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

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.

Who?
1. The impact of the Expedition's results for understanding fault slip behaviour and earthquake and tsnuami generation processes will be of direct benefit to many different communities. Firstly the results will impact those working on hazard assessment and mitigation in Indonesia and other countries surrounding the Indian Ocean. In addition, the results will have impact on countries bordering other subduction zones with very thick and/or highly indurated input sections. Such subduction margins include the Makran offshore Iran and Pakistan, the southern Lesser Antilles, Alaska, and Cascadia offshore northwestern USA and southern Canada. Finally, the results will also be relevant to the earthquake potential of other fault systems rupturing thick sedimentary sections with unexpected levels of induration and diagenetic strengthening (where those sections may be rarely directly sampled).

2. Outreach efforts and wide broadcast of the results of the Expedition will impact the general public, in terms of increasing understanding of Earth science processes, plate tectonics, subduction zone processes, and the formation of large earthquakes and tsunamis. These subjects have always been of great public interest, however interest in subduction zones and related hazards is at a very high level due to the recent devastating events, including the Indian Ocean earthquake and tsunami in December 2004 and the Japan Trench earthquake and tsunami in March, 2011. Awareness has been amplified because of the new levels of media and social media coverage following international natural disasters. Public awareness is important for several reasons: general scientific education of the population; understanding of the importance of scientific research for all; encouragement of the next generation to study science and take up scientific careers; and the need for awareness of natural hazards for tourists overseas.

How?
1. The drilling results and their implications for hazard potential will be directly made available to research organisations and relevant government bodies in Indonesia via Southampton University's and the IODP Expedition's contacts. This will be aided by participation of Dr Marina Frederik from BPPT Indonesia in the Expedition. Contacts of the PI, Science Party and the PI's wider subduction zone collaborator network will also be used to disseminate the results to organisations in other countries that may a) be impacted by the Sunda subduction zone hazard potential and b) by other subduction zones with potentially similar input section and hence fault and earthquake properties. This will also be facilitated by presentation of the results at worldwide conferences and workshops and by publication.

2. Science communication and public outreach is a major component of Expedition 362 and of IODP and two Education and Outreach (E&O) Officers will sail. The Co-Chief Scientist's role will be to assist in coordination of the education and outreach programme, to liaise with the two E&O Officers, and to promote and to encourage promotion amongst the science party of the objectives, results and implications. As a result, the general public will learn about processes at subduction zones, the potential hazards, the technology being developed and applied, and how the results can be used to improve mitigation of these natural hazards. Planned activities during the Expedition include: web-based materials following the Expedition's results and progress; interviews broadcast online with science party members about shipboard life and science; activities with school groups; video conference seminars from ship to shore; and ship tours in Singapore for local Universities and other groups. Post-Expedition these efforts will continue by the PI, E&O officers, and by other members of the science party, including: school visits and resources; public talks; University open day stands (including Southampton/NOCS); and potential museum exhibits