11 Mar 2011

Japan earthquake: planning for disaster

We can plan to lessen the impact of events like the earthquake and tsunami off the coast of Japan, but their effect depends on a range of circumstances, explains seismologist Dr Wayne Richardson.

We can plan to lessen the impact of events like Japan's earthquake/tsunami, but their effect depends on a range of circumstances (Reuters)

Earthquakes happen. The latest event in the news, a magnitude 8.9 event near the east coast of Honshu, Japan, follows others elsewhere in the world, with or without accompanying tsunami: whether in Christchurch, New Zealand, in Chile, or in Indonesia.

The Sanriku earthquake, magnitude 8.4, about 290km off the coast of Honshu, Japan, in March 1933 inflicted 2,900 deaths, with most of the casualties and damage caused by the large tsunami rather than the earthquake itself: about 5,000 houses were then destroyed, nearly 3,000 of which were washed away. And a 2.9m wave was recorded in Hawaii.

A number of factors
How the tsunami behaves when it hits land depends on a number of factors. "The size of the tsunami away from the epicentre will depend on how the wave propagates across the ocean, and that depends on the sea floor structure," says Professor David Tappin of the British Geological Survey.

"But when it strikes land, the height of the wave will be dependent on local coastal topography. For example, if it's in an inlet, then the energy could be focused and it could be dangerous. But I would stress that we don't know what the effect of the tsunami in the 'far field' is yet."

One thing that is sure is that when the tsunami hits land, it is not the sort of wave you see at the beach. The 2004 Indonesia tsunami, in which hundreds of thousands died, was a solid wall of water, picking up debris and carrying that debris along with it. "You just can't stop it. It just keeps going," says Wayne Richardson.

Read more: Tsunami science - a wall of water

Such events are etched in the memories of local inhabitants long after the trauma has dissipated. We learn and take precautionary measures, through earthquake building codes and sensible planning, to lessen the cost of whatever we might have at risk in the future.

But there is a limit to how strong or everlasting any structure can be reasonably designed. The impact also depends on the circumstances – there were no fatalities for the 2010 September event near Christchurch even though that event was larger than the “aftershock” six months later.

Much about modern building practices was learnt from the 1995 Kobe earthquake in Japan and incorporated accordingly into revised codes. These measures are for the benefit of local residents and visitors alike. Life goes on.

Dr Wayne Richardson works for the International Seismological Centre