Thermal structure of incoming sediments at the Sumatra subduction zone

2004年的节礼日海啸是由苏门答腊岛俯冲带的一场大地震引发的，印度洋沿岸的社区遭到了破坏。这是自20世纪60年代以来第一次9级以上的地震，因此也是第一次用现代技术进行研究的地震。这次地震是不寻常的，因为俯冲带板块边界断层上的运动延伸到接近地表，甚至可能已经到达海沟中的地表；在主要俯冲带地震中，断层以前被认为在大约10公里深的地方停止滑动。滑动向地表延伸的结果是地震震级更高，特别是海啸更大。地震以来的地球物理研究表明，发生浅层滑动的区域也具有不寻常的海底形状--从进入的板块向上有一个陡峭的斜坡，然后是一个大致恒定深度的宽阔高原，而不是通常存在的更陡峭的斜坡。这两个观测结果--浅层滑动和高原--都表明，进入这里俯冲带的沉积物异常坚固。俯冲带中沉积物的强度受存在的水量和组成颗粒的确切矿物的控制。粘土预计将构成沉积物的大部分，根据温度的不同，这些粘土具有不同的形式，具有非常不同的强度。如果温度高，那么存在的粘土就会更坚固，就像在其他俯冲带超过10-15公里的深度发现的那样。然而，在苏门答腊岛近海地区，沉积物在相对年轻的板块上缓慢积累，可能会使这些温度达到比正常水平低得多的温度。或者，沉积物中的水含量可能非常低。确定这些想法中哪一个是正确的唯一方法是在板块边界断层最终形成的深处钻探和采样沉积物。我们通过在俯冲带以外的位置钻探来做到这一点--在这里，我们需要采样的沉积物只在海床下1500米，而不是4000-4500米。我们将能够确定存在的沉积物的类型和沉积的年代，以及今天的水含量和温度。通过建立沉积物的堆积方式以及温度和水分如何随时间演变的计算机模型，我们将能够匹配当前的条件，然后能够预测未来沉积物的变化--随着我们钻探的地点最终成为俯冲带的一部分，温度和粘土的形态以及存在的水量将如何演变到未来。

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.