Mangroves could protect against earthquakes

Catherine de Lange Reuters AlertNet 14 Jun 11;

Buried mangrove forests act as earthquake-resistant foundations for buildings, a study in the Caribbean has found.

Ancient mangrove forests, such as those buried in the coastal regions of the Caribbean, can protect buildings against earthquakes, according to researchers working on the French island of Guadeloupe.

They suggest that building on top of buried mangroves could be preferable in regions of high seismic activity, especially where earthquake-resistant buildings are unaffordable.

Engineers designing earthquake-resistant buildings often add a soft layer, usually made of rubber bearings, between the ground and a building. During an earthquake, the building then moves as a whole structure, minimising damage.

It now seems that mangroves have a similar effect, said Philippe Gueguen, from Joseph Fourier University, in France, lead author of a study published in the June issue of the Bulletin of the Seismological Society of America.

His team used sensors and mathematical models to analyse ground motion during more than 60 earthquakes of magnitudes 2-6.4 at a test site in Guadeloupe. The team found that although the region has soft, sandy soil, which is highly prone to 'liquefaction' — where soil breaks up and acts as a liquid during an earthquake — the flexibility of the mangrove layer greatly reduces deformation of the soil.

This could explain anecdotal evidence from a major 7.4 magnitude earthquake in Martinique in 2007, when people in buildings built over mangroves experienced only minimal effects, Gueguen said.

The study's discovery that the mangrove layer shakes at a stable frequency could also help structural engineers design safer buildings, said Andrew Brennan, a lecturer in geotechnical engineering at the University of Dundee, United Kingdom.

"As a structural engineer, if you know what frequency your building is going to be shaking at then you could design it so that wasn't a problem," Brennan said.

Gueguen added that the findings might also apply to other sub-tropical regions with mangroves and high seismic activity, such as coastal regions in South America.

Brad Walters, professor of geography and environment at Mount Allison University, Canada, said that mangroves can also protect against hurricanes and tsunamis as they have a complex structure, with strong roots and trunks above and below ground that attenuate the impact of waves and wind gusts.

But planting new mangrove forests would not protect against earthquakes, as the mangroves studied in this latest research are old and buried deep underground, Brennan said.

Walters added that since tsunamis often follow earthquakes in coastal areas, wave protection may be the more important issue.

The protection ancient mangroves would offer during a strong earthquake is also not clear yet, as the study only looked at moderate seismic activity, said Gueguen.

Abstract in Bulletin of the Seismological Society of America
http://www.bssaonline.org/cgi/content/abstract/101/3/1073?stoc