現在地 HOME > 地震・天文12 > 221.html
★阿修羅♪
|
現在地 HOME > 地震・天文12 > 221.html
★阿修羅♪
|
|
ウースター大のマクロスキー教授らのグループは昨年の大地震がスンダ海溝付近に大きな歪みをもたらしていることを見出し、一年以内にマグニチュード8規模の大地震が発生すると予測している。
http://news.bbc.co.uk/2/hi/science/nature/4354217.stm
Last Updated: Wednesday, 16 March, 2005, 18:22 GMT
New Asian quake threat warning
By Paul Rincon
BBC News science reporter
A build-up of stress on faults in Sumatra following the Indonesian earthquake is likely to trigger another large quake and perhaps a tsunami.
That is the claim made in Nature by a team from the University of Ulster, UK.
The slip that caused last year's devastating quake placed increased stress on the Sumatran fault and on the adjacent undersea Sunda Trench.
A new rupture could trigger a magnitude 7-7.5 quake on land and a magnitude 8-8.5 quake beneath the sea, they say.
The 2004 earthquake occurred when the deep, flat Indian plate slipped under the Burma plate.
Major earthquakes tend to cluster in these subduction zones where two or more plates of the Earth's crust grind and overlap.
When a quake takes place, the displacement causes the surrounding crust to become distorted. This places stress on other fault lines and structures in the area.
Researchers from the University of Ulster at Coleraine used information about displacement following December's magnitude 9.0 quake to calculate the stresses it placed on the surrounding region.
Stress zone
They concentrated on the Sumatran fault, a so-called "strike-slip" fault which cuts through the island of Sumatra, and the Sunda trench, a continuation of the underwater subduction zone that ruptured to cause the tsunami last year.
"We found that both of them had been significantly loaded, in stress terms, by the 26 December quake," said lead author John McCloskey.
Their results show a stress increase of up to five bars in the 50km stretch of the Sunda trench located next to the rupture zone. They show an increase of up to nine bars for about 300km on the Sumatra fault near the city of Banda Aceh.
Based on these findings, the scientists predict a possible magnitude 7-7.5 earthquake on the Sumatran fault and a magnitude 8-8.5 quake at the Sunda trench.
"The huge changes in stress mean that I am comfortable talking about a significant increase in the risk of another quake. But that is as far as I am prepared to go," Professor McCloskey told the BBC News website.
The researchers stop short of predicting when another large quake will strike the region. But similar events elsewhere in the world have occurred within a few years of each other, or even a few months.
Another tsunami?
In Japan, at least five major quakes in the Nankaido segment of the Nankai subduction zone have been accompanied by similar events on the linked Toanakai/Tokai segment within five years - and three of the subsequent quakes ruptured in the same years as their precursors.
The magnitude 7.4 Izmit earthquake in Turkey in 1999 triggered the magnitude 7.1 Duzce earthquake three months later.
Some researchers believe large earthquakes occur at the Sunda trench on a cycle of 200 years, which is determined by stress loading at the subduction zone. The last big event occurred 150 years ago, but Professor McCloskey said the recent quake could have accelerated this cycle.
"The amount of extra stress could be equivalent to 50 or 60 years of loading. But I personally am not convinced by this theory," he said.
A large earthquake at the undersea Sunda trench had the potential to cause another tsunami, the University of Ulster researcher added.
But Professor Nick Ambraseys of Imperial College London, UK, warned against making predictions that could cause panic, when there was no way to tell when another earthquake would occur.
"False alarms and inaccurate timing could create more problems than already exist," he said.
"There is nothing in their article that enables, with any degree of certainty, the prediction of the immediacy of the next earthquake."
The authors of the report say this makes a tsunami early warning system for the Indian Ocean all the more urgent.
Second major earthquake could strike Indian Ocean within a year
http://image.guardian.co.uk/sys-files/Guardian/documents/2005/03/17/tsunamigraphic.pdf
Graphic: the picture under the sea
David Adam, science correspondent
Thursday March 17, 2005
The Guardian
Scientists analysing the aftermath of the Boxing Day earthquake under the Indian Ocean warned today that another devastating quake is now far more likely to strike the region.
The seismic slip off the coast of Sumatra that triggered the tsunami has piled dangerous levels of stress on to two vulnerable parts of the fault zone, significantly raising the chances of a magnitude 7.5 earthquake. The scientists cannot accurately predict how soon such an earthquake may occur, but they point out that previous examples of so-called "coupled" earthquakes have struck within a year of each other.
John McCloskey of the University of Ulster in Coleraine, who led the research, said: "Many of us are brought up to understand hazard as whenever you've had your bit of bad luck it doesn't happen again. Lightning never strikes twice. But one great indicator that you're going to have an earthquake is that you've just had one."
He added: "These are very significant and extensive increases in stress. We cannot say for certain it will result in an earthquake but it's the biggest stress increase over a large area that we've measured since we started doing this research."
A powerful undersea earthquake rocked parts of the region yesterday but there were no reports of damage or casualties. The quake, which registered 6 on the Richter scale, struck about 19 miles beneath the seabed off Aceh province in Indonesia but did not cause a tsunami.
The new study shows one of the regions at increased risk of a more powerful event is a 31-mile stretch of the undersea Sunda trench, next to the 745-mile long zone that ruptured on Boxing Day.
Earthquakes in the Sunda trench triggered fatal tsunamis in 1833 and 1861.
Not all big undersea earthquakes cause tsunamis but the scientists say their results emphasise the urgent need for a warning system in the Indian Ocean.
