Yet more Indian Ocean earthquakes? Sadly, yes

The shores of the Indian Ocean and the people who live near them will take years and maybe decades to recover from the awful events of 26 December 2004.  While relief and reconstruction efforts are underway, so too is the scientific analysis of what happened.  Throwing a malevolent shadow is the uncertainty of whether there may yet be more tsunamis so soon after the first in the region for 150 years.  The Sunda trench where the massive earthquake took place had remained stable for a long time.  Stresses built up, eventually to cause the subduction zone to fail catastrophically.  However stress relief in one place redistributes that which remains along other fault lines, and can create space in which new breaks might occur.  Geophysicists from the University of Ulster have analysed the likely disruption of stress in the eastern Indian Ocean (McCloskey, et al. 2005.  Earthquake risk from co-seismic stress.  Nature, v. 434, p. 291) following the distribution of about 20 m displacement on the Sunda subduction zone over a N-S length of around 500 km.  They feared that such a huge perturbation may activate other large faults.  A changed stress field seems to have been the cause of the Izmit earthquake that devastated central Turkey and also set in motion repeated seismicity along the subduction system off Japan in the past. McCloskey and colleagues foresaw two worrying possibilities for the Sunda subduction system: stress localised just to the south of the Boxing Day event could migrate southwards to trigger release again on the subduction zone; a large strike-slip fault that runs down the centre of Sumatra, itself linked to subduction, may fail soon. fear that the second is the more likely.  Since modern seismology emerged, so few earthquakes have occurred in the area compared with other large subduction settings that prediction is difficult.  The Ulster scientists were correct, very soon after their prediction was published.  On 28 March 2005, a magnitude 8.7 earthquake occurred on the subduction zone about 150 km south-west of that on Boxing Day 2004.  Its motion involved vertical displacement, so it was feared to trigger yet more tsunamis and sirens sounded throughout the previously devastated areas.  The warnings were heeded.  Apart from some panic that cause two deaths in Sri Lanka, people moved quickly to safe ground.  Thankfully, perhaps miraculously considering an energy release not far short of that at the end of 2004, there were no tsunamis of any consequence.  Yet the places on the nearby Indonesian island of Nias were devastated by the shock waves, killing upwards of a thousand people.  This is a grim warning that McCloskey and colleagues’ interpretation of stresses moving southwards along the main ocean floor fault system is happening.  The risk of further devastation soon is by no means over.


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