The deep-focus earthquake cycle

地震并不是在地球上的任何地方都发生。相反，它们集中在构成地球表面的海洋板块和大陆板块边缘的沿着地带。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，小于格拉斯哥和爱丁堡之间的距离。在会聚板块边界，例如日本东海岸和南美洲西海岸，洋壳俯冲到地球深处。在这些边界上，地震可能发生在近700公里（435英里）的深处，这与伦敦和因弗内斯之间的距离相似。在地球表面几十公里范围内发生的地震以周期性模式发生。两个板块粘在一起，并被迫在地球深处的力量向不同的方向移动，将建立应变。在某一点上，应变太大，岩石无法承受，地震就发生了。这类似于松紧带的拉伸和断裂。地震后，有一段时间的放松和故障愈合。对于地球深处的地震，由于压力和摩擦力随着深度的增加而增加，因此咬合过程的性质必须有所不同。这类似于沿着地板拖动箱子或雪橇，箱子中的重量越大，拉动就越难，如果箱子中的重量太大，则摩擦力太大，箱子无法移动。但是，由于地震持续发生在很深的地方，一定有某种类似于在盒子上装上轮子的过程，减少摩擦，允许运动。这个提议旨在理解的问题是：是什么物理机制提供了轮子，允许发生深地震？过去几年我一直在开发独特的实验装置来解决这个问题。在我的设备中，我将重现发生深地震的极端压力（20万个大气压）和高温（800-1000 ℃）。然后，我将应变我的样本，并听取了“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，我将能够了解什么样的物理过程在深地震中是活跃的，以及是什么提供了允许深地震发生的“轮子”。这里提出的问题的答案对我们理解地球是如何发展的以及它现在的行为有着令人兴奋的意义。如果我能确定驱动深层地震的过程，我还必须了解地球深处小区域的矿物学和应力。通过对深源地震的深入了解，我们可以深入了解为什么地球与其他岩石行星如此不同，以及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.