A major quake measuring around 8.6 magnitude is expected beneath Siberut Island, along the Sunda megathrust, where the Indo-Australian tectonic plate butts up against the Eurasian plate - one of the world's most active fault lines.
"The fault slipped up to 20 meters and that is like several hundred years of plate convergence. It should take several hundred years, if not longer, to accumulate the stress in the system to rupture on that particular strand."
The 2009 Padang earthquake on Sept 30, while large, has not relieved any pressure beneath the Mentawais, as it resulted not from a rupture of the megathrust, but was on a deeper fault.
"Because that (pressure) has not been released in the Padang region, we know the stress has been building and it must eventually be released. The sort of stress which ultimately led to the big rupture at the northern end of Sumatra on Boxing Day."
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