[JAKARTA] Differences in the structure of the ocean floor off the coast of Sumatra could explain why the 2004 Indian Ocean earthquake created a much larger tsunami than a similar magnitude quake the following year, according to a study.
The finding could have implications for tsunami early warning systems, said experts.
Both earthquakes were caused by subduction — the sliding of one section of the ocean floor under the adjacent section. But while the 2004 9.2 magnitude earthquake caused a huge tsunami that killed over 250,000 people, the 8.7 magnitude 2005 quake triggered a much smaller local tsunami, causing less damage.
Now researchers from Indonesia and the United Kingdom have found that differences in the structure of the sedimentary rock of the ocean floor could influence the scale of tsunamis caused by earthquakes of similar magnitude.
They used a technique called 'seismic reflection profiling' to produce images, similar to the ones made by sonar, of the ocean floor along the subduction zone.
They found that rock at the section of the fault that ruptured in 2004 was less dense and therefore weaker than the surrounding rock. This weaker rock deformed easily during the earthquake, displacing more water and producing a bigger tsunami, they said.
Yusuf Surachman Djajadihardja, one of the authors of the paper and a director of the Natural Resources at the Agency for the Assessment and Application of Technology, Jakarta, said the findings were being incorporated into the tsunami early warning system.
Wahyu Pandu, director of Indonesia's Tsunami Development and Operational Program, told SciDev.Net that the main significance of this research was in determining where to position early-warning buoys which relay sea-level changes via satellite.
"We will soon install the buoys in the tsunami-prone areas where the structure of seafloor is looser," he said. "The research told us that such an area stretches between Sumatran coastal cities Barak Padang and Bengkulu."
But he added that it "provides no push for big renovation to the warning system".
"We need just minor adjustments," he said.
Fauzi, head of the earthquake and tsunami division at Indonesia's Meteorology and Geophysics Agency, said the findings would not have a large impact on the reliability of the tsunami early warning system, because the size of tsunamis and the damage caused by them is affected by multiple factors.
The research was published in Science earlier this month (9 July).
Science 329, 207 (2010)