Indonesia’s quake

The massive earthquake off Indonesia surprised scientists: Usually this type of jolt isn’t this powerful.
The biggest earthquakes tend to occur in subduction zones where one plate of the Earth’s crust dives under another. This grind produced the 2004 magnitude-9,1 Indian Ocean disaster and the magnitude-9 Japan quake last year.
Wednesday’s magnitude-8,6 occurred along a strike-slip fault line similar to California’s San Andreas Fault. Scientists say it’s rare for strike-slip quakes, in which blocks of rocks slide horizontally past each other, to be this large.
“It’s clearly a bit of an odd duck” – said seismologist Susan Hough of the US Geological Survey in Pasadena, Calif.
As one of the world’s most seismically active places, Indonesia is located on the Pacific “Ring of Fire”, an arc of volcanos and fault lines encircling the Pacific Basin. Pressure builds up in the rocks over time and is eventually released in an earthquake.
Wednesday’s quake was followed by a magnitude-8,2 aftershock. Both were strike-slip quakes.

Indonesia's quake
Indonesia.

It was nothing like December 2004. Sirens wailed, warnings blared and police moved people away from coastlines around the Indian Ocean as a powerful earthquake off northern Indonesia sparked fears of another devastating tsunami.
Damage was light and big waves never came in the wake of Wednesday’s quake, not like nearly eight years ago when walls of water roared across the Indian Ocean and ploughed into coastal communities in 13 countries without warning.
“The reports were of people panicking but there was little damage. We need to check for sure directly though” – Eko Budiman, the deputy head of emergency mitigation, said at Medan airport in northern Sumatra, struggling to reach Simeulu island near the epicentre.
The alerts and evacuations mean a regional system passed a major test since it was set up after the massive quake and tsunami of 2004 that killed 230,000 people around the Indian Ocean, including 170,000 in northern Indonesia alone.
But luck may have helped avert disaster this time as much as the warning system, especially in Indonesia’s Aceh province, where roads were jammed with residents trying to flee.
“The simple message is that in any critical condition like this, it’s impossible to get everyone out in time” – said Keith Loveard, chief risk analyst at Jakarta-based security firm Concord Consulting.
“The tsunami alert system worked to a degree … While awareness has improved, reinforced by 2004, it still needs to get better through public education and government campaigns.”
The 2004 disaster swept in with sudden ferocity. Thailand’s southwestern beaches and hotels were packed with tourists on their Christmas vacations and people were out for a stroll on Chennai’s Marina Beach in southern India when the waves hit.

A huge tsunami, triggered by a 9,0-magnitude quake, rolled onto Japan’s east coast in 2011. In 2004, tons of water crashed onto Indonesia following a 9,1 quake. But no tsunami hit Sumatra Wednesday after a massive quake rocked the country. Why not?
Panic gripped Indonesia’s Sumatra on Wednesday: the government issued two tsunami warnings in three hours, as an 8,6 quake off the coasts was followed by an 8,2 aftershock. The wounds of the 2004 disaster, which took the lives of over 230.000 people, three quarters of them in Sumatra, were still raw – and many broke down trying to reach higher ground or fleeing to shelters.
Wednesday’s tremor was a strike-slip quake, says Roger Musson, seismologist at the British Geological Survey. Such earthquakes do not make the seafloor move up or down, they just slide against each other making the water vibrate.
“As soon as I discovered what type of earthquake it was, then I felt a lot better” – Musson told the UK Press Association.
The tsunami that left Japan’s east coast in ruins last year was a mega-thrust one. The sea floor moved vertically, displacing columns of water and releasing huge amounts of energy.
A thrust up earthquake needs to hit over 7,0-7,5 to trigger tsunamis, geophysicist Don Blakeman noted to Life’s Little Mysteries website.
The devastating 9,1 quake, which struck Indonesia in 2004, was significantly more powerful than Wednesday’s as such, but it also was a “vertical” type.
So the most Indonesia registered this time was a wave hardly building up to one meter. This is also due to the fact that waves tend to slow down by the time they reach shore, though at the initial point they may travel at a pace roughly close to the speed of a jet.
Indonesia is the world’s largest archipelago, spreading across 17.500 islands; it experiences some of the most powerful earthquakes and volcanic eruptions on the planet. The country sits between the world’s most active seismic zones – the Pacific Ring of Fire and the Alpide belt, jointly accounting for over 90 per cent of the world’s largest quakes. In the previous year alone, Indonesia went through at least twelve quakes with magnitudes greater than 6,0.

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