Everybody’s heard of Tohoku-Oki. Banda Aceh (or Sumatra at least) is also firmly on the map. These places achieved notoriety through being the locations of the largest and damaging recent subduction zone earthquake events. With a combined death toll of over 300,000 people and billions of dollars of damage, the Sumatra 2004 and Tohoku-Oki 2011 events firmly shook the dangers of megathrust earthquakes back to the top of public and scientific consciousness. But have you heard of the Makran? The Makran is a potentially dangerous subduction zone which is decidedly less well known. This margin was the subject of my PhD, so here is a little blog post about why we should keep an eye on it!
Damage from the Tohoku-Oki earthquake and tsunami
Source: http://www.eqecat.com
Prior to 2004, the Sumatra-Andaman subduction zone was considered by some to be unlikely to generate a very large (Mw>9) earthquake. One reason for this was that the oceanic plate heading into the subduction zone has a lot of sediment sat on top of it, partly due to the presence of the huge Bengal submarine fan to the north. Sediments like this were inferred to be too unconsolidated (mushy) to support the brittle rupture of an earthquake, and therefore sediment-rich margins such as Sumatra were historically considered to pose less of an earthquake hazard. Both the Sumatra and Tohoku-Oki events likely seismically ruptured the shallowest part of their plate boundaries, the parts closest to the trench and in the deepest water. This is part of what made their tsunamis so damaging – a rupture in deep water means that a larger body of water is affected, leading to higher tsunami heights. At Sumatra at least, this shallow part is also likely to largely consist of sediment.
This leads us to the Makran, the subduction zone which my PhD focused on. Off the coast of Pakistan and Iran, the Makran is often actually cut off the maps of ‘global’ subduction zones entirely (along with its salty friends the Mediterranean and Hellenic subduction zones). The number of papers I optimistically opened during my PhD only to find a map starting in eastern India and ending somewhere in the Atlantic, thereby neatly missing off Europe, Africa the Middle East and a good chunk of Asia, was irritating to say the least. So why has the Makran been so abandoned? I think the main reasons are a historical lack of data, relatively low earthquake activity (though this is hardly unique to the Makran), perhaps being located in a slightly problematic part of the world, and generally just being a bit of an odd one. Since the Sumatran earthquake however, there has been some rekindling of interest in the Makran, or ‘the other Indian Ocean subduction zone’.
A handy map of the location of the Makran.
Source: clasticdetritus.com
Just as the Sumatran subduction zone is loaded with sediments from the Himalayas courtesy of the Bengal Fan, the Makran subduction zone is loaded with sediments from the Himalayas courtesy of the Indus Fan. There are currently up to 7 km of sediment on top of the incoming Arabian Plate, at least 4 km of which are Indus related. In comparison, the region of Sumatra where the 2004 rupture occurred has 4-5 km of sediment and Tohoku-Oki has less than 1 km. The thick sediments in the Makran have created a subduction zone with no discernible trench and the largest accretionary prism on the planet.
The Makran has one significant recorded, historical earthquake on its record, a magnitude 8.1 event which occurred in 1945 and killed ~4000 people. This earthquake caused a large tsunami on nearby coastlines, with reported wave heights of up to 15 m. This event alone illustrates the potential of this region for destructive earthquakes, however even this earthquake is relatively poorly understood when compared to many other megathrust events. Key questions about the structure, shape and sediment properties of the Makran subduction zone also remain, creating a less than ideal situation for making hazard assessments. My PhD aimed to fill in some of the knowledge gaps regarding the Makran’s structure, shape and earthquake potential, therefore allowing it to be compared more meaningfully to other margins, and allowing better-informed seismic hazard assessments to be undertaken.
Through interpreting a large 2D seismic dataset (allowing us to see the plate boundary and associated faults under the seabed), and using our results (along with other information) to make a cross section of how heat is distributed down the subduction zone, we were able to learn a little more about how earthquake prone the Makran may be. We found that the Makran has a very simple, almost cartoon-like accretionary prism structure (vastly easier to interpret than many other margins!), with few obstacles along the east-west length of the megathrust which would stop an earthquake extending sideways (and increasing its magnitude). This is important as the length of a rupture is directly related to its magnitude: one of the extraordinary features of the Sumatra 2004 event was its rupture length of >1200 km.
An example 2D seismic line showing the structure of the Makran accretionary prism. Width of view ~70 km.
Source: Smith et al. 2012
From our thermal modelling we also found that the Makran has the potential to experience seismic activity in the very shallow parts of the subduction zone, in a Tohoku-Oki and Sumatra like fashion. This is due to the extreme thickness of sediments above the plate boundary in this location, leading to a compressed and strong body of sediment capable of earthquake (brittle) rupture, rather than a mushy mess. Combining these features led us to conclude that the Makran may be capable of generating earthquakes of over magnitude 9, with associated tsunamis. Tsunamis in this region could impact the large cities of Karachi, Muscat and Mumbai, with devastating results.
Sumatra tsunami damage.
Source: en.wikipedia.org
What the Sumatra and Tohoku-Oki events showed was that (to quote one of the less scientifically accurate geology movies out there) – ‘there’s no history of anything until it happens’, and that basing current hazard assessments on historical behaviour can be risky, especially when the historical record is short. The Cascadia subduction zone is another cautionary tale. Historically thought of as largely aseismic, it was only the discovery of drowned forests and an orphan tsunami recorded in Japan which led to our current understanding of it as a margin with a long recurrence interval between large megathrust events.
With plans underway for the construction of two nuclear reactors near Karachi, well within the reach of any Makran sourced earthquakes, keeping an eye on this subduction zone and respecting its potential is surely more important than ever. So if you do happen to come across a map of global subduction zones….do me a favour and check that the Makran is on there!
Earthquakes and other things 