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In June 1929, a strong earthquake
shook the South Island of New Zealand.

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In Copenhagen,

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a seismologist named Inge Lehmann studied
the records it left at stations around

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the world.

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Faint waves had reached stations
in Russia, in a zone where,

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by every theory of the time, the Earth's
core should have kept them out.

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She spent years on that puzzle.

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In 1936, she published her answer.

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Deep inside the liquid core,
there must be a second, inner core,

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where waves travel faster.

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That inner core had bent those
waves into the forbidden zone.

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No one has ever been there.

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An earthquake showed it to her, and by
the end of this lecture, you'll see how.

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In this lecture, we'll follow
an earthquake from start to finish.

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We'll see how a fault loads and breaks,
and how scientists read its waves.

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Then we'll see how one
earthquake leads to the next,

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and what the waves reveal
deep inside the planet.

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Earth's outer shell is broken into plates
that drift a few centimeters a year.

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Where two meet, the boundary
between them is a fault,

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and most of the time the two
sides are stuck together,

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even as the plates keep moving.

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The rock near a stuck fault slowly bends,
like a springboard under a growing weight.

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In 1910,

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a geologist named Harry
Fielding Reid showed

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this from land surveys made before and

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after the great San
Francisco earthquake of 1906.

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He called it elastic rebound: decades
of slow bending, then a sudden snap back.

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That's the earthquake cycle.

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The fault loads for decades or centuries,
breaks in seconds or minutes,

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then locks and begins to load again.

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Every large earthquake is
one turn of that cycle.

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Scientists can watch the loading happen.

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GPS receivers anchored to the ground track
their own position to within a fraction

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of an inch, a few millimeters.

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Far from a locked fault, stations
ride along with their plates,

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but the ones close to it lag behind.

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The ground is bending.

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How tightly a fault is stuck
is called its coupling.

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A fully coupled fault stores almost all
of the plates' motion, year after year.

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Off northeastern Japan,

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the great earthquake of March 2011 showed
the plates there had been locked right

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out to the trench, where
the Pacific plate dives under Japan.

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When that boundary let go, the seafloor
shifted, in places by about 50 meters,

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more than 150 feet.

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Not every fault stays
perfectly still while it loads.

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Beneath Washington State
and British Columbia,

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the deep part of the plate boundary
slips an inch or so, a few centimeters,

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over a couple of weeks,
about every 14 months.

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Nobody feels it.

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GPS revealed these slow slip
events around the year 2000.

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Scientists watch them closely,

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because each one adds load
to the locked part of the fault above.

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The cycle leaves a record in the ground.

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Geologists dig trenches across faults and
read the layers of soil like pages in a

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book.

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Where a layer is torn and offset,
an earthquake broke it.

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Dating those layers shows how often a
fault has broken over thousands of years.

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An earthquake doesn't break
the whole fault at once.

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It starts at one point, the hypocenter,
where the stress first beats the friction.

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From there, the break, or rupture,
races outward across the fault,

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typically at 2 to 3
kilometers every second.

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A few ruptures even outrun some
of the waves they send out.

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The larger the earthquake,
the longer the break keeps going.

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A moderate one is over in a few seconds.

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In the great Sumatra earthquake of 2004,

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the rupture kept tearing
northward for about eight minutes,

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along more than 1,300 kilometers of fault.

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How much fault breaks, and how far
it slips, sets an earthquake's size.

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Scientists multiply three things:
the area of fault that broke,

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how far its two sides slipped,
and how stiff the rock is.

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The result, the seismic moment, is what
the modern magnitude scale is built on.

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Each whole step on the scale means
about 32 times more energy released.

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Most earthquakes start within
about 70 kilometers of the surface.

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But where one plate dives beneath another,

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earthquakes follow the sinking plate
down, as deep as about 700 kilometers.

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In 1994,

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one of the largest deep earthquakes ever
recorded struck about 630 kilometers

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beneath Bolivia.

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It was felt as far away as Canada.

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The break sends out waves.

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P waves squeeze and stretch the rock,
like a push along a spring,

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and they travel fastest.

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S waves shake it from side
to side, and arrive later.

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Where they reach the surface, surface
waves roll outward along the ground.

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They're the slowest, and they
cause much of the damage.

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The gap between the P and S waves tells
a station how far away the earthquake is.

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For a nearby one, each second
of that gap means about 8 kilometers.

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Three stations are enough
to pin down the spot,

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and networks of hundreds
do it within seconds.

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Faults break in three main ways.

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In a normal fault, the crust
stretches and one block drops.

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In a reverse fault, it's squeezed
and one block rides up over the other.

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In a strike-slip fault, the two
sides slide past each other sideways.

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The waves themselves reveal
which way a fault broke.

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At some stations, the first motion of
the ground is a push; at others, a pull.

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Mapped around the globe,

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those pushes and pulls draw a pattern
seismologists call a beach ball.

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In February 2023, a magnitude
7.8 earthquake struck Türkiye.

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Its beach ball showed a strike-slip break,

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and its aftershocks traced that
break along the East Anatolian Fault.

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The size takes longer to pin down.

