Thermal structure of incoming sediments at the Sumatra subduction zone

Coastal communities across the Indian Ocean were devastated by the 2004 Boxing Day tsunami that was generated by a very large earthquake in the Sumatra subduction zone. This was the first earthquake greater than magnitude 9 since the 1960s, and hence the first to be studied with modern techniques. The earthquake was unusual because movement on the subduction zone plate boundary fault extended close to the surface, and may even have reached the surface in the trench; the faults in major subduction zone earthquakes were previously thought to stop slipping at about 10 km depth. The result of slip extending close to the surface is that the earthquake magnitude was higher, and the tsunami in particular was made larger. Geophysical studies since the earthquake show that the region where shallow slip occurred also has an unusual seabed shape - there is a steep slope up from the incoming plate, and then a broad plateau of approximately constant depth, rather than a more gradual slope that is generally present. Both of these observations - shallow slip, and the plateau - suggest that the sediments that go into the subduction zone here are unusually strong.

The strength of the sediments in subduction zones is controlled by the amount of water present, and by the exact minerals that make up the grains. Clays are expected to make up much of the sediment, and these take different forms, with very different strength, depending on the temperature. If the temperatures are high, then the clays present will be stronger, as are found at depths more than 10-15km in other subduction zones. However, in the region offshore Sumatra slow accumulation of sediments onto a relatively young plate may allow these temperatures to be reached much shallower than normal. Alternatively, the sediments may contain a very low amount of water. The only way to determine which of these ideas is correct is to drill and sample the sediments at depths where the plate boundary fault will eventually form. We do this by drilling at a location outside of the subduction zone - here the sediments we need to sample are only 1500m beneath the seabed rather than 4000-4500m. We will be able to determine the type of sediment present and what age it was deposited, as well as the present-day water content and the temperature. By building computer models of the way that the sediment builds up, and how the temperature and water content have evolved through time, we will match the present-day conditions, and then be able to predict what happens to the sediments in the future - how will the temperature and hence the form of the clays, as well as the amount of water present, evolve into the future as the site where we have drilled eventually becomes a part of the subduction zone.