Living with earthquakes: Know your faults

Over the last 20 years our understanding of how active faults move in earthquakes, and our ability to recognise them before they move, has increased dramatically. From a situation in the 1970s when even the most fundamental characteristics of earthquake faults, such as their depth and orientation, were known only crudely, we can now resolve precise details of their shape, position and even slip variation over their surface, without going anywhere near the epicentral region. These advances have come through the combined use of seismology and space-based surveying techniques, particularly GPS and radar interferometry, so that we are now concerned with apparent discrepancies between the results from these various techniques at a level that would have been dismissed as noise just a few years ago. At the same time we have become much better at identifying active earthquake-generating structures before they move, largely through appreciating the signature they produce in the landscape at the surface. This review outlines some of these developments and discusses where they are taking us. Perhaps we will be less embarrassed in the future than we have been in the recent past by large earthquakes that occurred on faults that were unknown or unappreciated at the time, but most of which could have been identified beforehand, given our better understanding of what to look for. We also have a much better idea of how to visualise the deformation of the continents which, after all, is where we live and where earthquakes cause most damage. The continents behave quite unlike the rigid plates in the oceans: they crumple or fragment over huge regions, so that it makes no sense to ask: "what plate is Greece, or Tibet, on?" The challenge has been to work out the overall motions in such places and see how they are achieved by slip on faults in earthquakes. Some of the processes we can see occurring today require the faulting to evolve with time and we are starting to recognise this in the landscape, identifying faults that grow, interact, or even die, becoming inactive. We are progressing beyond the goal of trying to understand how faulting achieves the present-day large-scale motions to the more profound question of how the fault configurations have evolved through time. Many of these developments have direct application in earthquake hazard assessment, in that they help us to identify active geological structures and to anticipate the likely surface deformations that accompany seismic slip on faults at depth.