Il 12 maggio 2008 NEL SICHUAN, IN PROSSIMITA’ DEL TIBET, IN CINA, un sisma di 7,9 gradi ha causato 87 mila tra morti e dispersi – …
Aumentano i legami tra il terremoto nel Sichuan e la costruzione di una diga
L’epicentro è a 3 km dal lago Zipingpu, una riserva capace di contenere 1,1 miliardi di metri cubi d’acqua
PECHINO – Aumentano le evidenze che lo spaventoso terremoto di 7,9 gradi della scala Richter, che il 12 maggio scorso ha provocato 87 mila tra morti e dispersi nella regione cinese del Sichuan, possa essere stato innescato dalla pressione dell’acqua contenuta nel lago artificiale creato dalla diga Zipingpu, finita nel 2006 e gravemente danneggiata dal sisma.
AGENTE – È più di un’ipotesi avanzata da studiosi americani e cinesi, sostenuta anche dall’ingegnere Fan Xiao, da 14 anni a capo del Servizio geologico e minerario del Sichuan, un ente governativo. Il lago infatti si è formato accanto un’importante linea di faglia, che il 12 maggio si è rotta provocando il terremoto. «Il lago Zipingpu è stato realizzato in una conosciuta area sismica ed è molto probabile che abbia avuto un ruolo sul movimento della faglia», ha dichiarato Xiao. Un’ipotesi simile è stata presentata da Christian Klose della Columbia University a un simposio dell’Unione geofisica americana nello scorso dicembre. Le conclusioni degli scienziati americani e cinesi (tra i quali il geofisico Lei Xinglin, del Dipartimento cinese dei terremoti, che ha pubblicato un rapporto in dicembre) è che sicuramente un sisma sarebbe prima o poi avvenuto in quella zona, ma molto probabilmente la pressione del lago è stata un agente scatenante e ha fatto anticipare i tempi di alcuni secoli.
LIVELLO DIMINUITO – Il terremoto è avvenuto in un momento in cui le autorità avevano deciso di far dimiuire rapidamente il livello dell’acqua contenuta nel lago. «Questo è il momento più pericolo per i terremoti indotti dai laghi artificiali», spiega Fan Xiao. I terremoti indotti dai laghi artificiali sono un fenomeno ben conosciuto da geologi e ingegneri. In India nel 1967 un terremoto di 6,5 gradi innescato dal lago formato dalla diga Koyna provocò circa 180 morti. E anche la frana del monte Toc, che causò la tragedia del Vajont, avvenne quando si cercò di abbassare rapidamente il livello del lago per evitare la grande ondata una volta compreso che la frana aveva iniziato a muoversi. Ma, paradossalmente, l’alto livello dell’acqua aveva in parte impedito che la frana scivolasse. Una volta tolto anche questo «muro di contenimento», la frana precipitò nel lago provocando il disastro.
RISCHI SOTTOSTIMATI – Come si può ben immaginare, questo in Cina è un argomento tabù. La diga Zipingpu sul fiume Minjiang fa infatti parte dell’enorme progetto di dighe per centrali elettriche nella Cina centro-meridionale, di cui fa parte anche la famosa diga delle Tre gole. La Zipingpu iniziò a essere costruita nel 2001, un progetto di 750 milioni di dollari per creare una centrale di 760 mila kilowatt, irrigare i campi, controllare le inondazioni e dare acqua alla vicina Chengdu, capoluogo di 10 milioni di abitanti del Sichuan. Ma già durante la costruzione alcuni esperti dissero che le autorità avevano sottostimato i rischi geologici della zona. Il lago iniziò a essere riempito nel 2004, ma già tra la fine del 2004 e la fine del 2005 nell’area vennero registrato 730 scosse sismiche di intensità inferiore a 3 gradi, ha reso noto Fan Xiao. Il 12 maggio si ruppe una parte della faglia accanto al lago lunga 335 km, proprio nella direzione che Klose ha indicato compatibile con la pressione esercitata dall’acqua. Ma la versione ufficiale cinese, sostenuta anche da importanti scienziati, è che il terremoto è dovuto a cause esclusivamente naturali che nulla hanno a che vedere con la diga.
http://www.corriere.it/scienze_e_tecnologie/09_febbraio_06/diga_terremoto_sichuan_95bfd262-f447-11dd-952a-00144f02aabc.shtml
February 6, 2009
Possible Link Between Dam and China Quake
By SHARON LaFRANIERE
BEIJING — Nearly nine months after a devastating earthquake in Sichuan Province, China, left 80,000 people dead or missing, a growing number of American and Chinese scientists are suggesting that the calamity was triggered by a four-year-old reservoir built close to the earthquake’s geological fault line.
