The deterministic behaviour of earthquake rupture beginning

Earthquakes are among the most destructive natural hazards. The energy released by an earthquake can be quantified by its magnitude. However, predicting how much energy the earthquake will release before the end of its rupture process represents a challenging question in geohazards. The way earthquake ruptures grow and arrest determines the final event size: small-to-moderate ruptures evolve in few seconds with typical lengths of few kilometers, while large-to-huge events develop in tens of seconds or more, involving lengths of several hundred kilometers. If the rupture process starts in the same way for small and large earthquakes, no deterministic prediction of the final size is feasible, until the process has finished. On the contrary, if the source mechanism starts differently from its early beginning, real-time proxies can be measured on seismic waves to discriminate the final event size. Here we show that the initial ground displacement growth is differently for small and large earthquakes, based on the analysis of an unprecedented catalog of seismic waveforms from worldwide earthquakes. The result supports the hypothesis of early predictable event magnitude for a wide range of different size earthquakes in diverse geological settings. This study confirms that the measure of the initial growth of displacement can be used as a parameter for a fast magnitude estimation, making it feasible for future implementation in early warning systems.