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San Francisco.

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October 17, 1989.

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More than 60,000 fans have packed

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Candlestick Park for game
three of the World

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Series.

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The Giants, against the Oakland A's.

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It's 4 minutes past five.

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The TV broadcast has just started.

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Then the stadium starts to move.

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And the live picture, going out
to millions of people, cuts to static.

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This is the story of the
Loma Prieta earthquake.

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The fault behind it.

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The warnings that came first.

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And how a whole region
rebuilt around the lessons.

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California sits on the edge
of two tectonic plates.

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The Pacific plate is grinding northwest,
past the North American plate.

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About 2 inches a year.

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Most of that motion happens
along the San Andreas fault,

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which runs for more than 700
miles through California.

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And around San Francisco Bay,
it splits into a family of faults.

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The San Andreas.

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The Hayward.

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And the Calaveras.

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And just like the plates under Japan,
these faults don't slide smoothly.

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They lock together.

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The ground bends,
for decades, even centuries.

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And then it all lets go at once.

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San Francisco learned that
the hard way, on April 18, 1906.

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A magnitude 7.9 earthquake ripped open
the San Andreas for nearly 300 miles.

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Broken gas lines and water mains turned
the shaking into a firestorm that burned

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for days.

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About 28,000 buildings were destroyed.

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And more than 3,000 people died.

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North of the city,

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roads and fences that crossed the fault
were torn apart and shifted sideways.

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Near the town of Olema, one side of the
fault moved about 21 feet past the other.

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A scientist named Harry Fielding
Reid studied those measurements,

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and worked out why earthquakes happen.

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The rock on either side
of a locked fault bends slowly.

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Then it snaps back.

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He called it elastic rebound.

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And it remains how geologists
explain earthquakes.

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All that rubble had to go somewhere.

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A lot of it was dumped along
the city's northern shore.

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In 1915, sand was pumped on top
of it for a world's fair.

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And that new land became
a neighborhood: the Marina District.

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By the 1980s,

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scientists were working out which parts
of the San Andreas were most likely to

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break next.

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One of the stretches they flagged was
the southern Santa Cruz Mountains,

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south of San Francisco.

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In 1988,

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a national panel of earthquake experts
gave that segment a 30% chance of a large

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earthquake within 30 years.

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The earthquake came the very next year.

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At 4 minutes past five,
the fault broke, about 12 miles down,

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beneath the Santa Cruz Mountains,
near Loma Prieta peak.

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Magnitude 6.9.

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The strongest shaking lasted
only about 15 seconds.

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The break never even reached the surface.

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But the waves spread
across the whole region.

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And in some places,
the ground itself made them worse.

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In Santa Cruz, close to the epicenter,

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old brick buildings on the Pacific
Garden Mall crumbled into the street.

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Three people were killed there.

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About 60 miles away, in the Marina
District, the shaking turned the loose,

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wet ground beneath
the houses almost to liquid.

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Buildings sank.

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Some collapsed.

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The rubble from 1906, 83 years later,
helped bring buildings down.

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Fires broke out from broken gas lines.

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Four people died in the Marina,
and seven buildings collapsed.

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And it wasn't just the Marina.

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All around the bay, buildings
and roads stood on soft mud and fill.

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In soft ground, seismic waves
slow down and grow taller,

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like jelly shaking in a bowl.

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Those sites shook two to five times
harder than solid rock nearby.

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On the Bay Bridge, a 50 foot section
of the upper deck broke loose,

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and dropped onto the deck below.

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One person was killed.

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The bridge was closed for about a month.

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The worst came in Oakland.

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The Cypress Street Viaduct
was a double-decker freeway.

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Its upper deck came
down onto the lower one,

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right onto cars in the evening commute.

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Forty two people died there.

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Traffic that evening
was lighter than usual.

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A lot of people had gone home
early to watch the World Series.

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Experts believe that kept
the toll from being far worse.

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Rescuers worked in the wreckage for days.

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Four days after the quake, they
found a man still alive in his car,

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beneath the fallen deck, after 90 hours.

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He survived the rescue,
but died weeks later.

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In all, 63 people died,
and more than 3,700 were hurt.

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More than 18,000 homes were damaged, and
some 12,000 people were left without one.

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The bill came to about $6 billion.

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At Candlestick Park, the stadium held.

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The fans walked out safely.

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The World Series picked
back up 10 days later.

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And the A's went on to sweep it.

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The earthquake reshaped San Francisco.

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The damaged Embarcadero Freeway,
a wall of concrete along the waterfront,

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was torn down in 1991.

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And the city's waterfront
opened back up to the bay.

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The eastern half of the Bay
Bridge was replaced entirely.

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The new span, built to ride out
a major earthquake, opened in 2013.

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It cost about $6.5 billion.

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Across California,

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engineers went back to freeways
and bridges built before modern codes.

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They wrapped old concrete columns
in steel, and tied spans together.

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And California built an earthquake
early warning system.

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Sensors catch the first, faster waves,

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and send an alert ahead
of the strong shaking.

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On Loma Prieta's thirtieth
anniversary, in 2019,

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the state started sending
those alerts to phones.

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But the plates haven't stopped.

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Many scientists say the most dangerous
fault in the region is the Hayward fault.

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It runs right under the cities
on the east side of the bay.

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And it last broke in 1868.

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You can actually watch it move.

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Parts of the Hayward fault
creep along all the time,

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about a sixth of an inch a year.

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In downtown Hayward, old curbs that
cross the fault have been bent sideways,

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bit by bit, over the decades.

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Its last big earthquake, in 1868,
wrecked almost every building in Hayward,

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and killed about 30 people.

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Until 1906, that was the one they called
the great San Francisco earthquake.

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The U.S. Geological Survey puts the chance
of at least one magnitude 6.7 or larger

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earthquake in the Bay Area,
between 2014 and 2043, at 72%.

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So every October, around
the anniversary of Loma Prieta,

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millions of people practice three simple
steps in the Great ShakeOut drill.

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Drop to your hands and knees.

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Take cover under a sturdy table.

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And hold on, until the shaking stops.

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Loma Prieta wasn't the big one.

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It was a reminder.

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The faults beneath the bay
are still storing up strain.

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An inch at a time.

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Thanks for watching Earth Explained.

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