Inside Planet Earth Page #7

like this site usually has.

But the interesting thing is,

it's all limited to

a very short period of time.

If you look here,

you can see the fault has been

locked for most of the year.

But suddenly, in the middle

of September, it takes off.

And it slips about 5mm

in the space of a couple of hours.

And it continues to slip

in the next few days.

In other words,

a whole year's budget of slip

occurs within the space

of a few days.

That might not seem much--

5mm--

but it's affecting

a long section of the fault,

maybe 5 miles long, involving

millions of tons of rock.

Now, it really

doesn't matter here.

This is a nice little field.

There aren't any people.

But further north,

this creep process

and the things that are

happening beneath the earth

are far more sinister.

San Francisco is home

to 2.5 million people

and lies right on the Hayward fault.

In its 200 years' history,

it's been hit by 3 killer quakes.

Scientists know

the slippage on the fault

has begun to slow down--

a warning sign.

But when will it strike?

In 1989, 67 people died

in an earthquake

at Loma Prieta,

part of San Francisco.

No one had foreseen it coming.

Earthquakes are

an inevitable consequence

of the movement of plates.

We cannot stop plate motions.

And therefore

we cannot stop earthquakes.

But we can begin

to live with them.

And the first thing

we must learn to do

is to build our cities better.

As the world population grows,

so does the danger.

There are now more than

100 cities around the world

with population of over 2 million,

and more than half of those

are on plate boundaries

or places where earthquakes

have already struck.

Los Angeles is the most vulnerable

of all the places on

the West Coast fault line.

This stretch of road shows why.

Not only is it on

the San Andreas,

but here the fault has been

forced around a bend.

Where this happens,

the rocks get compressed

with enormous force.

They cannot go sideways,

so they go up.

The movement curls the rocks,

bends, folds, and crunches them,

faulting and distorting

the land.

Many faults never surface,

although scientists

know they're there.

Known as blind faults,

they're probably

the most dangerous of all.

You come down the hills here

until you hit the plains--

the flat area where

11 million people live--

and across there, far behind,

the tall buildings

of downtown L.A.--

We think there's a fault

right there,

right underneath

the center of L.A.

At 4:
24 in the morning

on the 17th of January 1994,

the suburb of Northridge

was shaken to its foundations.

A blind fault had fractured

11 miles down,

causing some of the most intense

ground-shaking ever recorded.

Ground acceleration

exceeded gravity,

throwing buildings, furniture,

and people into the air.

61 died.

9,000 were injured.

Buildings, roads,

and bridges were destroyed.

Damages ran to $20 billion.

And the aftershocks went on

for days.

When it goes, like Northridge did,

seismic waves

are spread all over the basin

as big ripples

shaking all the buildings.

It's exaggerated

where the sediments

are concentrated

in the basin here

because the basin's

completely flat,

and it's flat because

it's washed in there

from the mountains by water.

So we have saturated sediments.

When the sand-and-water

combination is shaken together,

it turns liquid for 5 minutes

after the shocks,

causing tall buildings

to destabilize and fall.

It's called liquefaction

and presents an enormous

and unpredictable danger.

And there is another geological

trap waiting to spring.

The San Gabriel Mountains

stretch from

the San Andreas fault

down to the Los Angeles basin.

As the earthquakes crack

and shudder along the faults,

they are being

inched towards the city.

Here the fault is pink

and the mountains in yellow.

This is how

they have swung so far.

In some distant

geological future,

they will cover

the city completely.

What does the geological future

look like?

What clues are hidden

in the mountains, deserts,

and seas?

It's difficult to grasp

that the march of the Earth

is unstoppable.

We've been very lucky

because modern society

has actually developed at a time

when very little

has been happening geologically.

So we've been in a period

of geological quiescence.

Now, this isn't going to last.

We know that there are going

to be major global catastrophes

occur in the future--

a gigantic tsunami, big volcanic

eruptions, more impact events.

And on that basis, we know that

we're all living on this planet

simply by geological consent.

Ice is crucial in helping

to keep the Earth's climate

mild enough for life to survive.

But worldwide,

the temperature is rising

and the ice is melting.

If this keeps happening,

sea levels will rise.

A rise of only 15 inches

would spell catastrophe

for low-lying countries

and low-lying cities.

Major sea-level rise in itself

may be catastrophic

for the planet,

but actually there are

unexpected effects as well,

and that is

that rising sea levels

may trigger volcanic eruptions.

We know this because we've been

looking at the relationship

between changing sea levels

and volcanic eruptions

in the Ice Age,

when we had changes of 100

or 200 meters in sea level

over a few tens of thousands

of years.

McGuire's new work

on Etna seems to show

that the more rapidly

sea level rises or falls,

the more violently the volcano

tends to explode.

When sea levels rise

around coastal volcanoes

such as Etna,

they have

a rather interesting effect.

If you imagine the volcano

sitting on a plate,

with Etna here,

if you then load the other half

of the plate

by a sea-level change

of 100 meters or so,

it has the effect of bending it.

And what that does

is set up tensional stresses

within the volcano.

And any magma sitting there

will burst its way out in the

form of an explosive eruption.

In modern times,

we haven't really experienced

the full terror

of a volcanic catastrophe.

If Bill McGuire's theory

is right,

such an event could hurry in

the new Ice Age

because nature can wield

a 2-edged sword.

Rising sea levels

could cause volcanoes to erupt,

spewing debris and sulfur

into the atmosphere.

That will cut out

the heat from the sun

so the Earth will cool.

That will make

the ice form again,

freezing the oceans

so sea levels drop once more.

And that sudden fall may trigger

more volcanic activity,

pumping more material

into the atmosphere,

making the Earth even colder,

ushering in the dark and the ice

and the long winter.

There is evidence for many ice

ages during Earth's history.

But the last full

and most severe one

began 45 million years ago,

just as the Himalayas

were being formed.

One theory attributes

the dramatic cooling

to this tectonic turmoil.

The mountains are littered

with limestone rocks and rubble.

This debris would combine

with rain

and carbon dioxide in the air

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