Digging for answers: getting to the bottom of the Tohoku earthquake

On the 11th March 2011 the Tohoku earthquake struck off the north-eastern coast of the Japanese island of Honshu. An area of seafloor larger than Greater Tokyo moved eastwards by 5 metres and some parts of the fault moved by up to 50 metres. A total of nearly 19,000 people were killed, mainly as a result of the following tsunami. But the event came as a huge surprise to scientists the world over “This was a truly extraordinary earthquake and very unfortunately seismologists, including myself, did not expect this to happen,” said Japanese scientist Dr. Kiyoshi Suyehiro.

Why was the earthquake bigger than expected?

Now, a year later, an international collaboration of German, Swiss and Japanese scientists are carrying out research expeditions in the subduction zone off the coast of Japan to try and uncover exactly what happened and why the earthquake was so much bigger than expected.

The team used echo sounders to map the shape of the seafloor in the region surrounding the epicentre. The results of this mapping were published in a short article in the journal Science last December. By comparing this most recent survey with previous studies done in 1999 and 2004 the scientists were able to uncover how the underwater landscape had changed. In some areas the seafloor was 50 metres higher than before the earthquake. The team believes that these are large submarine landslides, or turbidity currents, which were triggered by the shaking during the earthquake and now sit on the floor of the ocean.

This spring, the German research vessel SONNE got a step closer by directly collecting sediment samples from the so-called accretionary prism, the wedge of sediments that sits in the trench off the coast of Japan. The results of this cruise were presented at the European Geosciences Union conference in April. “We acquired new data and samples that will help us to better understand the development of such enormous earthquakes,” explained Professor Gerold Wefer, chief scientist for the project and director of MARUM, the German Centre for Marine Environmental Changes at Bremen University.

The expedition took 16 sedimentary cores totalling 95 metres in length. They also sent a remotely operated vehicle (ROV) underwater to take photos and further samples at the bottom of the ocean. When pebbles are found in the sediment samples it is likely that they are from one of the submarine landslides. It is also possible to use microfossils called coccolithophores, tiny shells of calcium carbonate, to identify the landslides. “Usually they cannot be deposited deeper than 4000 or 4500 metres because they would get dissolved,” explained Professor Michael Strasser from the Swiss Federal Institute of Technology. So when scientists find coccolithophores in sediment samples from depths of 7 kilometres they know they must be looking at landslide deposits.

“Perhaps our most striking result,” continued Professor Strasser, “is that we were able to get a record of at least three, older sedimentary mobilisation events, which potentially suggests the occurrence of previous, large, Tohoku-like earthquakes.” In other words, the Tohoku earthquake might not be as unprecedented as thought. By making geochemical measurements on the sediments or by correlating ash layers with specific volcanic eruptions the scientists hope to date these older landslides. If this information is known, they will be able to calculate an earthquake recurrence interval for the region. Knowing how often these sorts of events occur is vital for assessing the seismic hazard in the future.

A third project is also underway to try and discover why the earthquake and tsunami were so much bigger than predicted. The Japan Trench Fast Drilling Project (JFAST) is being carried out on the Japanese drilling vessel ‘Chikyu’ as part of the global Integrated Ocean Drilling Programme (IODP). It had been thought that the soft sediments within the Japan Trench slip freely rather than storing stress and undergoing brittle rupture during earthquakes. But scientists now think that the tsunami was so devastating because slip also occurred in these sediments explained Dr. Kiyoshi Suyehiro, President of the IODP.

The JFAST cruise set sail on the 1st April to take samples of the sediments and work out why the material stored energy and behaved in a brittle fashion. “JFAST aims to drill through the very tip of the fault zone where the slipping occurred,” said Dr. Suyehiro. The JFAST project also hopes to make temperature measurements in the vicinity of the epicentre. When the earthquake occurred the surrounding material was heated due to friction on the fault. By measuring the leftover heat the frictional stress during the earthquake can be determined.

The remotely-operated vehicle used by the scientists on the German research vessel. Copyright: MARUM – Center for Marine Environmental Sciences, Bremen/Germany

The team of 28 international scientists on board also faces significant technical challenges. They are drilling in a water depth of 6.91 kilometres and are hoping to extract a further 1 kilometre of sediment core from below the seafloor; only one other deep sea drilling project has drilled successfully at a greater depth. But if the expedition retrieves the samples it needs, the research will have consequences for other locations around the globe that are at risk from these big earthquakes.

Many have been shocked by the seemingly large number of catastrophic earthquakes in the past decade. Earthquakes aren’t getting more frequent but more and more people are living close to the danger zones. With many of the world’s megacities at risk, this sort of research effort is vital. It will also be crucial to ensure that the results are communicated to countries that aren’t fortunate enough to have their own scientific expertise.

Featured image credit: RF Forschungsschiffahrt Bremen/Germany (The German research vessel SONNE, which collected samples from underwater landslides)

Tim Middleton
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