami's calling card.

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A layer of ocean sand,
kilometers from the coast.

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And beneath it, older layers,
left by even earlier giant waves.

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So giant tsunamis had
hit this coast before.

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Again and again, centuries apart.

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Official forecasts did expect
a large earthquake here.

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But they expected one around magnitude 8.

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Not nine.

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Farther north, on the rugged Sanriku
coast, the memories were fresher.

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In 1896, a tsunami ran up as high as 38
meters, and killed about 22,000 people.

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In 1933, another one
killed about 3,000 more.

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Afterward,

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the people of a small village called
Aneyoshi carved a warning into stone.

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It reads: Do not build your
homes below this point.

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On March 11, 2011, at 2:46 in the
afternoon, the fault finally let go.

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The break started about 70
kilometers off the Oshika Peninsula,

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roughly 30 kilometers down.

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From there, it tore along the trench
for roughly 400 kilometers.

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In places, the two plates slipped
as much as 50 meters past each other.

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Japan's earthquake early
warning system did its job.

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Sensors caught the first, faster waves,

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and alerts went out before
the strongest shaking hit.

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Tokyo, about 370 kilometers away,
had about a minute of warning.

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Three minutes after the quake,

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Japan's weather agency issued
its highest tsunami warning.

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But the first estimate of the quake's
size was far too small: magnitude 7.9.

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So the first wave forecasts
were too low, 3 to 6 meters.

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Then sensors out at sea
saw the real wave coming.

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The warnings jumped
to more than 10 meters.

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The final magnitude?

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Nine point zero by Japan's count.

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Nine point one by the U.S.
Geological Survey.

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Out in the deep ocean,
a tsunami moves as fast as a jet,

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around 800 kilometers an hour.

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And it's barely a meter high.

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But as the water gets shallower
near the coast, the wave slows down.

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And all that water has
nowhere to go but up.

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And this one spread out in every
direction, across the entire Pacific.

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Near Miyako, the water ran up
a hillside to 40 meters above the sea.

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Seawalls built for earlier
tsunamis were simply overtopped.

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Ships were carried into town.

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In Miyako, a ferry came
to rest among the houses.

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On the Sendai plain, the same
ground that flooded in 869,

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the sea swept across Sendai
Airport, and kilometers inland.

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In all, more than 500 square
kilometers of Japan went underwater.

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Whole fishing towns along
the coast were destroyed.

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In Aneyoshi, the wave stopped about
90 meters downhill from the old stone.

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Every home in the village
had been built above it.

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And every one was spared.

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At the Fukushima Daiichi nuclear plant,

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a wave about 14 meters high flooded
a site designed for about six.

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The plant lost power,
and three reactors melted down.

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It was the worst nuclear
accident since Chernobyl.

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And the tsunami kept going.

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It reached Hawaii about 7 hours
later, and California about ten,

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wrecking boats in their harbors.

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About a day after the quake,
it reached Chile.

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More than 18,000 people
died or were never found.

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Most of them drowned.

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And more than 120,000
buildings were destroyed.

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The World Bank estimated
the damage at up to $235 billion.

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At the time, it was the costliest
natural disaster ever recorded.

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The earthquake even
reshaped the land itself.

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The Oshika Peninsula moved more
than 5 meters toward the sea,

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and sank by more than a meter.

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It moved so much of Earth's mass
that, by one NASA calculation,

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the planet now spins a tiny bit faster.

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Each day became shorter by about
1.8 millionths of a second.

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Far too small to notice.

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But there are stories of survival, too.

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In the city of Kamaishi, schoolchildren
had practiced one simple rule for years.

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After a big quake, run for high ground.

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Right away.

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Don't wait for anyone.

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Almost all of the city's
nearly 3,000 students survived.

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Japan rebuilt its tsunami warnings.

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Now, when a quake is too
big to measure quickly,

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the first warning doesn't
guess at a number.

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It simply says the tsunami will be huge.

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Along the Japan and Kuril trenches,

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150 seafloor stations now listen
for earthquakes and tsunamis,

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wired to land by about
5,700 kilometers of cable.

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And more than 400 kilometers
of new seawalls now line the coast.

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Some are taller than 12 meters.

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The plates haven't stopped.

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In a typical year, Japan
feels about 2,000 earthquakes.

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That's five or six, every single day.

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And you can watch them happen,
live, on our globe.

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Scientists are watching another
fault, too: the Nankai Trough,

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off southwestern Japan.

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In 2025,

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Japan's government put
the chance of a great

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earthquake there within 30 years at 60

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to 90% or more by one method,
and 20 to 50% by another.

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The stone at Aneyoshi still stands.

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And its message is the lesson
of this whole story.

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The Earth keeps its own calendar.

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And it remembers what we forget.

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Thanks for watching Earth Explained.

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