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In 2011, Japan's first tsunami warning,

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about three minutes
after the shaking began,

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was based on a magnitude of 7.9.

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The fast method maxes out
for the very largest earthquakes,

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and the estimate was
raised, in steps, to 9.

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The quick numbers serve the warnings;
the final one comes from the whole record.

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That final number is the moment magnitude,

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worked out from the fault's area and slip.

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Unlike older scales, it doesn't stop
growing for the largest earthquakes.

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So a great earthquake can show
several magnitudes in its first hour,

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as quick estimates give way
to the moment magnitude.

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Radar satellites can map
a whole earthquake at once.

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By comparing images
taken before and after,

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they draw bands of color where the ground
moved, each band about 3 centimeters,

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about an inch, of motion
toward the satellite.

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The pattern traces the fault that broke,
even where no one can reach it.

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Earthquakes follow rules of their own.

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For each step down in magnitude,
there are about ten times as many.

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Worldwide,

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there are more than 1,000 earthquakes
of magnitude 5 or more in a typical year,

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but only about one of magnitude 8 or more.

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After a large earthquake come aftershocks,
and they fade in a regular way,

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a pattern the seismologist Fusakichi
Omori found in Japan in the 1890s.

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Ten days after the main shock,

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they come about a tenth
as often as on the first day.

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A hundred days after, about a hundredth.

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The largest aftershock is, on average,

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about 1.2 smaller in magnitude
than the main earthquake.

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So after a magnitude 7,

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a strong aftershock near
magnitude 6 is no surprise,

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and damaged buildings should
stay empty until they're checked.

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An earthquake doesn't only release stress.

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It moves it.

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Where the broken fault ends, the stress
on the rock just beyond it rises,

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and the next segment is
pushed closer to breaking.

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The North Anatolian Fault
in Türkiye shows it clearly.

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Starting in 1939,

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a series of large earthquakes marched
along it from east to west over 60 years.

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A study of the ten largest found that nine
had been pushed closer to failure by the

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ones before them.

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In August 1999, the march
reached Izmit, close to Istanbul,

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and raised the odds of a strong
earthquake beneath Istanbul.

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People can unlock faults too.

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In Oklahoma,

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oil and gas operations pumped huge volumes
of salty wastewater deep underground,

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raising the pressure inside
old faults until they slipped.

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Earthquakes of magnitude 3 or more jumped
from about two a year to more than 900 in

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2015.

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Oklahoma's largest, a magnitude
5.8 near Pawnee in 2016,

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was felt across several states.

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All of this goes into forecasting,
which gives odds, never a date.

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The small town of Parkfield, California,
sits on the San Andreas fault.

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Between 1857 and 1966,
six moderate earthquakes struck there,

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roughly one every 22 years.

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In 1985, scientists forecast
the next one before 1993,

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and filled the area
with instruments to catch it.

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It came in 2004, 11 years after
the forecast window closed.

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Faults are regular enough to forecast, but
not regular enough to keep a timetable.

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So modern forecasts combine the cycle,
the record in the trenches,

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and GPS measurements of the loading,

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into odds for each fault
over the coming decades.

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After Oklahoma's surge,

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the U.S. Geological Survey began issuing
one-year forecasts for earthquakes caused

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by people, too.

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The same waves that shake the ground
travel through the whole planet.

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Deeper rock is under enormous pressure,

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and waves generally
speed up as they go down.

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So their paths curve gently
back toward the surface,

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like the shimmer
of a mirage over a hot road.

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In 1909,

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a Croatian scientist named Andrija

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Mohorovičić noticed
that waves from a nearby

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earthquake arrived in two sets.

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Some had taken a faster
path through deeper rock.

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He had found the bottom of the crust, a
boundary still named after him: the Moho.

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Far deeper, the waves reveal the core.

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More than a quarter of the way around the
globe from an earthquake, S waves vanish,

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and they never return.

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P waves fade out too,
then reappear on the far side.

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S waves can't pass through liquid.

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The outer core is molten.

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And that brings us back to Inge Lehmann.

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The faint waves in Russia
had crossed the liquid core,

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and been bent by something
faster inside it.

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It was an inner core, with a radius
of about 1,200 kilometers.

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Later work showed it's
solid iron and nickel,

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about as hot as the surface of the Sun.

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Modern networks map the deep
Earth in three dimensions,

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from millions of these records.

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No one has drilled deeper
than about 12 kilometers.

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Most of what we know
about everything below

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comes from timing earthquake waves at

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00:11:30,090 --> 00:11:32,210
stations around the world.

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So, three things to remember.

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First, a fault loads for decades,
breaks in minutes, and locks again:

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the earthquake cycle.

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00:11:42,400 --> 00:11:45,060
Second, the waves reveal
where a fault broke,

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how it moved and how big the break was,

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and each earthquake passes
stress on to the next.

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And third, those same waves have
mapped the inside of the planet,

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down to its solid core.

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Every earthquake is a message
from inside the planet.

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Inge Lehmann read one from 1929,
and found a world no one will ever visit.

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Thanks for watching Earth Explained.

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Subscribe for the next lecture, and watch
the planet live, around the clock,

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