A Columbia University scientist who studied the quake has said that it may have been triggered by the weight of 320 million tons of water in the Zipingpu Reservoir less than a mile from a well-known major fault. His conclusions, presented to the American Geophysical Union in December, coincide with a new finding by Chinese geophysicists that the dam caused significant seismic changes before the earthquake.
Scientists emphasize that the link between the dam and the failure of the fault has not been conclusively proved, and that even if the dam acted as a trigger, it would only have hastened a quake that would have occurred at some point.
Nonetheless, any suggestion that a government project played a role in one of the biggest natural disasters in recent Chinese history is likely to be politically explosive.
The issue of government accountability and responsiveness has boiled over in China in the past year. The grieving parents of thousands of schoolchildren killed in the disaster have already made the 7.9-magnitude earthquake a political issue, charging that children died needlessly in unsafe school buildings approved by negligent or corrupt officials.
More public anger erupted last year when the government failed to prevent the sale of tainted milk powder that sickened nearly 300,000 children and killed six.
“Any kind of government-related disaster presently is very, very damaging and politically extremely sensitive,” said Cheng Li, the China research director at the Brookings Institution.
If it is proved that the earthquake “was related to a man-made situation and not just a natural disaster, the government will be very uncomfortable with that kind of report because of the whole issue of government accountability,” Mr. Li said.
Questions about the Zipingpu Dam are especially delicate because China is building many major hydroelectric dams in the southwest, a region which has abundant water resources but is considered prone to earthquakes.
In a petition to the government in July, a group of environmentalists and scholars said the fact that government scientists had underestimated the risk of the May earthquake raised questions about a host of other dams built in the same valley and along five other major rivers, according to an article published by Probe International, an environmental advocacy group. Chinese authorities have steadfastly dismissed any notion that reservoir-building in Sichuan Province placed citizens at any added risk, and they have blocked some Web sites of environmental groups that suggest that dangers have been overlooked.
In a December article in the Chinese magazine Science Times, two scientists from the Chinese Academy of Sciences strongly denied that the dam played any role in the earthquake. “The earthquake research community outside and inside China has widely accepted the notion that the May 12 Wenchuan earthquake was a huge natural disaster caused by massive crustal movement, because no reservoir triggered-quake with a magnitude eight has ever occurred in history,” said Pan Jiazheng, an expert in hydroengineering, according to a translation published by Probe International.
Scientists generally agree that a reservoir, no matter how big, cannot by itself cause an earthquake. But Leonardo Seeber, a senior scientist with the Lamont-Doherty Earth Observatory of Columbia University, said the impact of so much water could hasten an earthquake’s occurrence if geological conditions for a quake already existed. He said the best known example was a 1967 earthquake triggered by the Koyna Dam in a remote area of India, with a magnitude of about 6.5 and a death toll of about 180 people.
Mr. Seeber said that while the link between the Sichuan earthquake and the Zipingpu Dam was not yet proved, work by Christian Klose, a Columbia University researcher specializing in geophysical hazards, suggested the stress caused by the water’s weight might have hastened the quake by a few hundred years.
“It would have occurred anyway,” Mr. Seeber said. “But of course the people who were affected might think the timing is an important difference.”
Mr. Klose estimated that the weight of the water in the Zipingpu reservoir amounted to 25 times the natural stress that tectonic movements exert in a year. The added pressure, he wrote in an abstract to an unpublished study, “resulted in the Beichaun fault coming close to failure.”
Fifty stories tall and big enough to hold more than one billion cubic meters of water, the Zipingpu Dam astride the Minjiang River was billed as one of China’s biggest water control projects.
Officials said the $750 million project, part of a grand plan to develop regions in China’s south and west, would generate 760,000 kilowatts of electricity, irrigate more farmland, help control flooding and provide more water to industries and residents of nearby Chengdu, a city of more than 10 million.
Almost as soon as construction got under way in 2001, one expert, Li Youcai, voiced fears that officials were underplaying the risk of a major earthquake in the region, but government officials rejected his argument, according to an article published last year on China Dialogue, a Web site devoted to environmental news.
Officials allowed the reservoir to fill with water in late 2004. Fan Xiao, a chief engineer with the Sichuan Geology and Mineral Bureau, said that from late 2004 to late 2005, the data showed 730 minor earthquakes, with magnitudes of 3 or less.
When the major earthquake struck last May, it originated 3.4 miles from the reservoir. The rupture in the Earth’s crust stretched for 185 miles, initially moving in a direction that Mr. Klose said was consistent with the pressure from the water’s weight.
Mr. Fan, the chief engineer for the regional geology investigation team, told reporters soon afterward that he believed that the reservoir influenced the timing, magnitude and location of the earthquake.
