Sound

Programme Outline

00.00 - 01.00 Californian crisis

Los Angeles, California: a playground for the rich and famous. But beneath the surface is the unpredictable menace of earthquakes.

01.00 - 01.30 The 1989 earthquake

The last big earthquake in San Francisco was in 1989. It claimed 62 lives, caused 4000 injuries and 6000 million dollars' worth of damage. Unfortunately, there is reckoned to be worse to come.

01.30 - 01.53 San Andreas fault

We see a map showing the San Andreas fault which is 950 km long. It is a crack along the Earth's crust between the Pacific and the North American plates.

01.53 - 02.40 Vox pop

We hear a range of views on living with earthquakes, and see an earthquake drill.

02.40 - 03.20 Measuring movement

Mary Lou Zoback of the US Geological Survey at Manlo Park tells us that there is a small earthquake every 10 days.

Seismometers dotted around the area record the movements of the ground.

03.20 - 03.40 Earthquake ripples

An earthquake sends out ripples in every direction. The seismometer has to measure movements in all three directions - north-south, east-west, and up-down.

03.40 - 06.11 The Richter scale

The Richter scale is used to compare the strengths of earthquakes. The energy associated with an earthquake goes up by a factor of 30 for each unit on the Richter scale. Seismometers all over the world measure waves from earthquakes.

The San Francisco earthquake of 1906, one of the largest ever, measured 7.8 on the Richter scale. This is equivalent to 100,000 atom bombs. The Bay area was very badly affected as it was built on reclaimed land. It is still a very developed area today.

The 1989 earthquake measured only 7.1 on the Richter scale. A 7.8 like 1906 would be 16 times more energetic and the amount of glass from broken windows would reach a depth of 6 metres in the street.

06.11 - 07.06 Predicting earthquakes

Another big earthquake will almost certainly happen - but when? Can we predict them? We see an animation of past earthquakes on a world map. They occur on fault lines. The earth's crust is split into about 20 plates which are constantly on the move.

07.06 - 07.48 Plate movement

Nowadays, we can measure the movement of plates using satellites which are 18,000 km up. They measure the position of special transmitters which are fixed in the ground. Some transmitters move about 40 mm a year - about the speed of fingernail growth! Over the years this movement builds up.

07.48 - 08.30 Pangaea

It is thought that 250 million years ago the last of a number of super-continents, Pangaea, began its long transition towards the shape of the world's continents we know today. Millions of years from now, Los Angeles will probably be further north than San Francisco, and have a colder climate.

08.30 - 09.51 Plate movement in 1906

In 1906, the plates at the San Andreas fault moved 3 metres. We can see how a fence was displaced by this movement.

An earthquake is caused by the movement between two tectonic plates. They are constantly moving deep underground. There is a build-up in tension at the surface until suddenly it snaps and the ground moves.

09.51 - 10.35 The shifting Earth

We see a graphic of underground movement eventually causing the surface to snap. The San Andreas fault hasn't moved on the surface since 1906 - a cause for some worry since the surface has a lot of catching up to do. Other, more recent earthquakes have been due to the movement of other plates.

10.35 - 11.08 Focus and epicentre

Shock waves are produced at the focus of an earthquake, which is underground. The epicentre is the point on the surface directly above the focus. Seismologists can measure the time it takes for the waves to arrive at seismic stations and work out the position of the epicentre.

11.08 - 11.30 Earthquake waves

Seismograms show two groups of waves. We see a graphic showing body waves, which travel through the interior of the Earth, and surface waves, which move more slowly around the surface.

11.30 - 13.14 Body waves

There are two kinds of body waves: P (primary) and S (secondary). L waves travel around the surface. A seismometer measures movement in three dimensions. P waves make it move up and down; S waves make it move from side to side. L waves make it move in a rolling motion.

We see a graphic of waves travelling from Taiwan to California.

13.14 - 13.45 P waves and S waves

P waves are longitudinal (P for push and pull). S waves are transverse (S for shear).

13.45 - 14.08 Damage from waves

Close to the epicentre, the P and S waves cause the most damage. Further away from the epicentre it is the surface waves that produce more shaking.

14.08 - 14.45 Seeing inside the Earth

Earthquake waves allow us to probe inside the Earth. As with ultrasound, we can build up an image of the inside of the Earth using the waves that pass through it. We see a graphic of the Earth's structure: the crust, the mantle, the liquid core and the solid inner core. The radius of the Earth is 6000 km, yet we have only drilled 12 km into its crust.

14.45 - 15.50 Seeing with P waves

P waves travel through the Earth. They change speed and are are refracted at the boundary between the mantle and the liquid core. This leaves a whole zone opposite the epicentre with no P waves: the P shadow zone. This gives evidence that the core has a different density from the surrounding mantle.

15.50 - 16.40 Seeing with S waves

S waves are transerse waves, which will travel through a solid (or along the surface of a liquid) but not through the body of a liquid. The fact that S waves do not get through the core of the Earth tells us that some of the core must be liquid.

16.40 - 17.20 Difficulty of prediction - analogy

It is difficult to predict when an earthquake will strike. We see an analogy of a brick being dragged on sandpaper. Sometimes there are lots of small movements and sometimes a big one. It is impossible to know when the big movements will occur.

17.20 - 18.30 Preparing for earthquakes

What can the Californians do? They can't predict when an earthquake will strike. So they have to be prepared for it at any time. They have built shock absorbers into the foundations of their buildings. There are cross braces throughout the buildings to strengthen the upper structures and ensure that it is only the foundations that move.

18.30 - end Final vox pop

'They're a lot of fun. It's kinda like a retribution thing!'