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At 12 minutes past five
on a spring morning in 1906,

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San Francisco was still asleep.

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Then, without warning,
the ground began to roll.

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For about a minute, the whole city shook.

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Buildings fell, gas lines broke,
and fires burned for three days.

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What happened beneath San Francisco that
morning happens all over the planet.

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Somewhere on Earth,

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the ground shakes hard enough for people
to feel it about every five minutes.

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Most of those earthquakes are small.

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A few change history.

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In this lecture, we'll start from the
very beginning, with four questions.

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What makes the ground shake?

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Why do earthquakes strike where they do?

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How do we measure them?

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And what decides how much harm they do?

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To understand that morning,
start with the ground itself.

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Earth's outer shell isn't one solid piece.

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It's broken into giant plates that
fit together like a cracked eggshell.

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The plates are always moving, but slowly,
just a few centimeters a year.

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That's about as fast
as your fingernails grow.

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Where two plates meet,
their edges don't slide smoothly.

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They catch on each other and lock,
while the plates keep moving.

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The rock along the edge slowly bends,
like a ruler you push at one end.

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Year after year, it stores up energy.

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Eventually, the rock can't hold any more.

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It breaks along a crack called a fault,
and the two sides lurch past each other.

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All that stored energy rushes
out as waves through the ground.

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That sudden slip is an earthquake.

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That's what happened in 1906.

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The San Andreas fault slipped along
nearly 300 miles, about 480 kilometers.

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Roads and fences that
crossed it were torn apart.

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In places, the two sides moved
as much as 20 feet, about 6 meters.

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A geologist named Harry Fielding Reid

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compared land surveys
made before and after

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the earthquake.

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They showed that the ground had
been slowly bending for decades,

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and then snapped back,
like a spring let go.

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He called it elastic rebound.

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It's still how scientists
explain earthquakes.

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Put every earthquake on a map,
and a pattern jumps out.

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They line up along
the edges of the plates.

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The busiest edge circles
the Pacific Ocean: the Ring of Fire,

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which has a lecture of its own.

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Plates meet in three ways, and each
one shakes the ground in its own way.

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Where plates pull apart, as they do in
Iceland, the crust stretches and cracks,

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and blocks of rock drop down.

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These earthquakes are usually
shallow and fairly small.

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Where plates grind past each other
sideways, the ground lurches side to side.

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Most of these faults lie under the sea.

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But on land, like the San Andreas,
they can be very dangerous,

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because they run close to cities.

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Where plates push together,
one usually slides beneath the other,

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at what's called a subduction zone.

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The two can lock together
over a vast area.

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That's where the giants happen.

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Every earthquake of magnitude 9 or more
ever measured struck a subduction zone.

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Where two continents push
together, neither one sinks.

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Instead, the crust crumples and rises
into mountains, like the Himalaya.

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Some of the deadliest earthquakes
on land have struck along this belt,

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from Pakistan to China.

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A few earthquakes strike
far from any plate edge.

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In the winter of 1811 and 1812,

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a series of powerful earthquakes
shook the middle of the United States,

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around New Madrid, Missouri.

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They're rare, but they show that
plate edges aren't the whole story.

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Every earthquake starts at one spot
underground, where the fault first breaks,

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often less than 20 kilometers down.

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The break then races along the fault.

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The spot on the surface right
above the start is the epicenter:

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the dot on an earthquake map.

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Waves spread out in every direction,
like ripples in a pond.

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Instruments called seismometers
record those waves,

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and they always arrive in the same order.

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First come P waves,
which push and pull the rock.

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They're the fastest, and usually gentle.

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Next, S waves shake the rock
up and down or side to side.

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Last come surface waves,
rolling along the ground.

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The S waves and surface waves
bring the strongest shaking.

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To compare earthquakes,
scientists give each one a magnitude:

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a single number for how
much energy it released.

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Each step up the scale means
about 32 times more energy.

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So a magnitude 7 isn't
a little bigger than a five.

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It releases about 1,000 times more energy.

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You may have heard of the Richter scale,

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the first magnitude scale,
created in 1935.

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Scientists now use a newer scale,
called moment magnitude,

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that works even for the very
largest earthquakes.

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Magnitude describes the earthquake.

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Intensity describes the shaking: how hard
the ground moved at one particular place.

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One earthquake has one magnitude,
but many levels of shaking,

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strongest near the fault
and weaker farther away.

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Intensity depends on more than distance.

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Of two earthquakes the same size, the
shallower one shakes the surface harder.

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Soft ground, like loose sand or filled
land, can make the shaking far worse.

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That's what happened in San
Francisco's Marina District in 1989,

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the story told in our lecture
on the World Series earthquake.

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Engineers have a saying: earthquakes
don't kill people, buildings do.

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In January 2010, a magnitude 7 earthquake
struck just outside Haiti's capital,

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Port-au-Prince.

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Many buildings were poorly built concrete
and brick, with too little steel,

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and they collapsed.

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More than 100,000 people died.

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About six weeks later, a far
stronger earthquake struck Chile.

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At magnitude 8.8, it released
about 500 times as much energy.

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The shaking was strong,
but Chile had strict building codes,

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learned from many deadly earthquakes,
and most buildings stood.

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About 500 people died.

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Warning systems can help too.

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Because P waves arrive first,

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sensors near a fault can catch them and
send an alert before the strong shaking

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arrives.

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Japan and the U.S. West Coast
both have systems like this.

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The warning may last only seconds,
but that's enough to slow trains,

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open fire station doors,
and let people take cover.

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Right next to the fault,
there may be no warning at all.

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Be ready before it starts: fasten
tall, heavy furniture to the wall.

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When the shaking starts,
drop to your hands and knees.

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Cover your head and neck, under
a sturdy table if one is close.

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Hold on until the shaking stops.

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Don't run outside, and skip
the doorway: in a modern home,

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it's no stronger than anywhere else.

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Strong shaking can bring other
dangers: landslides on steep hills,

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fires from broken gas lines,
and near the coast, a tsunami.

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If you're near the coast
and the shaking is strong,

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or lasts more than about 20 seconds,
head to high ground or inland right away.

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Don't wait for an official warning.

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The earthquake itself
may be your only warning.

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After a big earthquake come aftershocks:

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smaller quakes nearby that can
go on for weeks, even years.

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No one has ever reliably
predicted the time,

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place and size of a major earthquake.

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Instead, scientists forecast the odds.

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California's official
forecast calls a strong

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earthquake somewhere in the state all

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but certain within 30 years.

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So, three things to remember.

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First, an earthquake is
a sudden slip on a fault,

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releasing strain that built
up slowly as the plates moved.

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Second, most earthquakes
strike where plates meet,

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and the largest where one
plate slides beneath another.

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And third, magnitude
measures the earthquake,

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intensity measures the shaking,

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and buildings decide
how much harm it does.

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The ground beneath you is moving
a few centimeters every year.

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Earthquakes are the moments
when it catches up.

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In Earthquakes two oh one,
we'll go deeper: inside the fault,

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down through the planet's core,
and into the science of forecasting.

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Thanks for watching Earth Explained.

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