Jon Copley eyes the sea floor

"Nothing in the world is softer and weaker than water" wrote the Chinese sage Lao Tze in the 6th century BC, "yet nothing is better at attacking the hard and the strong."

Tsunamis are the most destructive manifestation of this power.  Although they are often no taller than normal wind-driven waves – the Hollywood image of a skyscraper-sized wall of water is misleading – tsunamis are devastating because the water continues to surge relentlessly when it hits the shore, rather than receding.

The Indian Ocean tsunami on 26 December 2004 was one of the worst disasters in modern history, leaving more than 229,000 dead in its wake.  Although scientists cannot predict exactly when the earthquakes that trigger tsunamis will occur, science should be able to predict the impact of such events and help authorities to plan for them.  But this science is still in its infancy.

The Boxing Day tsunami was three times larger than predicted from the size of the earthquake alone.  So as aid agencies swung into action to help survivors, scientists also mobilised to investigate the earthquake zone off the coast of Sumatra where the tsunami originated.  And thanks to some unconventional sources of support, we were able to respond more rapidly than the usual processes of grant application and peer-review allow.

Earthquake on the ocean floor

Tsunamis are the product of events beneath the ocean floor, where the movement of plates of the Earth’s crust translates into movements of the seabed, which in turn disturb the ocean surface.  Seismologists can estimate the size of earthquakes from instruments around the globe and the Indian Ocean earthquake measured more than magnitude 9.0, making it one of the largest ever recorded.  But the subsequent chain of events that generates a tsunami can be complex.

The ocean floor either side of the fault where the earthquake occurs can lift up or drop down by several metres.  An earthquake can also trigger underwater landslides, for example, which disturb the ocean surface and contribute to the wave. So the key to understanding the birth of a tsunami is to understand how the earthquake has changed the ocean floor.

Investigations

One of the first scientific efforts on the scene of the Boxing Day earthquake was the Royal Navy’s survey ship HMS Scott, which was made available to scientists at short notice in response to the disaster.  HMS Scott is equipped with a sonar system that can map the seabed in great detail.  My colleagues Tim Henstock and Lisa McNeill of the National Oceanography Centre, Southampton, used HMS Scott’s sonar to identify seabed features such as underwater landslides that could have been formed during the earthquake.

But sonar can only tell you so much.  To really understand what happened, you have to inspect the seabed close-up.  So five months after the earthquake, I joined a team of geologists, geophysicists, wave modellers and biologists from the UK, US, Canada, France and India for another expedition to the same area.  We used a remotely-operated vehicle from a commercial survey ship to dive to the ocean floor and examine some of the features identified by HMS Scott.

Perhaps surprisingly, this is where biologists like me could contribute to the investigation.  Possible recent underwater landslides identified by HMS Scott turned out to be home to deep-sea creatures such as bamboo corals that were much too old to have grown since the earthquake.  But at one site, nicknamed ‘the ditch’, there were no obvious signs of life, consistent with recent disturbance of the seafloor during an earthquake.

Our goal is to obtain a better understanding of how earthquakes generate tsunamis.  This should help local authorities produce effective emergency plans for the next tsunami whenever and wherever it arrives.  And with 60 per cent of the world’s population living on or near a coast, it is ‘when’ and not ‘if’.

Funding footnote

Piecing together the events that generated the tsunami has a forensic feel and a colleague nicknamed our efforts CSI: Deep Sea in reference to TV’s popular Crime Scene Investigation series.  Our expedition was actually funded by the BBC and Discovery Channel to make a documentary.  We could not have got out there so quickly if the TV companies had not made it happen, as the carefully considered peer-review of research proposals usually takes months and itineraries for research ships are often fixed years in advance.  Thankfully science often offers a compelling narrative to match that of a detective story – and perhaps we should trade more on this asset for support.

Dr Jon Copley is a marine biologist at the National Oceanography Centre, Southampton