The Mentawai Islands: The rise and fall of a surfing wonderland
In this series Swellnet has been exploring the changing nature of coastlines. In the last installment we examined the sand movement that threatens to destroy the famous lefthander at Skeleton Bay, Namibia. This time we take a look at the Mentawai Islands and the volcanic processes causing the famed coral reefs to slowly sink and then rapidly rise.
In the early 1990's only a handful of surfers had travelled to the Mentawai Islands. The location and quality of the waves in the northern Indonesian island chain was a closely guarded secret, so much so that an early magazine trip withheld details calling them 'The Incredible Sinking Islands'. The name was fitting, for the Mentawai Islands are indeed sinking, it's happening at an alarming rate and the only thing that will stop it is a great and destructive earthquake.
The Mentawais lie off the West Sumatran coast in one of the most tectonically active regions on the planet, tenuously placed on the edge of a subduction zone called the Sunda megathrust. Subduction zones are characterised by one tectonic plate sliding under the other, with the area where the two plates make contact known as a megathrust fault. In the case of the Sunda megathrust, the Indo-Australian plate is moving north-east and subducting under the Sunda plate at a rate of six centimetres per year.
In reality one plate doesn't simply slide under the other like an escalator. The megathrust actually remains locked in position with the Earth's crust deforming and building up pressure around the area until finally it gives way. The plate on top dips down as it has nowhere else to go with the attached land masses dipping down with it. The visible result is coastal inundation past the intertidal zone.
This process occurs over many years until eventually the plate on top, bowing down with extreme pressure, can no longer hold and fails. When this happens the plate 'springboards' back in an upwards fashion, causing a large displacement of mantle and crust and huge under sea earthquakes. These earthquakes, which usually run from deep sub-sea trenches, in turn cause a terrifyingly destructive tsunami that can travel many thousands of kilometres.
The 2004 Boxing Day Tsunami that claimed hundreds of thousands of lives occurred when a 1,600km stretch of the northern section of the Sunda megathrust ruptured between Aceh and Myanmar. The epicentre of the resulting 9.2 magnitude earthquake was just north of Simeulue. Recordings of uplifted coral on the reefs surrounding Simeulue showed that the land had risen by as much as 1.5m when the pressure was released by the megathrust.
This event was followed by another earthquake in March 2005 that measured 8.6 with an epicentre just offshore from Nias. In that earthqauke the reef at Lagundri Bay, Nias, was lifted 2.9m transforming the quality of the wave. Many other waves in the vicinity were also altered when their reefs were were pushed upward.
One of the pioneers of the Mentawai Islands and captain of the Barrenjoey, John McGroder, has noticed the changes. "Reefs have risen and dropped in my time in the Shakey Isles," says McGroder. "Most notably up North in the Nias and Simelue region after the 2005 Nias earthquake. Waves like Grubbies you can no longer ride for over a minute. Bawa is also a different monster when it is big."
Yet before the reefs and land were jolted upward the region was slowly sinking. Low lying coconut fields, rice paddies and other inhabited areas of the coast were being inundated as the region bowed down towards the megathrust fault.
Scientists believe we won't see another great earthquake event occurring anytime soon in the Sumeulue and Nias area because it takes centuries for tectonic plates to build up the stresses again. The area to the south, however, is another story.
Earthquakes the size and intensity as the 2004 and 2005 events occur approximately every two hundred years and the last time the Mentawai region saw any significant earthquake activity was in 1833. This has been deduced by analysing ancient coral structures known as microatolls in a process not unlike reading the age of trees through tree rings.
Despite the activity to the north the whole middle section of the Sunda megathrust – the region containing the Mentawai Islands – is still sinking as pressure builds and the coastlines submerge. Macaroni's on North Pagai Island is a glaring example of coastal submergence. The headland behind the 'funnest left in the world' is slowly being swallowed up, with saltwater inundation causing the local flora to die; evident in the ghostly palms seen in the background of many a surf clip shot there.
There have been a few minor releases of pressure in recent times, with an 8.2 magnitude quake centred 200km south-east of the Mentawais in September 2007 raising selected regions of coastline. John McGroder was in the area and recounts his experience of the earthquake:
"There were two earthquakes. One at 6pm while Barrenjoey was anchored in the bay at Maccas. We got out of there in a hurry and steamed slowly down to Thunders. The next earthquake happened at 8am in the morning. There were four boats at Thunders and surfers in the water. The surf was very small. We all moved our boats into deeper water and got the guys out of the surf. They actually were getting shaken on their boards."
"After that it was all a matter of waiting to see if a tsunami happened. After a few hours we noticed the tide seemed a lot lower than normal. We went ashore and there were dying fish caught in puddles, live coral that should have been in the water, and great cartoon like cracks in the reef. The locals were scared and described the night as if something was coming out of the ground...pretty well sums it up for me."
These earthquakes, along with the more destructive 2009 quake which claimed over a thousand lives around Padang, are only small releases of pressure along the megathrust. The islands as a whole are still sinking with breaks like Macaronis, HT's, Rifles along with them.
The question of how long these famous waves will keep their current shape is anyone's guess. Perfectly shaped waves reflect the form of the reef below and even small changes can alter them. Owing to scientific predictions the next great earthquake may not be too far away, and following that we'll see a much different wave scape across the Mentawais.
Kerry Sieh is a former professor of Geology at the California Institute of Technology and current AXA-Nanyang Chair in Natural Hazards at the Earth Observatory of Singapore. He has warned that a rupturing of the Sunda megathrust off the Mentawai Island chain is high likely within the next few decades.
"The great earthquake could happen tomorrow or thirty years from now," Kerry has stated, "but it is not likely to be delayed much beyond the next few decades." Further research reveals that the pressure accumulated along this stretch of the megathrust has now approached or even exceeded the levels relieved in 1833 and the last big quake before that in 1797. This seemingly makes the Mentawai Islands a ticking time bomb.
When the megathrust does rupture the resulting tsunami could, and most likely will, be catastrophic. After the dust settles it's assumed that areas currently sinking will have been forced upward to a higher level as was seen at Nias and Thunders. Whether this results in future discoveries being made throughout the Mentawai Islands is merely a side note, let's just hope the loss of life associated with these events is minimal. //CRAIG BROKENSHA & STU NETTLE
SurfAid is a non-profit organisation helping the people of the Mentawai Islands. Image 1: Aerial view of Macaronis (Surfaid/Smith) Image 2: Coastal examples of seismic activity (Sieh) Image 3: Saltwater intrusion causing death of coconut tree (Brokensha) Image 4: Uplifted reef at Thunders (Brokensha)