California is in many ways a paradise on Earth. But there's a snag: it lies on one of the world's most dangerous earthquake zones, the famous San Andreas Fault, which runs the length of the state.

The Earth's crust is made up of tectonic plates, which join together like a giant jigsaw puzzle. The danger lies at the edges. Almost all earthquakes have taken place along these lines. The tectonic plates 'float' on a bed of moving liquid rock. Scientists think that pressure builds up between the edges of the plates until it suddenly bursts through as an earthquake.

On 17 April 1906, the San Andreas Fault violently ruptured. Many buildings, especially those made of wood, survived the shocks only to burn down in the fireball that followed. More than 3,000 people died and 28,000 houses were destroyed.

Since 1906, to help to predict tremors, scientists have put instruments into the ground all over California. These include laser beams that can measure earth movements of

one millimetre over a distance of six kilometres.

Architects have now developed buildings that move with the shock waves but don't collapse. All big cities that are threatened by earthquakes try to construct tremor-proof buildings.

California is always vulnerable and waiting for 'The Big One'. On 17 October 1989, while much of California watched the World Baseball Series on TV, they suddenly found themselves watching an earthquake. Terror struck throughout the city. The modern techniques of building against earthquakes proved effective. Few modern buildings were seriously affected and far fewer people died (just 62) than in 1906.

On the other side of the Pacific, but on the same tectonic plate, the Japanese had long been used to earthquakes. But when a massive earthquake struck the capital Tokyo on 1 September 1923, the effect was devastating. Three-fifths of the city was destroyed, mostly by fires caused by paraffin stoves or where buildings had been erected on swampy ground. 140,000 people died.

Kobe is the biggest container ship port in the world, 400 kilometres west of Tokyo. There hadn't been a major earthquake in the region for 400 years. On 17 January 1995, at 5.45 in the morning, the ground shook for 20 seconds. The earthquake measured 7.2 on the Richter Scale. (San Francisco in 1989 measured 6.9.) Almost every house on the outskirts of the city was flattened.

The Kobe Earthquake was particularly destructive for two main reasons:

1. The focus of the tremors was close to the main city.
2. It was not very deep: only 20 kilometres below the surface.

It was the most destructive earthquake in recent times. More than 5,000 people died, and many strong structures which were expected to survive did not. It cost Japan nearly 100 billion pounds.

Scientists still can't predict when or where earthquakes will happen. Although most earthquakes happen on or near the edges of tectonic plates, some don't. Maharashtra, in central India, is in the middle of the hard crystalline 'Indian' plate. Early in the morning on 1 September 1993, for 30 seconds, an unprecedented earthquake shook with a magnitude of 6.2. In the Latur district of Maharashtra State it was devastating. At least 22,000 people died. 150,000 were made homeless. The earthquake occurred under scattered villages of stone cottages. As the buildings shook to pieces and collapsed, thousands of people were killed in their beds.

Small earthquakes roll around the world all the time: at least 1,500 measuring over 5 on the Richter Scale every year. Even in the UK, about 20 small earthquakes a year are recorded, which can't be felt.