IODP Expedition 362 Sumatra Seismogenesis, Co-Chief activities and deformation structures of the subducting oceanic plate

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Most of the world's very large earthquakes happen on the plate boundary faults at subduction zones where two plates converge (these include Sumatra in 2004, 2005, and 2007; Chile in 2010; and Japan in 2011). Because the parts of these faults that move during the earthquake lie underwater, they can also be the source of major tsunami. However, different subduction zones are subject to different sizes of earthquakes, and different patterns of earthquake rupture, so that the hazards vary significantly. In most cases rupture on the plate boundary faults is limited to a zone where the fault lies from ~30-40km up to ~5-15km beneath the seabed. But in some recent earthquakes the fault rupture has been much more extensive, reaching much shallower depths than expected and potentially even as far as the seabed. The 2004 earthquake that ruptured offshore Indonesia (North Sumatra) and India (the Andaman and Nicobar Islands) is one example of shallow rupture but the reasons for this behaviour are not well known. Our understanding of the earthquake process has significantly advanced in the last few decades with improvements in the technology for recording earthquake waves. But opportunities to directly sample and take measurements within the fault zones which generate these earthquakes are rare and are particularly challenging where large sections of sediment have built up.The different fault behaviours depend on the physical properties of the faults themselves, controlled by the seabed sediments adjacent to the subduction zone, and factors such as the presence of fluids within the fault. One way to determine these properties, and presence of fluids, is to drill into the fault zone and directly take samples or measurements. But in places where the fault is very deep, for example where large thicknesses of sediment eroding from mountains and transported offshore to create large submarine fans have accumulated, as an alternative, we can drill the thinner sediments before they enter the subduction zone. We can measure their properties and then use laboratory experiments and numerical modelling to work out how the sediments change as pressure and temperature builds up as they are buried deeper and deeper and then enter the subduction zone. The Integrated Ocean Discovery Program (IODP) Expedition 362 will do just this, by drilling two boreholes on the Indian tectonic plate offshore Sumatra Island, Indonesia, the source of the 2004 earthquake and tsunami. Although the subduction zone that generated this earthquake extends for 5000 km, it has never before been sampled as part of a scientific ocean drilling programme. The two boreholes will enable scientists to sample and measure the properties of the sediments and sedimentary rocks to depths of ~1.5 km below the seafloor - these sediments and rocks then enter the subduction zone and they control the type of fault that develops, the way the fault slips during an earthquake, and hence the size of the earthquake and tsunami. Measurements during and after the expedition will focus on determining the strength and hardness of the material by making physical, chemical and thermal measurements. These measurements will help the scientific team to assess whether the fault-forming sediments and rocks are much stronger than those in other subduction zones because of the very thick sediment pile, and whether this is the reason for the earthquake extending to shallow depths below the seafloor. When the analyses have been completed, they can be used to work out which other subduction zones might have similar types of earthquakes and tsunami to the 2004 event. So the results will be very important for assessing natural hazards in Indonesia and in other countries around the world.