Japan earthquake: Can you tsunami-proof a country?
Japan's massive earthquake has sparked a tsunami which has caused further devastation. But what can be done to tsunami-proof a country?
It is a terrifying illustration of man's vulnerability to nature, and the tsunami that has struck the Japanese coast illustrates the difficulty, even for a prosperous nation, of preparing for such an onslaught.
Dr Tiziana Rossetto, a reader in earthquake engineering at University College London, says there is much that contingency planners can do to minimise damage and loss of life.
Precautions include designing buildings to withstand the impact of the waves as well as developing early-warning systems, public education programmes and evacuation strategies, Dr Rossetto adds.
But she warns: "How helpful an early warning system depends on how far away the tsunami strikes - if it's close by, it's not going to be very effective."
Confronted with an unstoppable force like a tsunami - the massive waves created by events like underwater earthquakes - the best course of action is to ensure that members of the public get inland and to higher ground as quickly as possible.
- Totally preventing tsunami-damage is impossible
- But an early warning system can allow evacuations and other precautions
- Buildings can be constructed to survive the wave with repairable damage
Sophisticated international warning systems mean impending tsunamis can be picked up by sensors. Countries can use radio and television broadcasts as well as loudspeaker networks to alert the public.
However, Dr Rossetto says that if, as in the case of the Japanese tsunami, the earthquake occurs very close to the coast, the authorities may only have five or 10 minutes to warn people.
As a result, countries like Japan have highly-developed public information campaigns to ensure that as many citizens as possible know what to do, using school lessons, television and radio broadcasts and leaflet campaigns.
Likewise, Hawaii has street signage indicating the quickest routes to higher, safer, ground in the event of an emergency.
Evacuating a city may not always be possible if the time-frame is tight, so people may be directed to the top floors of buildings or multi-storey car parks.
In Japan, authorities have tested vertical elevation structures - essentially, tall platforms that people can get onto to lift them above the likely height of any tsunami.
Sensitive buildings like nuclear power plants will be sited as far away from the coast as is possible and designed to shut down into "safe mode" as soon as an earthquake is detected.
Additionally, Dr Rossetto says, sea barriers are sometime put up around strategic areas like harbours and ports - but their effectiveness is not particularly well-established.
Evacuation is not an easy option on many low-lying islands.
In places where people can be got to safety structural engineering can do much to minimise the aftermath.
Dr Gopal Madabhushi, from Cambridge University's engineering department, co-authored a study on tsunami-resistant designs for houses after the 2004 disaster that devastated coastal areas in Indonesia, Thailand, Sri Lanka and elsewhere.
Testing a design from a group of architects at Harvard University, researchers used a large wave tank and found that allowing water through was better than trying to stop it.
"Instead of trying to obstruct the wave, you allow the wave to pass through the structure while causing minimum damage. Large doorways and windows offer a clear path to go through."
The goal of the design is to protect the building and make it easier to reconstruct afterwards, on the assumption that the occupants have fled to higher ground. Destroyed doors and windows are easy to replace.
"Internal doorways are aligned rather than staggered. If you have staggered doors, the wave gets 'locked' into the house."
Houses on piles are one answer, but might not be desirable in all regions for practical reasons.
"Hawaii has traditionally built reinforced concrete buildings - they might leave the ground floor clear like a parking bay, effectively like a building on stilts," says Dr Madabhushi.
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Another key facet in building design is making sure load-bearing walls are not hit head on by the wave, causing the collapse of the structure.
"You can see which direction any tsunami would come from, it is easy to make the load-bearing walls perpendicular to that direction."
Not all buildings will be so well-designed, however, warns Dr Adrian Chandler, an earthquake engineer at UCL's Hazard Research Centre.
"For your average residential structure, it's down to the owner as to whether they're going to put these preparations into place," he says.
"In many cases the costs will be excessive."
Indeed, Dr Chandler adds, the only way to be free of the risks associated with a tsunami is not to live near the coast in the first place.
It is advice that has been heeded in the past. After the Hawaiian city of Hilo was hit by successive tsunamis in 1946 and 1960, authorities moved its residents further inland.