Sydney: A massive undersea earthquake is long overdue beneath the Mentawai islands in Indonesia and could trigger another deadly tsunami, say scientists mapping one of the world's most quake-prone zones.
Unlike the 2004 Indian Ocean tsunami, which killed around 226,000 people, this tsunami is expected to be smaller but may be very deadly as it would hit Sumatra's densely populated coast.
"The size of the tsunami may not be as big, but the problem is the size of the population is about three times as great as Aceh," Kerry Sieh, director of the Singapore-based Earth Observatory, said.
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.
Exactly when the big quake will strike is not known.
"We say most likely in the next few decades. Thirty seconds to 30 years, somewhere in there," said Sieh, who has studied geological records showing that for the past 700 years, major quakes have occurred along the Sunda megathrust every 200 years.
There have been three major quake cycles: the late 1300s, the 1600s, and between 1797 and 1833.
"The timing between those three sequences is about two centuries," said Sieh, adding a section of the megathrust under Siberut has not ruptured for 200 years, so it is due to slip and cause a major quake.
The Sunda megathrust extends from Myanmar in the north and sweeps in a southeast arc through Sumatra, Java and towards Timor. The northern 1,600 kilometres section of the fault, from Myanmar to Aceh, ruptured in 2004 sending the deadly Boxing Day tsunami out into the Indian Ocean.
"The Boxing Day quake reset the [super earthquake] cycle for that segment of the fault," Mike Sandiford at the School of Earth Sciences, University of Melbourne, Australia, said.
"The fault slipped up to 20 metres 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 Sunda megathrust is made up of three distinct sections. In March 2005, a powerful quake hit the second section near Nias island, causing more than 11-metre deformations beneath the island.
In 2007, an 8.4 and a 7.8 earthquake hit the southern end of the third section, the "Mentawais Patch", but not the northern part of the "Mentawais Patch".
"Now we have another 300 kilometres that has not yet failed. It hasn't failed since 1797," Sieh said.
The 2009 Padang earthquake on September 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.
"It may have potentially loaded that segment that has not ruptured for a long time. It may have taken us closer to the big failure," said Sandiford.
"Because that [pressure] has not been released in the Padang region, we know the stress has been building and it must eventually be released."
