Thermal structure of incoming sediments at the Sumatra subduction zone

Lead Research Organisation: University of Southampton
Department Name: School of Ocean and Earth Science

Abstract

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.

Planned Impact

Expedition 362 will drill and sample materials entering the Sumatran subduction zone to determine the present composition and physical state of the sediments, and how they evolve to control the properties and hence behaviour of the plate boundary fault that generates very large magnitude earthquakes and tsunamis, such as that in December 2004. A further objective is how the incoming sediments control the unusual large-scale structure of the margin. The benefits therefore will be potentially wide reaching and not simply of relevance to the academic community.

Much of the impact will develop through the Expedition-specific activities rather than during the moratorium grant work, and therefore will be done at no additional cost to the UK-IODP programme.

Specific areas of expected impact include:
(a) Hazard assessment and more appropriate mitigation in the Indian Ocean region and elsewhere. This will be accomplished by wide distribution of the scientific results and by maintaining links with relevant communities, eg through the involvement of Indonesian scientists in the Expedition and the post-cruise science efforts.
(b) Public education and outreach to those across a range of ages and locations who are interested in processes within the Earth. Two Education and Outreach officers will sail (supported by IODP) on the Expedition, and will coordinate activities including real-time links with school and public groups, and web-based materials including interviews with scientists about life on board and the science being carried out. Previous IODP expeditions have received strong media coverage within the UK and globally.

Publications

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Description The large thickness of sediments on the Indian plate approaching the Sumatra subduction zone means that high temperatures >100C can be reached offshore of the subduction zone. The deeper sediments within the section experience temperatures which are typically only present beneath the subduction zone, and are likely to lead to dehydration of clay minerals, changing the strength and frictional properties. There are ranges in the possible depth history from sound speeds that are imperfectly known, and from variations in the water content with depth; ranges in the history of temperatures come from uncertainty in the depth and from different estimates of the thermal conductivity of the sediments. However our key conclusion about the high temperatures being likely to cause changes to the strength is unaffected.
Exploitation Route Temperature history could be used in modelling reactions within the sediment sequence.
Sectors Education