Early rupture signals predict the final earthquake size

When a seismic rupture starts, the process may evolve into multiple ways, generating different size earthquakes. Contrasting models have been proposed to describe the evolution of the rupture process while limited observations at the scale of real earthquake data are available, so that a unifying theory is still missing. Here we show that small and large earthquake ruptures are different before the arrest and they do not exhibit a common, size-independent, universal behaviour. For earthquakes with magnitude 4 < M < 9 occurred in Japan, we measure the initial rate of the P-wave peak amplitude and show that this quantity is correlated to the final event magnitude and not affected by distance attenuation, thus being a proxy for the initiation time of the rupture process. While opening new views on the rupture preparation process, our findings can have significant implications on the effective development of fast and reliable methods for source characterization and ground shaking prediction.