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

世界上大多数特大地震都发生在两个板块交汇的俯冲带的板块边界断层上（包括2004年，2005年和2007年的苏门答腊; 2010年的智利; 2011年的日本）。由于这些断层在地震中移动的部分位于水下，它们也可能是大海啸的来源。然而，不同的俯冲带所遭受的地震规模不同，地震破裂的模式也不同，因此地震的危害也有很大差异。在大多数情况下，板块边界断层上的破裂仅限于断层位于海底以下约30- 40公里至约5- 15公里的区域。但在最近的一些地震中，断层破裂的范围要大得多，深度比预期的要浅得多，甚至可能远至海底。2004年发生在印度尼西亚（北苏门答腊）和印度（安达曼和尼科巴群岛）近海的地震就是一个浅层破裂的例子，但这种行为的原因还不清楚。在过去的几十年里，随着记录地震波技术的改进，我们对地震过程的理解有了很大的进步。但在产生这些地震的断层带内直接取样和测量的机会很少，在沉积物堆积的大部分地区尤其具有挑战性。不同的断层行为取决于断层本身的物理性质，这些物理性质受俯冲带附近海底沉积物的控制，以及断层内是否存在流体等因素。确定这些性质和流体存在的一种方法是钻入断层带并直接取样或测量。但是在断层很深的地方，例如，从山上侵蚀下来的大量沉积物被运到海上形成了巨大的海底扇，作为一种替代方案，我们可以在它们进入俯冲带之前钻取较薄的沉积物。我们可以测量它们的性质，然后使用实验室实验和数值模拟来计算沉积物如何随着压力和温度的增加而变化，因为它们被埋得越来越深，然后进入俯冲带。综合海洋发现计划（IODP）第362远征队将通过在印度尼西亚苏门答腊岛近海的印度构造板块上钻两个钻孔来做到这一点，苏门答腊岛是2004年地震和海啸的源头。尽管引发这次地震的俯冲带延伸了5000公里，但作为科学海洋钻探计划的一部分，它以前从未被采样过。这两个钻孔将使科学家能够采样和测量海底以下约1.5公里深处的沉积物和沉积岩的性质-这些沉积物和岩石然后进入俯冲带，它们控制着发展的断层类型，地震期间断层滑动的方式，因此地震和海啸的规模。考察期间和之后的测量将侧重于通过进行物理、化学和热测量来确定材料的强度和硬度。这些测量将有助于科学团队评估断层形成的沉积物和岩石是否因为非常厚的沉积物堆而比其他俯冲带的沉积物和岩石坚固得多，以及这是否是地震延伸到海底以下浅层的原因。当分析完成后，它们可以用来计算出其他俯冲带可能有类似类型的地震和海啸的2004年事件。因此，这些结果对于评估印度尼西亚和世界其他国家的自然灾害非常重要。

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.