'Caught in the vice' between tectonic plates
An international drilling expedition to the Nankai Trough off Japan's southwest coast, one of the most volatile earthquake zones on Earth, has discovered that there is strong variation in the tectonic stresses in the area. The NanTroSEIZE (Nankai Trough Seismogenic Zone Experiment) expedition 314 measured geological formation properties of rock and sediment density, porosity, velocity of sound, natural gamma ray and resistivity. This first part of the experiment was successfully completed in November after 56 days of drilling at five sites along the trough's accretionary wedge (the undersea mountain range formed where the tectonic plates meet). Drilling went as deep as 1,400 metres below the seafloor. NanTroSEIZE is a multidisciplinary, multinational venture involving earth scientists, paleoceanographers, sedimentologists, geophysicists and geologists from 21 countries. Europe's participation in the experiment is coordinated by the European Consortium for Ocean Research Drilling (ECORD), funded under the EU's Sixth Framework Programme (FP6). 'The rock caught up in the tectonic plate boundary is literally falling apart as a result of intense stresses of tectonic plate convergence,' explains co-chief scientist Harold Tobin of the University of Wisconsin-Madison, USA. 'By drilling a transect spanning the area of tsunami generation, we found that the region that lies above the earthquake-producing zone exhibits very different stress conditions than other parts of the plate boundary.' 'Caught in the vice between two converging rigid tectonic plates, the wedge was found to be undergoing strain in preparation for the next earthquake,' says co-chief scientist Masataka Kinoshita of the Institute for Research on Earth Evolution (IFREE) at the Japan Agency for Marine Earth Science and Technology (JAMSTEC). The research team aboard the scientific drilling vessel Chikyu also found a methane hydrate-rich zone at one drill site, spreading out just 220 to 400 metres below the seafloor. This solid form of water, also known as methane ice, is believed to develop on the seafloor and deep in the sedimentary structures due to migration of gas from the depth of the Earth along geological faults that then crystallises on contact with cold sea water as a result of temperature and pressure. The ice contains large amounts of methane and is considered by some as a future source of fossil energy. However, critics stress that methane is a powerful greenhouse gas, which might have been responsible for past climate changes and might contribute to future climate changes. The experiment is projected to continue until 2012 in a total of four stages. The second has already started, as drilling expedition 315 set sail for the Nankai Trough immediately after expedition 314 returned. This as well as the following mission will revisit the drilled sites in order to take continuous samples from the upper section of the active accretionary prim and across the plate boundary faults.