The deep-focus earthquake cycle

地震并不是在地球上的任何地方都会发生。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地带。大约四分之三的地震发生在地表60公里（37英里）以内——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。在汇合的板块边界上，例如在日本东岸和南美洲西海岸附近，海洋地壳俯冲到地球深处。在这些边界上，地震可以发生在近700公里（435英里）的深度，这一距离与伦敦和因弗内斯之间的距离相似。发生在距离地球表面几十公里范围内的地震以周期性的模式发生。两个板块被粘在一起，在地球深处的力的作用下被迫向不同的方向移动，就会产生张力。在某一点上，岩石承受的压力太大，地震就发生了。这类似于橡皮筋的拉伸和断裂。地震过后，会有一段放松期和断层愈合期。对于地球深处的地震来说，断裂过程的性质肯定是不同的，因为压力和摩擦力会随着深度的增加而增加。这类似于在地板上拖着一个盒子或雪橇，盒子里的重量越大，就越难拉动，如果盒子里的重量太大，摩擦就太大，盒子就无法移动。但是，由于地震继续发生在极深的地方，一定有一些类似于在盒子上安装轮子的过程，以减少摩擦并允许运动。这个提议旨在理解的问题是：是什么物理机制提供了轮子，允许深层地震？在过去的几年里，我一直在开发一种独特的实验装置来解决这个问题。在我的仪器中，我将重现深层地震发生时的极端压力（20万大气压）和高温（800-1000摄氏度）。然后，我将对样品进行过滤，并聆听“实验室地震”发出的声音。通过分析“实验室地震”的大小和数量，我将能够了解深层地震中活跃的物理过程，以及是什么提供了“轮子”，使深层地震发生。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令人兴奋的意义。如果我能确定导致深层地震的过程，我还必须了解地球深处小区域的矿物学和应力。通过对深层地震的深入了解，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生存。

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.