Indonesian quakes: Horizontal slip prevented tsunami

By Paul Harper

Residents flee on motorcycles to high ground after receiving tsunami warning signals after an earthquake off Indonesia's western coast shook this Thai southern resort province of Phuket. Photo / AP
Residents flee on motorcycles to high ground after receiving tsunami warning signals after an earthquake off Indonesia's western coast shook this Thai southern resort province of Phuket. Photo / AP

With memories of 2004's Boxing Day Tsunami fresh in the mind, last night's magnitude 8.6 earthquake sent people along the Indonesian coastline heading for higher ground.

Tsunami watches were issued across the Indian Ocean, however no damaging tsunami materialised from the tremor or any of its large aftershocks.

This was because the earthquake was the result of horizontal movement on a strike-slip fault, rather than a vertical displacement of the sea floor, British Geological Survey seismologist Dr Susanne Sargeant says.

"Strike slip earthquakes are caused when two blocks move horizontally past each other. Such an earthquake would not lead to the vertical displacement of the sea floor that would be required to generate a tsunami. Consequently, the potential for a large tsunami from this earthquake is likely to be low," she said.

While the earthquake was considerably large and shallow - striking at a depth of only 22km, because it was centred some 400km from Banda Aceh the potential for significant damage caused by ground shaking was "relatively low", Dr Sargeant said.

Professor Kevin Furlong from Pennsylvania State University said that although the earthquake struck with the Indo-Australian plate, its occurrence is "almost certainly linked to the plate interactions between Indo-Australian plate and Indonesia".

"This earthquake reflects a style of faulting (strike-slip) which involves principally horizontal motion, and thus is unlikely to generate a significant tsunami; although very strong ground shaking would be felt on Sumatra. This is also an extremely large magnitude earthquake for this style of faulting, meaning that it likely involved substantial fault movement, and the fault likely extends for 200+km."

Dr Furlong, who has recently returned to the US after a sabbatical at the University of Canterbury, said the quake was similar to a magnitude 7.2 earthquake which struck the same location on January 10 of this year.

"Although this earthquake was within the Indo-Australian plate, any earthquake of this size will change the stress regimes acting on the nearby plate boundaries. The result is that stress conditions on the subduction plate boundary beneath Sumatra have changed, although the implications of that change are uncertain."

At least 34 aftershocks have struck the region since the first tremor, the largest a magnitude 8.2.

Dr Bruce D Malamud, reader of Natural and Environmental Hazards at King's College London's Department of Geography, said aftershocks will occur along the fault for weeks and months after the main shock, and perhaps even years later.

"After an earthquake occurs along a fault, stress is released in parts. But then, part of this stress is redistributed to other parts of the fault. This means that they are now more likely to become unstable, with many subsequent earthquakes."

Dr Malamud said, despite several large earthquakes in recent years, the number of earthquakes larger than magnitude 7 is not increasing.

"The number of earthquakes per year with moment magnitude greater than or equal to 7 varies certainly, year to year, but the average from 1900 to present is about 17 magnitude 7 or greater earthquakes per year (compared to about 1 magnitude 8 or greater earthquake). If we just look at 1990 to 2010, then the average was about 15 magnitude 7 or greater earthquakes per year. And if we look at the last three years, then the average is also 15 of this size earthquake per year. So, no, the actual number of very large earthquakes is not increasing over time. It fluctuates year to year, with some years less and some years more."

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