WEBVTT

00:00:04.530 --> 00:00:05.530
San Francisco.

00:00:05.870 --> 00:00:08.220
October 17, 1989.

00:00:08.660 --> 00:00:10.694
More than 60,000 fans have packed

00:00:10.734 --> 00:00:13.460
Candlestick Park for game
three of the World

00:00:13.500 --> 00:00:14.500
Series.

00:00:14.580 --> 00:00:16.430
The Giants, against the Oakland A's.

00:00:18.800 --> 00:00:20.180
It's 4 minutes past five.

00:00:20.700 --> 00:00:22.680
The TV broadcast has just started.

00:00:23.210 --> 00:00:24.670
Then the stadium starts to move.

00:00:25.310 --> 00:00:28.990
And the live picture, going out
to millions of people, cuts to static.

00:00:31.320 --> 00:00:33.840
This is the story of the
Loma Prieta earthquake.

00:00:34.450 --> 00:00:35.450
The fault behind it.

00:00:36.070 --> 00:00:37.500
The warnings that came first.

00:00:37.850 --> 00:00:40.370
And how a whole region
rebuilt around the lessons.

00:00:44.310 --> 00:00:46.830
California sits on the edge
of two tectonic plates.

00:00:47.840 --> 00:00:51.800
The Pacific plate is grinding northwest,
past the North American plate.

00:00:52.370 --> 00:00:53.520
About 2 inches a year.

00:00:55.810 --> 00:00:59.553
Most of that motion happens
along the San Andreas fault,

00:00:59.593 --> 00:01:03.240
which runs for more than 700
miles through California.

00:01:03.810 --> 00:01:07.440
And around San Francisco Bay,
it splits into a family of faults.

00:01:08.150 --> 00:01:09.150
The San Andreas.

00:01:09.660 --> 00:01:10.660
The Hayward.

00:01:10.790 --> 00:01:11.790
And the Calaveras.

00:01:13.910 --> 00:01:17.720
And just like the plates under Japan,
these faults don't slide smoothly.

00:01:18.260 --> 00:01:19.260
They lock together.

00:01:19.700 --> 00:01:22.340
The ground bends,
for decades, even centuries.

00:01:23.000 --> 00:01:24.400
And then it all lets go at once.

00:01:28.360 --> 00:01:32.600
San Francisco learned that
the hard way, on April 18, 1906.

00:01:33.310 --> 00:01:39.060
A magnitude 7.9 earthquake ripped open
the San Andreas for nearly 300 miles.

00:01:41.440 --> 00:01:46.092
Broken gas lines and water mains turned
the shaking into a firestorm that burned

00:01:46.132 --> 00:01:47.132
for days.

00:01:47.360 --> 00:01:49.740
About 28,000 buildings were destroyed.

00:01:50.330 --> 00:01:52.220
And more than 3,000 people died.

00:01:54.470 --> 00:01:55.425
North of the city,

00:01:55.465 --> 00:01:59.720
roads and fences that crossed the fault
were torn apart and shifted sideways.

00:02:00.370 --> 00:02:05.460
Near the town of Olema, one side of the
fault moved about 21 feet past the other.

00:02:07.710 --> 00:02:11.450
A scientist named Harry Fielding
Reid studied those measurements,

00:02:11.490 --> 00:02:13.700
and worked out why earthquakes happen.

00:02:14.190 --> 00:02:16.800
The rock on either side
of a locked fault bends slowly.

00:02:17.260 --> 00:02:18.260
Then it snaps back.

00:02:18.820 --> 00:02:20.240
He called it elastic rebound.

00:02:20.700 --> 00:02:23.640
And it remains how geologists
explain earthquakes.

00:02:26.090 --> 00:02:27.860
All that rubble had to go somewhere.

00:02:28.360 --> 00:02:30.950
A lot of it was dumped along
the city's northern shore.

00:02:31.450 --> 00:02:35.160
In 1915, sand was pumped on top
of it for a world's fair.

00:02:35.680 --> 00:02:38.850
And that new land became
a neighborhood: the Marina District.

00:02:42.700 --> 00:02:43.418
By the 1980s,

00:02:43.458 --> 00:02:47.968
scientists were working out which parts
of the San Andreas were most likely to

00:02:48.008 --> 00:02:49.008
break next.

