Indonesia’s next big quake due under Mentawais

SYDNEY, (Reuters) – 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, told Reuters.

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.
SUPERQUAKE CYCLES

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 km (1,000 mile) section of the fault,  from Myanmar to Aceh, ruptured in 2004 sending the deadly  Boxing Bay tsunami out into the Indian Ocean.

“The Boxing Bay quake reset the (super earthquake) cycle  for that segment of the fault,” Mike Sandiford at the School of Earth Sciences, University of Melbourne, Australia, told  Reuters.

“The fault slipped up to 20 metres (60 feet) 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 (33-feet) 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 km that has not yet failed. It  hasn’t failed since 1797,” Sieh said.

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.

“It may have potentially loaded that segment that has not  ruptured for 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. The sort of stress which ultimately led  to the big rupture at the northern end of Sumatra on Boxing  Day.”
QUAKE SCENARIOS

One reason the Sunda megathrust generates major quakes is  because it has very long fault planes that can slip as one. But  because it bends as it runs south through Indonesia, scientists  believe big quakes are limited to each section of the  megathrust. Sandiford says a 30-km (18-mile) fault could generate a  maximum 7 magnitude quake, a 300-km (190-mile) fault a maximum  8 magnitude, and a 1,000-km (62-mile) fault a maximum 9. The  2004 quake was over a fault some 1,600 km (1,000 mile) long.

“The Boxing Day earthquake was huge. We have only had three  or four of those quakes in the last 100 years or so,” he said.

Singapore’s Sieh paints two scenarios for the next big  quake. The first is an 8.6 quake on the northern section of the  “Mentawais Patch”.

“There is only one piece of data that tells us it last  broke in 1797. If we are wrong, it may be that the last event  was the 1680s. If that is the case, we could have a  significantly greater uplift and significantly larger tsunami,”  he said. Recent studies by Sieh suggest a second scenario, where  another big quake could occur along the same section of the  megathrust that caused the 2004 Boxing Day tsunami.

While the middle of this section ruptured up to 25 metres  (76 feet) in 2004, the lower part only slipped 10 metres (30  feet), leaving hundreds of years of stress still to be  released.   “We are wondering whether there could be another big  earthquake down in the south end of the 2004 section, which  could break sometime in the next few decades as well,” Sieh  said.

The Mentawais fault line runs under the sea and any major  quake is expected to rupture the ocean floor causing a tsunami.

In 2004 the quake spread a tsunami across the Indian Ocean  to India and Africa. A tsunami generated from a Mentawais quake  would send a wave southwest out into the empty Indian Ocean. But the wave would also hit Sumatra’s densely populated  coast between Padang and Bengkulu, although the Mentawais  island chain would help dissipate the wave’s energy before it  hit shore.

Scientists say there is little data linking a major quake  with Indonesia’s super volcanoes, like Sumatra’s Lake Toba, saying a volcano must be ready to erupt in the first place.

Toba erupted around 74,000 years ago in what is believed to  be the largest volcanic eruption in the last 2 million years.

Some scientists suggest the mega eruption may have  accelerated a glacial shift in climate, by spewing 2,800 cubic  kms of debris into the atmosphere, dramatically dropping the  earth’s surface temperature and sparking an ice age.