On the 11 April 2012, an 8.6 to 8.7 Mw magnitude Earthquake happened at sea at about 400km off the coast of Sumatra (Indonesia). It was followed by an extremely intense 8.2 magnitude aftershock, as well as a large number of weaker aftershocks.
The two events of a magnitude larger than 8 were localized in the south of the trench marking out the boundary between the Indo-Australian plates and the probe (SE part of Eurasia), as well as the emergence of the subduction megathrust. It is this megathrust that broke off on a length of more than 1000km, during the huge earthquake of 2004 (Mw superior to 9) and which was responsible for a destructive and lethal transoceanic tsunami, as well as on other segments further south in 2005 and 2007.
The two earthquakes on 11 April 2012 are intraplaque events which operate differently from mega-thrusts. They broke off the Indo-Australian oceanic plate with a strike slip fault. This was probably a sinistral motion on strike faults near the north-south, although a more complicated break off activating several conjugate faults should not be excluded.
The north-south faults are consistent with the reactivation of the fracture zones of the Wharton Basin, the latter being old transform faults. These faults were mapped out in detail further south (Deplus et al., Geology 1998) using bathymetric data that were already clearly showing they were active. The almost pure coseismic strike slip movement generated very limited vertical movements. The bottom of the ocean did not lift up abruptly during the earthquake, which explains the absence of a significant tsunami despite the extremely high magnitude.
The reactivation of faults from the Wharton Basin is due to the internal distortion of what formerly constituted a single stiff plate, the Indo-Australian plate. Due to the collision between India and Eurasia, this plate tends to split into two over a wide distortion zone. Here is a simplified explanation: since colliding with Eurasia about 50 million years ago, the movement of India has slowed down. However, Australia keeps moving to the north at higher speed. Therefore, a deformation and sinistral slippage zone appeared between the two lower plates. The movement on the faults which broke off on the 11 April contributed to this sinistral slippage.
For the 8.6 magnitude earthquake, the scaling laws linking the magnitude with the length of the zone that broke off along the fault imply either a very long break if the rupture front is restricted to the oceanic crust (therefore a very elongated and rare aspect ratio), or a more classical break affecting both the crust and a part of the lithospheric mantle. This last theory seems to be confirmed by the depth of the hypocentre, estimated to 20-25km, and the first models calculated by kinematic inversion for the seismic source.
For more information:
• GEOSCOPE page describing the earthquake
• Page about the earthquake on the INSU/CNRS website
• Deplus, C., M. Diament, H. Hébert, G. Bertrand, S. Dominguez, J. Dubois, J. Malod, P. Patriat, B. Pontoise, et J.-J. Sibilla, Direct evidence of active deformation in the eastern Indian oceanic plate, Geology, 26, 2, 131-134, 1998.
• Deplus C. et al., Indian Ocean actively deforms, Science, 292:1850-1851, 2001
• Singh, S., H. Carton, A.S. Chauhan, S. Androvandi, A. Davaille, J. Dyment, M. Cannat et N.D. Hananto, Extremely thin crust in the Indian Ocean possibly resulting from Plume–Ridge Interaction, Geophysical Journal International, 184, 29-42, 2011.
Page prepared with the contribution of Christine Deplus, Robin Lacassin and Satish Singh.
Translation / revision carried out by the Masters 2 ILTS team