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COLLIDING PLATES: Wednesday’s temblor a once-in-2,000-years event
KUALA LUMPUR: WEDNESDAY’S 8.6-magnitude earthquake off Sumatra was a once-in-2,000-years event, and although it resulted in only a few deaths, it increases the risks of a killer quake in the region, Earth Observatory of Singapore director Kerry Sieh said yesterday.
Reuters reported the seismologist as saying the quake and powerful aftershock were strike slip quakes.
“It’s an exceptionally large and rare event. Besides it being the biggest strike-slip earthquake ever recorded, the aftershock is the second biggest ever,” said Sieh, who has studied the seismically active, and deadly, fault zones around Sumatra for years.
Strike-slip quakes involve the horizontal movement of colliding earth plates, and are typically less powerful than those where there is vertical movement.
They are also less likely to trigger big tsunamis, or tidal waves.
A 9.1-magnitude quake in roughly the same region on Boxing Day in 2004 decimated Aceh province and killed more than 230,000 people in 13 countries around the Indian Ocean.
Sieh said preliminary data suggested that the strike-slip fault involved a sudden horizontal movement of the Indian and Australian plates along hundreds of kilometres, and that the plates were moving relative to each other horizontally at about a centimetre a year.
“If all of that is taken up on this one fault and if you make crude calculations about how much slip occurred during this earthquake, say 20m, it means that this earthquake shouldn’t happen more than once every 2,000 years.”
Despite its magnitude, Sieh said Wednesday’s quake caused only a few casualties and triggered small waves, but added the main problem was that it was likely to have increased stress on the plate boundaries near Aceh, increasing risks of another major earthquake in the same area as the 2004 disaster.
He said according to research carried out by him and his colleagues which was published in 2010, the 2004 Aceh quake had relieved only about half the stress that had built up over the centuries along a 400km portion of the Sunda megathrust fault line.
That made another major quake in the area a matter of time, said Sieh.
Over the centuries, repeated 8.0 and 9.0-magnitude quakes have struck along portions of the megathrust zone off Sumatra, flattening towns and killing thousands of people.
Malaysia’s Tsunami Early Warning Systems (MNTEWS) functioned well during Wednesday’s quake, said Malaysian Meteorological Department geophysics and tsunami division director Dr Mohd Rosaidi Che Abas.
He said MNTEWS had sent out earthquake information and early tsunami warnings within 10 minutes after the quake.
“Within half an hour, full details regarding the quake were obtained.”
MNTEWS was set up after the 2004 quake to provide early warning of tsunamis for those living along the coast.
The Science, Technology and Innovation Ministry implemented it up through the department in 2005.
Rosaidi said: “Early warning or information is disseminated through sirens, SMS, telephone, fax, online, mass media broadcasting system and public announcements.”
He said among the equipment installed were broadband and short-term seismic sensors, coastal cameras, tidal gauge stations, tsunami buoys and warning sirens in strategic locations. Rosaidi said 23 sirens had been installed.
“The sirens are activated only when an earthquake is detected, where people living in coasts can be evacuated to higher ground.”