October 31, 2008

New Earthquake sensors on the Acropolis

Posted at 2:25 pm in Acropolis, Greece Archaeology

Works on the Acropolis Restoration will include the installation of new sensors to measure the effects of earthquakes on the monuments.

Associated Press

Scientists to measure quake effect on Acropolis
By ELENA BECATOROS – 58 minutes ago

ATHENS, Greece (AP) — For thousands of years the Acropolis has withstood earthquakes, weathered storms and endured temperature extremes, from scorching summers to winter snow.

Now scientists are drawing on the latest technology to install a system that will record just how much nature is affecting the 2,500-year-old site. They hope their findings will help identify areas that could be vulnerable, allowing them to target restoration and maintenance.

Scientists are installing a network of fiber optic sensors and accelerographs — instruments that measure how much movement is generated during a quake.

“The greatest danger for our monuments at the moment is earthquakes,” Dimitrios Egglezos, chief civil engineer in charge of the Acropolis’ defensive circuit wall, told The Associated Press. So understanding how the structures react to the earth’s movement is paramount.

Egglezos said six accelerographs are to be installed starting next week at various parts of the Acropolis: at the base of the hill, part of the way up where the geology changes, and on the Parthenon, the Acropolis’ most famous monument, built between 447 and 432 B.C. in honor of the goddess Athena.

“The measurement of earthquakes and their consequences on the monuments is essential,” said Maria Ioannidou, who supervises restoration work on the Acropolis.

The fiber optics are installed on parts of the wall to measure subtle changes caused by changing weather conditions or earthquakes, while the accelerographs can help determine how the earth’s movement affects the monuments.

“This is the first system that we’ve installed to record the (natural) activity that affects our monuments,” Egglezos said.

They don’t look like much: a nondescript small metal box at the foot of a column, barely visible wires snaking across outer walls. But the insight they could give into potential problem areas is invaluable.

The first accelerograph was placed on the hill about two years ago as a pilot program. Another two were installed in late September on the Parthenon, one at its base and one on the top of the columns on the architrave, as part of a study by Japan’s Mie University and the National Technical University of Athens.

Greece is one of the most seismically active countries in the world, and while most of its earthquakes are relatively small and cause little or no damage, some have been fatal. In June, a 6.5 magnitude quake in western Greece killed two people and injured more than 200, while a 5.9 magnitude quake near Athens in 1999 killed 143 people.

Neither seriously damaged the Acropolis.

Indeed, some parts of the ancient citadel have weathered the forces of nature remarkably well. The Parthenon survived virtually intact until the late 17th century, when an Ottoman garrison used it as a gunpowder store; it was targeted by Venetian cannon fire and exploded, damaging parts of the temple.

The accelerographs could also give experts more insight into how the Parthenon has withstood earthquakes so well, Egglezos said.

“The earthquakes that don’t cause damage are very useful because we have a natural … experiment which doesn’t destroy the monument but gives us valuable information about how these structures behave.”

The two accelerographs funded by Mie University will be in place for three years, while the other seven will remain on the Acropolis permanently, Ioannidou and Egglezos said.

The fiber optic sensors, meanwhile, can detect even minor changes in the structure: slight expansion during hot weather, contraction in the cold of winter, the buildup of pressure from a particularly heavy rainfall. And, of course, shifts caused by earthquakes.

They have been installed on two outer parts of the perimeter wall: the southeast which is the highest and most vulnerable point, and on the north wall which shows evidence of damage, probably from an 18th Century quake.

Egglezos said the experts need about one or two years’ worth of data from the fiber optics before they can draw any concrete conclusions

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