The deterministic behaviour of earthquake rupture beginning

Earthquakes are among the most destructive natural hazards whose released energy can be quantified by their magnitude. Predicting how much energy will be released before the end of the rupture process represents a challenging question. The way earthquake ruptures grow, propagate and arrest determines the final size: small-to-moderate ruptures evolve in few seconds within few kilometers, while large-to-huge events develop in tens of seconds and 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 completed. Instead, if the source mechanism starts differently from its beginning, real-time proxies can be measured on early radiated waves to discriminate the final event size. Here we show that the initial ground displacement grows 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 magnitude for a wide range of different earthquakes in diverse geological settings. This study confirms that the initial growth of displacement can be used for a fast magnitude estimation, making it potentially feasible for future implementation in early warning systems.