Imaging the African Superplume

自从地球在45亿年前形成以来，它只有一个目标，那就是冷却。地幔位于我们居住的5-50公里厚的地壳和3000公里深的铁核之间，本质上是岩石。然而，在数亿年的时间里，表面的冷物质沉入地球，而内部的热物质上升。这种对流过程是板块构造的引擎，并塑造了我们今天所知道的地球。一个经常被争论的观点是，一些向上流动的物质集中在从地核边缘升起的圆柱形柱体中，即“地幔柱”。地幔柱被认为是造成现今远离板块边界的火山活动的原因，比如所谓的热点地区，如夏威夷和东非的阿法尔。羽流也被认为是过去可能分裂大陆并改变气候的更大爆发的原因。为了找到羽状物，我们可以用地震学来观察地球内部。发生在世界各地的地震通过我们的星球发送振动（如声波）。我们用一种叫做地震仪的灵敏仪器记录下世界各地的震动。如果地震能量通过一个炎热的地区，它会慢下来，如果它通过一个寒冷的地区，它会加快。如果我们从世界不同地区记录足够多的地震波，我们就可以建立地球内部的3D图像（就像医生使用X射线一样）。通过全球地震仪网络，我们成功地对下沉到地球深处的冷物质进行了成像，因为这些物体的大小为数百公里。然而，到目前为止，还没有人最终拍摄到热的上升羽流，因为这些羽流被认为非常狭窄（10公里宽），太小了，无法看到稀疏的全球地震仪分布。密集的区域地震网络将有所帮助，但大多数热点位于海洋中，部署地震仪具有挑战性且昂贵。拟议项目的目的是研究少数几个主要大陆热点区域之一的东非大裂谷。在过去的20年里，从南非到沙特阿拉伯的整个地区部署了许多临时地震网络，以了解大陆分裂。由于它们的体积很小，每一个都只能拍摄地球顶部几百公里的图像。为了更深入地研究，不同的网络组织已经同意与我分享他们的数据，以创建一个大型阵列。这意味着我将能够从地球深处的源头到地表成像一个可能的羽状物。这些图像将使我能够研究羽流是由什么组成的，它有多热，以及它在板块构造和地球冷却中扮演的角色。制作高分辨率地球内部模型的一个直接好处是，我们将能够更好地定位全球地震和核爆炸。地幔柱也被认为是非洲异常高海拔的原因，海拔高出海平面数公里。通过更好地了解羽状流，我们可以预测隆起的历史，这反过来又控制了东非河流盆地中石油的聚集地。这些地震图像还将有助于回答一个根本性的问题，即地幔柱在地球内部引擎中扮演什么角色，以及它们如何影响非洲大陆沿着裂谷的分裂。有了这一认识，我们就可以更好地评估与这一过程有关的火山和地震危险，这是东非合作者目前关心的问题。

Since the Earth formed some 4.5 billion years ago, it has had one objective, to cool down. The Earth's mantle, found between the 5-50 km thick crust on which we live, and the iron core at 3000 km depth, is rocky in nature. However, over hundreds of millions of years cold material at the surface sinks into the Earth, while hot material from the interior rises. This convective process is the engine for plate tectonics, and has shaped the Earth as we know it today. One, often debated suggestion is that some of the upward flow of material is focused in cylindrical columns, 'mantle plumes', rising from the edge of the core. Mantle plumes have been proposed to cause present-day volcanism away from plate boundaries, at so-called hotspots like Hawaii and Afar in eastern Africa. Plumes have also been held responsible for much larger past eruptions that may have split continents, and changed the climate. To find plumes, we can use seismology to look inside the Earth. Earthquakes occurring around the world send vibrations (like sound waves) through our planet. We record these vibrations all over the world using sensitive instruments called seismometers. If earthquake energy passes through a hot region, it will slow down, and if it passes through a cold region it will speed up. If we record enough earthquake waves from different parts of the world, we can build up a 3D image of the Earth's interior (much like a medical doctor uses X-rays). With global networks of seismometers, we have successfully imaged cold material sinking to large depths in the Earth, as these bodies are hundreds of kilometers in size. However, to date no one has conclusively imaged hot rising plumes, as these are thought to be very narrow (10s of kms across), too small to see with the sparse global distribution of seismometers. Dense regional seismic networks would help, but most hotspots are located in the oceans where deploying seismometers is challenging and expensive. The aim of the proposed project is to study one of the few major continental hotspot regions, the East African Rift. Over the past two decades, many temporary seismic networks have been deployed across the area from South Africa to Saudi-Arabia, to understand continental break-up. Due to their small size, each individually can only image the top few hundred kilometers of the Earth. To look deeper, the different network groups have agreed to share their data with me to create one large array. This means I will be able to image a possible plume from its source deep in the Earth to the surface. These images will allow me to investigate what the plume is made of, how hot it is, and what role it plays in plate tectonics and the cooling of the Earth. A direct benefit of producing high resolution models of the Earth's interior is that we will be able to better locate earthquakes and nuclear explosions globally. Mantle plumes are also held responsible for Africa's unusually high elevation, kilometers above sea level. By better understanding plumes, we can predict uplift history, which in turn controls where oil may collect in East Africa's river basins. The seismic images will also help to answer the fundamental question of what role plumes play in the Earth's internal engine, and how they affect the breaking up of the African continent along the rift. With this understanding, we can improve assessment of the volcanic and seismic hazards associated with this process, a current concern to collaborators in East Africa.