00:02:49.410 --> 00:02:53.501
One of the stretches they flagged was
the southern Santa Cruz Mountains,

00:02:53.541 --> 00:02:54.860
south of San Francisco.

00:02:57.100 --> 00:02:57.685
In 1988,

00:02:57.725 --> 00:03:03.939
a national panel of earthquake experts
gave that segment a 30% chance of a large

00:03:03.979 --> 00:03:06.090
earthquake within 30 years.

00:03:08.480 --> 00:03:10.090
The earthquake came the very next year.

00:03:16.450 --> 00:03:20.352
At 4 minutes past five,
the fault broke, about 12 miles down,

00:03:20.392 --> 00:03:24.010
beneath the Santa Cruz Mountains,
near Loma Prieta peak.

00:03:24.650 --> 00:03:26.170
Magnitude 6.9.

00:03:28.420 --> 00:03:31.660
The strongest shaking lasted
only about 15 seconds.

00:03:32.350 --> 00:03:34.220
The break never even reached the surface.

00:03:34.890 --> 00:03:37.150
But the waves spread
across the whole region.

00:03:37.630 --> 00:03:40.370
And in some places,
the ground itself made them worse.

00:03:42.760 --> 00:03:45.028
In Santa Cruz, close to the epicenter,

00:03:45.068 --> 00:03:49.440
old brick buildings on the Pacific
Garden Mall crumbled into the street.

00:03:50.140 --> 00:03:51.410
Three people were killed there.

00:03:53.630 --> 00:03:58.281
About 60 miles away, in the Marina
District, the shaking turned the loose,

00:03:58.321 --> 00:04:01.300
wet ground beneath
the houses almost to liquid.

00:04:01.850 --> 00:04:02.850
Buildings sank.

00:04:03.190 --> 00:04:04.190
Some collapsed.

00:04:06.220 --> 00:04:10.770
The rubble from 1906, 83 years later,
helped bring buildings down.

00:04:11.270 --> 00:04:13.340
Fires broke out from broken gas lines.

00:04:14.000 --> 00:04:17.140
Four people died in the Marina,
and seven buildings collapsed.

00:04:19.590 --> 00:04:20.980
And it wasn't just the Marina.

00:04:21.510 --> 00:04:25.200
All around the bay, buildings
and roads stood on soft mud and fill.

00:04:25.710 --> 00:04:29.333
In soft ground, seismic waves
slow down and grow taller,

00:04:29.373 --> 00:04:31.270
like jelly shaking in a bowl.

00:04:31.780 --> 00:04:35.380
Those sites shook two to five times
harder than solid rock nearby.

00:04:37.650 --> 00:04:41.427
On the Bay Bridge, a 50 foot section
of the upper deck broke loose,

00:04:41.467 --> 00:04:43.290
and dropped onto the deck below.

00:04:43.860 --> 00:04:45.020
One person was killed.

00:04:45.560 --> 00:04:47.200
The bridge was closed for about a month.

00:04:49.620 --> 00:04:50.900
The worst came in Oakland.

00:04:51.430 --> 00:04:54.400
The Cypress Street Viaduct
was a double-decker freeway.

00:04:54.900 --> 00:04:57.251
Its upper deck came
down onto the lower one,

00:04:57.291 --> 00:04:59.410
right onto cars in the evening commute.

00:05:00.130 --> 00:05:01.550
Forty two people died there.

00:05:03.870 --> 00:05:06.080
Traffic that evening
was lighter than usual.

00:05:06.590 --> 00:05:09.660
A lot of people had gone home
early to watch the World Series.

00:05:10.280 --> 00:05:13.230
Experts believe that kept
the toll from being far worse.

00:05:15.530 --> 00:05:17.450
Rescuers worked in the wreckage for days.

00:05:18.210 --> 00:05:22.159
Four days after the quake, they
found a man still alive in his car,

00:05:22.199 --> 00:05:24.580
beneath the fallen deck, after 90 hours.

00:05:25.330 --> 00:05:27.820
He survived the rescue,
but died weeks later.

00:05:31.540 --> 00:05:35.970
In all, 63 people died,
and more than 3,700 were hurt.

00:05:36.570 --> 00:05:41.760
More than 18,000 homes were damaged, and
some 12,000 people were left without one.

