Most people remember the M9.1 Boxing Day earthquake off Sumatra in 2004, and the devastation caused by the ensuing tsunami.
Far fewer will be aware that, less than a decade later, another major earthquake – large enough to make number 11 in the United States Geological Survey’s list of major earthquakes – struck in much the same area.
The April 2012 M8.7 Sumatra earthquake (originally recorded as M8.6) passed below the news radar because of the limited damage which resulted from the quake.
Now, however, new research conducted by a team of scientists at the University of Utah has identified what makes the more recent earthquake unusual – and, in so doing, throws new light on our understanding of major seismological events.
The Sumatran Earthquake of 2012
The Boxing Day earthquake was larger than that of 2012 (the magnitude scale is logarithmic and the USGS earthquake calculator shows it as almost 4 times bigger in terms of energy released) but the amount of damage attributable to the Boxing Day quake was proportionately very much greater, largely because of the mechanisms involved.
In 2004 the fault movement was vertical and so displaced a huge amount of water, causing a tsunami: in 2012 the movement was lateral (horizontal) and very little water was displaced.
The nature of faulting is one of the things which make the 2012 event stand out: it is, according to the research, “one of the largest strike-slip earthquakes ever recorded.” Not only that but the crust ruptured not along one fault but across four. “We have never seen a large earthquake like this jump across so many faults,” the study’s lead author, Professor Keith Koper, told Decoded Science.
This significance is enhanced by the earthquake’s location: it took place within one of the earth’s tectonic plates rather than at a plate boundary, which is where large earthquakes are most likely to occur. And the reason for that? Well, scientists have identified these ruptures as part of the breakup of the crustal plate underlying the Indian Ocean, the Indo-Australian plate.
Plate Breakup and the Indo-Australian Plate
The tectonic plates that we see marked on a map are in a transient state. So-called ‘supercontinents’ fragment and reform over cycles of hundreds of millions of years (the Atlantic Ocean is currently still opening, and the Mediterranean basin represents the death throes of what geologists know as the Tethys Sea). In the light of this, perhaps it’s not surprising to learn that the Indo-Australian Plate is slowly – very slowly – breaking up.
The causes of this fracture are complex, and associated with the relative and conflicting directions of plate movements in the Indian Ocean. The crust is being both extended and compressed. “In the north India is meeting resistance caused by its collision with Eurasia,” says Professor Koper, “while to the south the Australian section of plate is subducting relatively smoothly beneath the Sunda plate and other associated microplates.”
Earthquake Hazards: Importance and Implications of the Research
We can usually explain earthquakes in terms of fairly straightforward movements – thrusting at subduction zones, lateral movements along transform boundaries and so on – but the unusual nature of the April 2012 Sumatran earthquake in terms of its size and location throws up questions. “We have a pretty good idea of where large earthquakes are possible, but the past several years have been humbling for seismologists,” notes Professor Koper.
“The 2004 Sumatra event was largely unexpected and the size of the recent Tohoku-oki earthquake was unexpected as well. There is a vigorous debate in the seismology community right now about whether our method of producing seismic hazard maps needs to be modified in light of these recent giant earthquakes.”
The 2012 Sumatran earthquake, then, may prove fortuitous not just in the sense that it killed so few people (USGS puts it at at least ten, compared with over almost 250,000 after the Boxing Day tsunami) but helps fuel discussion about our understanding of seismic hazard in terms of predicting the potential size of major earthquakes and the damage they might cause.
Koper, K.D. et al. En echelon and orthagonal fault rupture of the 11 April 2012 great intraplate earthquakes. (2012). Nature. Accessed September 26, 2012.
USGS Magnitude 8.6 – off the west coast of northern Sumatra. (2012). Accessed 26 September 2012.