Risk Of Major Earthquakes, Tsunami Along Arabian Sea Greater Than Thought

The risk of earthquake and tsunamis devastating the coastlines along the western Indian Ocean is greater than previously thought, according to a new report published in the journal Geophysical Research Letters.

Coastlines of Pakistan, Iran, Oman, India and potentially further afield could be at an increased risk, the researchers report, suggesting than an earthquake similar in magnitude to the 9.3-magnitude shocker in Sumatra, Indonesia in 2004 could occur beneath the Arabian Sea and the Makran subduction zone.

The Makran subduction zone, which lies along the Western Indian Ocean, was previously thought to be incapable of generating major earthquakes because of relatively low seismic activity recorded over the years. The area has shown little seismic activity since an 8.1 magnitude earthquake in 1945 and a 7.3 in 1947.

Subduction zones are areas where two of the Earth's tectonic plates collide and one is pushed beneath the other. When an earthquake occurs here, the seabed moves horizontally and vertically as the pressure is released, displacing large volumes of water that can result in a tsunami. 

Tectonic plates are prone to slip and cause earthquakes when temperatures are between 150 and 400 degrees Celsius. Researchers calculated temperatures where places meet and found the potential earthquake zone to be much greater than expected based on their thermal observations.

"Thermal modeling suggests that the potential earthquake rupture zone extends a long way northward, to a width of up to 350 kilometers which is unusually wide relative to most other subduction zones," said Gemma Smith, lead author and PhD student based at the National Oceanography Centre Southampton, according to a news release. 

Smith said the thickness of the sediment on the subducting plate could be a contributing factor to the magnitude of an earthquake and tsunami there.

"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," she said.

"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.

"Smith said those 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," she said.

The photo 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).