Asia risks second mega-tsunami

The Indonesian island of Sumatra is now at risk from two potentially major quakes, one of which could generate waves 10 metres high, seismologists warn.

The research team is headed by the same expert who predicted a quake that struck Sumatra on 28 March, barely three months after the 26 December mega-quake, the second biggest ever recorded.

The research is published today in the journal Nature.

Lead author John McCloskey, a professor of environmental sciences at the University of Ulster, and team fear the next quakes may be as high as 7.5 and 9 respectively on the Richter scale.

In the latter case, cities along much of Sumatra's west coast would be exposed to a tsunami.

"I think it would be irresponsible for those in charge of preparing people in this area to ignore the possibility that the earthquake could happen in a year," McCloskey says.

A seismic mosaic
The study takes a fresh look at Sumatra's seismic mosaic in the light of the last two great quakes, focusing on the two biggest fault lines.

One fault line runs on the land down the western side of Sumatra, and has lateral friction, with one side trying to head northwest and the other trying to move southeast.

Stress on this so-called Sumatra fault, especially in the northwest, in the region of Banda Aceh, remains high, the researchers warn.

"The threat of an earthquake of magnitude 7.0 to 7.5 on the Sumatra fault north of 4° north [of the equator] has not receded," they write.

An even greater threat lies in the second fault line, the so-called Sunda Trench, a notorious seabed crack that runs about 200 kilometers to the west of Sumatra.

This area has a different and more perilous profile than the Sumatra fault as it has vertical movement, the kind capable of creating big waves by thrusting up sections of the sea bed.

It lies in a so-called 'megathrust' area, in which the Australian plate is trying to push its way under the Eurasian plate to the northeast.

The Sunda Trench has been a flashpoint of seismic activity for centuries.

Part of its northern section, at the conjunction with the tongue-shaped Burma microplate, was the epicentre for the 26 December quake, at 9.3 the second highest ever recorded.

The ocean floor ruptured along 1200 kilometres, creating a wave in which 217,000 people around the Indian Ocean's rim were killed or left unaccounted for.

That massive event had a domino effect, placing further stress to the south.

On 28 March, just 160 kilometres to the south, an 8.7-magnitude earthquake killed more than 900 people.

In turn, this quake has created a new spot of high vulnerability about 500 to 600 kilometres further south underneath the Mentawai Islands, between 0.7 and 5.5° south of the equator, according to the computer modelling.

On average, the Mentawai Islands produce a very big quake every 230 years, says McCloskey's team.

The last big one in the southern part of this section was in 1833, with an 8.5 quake that unleashed a damaging tsunami up to 10 metres high.

By comparison, the wave that scoured the coastline of the northern Indian Ocean on 26 December varied in height from 10 to 15 metres.

In the northern part of this section, there has not been any big quake since 1797, when there was a small slip of only a few metres under the main island of Siberut.

"Slip on the northern part of this section could be greater than 10 metres depending on the timing of the last rupture, [and] slip on the southern portion could be as great as in 1833," the researchers say.

When could this happen?
McCloskey says no one knows for sure when this could happen. But he says time is already short before the next expected event and may now be even shorter because of the cascade of recent thrusts on this highly tensed region.

Two "minute ... tiny" changes in stress, of one-tenth of an atmosphere pressure were sufficient to unleash the December and March events, he notes.

"If this earthquake [under the Mentawai Islands] were to happen within a year, it would not surprise me," he says.

"Even if the earthquake doesn't happen for 10 years, it's still better to start moving towards a culture of preparedness for these things, because they will happen.

"An earthquake under the Mentawai Islands which produced a tsunami would be felt strongly along the cities along the west coast of Sumatra.

"The tsunami will be generated 200 kilometres offshore, moving at 750 kilometres an hour, which gives you 15 to 20 minutes for people to get on to higher ground."

Source: abc.net.au