Citizens' smartphones unravel earthquake shaking in urban areas

Seismic shaking maps (known as ShakeMaps) have become an essential tool for assessing the impact of a recent earthquake or for scenario studies of future events. The spatial resolution of ShakeMaps remains low, particularly in urban areas where spatial variations can have a major impact on damage due to varying population density. Smartphones, with their internal accelerometers and widespread use, help to increase the density of seismic recording precisely where the potential impact of the earthquake may be greatest. In this work, we use statistical spatial modelling to show that smartphone measurements collected by the Earthquake Network citizen science initiative allow us to map site amplification at high spatial resolution, generate high-resolution ShakeMaps, and improve existing ground motion models. We apply the method to the red zone of Campi Flegrei, Italy, a high volcanic and seismic risk area with around 500,000 inhabitants and characterised by a complex spatial pattern of site amplification.