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|Sujet: Nouvelles définitions sur le séisme du 12 Janvier. Situation inédite. Mar 19 Oct 2010 - 14:56|| |
October 18, 2010
In Studying Haiti, a New Angle on an Earthquake’s Intensity
By HENRY FOUNTAIN
When the magnitude 7.0 earthquake struck the Haitian capital, Port-au-Prince, last January, the enormous destruction and loss of life were attributed largely to two factors: the proximity of the city to the fault that caused the shaking, and shoddy construction that allowed thousands of buildings to easily crumble.
Seismologists know that local geology can also affect the severity of an earthquake, by increasing the seismic forces under certain conditions. This was thought to have happened in the Haitian earthquake, as large areas of Port-au-Prince lie on layers of relatively soft sedimentary rock that is conducive to amplifying the seismic waves.
Now a new study finds that in addition to the underlying geology, the geometry of local surface features contributed to the earthquake’s intensity as well.
Susan E. Hough, a seismologist with the United States Geological Survey, and her colleagues found evidence that the shaking was amplified along a narrow ridge of hard rock south of the central city. The ridge was home to a popular hotel and other relatively well-built structures that were destroyed.
The finding, published online Sunday in the journal Nature Geoscience, should help scientists and planners working to map areas of the city at risk in future earthquakes, a process called microzonation.
Dr. Hough said seismologists have long known that what is referred to as topographic amplification can occur, but it has often been dismissed as “kind of a fluke thing.”
“It’s not something that scientists have been able to develop systematically,” she said. “Sedimentary layers are what people understand.”
There were indications from the earthquake damage that the ridge, in the suburb of Pétionville, had undergone severe shaking. Dr. Hough said that in addition to the destruction at the Hotel Montana, home to many foreign visitors, 7,000-pound battery racks at a cellphone facility farther west on the ridge shifted more than a foot.
But there was little data on local ground motion during the earthquake, which killed 230,000 people, according to official estimates. At the time, Dr. Hough said, Haiti had just one seismometer, an educational instrument that was improperly mounted. “When the earthquake happened, it went dancing around on its little feet,” she said. “It did make a record, but it wasn’t very useful.”
So Dr. Hough, with help from scientists with the Bureau of Mines and Energy in Haiti, installed eight portable seismometers, including two on the ridge and two in an adjacent valley, and used them to measure ground movement during some of the many aftershocks that have followed the earthquake. They found that the shaking along the ridge was more severe than in the valley, so it cannot be explained by amplification in sedimentary rocks that underlie the valley.
Dr. Hough likened the shaking along the ridge to that which can occur in a skyscraper. “If you start shaking a long, skinny ridge, it sways back and forth, like a big building,” she said. The seismic waves reflect internally within the wedgelike geometric structure of the ridge, combining to produce higher peak forces, a process called constructive interference.
Dominic Assimaki, a professor at Georgia Tech who reviewed Dr. Hough’s paper for Nature Geoscience but was not involved in the research, said the findings should help in developing more accurate models of amplification processes during earthquakes. “Analytically the problem has been studied quite extensively, but the models are very idealized,” she said.
As computer simulations become more detailed and more accurately match data from the real world, they can be used to develop guidelines for earthquake-resistant construction — how much ground movement can be expected on a ridge of a certain height or slope, for example. “The objective is to translate what we find in research into simple parameters that a designer can plug in,” Dr. Assimaki said.
In Haiti specifically, Dr. Hough said, scientists developing microzonation maps can now incorporate the topographic effects seen along the ridge in their work to help the country rebuild properly and better survive the next earthquake. And future earthquakes on or near the same fault are inevitable, seismologists say. “Potentially you can say, ‘You should build over here, and not there,’ ” she said.
Dr. Hough said that even on the ridge, with its severe shaking, some well-built and well-anchored homes and other buildings survived the earthquake practically undamaged. “It shows that you can build safely, even in zones like that,” she said. “You just have to know what you’re up against.”
“The good news is that we can characterize the shaking,” she added. “We can design for it.”