The deep-focus earthquake cycle

地球上并不是所有地方都会发生地震。相反，它们集中在构成地球表面的海洋和大陆板块边缘的带状区域。大约四分之三的地震发生在距离地表60公里(37英里)的范围内--大约是曼彻斯特和利兹之间的距离，而不是格拉斯哥和爱丁堡之间的距离。在汇聚的板块边界，例如日本的东海岸和南美洲的西海岸，洋壳被俯冲到地球深处。在这些边界处，地震可以发生在近700公里(435英里)的深处，这一距离类似于伦敦和因弗内斯之间的距离。发生在地球表面几十公里以内的地震是以周期性模式发生的。两个板块粘在一起，在地球深处的力量的作用下，被迫向不同的方向移动，将会产生张力。在某种程度上，岩石承受不了太大的应变，地震就发生了。这类似于橡皮筋的拉伸和断裂。地震发生后，有一段时间是放松和断层愈合的时期。对于地球深处的地震，咬合过程的性质必须不同，因为压力，因此摩擦，随着深度的增加而增加。这类似于沿着地板拖拽箱子或雪橇，箱子里的重量越大，拉起来就越难，如果箱子里的重量太大，摩擦力太大，箱子就无法移动。但是，由于地震继续在很深的地方发生，一定有一些类似于在盒子上放置轮子的过程，以减少摩擦并允许运动。这项提议旨在理解的问题是：提供轮子的物理机制是什么，从而允许深层地震？我在过去几年里一直在开发独特的实验设备来解决这个问题。在我的仪器中，我将再现发生深部地震时的极端压力(200,000个大气压)和高温(800-1000摄氏度)。然后，我会过滤我的样本，听一听“实验室地震”发出的声音。通过分析“实验室地震”的规模和数量，我将能够理解哪些物理过程在深部地震中是活跃的，以及是什么提供了允许深部地震发生的“车轮”。对这里提出的问题的答案对于我们理解地球是如何发展以及它现在的行为具有令人兴奋的启示。如果我能确定驱动深部地震的过程，我还必须了解地球深部小区域存在的矿物学和应力。通过对深部地震的透彻了解，我们可以深入了解为什么地球与其他岩石行星如此不同，为什么地球适合生命。

Earthquakes do not occur everywhere on Earth. Instead they are concentrated in bands along the edges of the oceanic and continental plates that make up the Earth's surface. Approximately three quarters of earthquakes occur within 60km (37 miles), of the surface - about the distance between Manchester and Leeds and less than the distance between Glasgow and Edinburgh. At convergent plate boundaries, for example off the East cost of Japan and the West coast of South America, oceanic crust is subducted deep into the Earth. At these boundaries earthquakes can occur to depths of almost 700 km, or 435 miles, a distance similar to that between London and Inverness.The earthquakes that occur within a few 10s of kilometres of the Earth's surface occur in a cyclic pattern. Two plates that are stuck together and being forced to move in different directions by forces deep in the Earth, will build up strain. At some point the strain is too much for the rocks to hold and an earthquake occurs. This is similar to the stretching and breaking of an elastic band. After the earthquake, there is a period of relaxation and fault healing. For earthquakes deep in the Earth the nature of the snapping process has to be different because pressure, and therefore friction, increases with depth. It is analogous to dragging a box or sled along the floor, the more weight there is in the box the harder it is to pull and if there is too much weight in the box, the friction is too great for the box to be moved. But, because earthquakes continue to occur at great depths, there must be some process analogous to putting wheels on the box that reduces friction and allows motion.The question that this proposal aims to understand is: what is the physical mechanism providing the wheels, permitting deep earthquakes?I have spent the past few years developing the unique experimental apparatus to tackle this question. In my apparatus I will recreate the extreme pressures (200,000 atmospheres) and elevated temperatures (800-1000 C) under which the deep earthquakes occur. I will then strain my samples and listen for the sound emitted by "lab-quakes". By analysing the size and number of "lab-quakes" I will be able to understand what physical processes are active in deep earthquakes and so what provides the 'wheels' allowing deep earthquakes to happen.The answers to the questions posed here have exciting implications for our understanding of how the Earth developed and how it behaves now. If I can determine the processes that drive deep earthquakes, I will also have to understand the mineralogy and stress present in small regions of the deep Earth. With a thorough understanding of deep earthquakes, we can gain insights into why the Earth is so different from the other rocky planets and why the Earth is hospitable to life.