Continued Increased Seismic Risk in Sumatra
The illustration above shows the rupture plane of the December and March earthquakes as well as the stress changes due to both events. Note the area of stress increase (locally up to 8 bars) on the Sunda Trench as well as high stress along the Sumatra fault (with stresses of up to 8 bars near Banda Aceh).
|Global coverage of this story:
Asia Risks Second Mega-tsunami [ABC Science Online, Australia]
Sumatra Poised for Another Tsunami [National Geographic USA]
Stress changes due to the Sumatra-Andaman earthquake of 26 December 2004 triggered the 28 March M=8.7 Simeule-Nias event, according to new research findings by scientists from the Geophysics Research Group, School of Environmental Sciences, University of Ulster, and the Seismology Laboratory of the California Institute of Technology, published in science journal Nature today.
The scientists also say that stresses from the March earthquake have significantly increased the risk of further large earthquakes to the south along the Sunda Trench – and on the island of Sumatra.
On 17 March 2005, the University of Ulster group published a Brief Communication in Nature in which they identified two areas where the stress, and consequent earthquake risk, had increased as a result of the 2004 Sumatra-Andaman earthquake. These regions were the Sunda Trench south of the December rupture and the Sumatra Fault near Banda Aceh.
On 28 March 2005, an earthquake measuring approximately 8.7 on the Richter Scale ruptured the Sunda mega thrust south of the December 26th rupture zone corresponding almost exactly with that area which had been pointed out on the 17th March paper. Fortunately and due almost entirely to the depth of maximum slip in this earthquake, the tsunami generated was very small and the number of people killed was relatively low at approximately 2000.
Following this earthquake, the team led by Professor John McCloskey developed collaboration with Professor Kerry Sieh’s Group in the California Institute of Technology. The aim of the collaboration is to integrate stress information and calculations from the University of Ulster group with the Caltech record of historical earthquakes that have occurred on the Sunda mega thrust over the last 200 - 300 years. This combination of stress information and historical records has exposed some worrying stress increases on parts of the megathrust which have not slipped for over 200 years.
“It would seem that the Sunda trench subduction zone is ripe for triggering given the long period of time since the last large earthquake on this segment,” said Professor Kerry Sieh of Caltech.
Professor McCloskey noted that similar increases in stress on other faults in the past have resulted in the triggering of large and sometimes devastating earthquakes. “We know that earthquakes communicate and teams of scientists around the world have been working on the understanding of the language of that communication,” said Professor McCloskey. “There is little doubt now that earthquakes communicate in the language of stress. When an earthquake happens in the crust, other faults in the area feel the effect which can either increase the likelihood of another event or decrease it. In the case of both the Sunda Trench subduction zone and the Sumatra fault, which runs along the centre of the island of Sumatra to the city of Banda Aceh, it would appear that the previous earthquakes in this sequence have significantly increased stresses on areas which have not experienced earthquakes in the recent past. Just as in the case of the stress from the Boxing Day earthquake and its triggering of the 28th March earthquake, we have significant concerns that these increase in stresses could result in further earthquake activity in the near future in this area.”
Dr Sandy Steacy of the Geophysics Research Group commented on the increased risk of further earthquakes along the Sumatra fault that runs along the centre of the island of Sumatra. “We have noted increases in stress on this structure over a total distance of about 900 km. This fault is known to have produced earthquakes of magnitude 7 or 7.5 in the past. While these events would not create a tsunami since the fault runs on land, they could be devastating if they occurred near centres of population. Unfortunately, the city of Banda Aceh lies on one particularly strongly stressed part of this fault.”
Commenting on what may happen in the future, the team were very careful to avoid any discussion of the likely timing of a following event. “The estimation of timing from these stress studies is as yet beyond the reach of earthquake scientists,” said Dr Suleyman Nalbant of the team from University of Ulster.
“It may be that sometime in the future we may be able to arrive at a relationship between these values of stress increase and the probability of an earthquake following of a given size within a particular time. We are not there yet and it is probable that it will be many years before earthquake scientists can do this reliably,” said Professor McCloskey. He added that, “at present there is no possibility of earthquake prediction which includes timing” and suggested that this may be completely forbidden by the underlying physics of the earthquake interaction.
What are the implications of these findings for people who live in the Indian Ocean region? It is clear that the possibility of another large earthquake in this area cannot be excluded. The earthquake that the Group most fear at present would be on the order of magnitude 8 to 8.5 on the Sunda Trench and would be potentially tsunamigenic.
“A reasonable model for the earthquake we fear is the 1833 earthquake,” said Professor Kerry Sieh of California Institute of Technology. It is known that the 1833 earthquake produced a large tsunami which resulted in many casualties on the island of Sumatra. Recent numerical simulations of this tsunami show significant wave energy on the north western and western coasts of Australia as well as, to a lesser extent, on the shores of Sri Lanka and some of the islands in the middle of the Indian Ocean. While much of the wave energy dissipates harmlessly on the southern ocean, there are significant areas of wave impact within the Indian Ocean.
What can be done to mitigate the effects of this potential earthquake? Clearly the implementation of a tsunami warning system is vital and this appears to be moving ahead rapidly. Also extremely important are education and the preparedness. In particular, individuals need to know to immediately head inland if they’re near the coast and feel a strong earthquake since a nearby earthquake can trigger a tsunami that arrives more quickly than any official warning.