Africa Speaks Reasoning Forum

Second quake likely, scientists warn

By Deborah Smith, Science Editor
March 18, 2005


The risk of another devastating earthquake in Indonesia has been greatly increased by the quake that caused the December tsunami.

In a new study of the region, seismologists have warned that the biggest immediate threat is of an earthquake of magnitude 7 to 7.5 near Banda Aceh in northern Sumatra.

John McCloskey of the University of Ulster said people tended to think that lightning never struck twice in the same place. "But with earthquakes it's exactly the opposite."

The December 26 earthquake ruptured 250,000 square kilometres of the Sunda trench region under the Indian Ocean, where the Indian and Australian plate is being forced down under the Burma plate.

Professor McCloskey's team calculated the stresses caused by the December event on the remainder of the Sunda trench and on the Sumatra fault, which runs along the centre of the island.

Their results were published yesterday in the journal Nature.

They found stress levels had increased in the 50 kilometres of the Sunda trench next to where the rupture stopped.

The "really disturbing" finding, however, was that very high levels of stress had built up for about 300 kilometres along the Sumatra fault, peaking under Banda Aceh.

The prediction of an earthquake of magnitude 7.5 near there was an educated estimate. "I quite honestly hope we're completely wrong," Professor McCloskey said. If it did happen it would be beneath land and so would not cause a tsunami.

Predicting when any seismic activity might occur was impossible, he said. While the new stresses had been accurately calculated, the underlying stresses in the earth's crust they were adding to were not known.

However, the underlying stresses may have been high near Banda Aceh, because it had not had any large earthquakes for at least 100 years.

History also showed that earthquakes in subduction zones such as the Sunda trench usually came in pairs.

Five of the seven large earthquakes in the Nankai trough to the south-east of Japan in the past 1500 years had been followed by similar-sized earthquakes within five years. Three occurred in the same year as the first one.

Earthquakes on the Sunda trench in 1833 and 1861 caused fatal tsunamis.

Professor McCloskey said the findings about the Sumatra region had to be put into perspective: "I would go for a holiday, but I wouldn't like to live there," he said.

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