00:05:42.320 --> 00:05:44.360
The bill came to about $6 billion.

00:05:46.720 --> 00:05:48.790
At Candlestick Park, the stadium held.

00:05:49.340 --> 00:05:50.810
The fans walked out safely.

00:05:51.280 --> 00:05:53.650
The World Series picked
back up 10 days later.

00:05:54.160 --> 00:05:55.480
And the A's went on to sweep it.

00:05:59.270 --> 00:06:01.230
The earthquake reshaped San Francisco.

00:06:01.730 --> 00:06:06.923
The damaged Embarcadero Freeway,
a wall of concrete along the waterfront,

00:06:06.963 --> 00:06:08.540
was torn down in 1991.

00:06:09.120 --> 00:06:11.610
And the city's waterfront
opened back up to the bay.

00:06:13.910 --> 00:06:17.140
The eastern half of the Bay
Bridge was replaced entirely.

00:06:17.670 --> 00:06:22.150
The new span, built to ride out
a major earthquake, opened in 2013.

00:06:22.720 --> 00:06:25.040
It cost about $6.5 billion.

00:06:27.300 --> 00:06:28.373
Across California,

00:06:28.413 --> 00:06:32.740
engineers went back to freeways
and bridges built before modern codes.

00:06:33.390 --> 00:06:37.130
They wrapped old concrete columns
in steel, and tied spans together.

00:06:39.370 --> 00:06:42.500
And California built an earthquake
early warning system.

00:06:43.020 --> 00:06:45.251
Sensors catch the first, faster waves,

00:06:45.291 --> 00:06:48.040
and send an alert ahead
of the strong shaking.

00:06:48.520 --> 00:06:51.578
On Loma Prieta's thirtieth
anniversary, in 2019,

00:06:51.618 --> 00:06:54.780
the state started sending
those alerts to phones.

00:06:58.720 --> 00:06:59.850
But the plates haven't stopped.

00:07:00.490 --> 00:07:05.000
Many scientists say the most dangerous
fault in the region is the Hayward fault.

00:07:05.550 --> 00:07:08.650
It runs right under the cities
on the east side of the bay.

00:07:09.210 --> 00:07:11.050
And it last broke in 1868.

00:07:13.330 --> 00:07:14.690
You can actually watch it move.

00:07:15.350 --> 00:07:18.052
Parts of the Hayward fault
creep along all the time,

00:07:18.092 --> 00:07:19.780
about a sixth of an inch a year.

00:07:20.310 --> 00:07:24.859
In downtown Hayward, old curbs that
cross the fault have been bent sideways,

00:07:24.899 --> 00:07:26.650
bit by bit, over the decades.

00:07:28.920 --> 00:07:34.159
Its last big earthquake, in 1868,
wrecked almost every building in Hayward,

00:07:34.199 --> 00:07:36.100
and killed about 30 people.

00:07:36.630 --> 00:07:40.870
Until 1906, that was the one they called
the great San Francisco earthquake.

00:07:43.480 --> 00:07:50.480
The U.S. Geological Survey puts the chance
of at least one magnitude 6.7 or larger

00:07:50.714 --> 00:07:55.830
earthquake in the Bay Area,
between 2014 and 2043, at 72%.

00:07:58.250 --> 00:08:01.715
So every October, around
the anniversary of Loma Prieta,

00:08:01.755 --> 00:08:06.450
millions of people practice three simple
steps in the Great ShakeOut drill.

00:08:06.970 --> 00:08:08.190
Drop to your hands and knees.

00:08:08.960 --> 00:08:10.540
Take cover under a sturdy table.

00:08:11.020 --> 00:08:12.910
And hold on, until the shaking stops.

00:08:15.370 --> 00:08:17.160
Loma Prieta wasn't the big one.

00:08:17.670 --> 00:08:18.670
It was a reminder.

00:08:19.090 --> 00:08:21.710
The faults beneath the bay
are still storing up strain.

00:08:22.130 --> 00:08:23.250
An inch at a time.

00:08:24.120 --> 00:08:25.740
Thanks for watching Earth Explained.

00:08:26.420 --> 00:08:31.266
Subscribe for the next story, and watch
the planet live, around the clock,

00:08:31.306 --> 00:08:32.560
on EarthWatch 24-7.