quinta-feira, 21 de maio de 2009

2009 L'Aquila Earthquake


Physics Today: Updated 4/8/2009 An earthquake measuring 6.3 on the Richter scale hit central Italy at 1:32 GMT. last night causing thousands of people to lose their homes more than 250 deaths (see USGS map below). "It was felt across the whole of Italy, but most strongly in central Italy," said Stuart Sipkin, a geophysicist at the US Geological Survey's (USGS's) National Earthquake Information Center.

The region had experienced a large number of minor tremors over the past four months, including a 4.0-magnitude event on 30 March.Seismic map of the 2009 L'Aquila Earthquake

Most of the damage surrounds the city of L'Aquila, which includes one of the oldest centers of learning in Europe, the University of L'Aquila. More than 4,000 buildings in the city have collapsed.

"We did know there would be quite a lot of damage because of the USGS Pager (Prompt Assessment of Global Earthquakes for Response) system," said Sipkin. "We can take our estimates of ground shaking and basically overlay this on a population density map, and we could see that [with the earthquake] a lot of people were exposed to large ground shaking," he added.

Enzo Boschi, the chairman of Italy's National Institute for Geophysics and Vulcanology, (INGV) told ANSA, an Italian news service, that the damage was extensive because the buildings were not designed to withstand earthquakes.

Photo courtesy of European-Mediterranean Seismological Centre

According to the New York Times, one of the four-story-high student dormitories collapsed, with one person dead and several missing. "This shouldn't have happened," said Gabriele Magrini, a physics student at the university. He told the New York Times that he was lucky enough to have been at a friend's house when the quake struck and that he had been waiting at the university since 4am, adding, "We've only seen two people come out. We're still waiting for 10."

Geographical history

Italy is a well-known complex earthquake zone, said Sipkin. "It's not a simple area like the West Coast of California where you have two large plates sliding against each other along the San Andreas fault.... You have the collision of Africa and Europe, its highly fractured and broken up, there's a lot of microplates moving around, which creates a lot of different types of fault action. This particular fault zone usually gives extensional earthquakes, but there's lots of different types of earthquakes that could happen."

There is a major fault line that runs north–south along Italy's Apennine Mountain Range and a minor east–west faultline that runs across the center of the country that produces frequent small earthquakes.Earthquake Hazzard Map from USGS

According to the US Geological Survey, the earthquake struck at a depth of 10 kilometers (6.2 miles), with an epicenter approximately 95 kilometers (60 mi) north–east of Rome, close to L'Aquila. The city has experienced major earthquakes in the past, but nothing on this scale since 1703.

"The duration of ground shaking depends on where you are. If you're on a hard surface, it ends pretty quickly, but if you're in a sedimentary built-up valley, then it would last longer," said Sipkin.

Two smaller quakes—(one at 4.8 on the Ritcher scale)—had hit the region the day before, which weakened many buildings before the main earthquake hit. Smaller aftershocks, which frequently occur after a significant earthquake, are still continuing.

''It was a common tremor for the Apennine mountain chain, one which occurs when underground shelves shift by ten centimeters or so,'' said Boschi. But it is impossible to predict when such tremors will happen, Boschi told ANSA, ''because the parameter variables change constantly. However, in the near future there should be no other ones similar in magnitude to the one last night, although we can expect aftershocks to continue in addition to the over 100 we have already recorded."

Controversy has erupted over Italian television reports that Gioacchino Giuliani, a laboratory technician, had predicted the earthquake but was told by authorities to take down his findings from a website.

Giuliani used a radon gas technique to make the earthquake prediction. Ignazio Guerra of the University of Calabria said that it is impossible to rely on that technique to predict an earthquake: ''There have been earthquakes without the emission of radon gas just as there have been emissions of radon gas without earthquakes. Thus this method is far from perfect."

Nenhum comentário:

Postar um comentário

Seismic Monitor - IRIS

Analytics com meu código