Last fire alarm

The fire alarm booms out over the ship’s loudspeakers at 10 o’clock every Sunday morning: “This is a fire drill. Please proceed to your muster station.” With helmet, safety goggles, steel toe-capped shoes and life-jacket, we all gather in front of the lifeboats while the crew members practice fighting a fire or dealing with an emergency. Safety must be guaranteed on a ship as large as the Chikyu. Looking like clowns and waiting for the safety officer’s OK, those who have just woken up prolong their sleep on the deck, while those who have just finished their shift and are tired from work also have a little snooze. These “Fire Drills” are like an anchor for us in our gradually blurred time-scale. An anchor that we can hold on to and that lets us know which day it is. We count the weeks on board by the number of fire drills lying behind or in front of us. One more remains before we put into port. It will be the very first time the new Japanese research ship Chikyu has returned from a scientific expedition. Then we will have dry land under our feet for the first time in almost two months.

We already know that the main scientific goals of our expedition were achieved. Boreholes were drilled successfully through the mega splay fault and into the frontal thrust of the Nankai Trough subduction zone at two sites (ETH Life Report of 8.1.08). Despite several difficult situations where for example the borehole’s stability was endangered and it threatened to collapse into itself, or only small fragments were brought to the surface instead of beautiful complete core sequences, we collected enough information and data for further research projects. It is unbelievably impressive to think that we drilled down into the ocean floor to a depth of 600 metres in water up to 4000 metres deep, and did so in a sea bed severely deformed by tectonic forces and characterised by a complicated rock sequence from fine-grained clays to massive gravels.

Geo-detectives

One of the highlights occurred last week when we drilled into the frontal thrust, the plate boundary between the Philippine and Eurasian plates. The excitement at seeing these cores finally opened out on the logging table was very great. We were surprised and fascinated to see that the critical zone is a sequence of several discrete thin layers only a few centimetres thick. Like “geological detectives”, we are now carrying out various precise analyses on the drill core samples (ETH Life Report of 22.1.08) to try to string together the individual jigsaw pieces so as to understand the processes along the plate boundaries and to reconstruct the development history of this active continental margin. Our data allows us to look back about five million years and to study the processes that are still active today and are partly responsible for the earthquakes in Japan, both in earlier times and today, in the deeper levels of the subduction zone.

We have much work still to do before the individual jigsaw pieces yield a coherent picture. After the expedition, the samples that were collected will be analysed in the laboratories of the scientific participants throughout the world to refine the data gathered on board. Firstly the data from our studies will be input into a micropaleontological dissertation project by ETH Zurich. Secondly we will investigate in our own laboratories how the drill core rocks and sediments behave mechanically when exposed to the tremors caused by earthquakes. We do this by clamping sample material in a pressure cell in which it is exposed to a cyclically changing load. This will simulate the stress variations in the underground strata during an earthquake, and we can analyse the deformations or fracture behaviour of the material that they cause. We hope this will give us important information about how the ocean floor behaves during a major earthquake. For example whether and how the fracture zones in the ocean sediments propagate up to the surface or whether the ocean floor slope, which is relatively steep in this region, collapses and the sediments trigger large underwater avalanches. Nowadays it is assumed that both mechanisms can trigger tsunamis caused by the sudden displacement of water resulting from the jolting movement of the ocean floor.

Other scientists will study further geological aspects of the region under investigation and will compare them with other studies or will use computer modelling to extrapolate the knowledge into the deeper regions of the subduction zone. This will enable a generic study of the processes operating during the origin of earthquakes or of how subduction zones develop. The studies will then form the basis for the second and third phases of the Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE; ETH Life Report of 8.1.08), because the Chikyu will set out on further expeditions as early as the end of the year. The plan is then to drill down to an even greater depth into the mega splay fracture zone and the plate boundaries - into the regions in which earthquakes originate.

Unique experience

The prospect of continuing the scientific studies in the laboratory and on the computers of our home institutes, and the anticipation of having dry land under our feet again at last, spurs us on to bring the last week here on board to a successful conclusion. No matter what the remaining days may yet bring, participating in this major ocean drilling project was certainly a unique experience for us young Swiss scientists. We have learnt a great deal from the experts on board, have made good contacts and are enthusiastic about the new scientific insights. Hopefully in the future this will allow us a better understanding of the geological processes occurring at subduction zones.