Results from the MEGA-TERA expedition point to a new fault system that may be a sign of the Indian and Australian plates breaking up
On 11 April 2012, a strong quake ruptured in the Wharton Basin of the Indian Ocean, west of Sumatra. Remembering the devastating 2004 tsunami, residents near the shore immediately sought high ground. Thankfully this earthquake was different. At a staggering 8.6-magnitude, it produced a maximum wave height of about one metre.
Why didn’t this earthquake produce a tsunami? Instead of being a thrust earthquake, where plates lurch up and down and displace water, the 2012 quake was a strike-slip, with the crust moving horizontally.
Wharton Basin Earthquake: Record-Smashing
The previous record holder for the largest strike-slip earthquake ever recorded had been the 7.8-magnitude Kunlun quake on the Tibetan Plateau in 2001. The energy released by the 2012 rupture in the Wharton Basin was 10 times stronger, and its aftershock was a massive 8.2-magnitude. It holds another record: largest intraplate earthquake on record. Most earthquakes occur at plate boundaries, but this quake ruptured hundreds of kilometres away from where the Indian-Australian and Sunda plates meet. How these great earthquakes occur away from plate boundaries has long been a mystery in plate tectonics.
The 2012 Wharton Basin earthquake was unique in both magnitude and location. It also stood out from the way that it ruptured. When seismologists began analysing the quake, they found that it fractured in a zigzag pattern along several faults (Figure 2). But according to the existing ocean seafloor maps, these faults didn’t exist.
Exploring the Wharton Basin
We set out to learn more about this unique earthquake and the area where it ruptured. For a week in the summer of 2015, we sailed on the research vessel Falkor of a month long cruise known as the MEGA-TERA expedition, taking high-resolution bathymetric and seismic reflection data. The bathymetric data (Figure 3) gave us a detailed view of the sea floor’s topography and showed scars of past earthquakes. The seismic reflection profiles allowed us to see deformation below the ocean floor’s surface. We also analysed seismic data from before, during and after the 2012 event.
From these data, we discovered the location of the 8.2 event’s fracture zone. The results were published in Science Advances on 4 January 2017. Based on these data, we suggest that the 8.6 earthquake ruptured along a series of closely separated, parallel faults instead of in the zigzag pattern that was previously proposed. Each fault segment was 75-100 kilometres long (Figure 4).
Evidence for a New Plate Boundary
The Australian-Indian plate is moving northwest at a rate of about 5 centimetres a year. But as Australia continues to sail ahead, India is running into a barrier: the Eurasian plate at the Himalayan front. This difference in speed is causing India and Australia to slowly split apart. We detected shear zones on both sides of the fracture zone. As deformation happened along these shear zones from the plates splitting apart, stress built up on each side of the fault. The fracture zones ruptured perpendicular to the shear zones, relieving the stress. The combination of the fracture zones and the shear zones forms what we call a “conjugate system of faults.”
When a plate breaks in two, there are many ruptures in a deformation to relieve the stress. After millions of years and many more earthquakes, the quakes will begin to fall into line and occur along a particular path. With the Wharton Basin earthquake, the path may be getting clearer, signalling a plate boundary that’s being born under our watch. Over the next few million years, we may see more and more strike-slip earthquakes along this stretch separating the Indian and Australian plates.
