Earthquake Seismology

Earthquakes have a strong impact on society, producing casualties and extended damage to buildings and infrastructures. Understanding how the earthquake machine works, from its generation, to propagation and arrest, is thus fundamental for Earth Science. Here we present the state-of-the-art knowledge about earthquakes; we move from recent techniques to infer earthquake physical parameters from observations to actual findings about the nature and origin of seismic events. The improvement in earthquake knowledge over recent years is grounded on the increasing capacity of observation systems. These systems are based on surface and borehole sensors, resulting in dense arrays, and they can deliver multi-scale, high-resolution data that allow details of large earthquakes and source properties of microseismicity to be determined. In this respect, we illustrate here the models that explain how a fault slips during large events, how fast a seismic rupture progresses and what are the mechanisms for its arrest. We also examine, by simulation approaches, how geological structures control earthquake ruptures. Though being the main interest of seismology, large events are rare; to complement the view on earthquake faulting seismologists investigate microseismicity based on the evidence of earthquake self-similarity over many orders of magnitude. We discuss here how source properties of small earthquakes contribute to an understanding of the role of fluids, the geometry and mechanics of fault systems, and the preparatory phase of larger magnitude events. We conclude the paper by presenting how these methods are tuned for real-time applications, such as earthquake early warning.