Thermal structure of incoming sediments at the Sumatra subduction zone

2004年节礼日，苏门答腊岛俯冲带发生了一场大地震，引发了海啸，印度洋沿岸的社区被海啸摧毁。这是自20世纪60年代以来第一次震级超过9级的地震，因此也是第一次用现代技术研究的地震。这次地震是不寻常的，因为俯冲带板块边界断层的运动向地表延伸，甚至可能通过海沟到达地表；以前认为，主要俯冲带地震中的断层在深度约10公里处停止滑动。滑动向地表延伸的结果是地震震级升高，特别是海啸的规模更大。地震后的地球物理研究表明，发生浅层滑动的地区也有一种不寻常的海底形状——从进入的板块开始有一个陡峭的斜坡，然后是一个大致恒定深度的广阔高原，而不是通常存在的更平缓的斜坡。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强烈。俯冲带沉积物的强度是由存在的水量和构成颗粒的确切矿物质控制的。粘土被认为是沉积物的主要组成部分，根据温度的不同，它们以不同的形式存在，具有不同的强度。如果温度高，那么存在的粘土就会更坚固，就像在其他俯冲带中在深度超过10-15公里处发现的那样。然而，在苏门答腊岛近海地区，沉积物缓慢堆积到一个相对年轻的板块上，可能会使这些温度达到比正常温度低得多的水平。或者，沉积物可能含有非常少量的水。要确定哪一种观点是正确的，唯一的方法是在板块边界断层最终形成的深度对沉积物进行钻探和取样。我们通过在俯冲带以外的地方钻探来做到这一点——在这里，我们需要采样的沉积物仅在海床下1500米，而不是4000-4500米。我们将能够确定目前沉积物的类型和沉积的年代，以及现在的含水量和温度。沉积物通过建立计算机模型的建立,以及温度和含水量随时间进化,我们将匹配的条件,然后能够预测将来沉积物发生了什么——如何将温度,因此黏土的形式,以及现在的水量,演变成未来我们有钻的地方最终变成了俯冲带的一部分。

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.