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 earthquakes. However, 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. The ubiquity of smartphones in these areas offers the opportunity to increase the density of seismic recordings precisely where the potential impact of the earthquake may be greatest. Here, it is shown that vibration data recorded by citizen’s smartphones in the red zone of Campi Flegrei, Italy, allows us to map local amplifications of seismic movements and to produce locally calibrated seismic ground-motions prediction models and high-resolution ShakeMaps of peak ground acceleration. The proposed method merges conventional seismological and crowdsourced smartphone data, integrating the uncertainties and peculiarities of each type of data. The integration of smartphone data enables the mapping of spatial variations in seismic amplification that show values between 2.83 and 0.25 with respect to the average amplification in the red zone. Consequently, about 19.4% of the population and about 24% of the buildings in the red zone are exposed to an amplification higher than 1.5.