May 14th, 2013 by Joshua S Hill
On Boxing Day of 2004 magnitude 9.1 undersea megathrust earthquake off the west coast of Sumatra, Indonesia, sparked a tsunami that took the lives of over 280,000 thousand people. The quake was caused when the Indian tectonic plate subducted underneath the Burma Plate, causing 1,600 kilometres of fault surface slippage in two phases over a period of several minutes.
New research carried out by scientists from the University of Southampton based at the National Oceanography Centre Southampton (NOCS), and the Pacific Geoscience Centre, Natural Resources Canada, have published research which finds earthquakes in similar magnitude could occur in an area beneath the Arabian Sea at the Makran subduction zone.
This research is important as it sheds light on a previously underestimated tectonic region. The study suggests that the risk of undersea earthquakes and subsequent tsunamis in the region is greater than previously thought. Such a discovery highlights the need for further investigation into the pre-historic tectonic activity of the region, and should be taken note of in hazard assessment for countries that could be affected, including Pakistan, Iran, Oman, and India, amongst others.
The Makran subduction zone (as seen in the image to the left) has shown very little tectonic activity since it erupted earlier last century, first in 1945 with a magnitude 8.1 earthquake, and then in 1947 with a magnitude 7.3 quake. As a result of this minimal historical activity scientists have generally classified the Makran subduction zone as being incapable of producing major earthquakes.
The scientists mapped out the area of a potential fault rupture zone beneath the Makran by calculating the temperatures where the plates meet. Temperatures at these locations are expected to be between 150 and 450 °C if the region is likely to generate rupture generating earthquakes.
“Thermal modelling suggests that the potential earthquake rupture zone extends a long way northward, to a width of up to 350 kilometres which is unusually wide relative to most other subduction zones,” says Gemma Smith, lead author and PhD student at University of Southampton School of Ocean and Earth Science, which is based at NOCS.
The team also discovered that the sediment thickness found on the subducting plate could be a contributing factor to the magnitude of a rupture.
“If the sediments between the plates are too weak then they might not be strong enough to allow the strain between the two plates to build up,” says Smith. “But here we see much thicker sediments than usual, which means the deeper sediments will be more compressed and warmer. The heat and pressure make the sediments stronger. This results in the shallowest part of the subduction zone fault being potentially capable of slipping during an earthquake.
“These combined factors mean the Makran subduction zone is potentially capable of producing major earthquakes, up to magnitude 8.7-9.2. Past assumptions may have significantly underestimated the earthquake and tsunami hazard in this region.”
The image above shows the location of the Makran subduction zone of Pakistan and Iran and locations of recorded earthquakes including the 1945 magnitude 8.1 earthquake (red dot to the north indicates the 1947 magnitude 7.3 earthquake). The profile for the thermal modelling of this study is the north-south trending black line, with distance given along the profile from the shallowest part of the subduction zone in the south (0 kilometres) to the most northern potential earthquake rupture extent (350 kilometres).
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