“The main lesson is that in building these kinds of projects we need to give more consideration to scientific planning and not simply consider the electricity or water or the economic interests,” Mr. Fan said.
The debate reignited in December when two scientists with the China Earthquake Administration and three other researchers published a study in the Chinese journal Seismology and Geology. They concluded only that the weight of the reservoir’s water and diffusion of water from the reservoir below the Earth’s surface “clearly affected the local seismicity” over a period of nearly four years before the fault ruptured.
The Chinese researchers called for further study to see whether the reservoir helped trigger the earthquake. One of them, Du Fang, with the Sichuan Earthquake Administration, said Thursday that it was impossible to know whether the reservoir influenced the earthquake without more research. “The possibility exists,” she said.
Ms. Du said she and other scientists were free to research the issue fully. “We scientists are free to research the topic we proposed, as long as it is worth studying,” she said. “I don’t feel any restrictions on access to the data from the government.”
http://www.nytimes.com/2009/02/06/world/asia/06quake.html?_r=1&th&emc=th
U21C-08; TI: The 2008 M7.9 Wenchuan earthquake – Result of Local and Abnormal Mass Imbalances?
AU: * Klose, C D
EM: ck2204@columbia.edu
AF: Columbia University, School of Engineering and Applied Science, 351 Mudd Building, New York, NY 10027, United States
AB: The May 12, 2008 M7.9 Wenchuan earthquake occurred along the Longmen Shan margin of the eastern Tibetan plateau in the Sichuan province of the People’s Republic of China. A complex and NNW dipping reverse fault system including the Beichuan fault ruptured 250-300 km parallel to the Longmen Shan thrust belt. This region has been tectonically loaded for >10kyr. It has low deformation rates of less than 1.0±1.0 mm yr-1 resulting in no major seismic activity during the Quaternary period. Several geophysical observations suggest that this M7.9 earthquake was triggered by local and abnormal mass imbalances on the surface of the Earth’s crust. These observations include (1) elastostatic response of the crust to the mass changes (2) slip distribution of the main rupture, and (3) aftershock distribution. Initially, approximately 2 years prior the nucleation of the mainshock, at least 320 million tonnes of water accumulated within the upper Min river valley. It enters the Chengdu plain of the Sichuan basin, a stable continental region (SCR). The water volume amplified the strain energy on the Earth’s crust. Shear stresses increased by >1kPa on the Beichuan fault at the nucleation point in about 20km depth. Normal stresses decreased by <-4kPa and weakened the fault strength. Pore pressure increases might have additionally destabilized the fault locally due to pore pressure diffusion. This effect, however, might be minor in 20km depth, because of low lateral fracture connectivity and permeability between the area of water accumulation and the Beichuan fault. Overall, the stress alterations within a 120±70km2 large area resulted in the Beichuan fault coming closer to failure. Such an area ruptured would account for a M7.2±0.1 earthquake assuming only 10 MPa stress drop. Secondly, a reverse fault focal mechanism dominated, in particular, during the first 50 seconds of the main M7.9 rupture. The Beichuan fault slipped up to 7m upward peaking at shallow depth (<7km) (Nishimura and Yagi 2008). A third reason is that the aftershock distribution (M>3) along the ruptured fault zone pointed toward a very shallow seismicity in the uppermost part of crust. Data show that about 87% of the total seismic moment has been released in the upper third part of the crust (<10km), 1% in the middle third (10-20km), and 12% in the lower third (>20km). Such a bimodal depth distribution with an aseismic mid crust is typical for earthquakes in stable continental regions (Klose and Seeber 2007, SRL 78:554-562). Thus, high spatiotemporal correlatives can be observed at shallow depth between both the seismic moment release of the aftershocks and the rupture slip distribution of the mainshock. This indicates that Mohr-Coulomb failure stress states were much higher in the uppermost part of crust than near a possible nucleation point of the mainshock in 19km depth. In conclusion, the ensemble of geophysical observations suggests that the root cause of triggering the M7.9 Wenchuan earthquake may have stemmed from local and rapid mass changes on the surface.
UR: http://www.geol.tsukuba.ac.jp/~nisimura/20080512/
DE: 0468 Natural hazards
DE: 7200 SEISMOLOGY
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
DE: 8164 Stresses: crust and lithosphere
SC: Union [U]
MN: 2008 Fall Meeting
http://www.agu.org/cgi-bin/SFgate/SFgate?&listenv=table&multiple=1&range=1&directget=1&application=fm08&database=%2Fdata%2Fepubs%2Fwais%2Findexes%2Ffm08%2Ffm08&maxhits=200&=%22U21C-08%22
