The Wharton Basin – A Site for Rare Geological Events

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The Wharton Basin – A Site for Rare Geological Events

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When a magnitude-8.6 earthquake struck the Wharton Basin on 11 April 2012, followed a few hours later by a M 8.2 shaker, it did not trigger devastating tsunamis.

This was unlike the M 9.2 earthquake that struck off the coast of Sumatra in 2004 and caused enormous property damage and the tragic loss of thousands of lives.

Prof Paul Tapponnier (left) and Prof Satish Singh pointing out the site that will be surveyed (Source: Lester Kok)

So why, you might ask, are so many scientists interested in the Wharton Basin and its comparatively harmless seismic events?

Firstly, there is a lot of curiosity about the Great Earthquake of 2012, whose size was unprecedented for an intraplate earthquake.

Also of keen interest is its epicentre some 50 kilometres (km) below the surface of the Wharton Basin. This is not an uncommon depth for an earthquake occurring within a subduction zone, but it is rare—if not unknown—for an earthquake to strike inside a tectonic plate at such a great depth.

In addition, the two 2012 quakes appear to have occurred along a newly indentified fault line. But there has been a lack of unanimity among scientists about its location.

And finally, many wonder if the rupture along the newly indentified fault line in the Indo-Australian plate is significant enough to suggest the formation of an entirely new plate boundary, which has never before been witnessed.

Dr Nugroho Dwi Hananto, a marine geophysicist at the Research Centre for Technology at the Indonesian Institute of Sciences (LIPI) and leader of the MIRAGE expedition, is intrigued by all of these questions, including one that links the current survey back to the destructive M 9.2 earthquake of 2004.

From left: Dr Helen Carton, Dr Jerome Dyment and Dr Nugroho Hananto. Dr Hananto, who is leading the MIRAGE, believes that the 2012 earthquakes in the Wharton Basin, and the movement of the intraplate fault lines there, may be putting enough pressure on the Sumatra subduction zone to trigger an event as large as 2004’s M 9.2 earthquake.

“The Wharton Basin is an important geological site because in 2012 we had two very large earthquakes, M 8.6 and M 8.2, occur there. The sizes of these earthquakes were big — the M 8.6 was the largest intraplate earthquake ever recorded,” said Dr Hananto.

“The other interesting thing is that these earthquakes happened far away from the plate margin, in the middle of the Indo-Australian plate, a great distance from the Sunda subduction zone in Indonesia,” he added. “For these reasons, it is very important for us to find out the cause of these earthquakes and their potential consequences.” 

So why has it been so difficult all this time for scientists to pinpoint the exact location of the fault line?

“The scientists who have identified fault lines so far have based their proposals on seismological data-collection sources located far away from the Wharton Basin,” Dr Hananto explained. “That is why their conclusions are not coherent. It is impossible to be precise when you are relying on remote detection.

“The Earth Observatory of Singapore (EOS) and LIPI co-manage 56 GPS stations on Sumatra and the Mentawai Island. They are accurate locally, but these stations are more than 450 km from the Wharton Basin, which accounts for the different proposed fault-line locations.

“Last year, we surveyed a part of this area, but the MIRAGE is the first survey of its kind to be conducted directly above the epicentre of the 2012 earthquakes, producing data that reflect the geological structure of the seafloor and sub-seafloor,” he said.

Mr Praditya Avianto, a technician of Marine Geology at the Research Center for Oceanography, speaking to students about the bathymetry being acquired while we are in international waters.

Dr Hananto is particularly interested in how the fault line fits into the geological system within the Sumatra subduction zone.

“It seems to me that the M 9.2 earthquake in 2004 induced enough stress on the Indo-Australian plate that it may have triggered the 2012 earthquakes,” Dr Hananto said. “The 2012 earthquakes then created seismic movements so large that they may have caused even more stress to the subduction zone.

“So, because of the 2012 earthquakes, energy may be building up right now in the subduction zone that could affect its future activity in this region.”

Prof Singh (left) and Prof Tapponnier believe that a part of Fracture Zone F6 in the Wharton Basin was re-activated during the 2012 earthquakes (Source: Lester Kok)

In other words, while the effects of the 2012 earthquakes may have appeared benign then, the impact they might have on the Sumatra subduction zone probably won’t be, which gives earthquake-preparedness agencies throughout the region yet another eventuality to plan for.

When ask about the possibility that we might be witnessing the creation of a new tectonic plate, Dr Hananto replied, “We don’t really know what will happen in the next decade. In the geophysical sciences, we aim to understand. We do not predict.” 

 

To continue to follow the progress of MIRAGE, please check the EOS blog throughout the month of July, and spread the word using #MIRAGEcruise.

All photographs are taken by Ben Marks, unless otherwise stated.

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