The earthquake that triggered December's devastating Indian Ocean tsunami caused a 1,000 km (620 mile) rupture in the sea floor, scientists said on Wednesday.
Using data from 60 Global Positioning System monitoring sites in southeast Asia, scientists at ENS/CNRS research institute in Paris calculated the unprecedented scale of the quake.
"We show that the rupture plane for this earthquake must have been at least 1,000 kilometers long," said Christophe Vigny who headed the research team.
The magnitude 9.15 earthquake, the biggest in 40 years, erupted off the coast of Indonesia's Sumatra island. It triggered a tsunami that left up to 232,000 people dead and missing in 13 Indian Ocean countries.
Indonesia, Sri Lanka, India and Thailand were most affected.
Vigny and his colleagues used displacement data recorded at the GPS sites across southeast Asia to construct and test models of the length of the rupture and the direction of thrust.
The satellite navigation network sites in Indonesia, Malaysia and Thailand were between 400 to 3,000 km from the epicenter of the December 26 quake.
"Small but significant seismic jumps are clearly detected more than 3,000 kilometers from the earthquake epicenter," Vigny said in a report in the science journal Nature.
Scientists have warned that a second earthquake in South Asia on March 28 increased stress on fault lines in the region, making it more vulnerable to another rupture and a tsunami.