Countries in the region and UN experts agreed plans for a system last week at a meeting in Paris, but it will not be completed until the end of next year. Until the network of tidal gauges and seabed sensors is in place, Japan and the US will issue alerts on seismic activity in the Indian Ocean.
The second area of concern identified in the new research is a 185-mile region of fault running directly beneath the island of Sumatra, close to the city of Banda Aceh, which was devastated in December and where rebuilding work is under way.
The scientists estimate that stress in the Sunda trench region has increased by up to 5 bars; in the Sumatra fault it has been forced up by as much as 9 bars.
Precedent
Prof McCloskey said there was a worrying recent precedent to consider: in 1998 the seismologist Suleyman Nalbant, one of the authors of the new study, used the same technique to show that local seismic activity had increased stress by 2 bars on a 31-mile stretch of the Anatolian fault in Turkey, which has a very similar structure to the Sumatra fault.
Less than 18 months later, the Turkish fault gave way near the city of Izmit and triggered a magnitude 7.4 earthquake that killed 20,000 people.
He also warned that earthquakes in subduction zones - where one continental plate passes underneath another, as happens at the Indian Ocean boundary - frequently struck in pairs.
In the Nankai trough to the south-east of Japan, five of the seven large earthquakes over the last 1,500 years have been followed by a similar event within five years. Three occurred within 12 months.
Prof McCloskey said: "It's by studying and reanalysing what happened in the past that we are able to have some confidence in the relationship between the stresses we're measuring and the occurrence of other earthquakes.
"But it should be stressed that the mapping is not one to one. It does not mean there will be another earthquake within a year or two, but certainly the risk has increased significantly as a result of what happened in December."
His team used computer simulations of the Boxing Day earthquake prepared by an American team at Caltech in Pasadena to recreate movement of the surrounding area.
Because some regions of the affected fault slipped further than others, the resulting redistribution of stress through the ground was patchy and uneven - meaning seismologists could not be sure at first whether the risk of a second quake was raised or lowered.
To find out, Prof McCloskey's group used mathematical models of elastic materials, which essentially view the Earth as a giant rubber ball. For several points along the faults in the two danger zones they worked out whether the movement of the surrounding rocks freed the two surfaces to slide past each other or clamped them together, making an earthquake more or less likely. The results appear in the journal Nature.
Peter Styles, the president of the Geological Society, said: "It has become apparent over the last 10 years that when a major earthquake occurs it changes the stress in adjacent areas. Sometimes this can serve to lock the fault, but sometimes it can make another failure more likely. Every effort should be made to ensure that appropriate monitoring technologies and communication protocols are put in place to monitor the Indian Ocean."
Nick Ambraseys, a seismologist at Imperial College London, said: "There is nothing in [the new study] that enables, with any degree of certainty, the prediction of the immediacy of the next earthquake - except that an earthquake such as those of 1833 and 1861 is likely to occur sometime in the future. False alarms and inaccurate timing could create more problems than already exist."
In separate research, marine scientists in the US have highlighted the risk of a tsunami in the Caribbean, where there is no warning system.
The team from the University of North Carolina and the University of Texas say more than 35 million people could be affected if a powerful earthquake struck along the boundary between the North American and Caribbean tectonic plates. At least 10 significant tsunamis have been recorded in the region since 1492, six of which are known to have caused loss of life.
The recovery so far
India
A World Bank report says tsunami losses in Tamil Nadu state were $815m (£423m) and about $100m in each of Kerala, Pondicherry and Andrah Pradesh. Relief work is expected to continue for several months but reconstruction is beginning in some areas. India has given up hope of rebuilding on six islands in the badly-hit Andaman and Nicobar archipelago where 5,764 are still missing, 40,000 are in camps, and the infrastructure is in chaos. It could take up to eight years for hospitals, power stations, and resorts to be rebuilt, says the government.
Thailand
Thailand's death toll was 5,303, including 2,159 unidentified people. Another 3,396 people are still missing and another 8,457 injured. Assistance is shifting from relief to reconstruction, as displaced people start to move back from emergency camps to their communities. Fishermen have started returning to work in boats donated by western aid organisations.
Indonesia
In Aceh province, 125,996 bodies have been buried, and 94,105 people are still missing. The number displaced is estimated at 400,156. Indonesia aims to take over relief efforts in Aceh by the end of this month, and has said it wants foreign troops from more than 12 countries to leave Aceh within the next fortnight. More than $4.5bn (£2.4bn) is needed to rebuild roads, power plants, houses, office buildings, farm lands and other facilities destroyed.
Sri Lanka
Latest official figures say 31,000 died, 5,000 are missing, 15,000 were injured, and 443,000 displaced. Some 75,000 houses were destroyed, as were 150,000 vehicles. Roads, railway, power, and water supplies were badly damaged. More than half of the damaged hotel and phone capacity has been repaired and the rail track is back to normal. Sri Lanka will need $1.5bn (£780m), and has asked for a longer period of relief from debt payments to rebuild its economy. Total losses are estimated by the World Bank and the government to equal 4.4% of GDP, with $500m needed in external financing in the short-term for 2005.
Somalia
A relatively small total of 298 were killed and a similar number injured. But containers of hazardous waste, radioactive waste, chemical waste and other substances, previously dumped on the coastline, are spawning illnesses after the containers were damaged by the tsunami. Reports suggest 18,000 households were affected, while an estimated 20,000 people are still living in caves and under trees.
題名には必ず「阿修羅さんへ」と記述してください。