The deep-focus earthquake cycle

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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.