All rocks have holes within them (called pores) that are filled with either gas or water (which together are referred to as fluids). These fluids are able to flow slowly along connected paths through the rock and can move chemicals with them. A common connected pathway occurs through geological faults. These are zones with lots of fractures which, if they are not filled with other material, provide lots of connected pore space for fluids to flow. Being able to predict the rate of fluid flow in fractured rocks is critical to extracting groundwater, oil and gas and geothermal energy. Deep rocks are also used for burial of waste materials, like radioactive waste or greenhouse gases. In this case an important part of assessing the safety of underground waste disposal sites is to predict the time taken for leaking waste dissolved in water to travel to the surface. If we can understand the physical laws that govern the formation of fractures in rocks, then we can make better predictions of flow paths beneath the ground. When earthquakes occur along geological fault lines they damage rocks, producing new paths for fluids to flow through. However at the same time, if fluids flow into a geological fault line, it can make it weaker and cause it to slip. Fluid can affect earthquakes by causing faults to 'heal' between earthquakes. This occurs when water flowing through a fault dumps the chemicals it's carrying to make new minerals (like cementing the two sides of the crack together). Next time there is an earthquake it will rupture this cement. If there is no healing, the fault will just slide without making an earthquake. The linking of the two processes of fluid flow and earthquakes has been suggested by scientists before, but we don't really understand how they are linked well enough to be able to make predictions. This proposal uses data taken from micro-earthquakes that have been caused by the construction of a large surface water reservoir in Brazil. Every year the water level in the reservoir rises and a few months later small earthquakes occur. This shows that the water is flowing into the faults and causing earthquakes to rupture along the fault zone. We are very lucky that in this case some earthquake scientists collected really accurate data to pin point the earthquake locations, often we can feel the earthquakes but don't know exactly where they are. By analysing the time it takes for the earthquakes to start and where they started we can learn something about the path the water has taken and how long it has taken for the faults to 'heal'. We will use new techniques to accurately find the position of each earthquake which will tell us how the earlier earthquakes have created new cracking and rearranged the paths for water to flow. To understand how this happens we will use a model we have developed that can simulate the growth of cracks while there is fluid moving through them. We'll need to check that these models are producing cracks that look like real faults in rock, so we will map out the shapes and sizes of faults in rocks that are exposed near the dam site in Brazil. We will also check that our model works by testing to see if it matches the earthquake patterns on two other faults at the same reservoir. By carefully checking that our model is making realistic looking earthquake patterns we'll be able to use these results to predict the affect of earthquakes on crack evolution at other locations.