Japan's earthquake watch

Japan holds regular earthquake drills

Japan is one of the most earthquake-prone countries in the world, perched on top of several converging tectonic plates.

Geological instability causes around 1,000 tremors in the country each year, although many of these go undetected by the public.

Every time they do strike they are a reminder to the country's capital, Tokyo, that it is long overdue "a Big One".

The last major quake in the city was in 1923, and records suggest the geologically precarious Kanto region - where Tokyo in located - will experience one of a similar size about every 70 years.

The 1923 quake, known as the Great Kanto Earthquake, killed more than 100,000 people. Although building and safety standards have greatly improved since then, experts predict that a major quake in the capital could still kill several thousand people and shake the world's financial markets.

But Tokyo is not the only Japanese city at risk.

The 1995 quake in Kobe, western Japan, jolted those who had been focusing quake prevention efforts only on the Kanto area. The Great Hanshin Earthquake killed more than 6,400 people and injured more than 400,000 and was the most powerful tremor in the country since 1923.

The devastation wreaked by an earthquake is not simply connected to its strength.

A tremor of magnitude 8 hit northern Japan last September - the strongest anywhere in the world in recent years. But because its epicentre was far offshore, and because the quake hit hardest in under-populated areas, it caused no fatalities and only seriously injured a handful of people.

Likewise, the first of a string of quakes to shake Niigata this weekend was a sizeable 6.8 magnitude. These tremors killed at least 25 people, but deaths could have run into the thousands if their focus had been in a built-up area - Niigata is largely rural.

The 1995 Kobe quake was only slightly stronger. But it was so devastating because its epicentre was only 20km from a city of 1.5 million people.

Annual drill

The urban density of Tokyo - home to more than 12 million people crammed into an area of just over 2,000 sq-km - therefore puts it at great risk.

A government earthquake panel said in August that there was a 70% chance of a quake around magnitude 7 hitting Tokyo in the next 30 years. The city government has predicted a quake measuring 7.2 could kill more than 7,000 people and injure around 160,000.

Another major factor in determining the death toll is timing.

Experts say many more people would have died in the Kobe quake if it had been later in the day - it struck at dawn, before most people were using cars or public transport.

Fires also pose a major risk after quakes

It could also be argued, however, that a quake that strikes while people are sleeping puts them at greater risk of collapsing buildings and ensuring fires. Fires killed the majority of victims in the Great Kanto Earthquake.

Because of its geological vulnerability, the Japanese authorities make every effort to prepare for a major earthquake.

New buildings are designed to withstand the strongest tremors and the army and emergency services go through elaborate drills every year, on the anniversary of the Great Kanto earthquake.

The difficulty lies with older buildings. A 1997 report by Tokyo's government showed that more than 1.6m houses in the capital were built before 1981, when new recommendations for quake-proofing came into force.

The local government offers all of the city's citizens a structural health check on their homes.

But engineering work to strengthen them is not cheap, and not everyone can afford it.