PARIS: Lightning does strike twice in the same place. That is the warning of seismologists as they assess the risk of another Indian Ocean tsunami.
That may seem an exaggeration. Surely the gods of nature vented all their anger when they ripped open the Earth’s crust last year and extinguished more than 200,000 lives?
The Dec 26 earthquake off the northwestern coast of Sumatra had a magnitude of 9.3, making it the second most powerful earthquake ever recorded, surpassed only by a 9.5 monster in Chile in 1960.
The quake wrenched up the sea floor along a 1,200-kilometre line, unleashing waves that towered up to 15 metres and scoured the coastlines of the northern Indian Ocean.
More than 220,000 people were killed, almost two million left homeless and economic damage ran into billions of dollars.
The energy release was so extraordinary that exceptional waves even washed up on beaches in Peru and Mexico, 20,000 kilometres from the epicentre. The Earth rang like a bell: vibrations were measurable for weeks afterwards.
Could it happen again?
Earthquake experts are usually loath to make predictions, but on this question they ditch their traditional reserve.
They say: Not only will a tsunami-generating quake happen again, it is likely to occur at almost the same place — and at any time.
“All the warning lights are flashing bright red,” says Paul Tapponnier, a researcher at the Paris Institute for Planetary Physics (IPGP).
Earthquakes may seem random but the evidence shows that they come in clusters. One big temblor exerts stress on an adjoining part of the fault, bringing it that much closer to rupture.
“It’s like a shirt which rips open. When one button pops, all the others are placed under greater pressure,” Tapponier says.
Kerry Sieh of the California Institute of Technology’s Tectonic Observatory notes that seven of the 10 giant earthquakes of the 20th century occurred between 1950 and 1965 — and five of these occurred around the northern Pacific margin.
What made the Dec 26 quake so powerful was that it was a type called a megathrust.
These quakes are not the cause of surface faults that rub along side by side, but of mighty tectonic plates that slide over each other at their boundaries on the Earth’s crust.
In this instance, it happened near a spot on the Pacific Rim of Fire that is notorious for huge earthquakes and tsunamis.
The plates meet at oblique angles, which means the movement is not a smooth, frictionless glide but a torturous buildup of stresses that can result in dramatic perpendicular slips.
‘MINUTE TRIGGER’: All it takes is a ‘minute’ trigger to unleash a massive quake to ease the rocky stress, says John McCloskey, a professor of environmental sciences at Britain’s University of Ulster.
The Dec 26 quake occurred at a point where the Indian plate, a part of the bigger Indo-Australian plate, is sliding beneath the Burma microplate, a tongue-shaped part of the bigger Eurasian plate.
It was triggered by a stress change equivalent to just one-tenth of one atmospheric pressure, according to McCloskey’s estimate.
The quake placed stress on the next section in the fault. As a result, a similar tiny increase in load triggered an 8.7-magnitude quake on March 28 just 160 kilometres to the south, killing 900 people. A major quake last occurred there in 1861.
This, in turn, has created a new spot of vulnerability about 500-600 kilometres farther south, underneath the Mentawai Islands, and seismologists are deeply worried.
These islands lie on the Sunda Trench, a long, deep ditch that runs to the west of Sumatra and marks the battleground where the Indo-Australian and Eurasian plates wrestle.
On average, the Mentawai Islands produce a very big quake every 230 years, according to McCloskey. The last big one was in 1833: an 8.5-magnitude quake that produced a 10-metre tsunami.
A wave generated some 200kms offshore will move at 750 kilometres per hour, which means it will strike within 15 or 20 minutes — barely enough time even if the region’s tsunami warning system is ready and operates perfectly.
“If there is a nine-magnitude quake in southern Sumatra, it will be terrifying,” says Tapponnier. “People should take shelter when they feel the first long tremor.” —AFP
