The Earth's most wave rich islands are also some of the most seismically active. Is there a relationship? 

In the last article we looked at the many different types of islands, we are now going to narrow the focus to some islands that are back in the news again. A few days ago there was an earthquake registering 7.8 on the richter scale struck a few hundred kilometres south west of the Mentawais. Almost instantaneously the surfing community started thinking about a major tsunami and possible destruction across one of the worlds most wave-rich regions.

Thankfully there was no major tsunami and life has returned to normal, a normal that includes a fair bit of watchfulness and possibly paranoia. Sometimes paranoia can be well founded, as the region is one of the planets most seismically active. But if the islands south and west of Sumatra are known as world class wave parks as well as a seismic free-fire zone, is it possible that there is a relationship between seismicity and good surfing reefs? The simple answer is yes, but first lets zoom out and look at some big picture tectonics.

Most readers are aware the Earth’s surface is made up of tectonic plates and these are shifting slowly, yet continuously. Because the Earth is a sphere, if a plate moves relative to the other plates, it must be either spreading apart, colliding or sliding past. Where plates spread apart, oceans open wider, such as the Mid Atlantic ridge. To simplify the process, when the plates move away from each other, magma floods in to fill the gap. Over time this builds to a thick sheet of basalt ocean crust. The spreading ridges are volcanically active and as a result there are some earthquakes, but they are relatively shallow and small so unlikely to create tsunamis.

Over time  the plate will move across the ocean basin, away from the spreading ridge, accumulating a layer of sediment. 

If the plate is sliding against (past) another plate you get a major fault called a transform fault. Land based examples include the San Andreas and NZ Alpine Fault. When plates slide past each other they tend to do it in jerky movements, these are major earthquakes and sometimes the movement is measured in many metres in just a few seconds. Movements are not always just sideways, there may be a little bit of upward/downward relative movement and so major ruptures can cause tsunamis, but not always. The 7.8 earthquake the other day was probably located on a deep transform fault so there was little or no throw or thrust on the ocean floor that would generate a tsunami.

Of course it's obvious that when plates collide is when big quakes and major movements are most likely - the kind that displace huge amounts of ocean and generate tsunamis. The Australian plate is moving north at almost six centimetres per year, a similar rate to the growth of your fingernails. However, the Asian plate and some of the other microplanes to our north are not moving out of the way - it's a tectonic collision! This collision has created the massive Sunda Megathrust, a fault complex responsible for the big 2004 Boxing Day Tsunami.

Thunders in the Mentawais showing once-submerged reef thrust upwards from a recent earthquake. Photo Craig Brokensha

Ocean crust is relatively dense and tends to be pushed down under the overriding plate, the old basalt crust slides down in to the Earths mantle to be recycled. As the ocean crust goes down, it takes a bit of sediment and water with it, and this acts a flux in the mantle, generating magma which then heads toward the surface creating awesome volcanoes. These volcanoes can grow pretty large, creating arcs of volcanic islands such as Bali and Lombok. Than can also create massive explosive eruptions like Krakatoa. The biggest eruptions leave just a huge, deep crater lake such as Lake Toba in Sumatra or Lake Taupo in NZ.

However, what gets interesting is that not everything on the surface of the ocean crust gets pushed under into the mantle. Oceanic Islands, even micro-continents and huge thicknesses of sediment get scraped off the down going plate and wedged on to the overriding plate. This is how Island arcs grow and evolve. Sumatra is a classic example, an island made of scraped off oceanic islands and sediments.

So what has this got to do with surf in the Mentawais? Probably a lot because all the islands from Simeulue all the way south east to Enggano are created by this process. If Sumatra is the bulldozer blade, the outer islands is the pile of dirt pushed ahead of it. If you have ever watched a dozer you would recognise that some times dirt is pushed up, and sometimes it tumbles or slides down, ahead of the pile. Interestingly, the reason why there is significant outer arc off Sumatra, but not Java and further east is that there is a heap of sediment in the Northern Part of the archipelago. Massive sediment loads are dumped at the head of the Bay of Bengal by the Ganges Delta, huge underwater landslides carry this sediment great distances across the ocean floor and a lot of it slides along the Andaman and Sunda trench, dumping it near Sumatra. The sediment does not reach further south and east, so we don't see fore arc islands off Java or the Eastern islands of the Indonesian Archipelago.

Simeulue, Banyaks, Nias, and the Mentawais are all the top of the pile as great wedges of sedimentary rocks have been scraped off the down going Indo-Australian Plate. Sometimes the wedges are pushed up, sometimes they are dragged down. Often there is sideways movement and the major Mentawai Fault leads to a swarm of North-South trending faults that move to accommodate some of the transform movement in this region. As the Australian plate is moving north-northwest it is actually sliding under Sumatra at an oblique angle. To accommodate the stresses, there is a long series of north/south trending transform faults in the down going Australian Plate. With the earthquake last week a couple of hundred kilometres south of the Sunda Trench, it was probably one of these north/south faults that ruptured rather than the main subduction zone.

A schematic cross section of Sumatra showing the subduction complex in the region. Image courtesy Herman Darman

To a casual observer, all the earthquake action seems to be about the Sunda Megathrust itself. However there is so many more faults accommodate the differential movement of the plates and the slices of rock as they get shoved around. So the islands are actually made up of a bunch of blocks constantly jostling, being thrust up, or dropping down such that the coast is rarely at the same relative height for very long (in geological time). My theory (as yet untested by research) is that the coast of the fore arc islands is so dynamic, the coral reef do not grow in equilibrium with a sea level for long. Some reefs may be sinking (relatively) as the crust is down warped by the subsiding plate. In other places reefs are thrust up as blocks are pushed up along faults or as the crust rebounds. The reefs may have grown in well shaped ramps and lozenges just as a function of normal coral growth (see Coastal Creationism Part 8), however I suspect the constant change may augment the process leading to a high density of good waves.

There is lot that we don't know about about the processes of reef growth and adaptation to change in relative sea level. I think that the surfers are sitting on an incredible body of knowledge through observation. For this reason I want to throw open the discussion and ask you to contribute your own observations and thoughts.

This article builds on an excellent article by Craig Brokensha here on Swellnet and it should be read in conjunction. Also, blindboy has written a great article looking at seismic risk in the region.

Coastal Creationism is an ongoing series written by Chris Buykx. Chris is a geologist, traveller and lifelong surfer. Specialising in eco-tourism, his passion is interpreting nature and the environment. Chris is a resident of Sydney’s Northern Beaches.

Click here to visit every instalment in the Coastal Creationism series.