The Philippine plate of the Pacific slides under the Eurasian continental plate at the Nankai Trough subduction zone off Japan’s east coast. In this process the tectonic plates can become interlocked and build up stress. Earthquakes happen when such stress frees itself with a sudden jolt. On average an earthquake with a magnitude greater than eight occurs in the region every 90 to 150 years. Quakes of this intensity can trigger devastating tsunamis, the last of which took place in 1944. At that time the quake, which created a tsunami, cost more than one thousand human lives. The next strong quake is expected in the middle of the present century. In the context of the Integrated Ocean Drilling Program (IODP), an international marine research program, the Japanese research ship Chikyu is now drilling for the first time in what is known as the seismogenic zone of a subduction zone, the Nankai Trough. The seismogenic zone is the region in which earthquakes are generated.

Swiss researchers on board the research ship

Up to 25 international scientists from various disciplines have been on board the Chikyu continuously since September. They are replaced every eight weeks. Two Swiss researchers went on board in December; France Girault, a doctoral student in the Department of Earth Sciences at ETH Zurich, and Michael Strasser, a geologist who recently completed his doctor’s degree at ETH Zurich and now works as a post-doc at the Research Center Ocean Margins of the University of Bremen with a grant from the Swiss National Science Foundation.

The aim of the research borehole project is to gain a deeper insight into the processes responsible for the formation of earthquakes and tsunamis at subduction zones by recording physical and sedimentological parameters. This is very important because 90 percent of the energy released by earthquakes world-wide is generated at subduction zones.

In addition the plan is that one day special instruments that will be installed in two of the total of six boreholes during the subsequent expeditions will enable the continuous measurement of for example the pressure and temperature in the region of the seismogenic zone. Thus the on-line monitoring could lead to improved forecasts of earthquakes that are to be expected.

The Chikyu’s first research voyage

The Chikyu put to sea on 21 September for the project which is called “The Nankai Trough Seismogenic Zone Experiment” (NanTroSEIZE). It is the Chikyu’s first research expedition. The design of the ship, which was built in Japan, is similar to that of the oil industry’s drilling ships. Thus it is the first research ship with the ability, even in the deep ocean, to drill down several thousand metres into sediments that are under high gas or liquid pressure. The plan is to drill up to 6000 metres down into the ocean floor during the NanTroSEIZE Project.

Several boreholes in which only physical properties such as the density of the rock and the propagation speed of seismic waves were measured were drilled at the start of the voyage. The next step will now involve drilling out sediment cores from which firstly information about sedimentological and tectonic events will be obtained, and secondly micro-fossils that allow the age of the sediments to be dated will be extracted from the sediments and identified. The latter will be France Girault’s task for the next two months. She was flown out to the Chikyu off the coast of Japan by helicopter on 19 December and now, working in shifts round the clock with another palaeontologist until 5 February, she will examine the micro-fossil content of all the sediment cores that are brought to the surface by the drilling tower during this period of time.

A happy coincidence

She says she has dreamt of taking part in this kind of research voyage ever since she heard a lecture by Michael Strasser about four years ago. Girault explains that he aroused her enthusiasm at that time with his descriptions of a voyage on the research ship “Joides Resolution” off the coast of Costa Rica. They now both see it as a lucky coincidence that they are travelling on the Chikyu together. The fact that only seven of the international scientists on board are Europeans and two of these are Swiss is a small sensation.

Girault regards the voyage as a great opportunity to allow her to interact with an international, interdisciplinary research community. She says that a possibility of this kind occurs only rarely. She thinks the big responsibility would allow her to gain much experience and to learn to take decisions independently. She sees the biggest problem as the two-month stay on a ship, during which the life she is accustomed to will move into the background.

Together with three other researchers, Michael Strasser will be responsible for describing the sediments. Above all he is keen to see what these will look like. This is because one of the boreholes is expected to bring to the surface sediments from the point where the sediment covering of the oceanic plate is pushed underneath the sediment package of the continental plate. “Although we have a pretty good idea of what these should look like, they have never yet been seen from this region before.”

Researching the origin of tsunamis

The plan is for the project to be completed by 2013 at the latest. Until then it will consume more than 100 million dollars a year. The researchers expect great things from the data that will be obtained in the meantime. For his personal research work on board, Strasser’s main hope is for new knowledge about the conditions at what is known as the “Megasplay Fracture Zone”, a kind of branching-off from the main disturbance zone. This branching, which will be drilled into directly, runs from the depth of the subduction zone to the surface of the ocean floor. Strasser explains that this means that energy, gases and liquids that are mobilised during an earthquake can travel along it to the surface of the sea bed. Therefore it has been suspected that conditions of this kind play a decisive part in the formation of tsunamis. A disturbance zone of this kind is also located off the coast of Sumatra, and caused the devastating seaquake and tsunami at Christmas 2004 that caused the deaths of tens of thousands of people. Therefore an exact knowledge of the processes that lead to catastrophic events of this kind could bring about significant progress in tsunami research and forecasting.

However, some obstacles still remain to be surmounted before that point is reached, because not only is it very costly in terms of time and money to drill deep into the ocean floor in water that is up to 4000 metres deep in some places. For example special “lateral thrust propellers” are needed to prevent the ship being carried away by the current during a drilling operation. Strasser explains that these are controlled via the Global Positioning System. However, he says there are occasional breakdowns that only recently forced the ship to leave expensive equipment behind on the ocean floor.

During their stay on the ship, Girault and Strasser will report in ETH Life in the coming weeks about events on the Chikyu.