Steve Connor: Deadly quakes may have been connected

Tremors in Samoa may have caused critical weakness in fault lines further to the west

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Earthquakes are one of the most violent and unpredictable forces of nature and a huge amount of scientific effort has gone into studying the factors that can trigger the sudden ground movements leading to the sort of death and destruction seen in Indonesia and the South Pacific.

In general, earthquakes occur on a geological fault line where two or more immense slabs of rock grind up against one another as part of the global tectonic movements of the Earth's crust. When friction causes this grinding motion to get stuck, the resulting build-up of stress can be released suddenly as an earthquake – a slip along the geological fault line.

One of the unanswered questions is whether yesterday's devastating earthquake off the coast of the Indonesian island of Sumatra, near the city of Padang, was in any way connected to the earlier quake that killed up to 100 people on the Pacific island of Samoa. The answer could turn out to be "yes".

A study published in the journal Nature has found that an earthquake at one fault can affect the strength of another fault line else-where in the world. The study did not, of course, look at the latest earthquakes, but investigated the effects of two earlier quakes – the 1992 Landers earthquake in California and the 2004 earthquake off Sumatra that resulted in the devastating tsunami that killed 230,000 people around the coasts of Thailand and Indonesia and across the Indian Ocean.

The seismologists found that both of these earthquakes had affected the strength of the San Andreas fault in California, which is the most studied fault line in the world. In other words, a large earthquake in one part of the world can affect the strength, and therefore the probability of a subsequent earthquake on a fault line in another part of the world.

A fault line's strength could turn out to be important in terms of predicting future earthquakes, but until now it has not been possible to measure it in any meaningful way. The American seismologists behind the latest study got round the problem by monitoring fluid-filled fractures deep within the San Andreas fault zone nearly two miles below the surface.

It is possible that the fluid in these fractures can act as a lubricant within the fault, thereby weakening the fault line's strength by making it more "slippy", and so increasing the likelihood of an earthquake. By monitoring these fluid-filled fractures with highly sensitive seismic instruments, it may be possible to detect critical changes to the strength of the fault just prior to a quake.

The scientists, led by Taka'aki Taira and the late Paul Silver of the Carnegie Institution for Science in Washington, believe that monitoring the movements of these fluid-filled fractures can become a proxy for measuring the strength of a geological fault. They said it may be possible one day to detect the sort of changes that occur before an earthquake strikes – in other words a method of earthquake prediction.

"Fault strength is a fundamental property of seismic zones. Earthquakes are caused when a fault fails, either because of the build-up of stress or because of a weakening of the fault," Dr Taira said.

"Changes in fault strength are much harder to measure than changes in stress, especially for faults deep in the crust. Our result opens up exciting possibilities for monitoring seismic risk and understanding the causes of earthquakes," he said.

But an equally intriguing conclusion of this study is that a fault line's strength can be affected by an earthquake many hundreds or even thousand of miles away – as far away as Sumatra is from California for example – according to team member Fenglin Niu of Rice University in Houston, Texas. "It is possible that the strength of faults and earthquake risk is affected by seismic events on the other side of the world," Dr Niu said. This finding could explain why the earthquake off Sumatra yesterday occurred just hours after another earthquake and its tsunami hit Samoa.

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