Seismic shift

Indonesian Meteorology and Geophysics Agencystaff monitor the seismic activities in Sumatra island at their Jakarta headquarters on April 12, the day after an 8.7 magnitude quake hit the Indian Ocean 431 km off the city of Banda Aceh.-

THE massive earthquake in the Indian Ocean south-west of Sumatra on April 11 has revealed a complicated faulting and rupturing process. The 8.7 magnitude quake was both the largest strike-slip earthquake, involving horizontal motion at the fault region, and the largest intraplate one ever recorded. This significant analysis of the seismic waves was published in recently in the journal Nature.

During the quake, the Indo-Australian plate ruptured over a complex network of at least four faults lying at right angles to one another. The energy released in each of the fault corresponded to about magnitude 8 and added up to a total event magnitude of 8.7. The initial shock was followed two hours later by an 8.2 magnitude aftershock on another fault lying to the south. According to geologists, most earthquakes of magnitude 8 and above occur at the edges of plates in subduction zones, where one plate dives under an adjoining plate and the motion along the fault causes vertical movement of the surface with the potential to cause a tsunami. The Sumatra quake, however, involved horizontal motion on a series of faults in the middle of the plate.

The faults broke through the upper part of the plate and appear to have slipped as much as 30-40 metres during the quake, the researchers said. Strike-slip quakes do not result in tsunamis.

While fragmentations on a small scale have been seen, this event, according to them, is evidence of the breaking up of the Indo-Australian plate into two separate plates, at an unprecedented scale of a giant tectonic plate. The break-up is apparently caused by stresses within the plate resulting from collision with Asia in the north-west, which slows down the western part of the plate, while the other side continues to move northwards, going under Sumatra to the north-east. The Indian sub-plate will eventually separate from the Australian sub-plate, they believe. But when that new plate boundary will form is not clear; it may take millions of years requiring hundreds, if not thousands, of such huge earthquakes.