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Natural Earthquake Protection System Discovered in Caribbean

Ancient buried mangrove swamps seem to absorb much of the energy from tremors in some parts of the Caribbean, say seismologists

Liquefaction is one of the major hazards associated with earthquakes. This phenomenon occurs when the shaking generated by a quake causes soil or sand saturated with water to lose its mechanical strength and behave like a liquid.

The results can be devastating. There are numerous reports of the catastrophic damage that liquefaction does to buildings, roads, sewerage systems and so on. This is a particular problem for towns and cities built on sand or near the sea.

Today, Philippe Gueguen at the University Joseph Fourier in Grenoble and a few buddies say they’ve discovered a natural mechanism that protects certain areas from earthquake damage and liquefaction.

These guys placed an array of sensitive accelerometers on the island of Guadeloupe in the Caribbean at a place called the Belleplaine test site. They then began to take data on the various tremors that were taking place, some 62 of them.

What they found surprised them. Compared to nearby areas that also experienced these quakes, the shaking at Belleplaine was significantly reduced.

The question is why: to a casual eye, Belleplaine is just like many other places. The top layer of soil here is a stiff type of sand, the type of ground that is particularly susceptible to liquefaction.

So Gueguen and co decided to investigate further by drilling bore holes. What they found is that Belleplaine sits on top of an ancient mangrove swamp that, over millions of years, has become embedded in limestone.

This buried mangrove layer is remarkably flexible and this has an important effect on the energy generated by earthquakes. “The buried mangrove layer plays the role of an isolation system,” say Guegen and co. It absorbs energy and protects the ground above from the worst effects of an earthquake. “The flexibility of the mangrove layer reduces the distortion and the stress in the sandy upper layer, and consequently reduces the potential of liquefaction of the site,” they say.

In fact, Guegen and co point out that today’s engineers use exactly the same mechanism to protect modern buildings from earthquake damage.

All this has important implications for Caribbean islands, many of which are at great risk of earthquake damage (think Haiti last year).

Guegen and co say a similar kind of soil structure occurs on many islands and this could be exploited to help protect buildings.

But there is one problem. Although the mangrove swamp dampens most waves, it still has a specific resonant frequency at which earthquakes can cause significant damage. But Gugen and co point out that once this frequency is known, it ought to be possible to build houses cheaply that do not resonate at the same frequency and so will avoid damage.

That’s not beyond the realms of possibility although organising a way in which it can be carried out in practice is a formidable challenge.

Nevertheless, Guegen and co have a fascinating new insight into the way buildings in this region can be protected from damage. It’s an idea that could save many lives.

Ref: http://arxiv.org/abs/1106.1268: A Natural Seismic Isolating System: The Buried Mangrove Eects

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