Does The Slip In The Early Steps Of The Rupture Scale With The Final Magnitude Of The Event ?

Earthquake early warning systems are real-time monitoring infrastructures designed to provide a rapid notification of the potential effects of an impending earthquake at critical distant targets. The ability in predicting if an earthquake is going to be large from the first few seconds on the accelerograms is crucial in resolving the trade-off between the lead-time of the system and the accuracy in the evaluation of the missed/false alarm probabilities. Here we investigate the scaling of the magnitude with the ground velocity integral computed from the first 4s of P wave and 2s of S wave. We come up with such a new parameter because it is expected to be more stable than peaks and it is directly linked to the energy radiated by the propagating rupture. Therefore it can give insights into the physics of the early stages of the rupture. We analyze more than 2000 high-resolution strong motion records from the Japanese databases K-net and Kik-net with earthquake magnitude ranging between 4 and 7. After normalization by the hypocentral distance, assuming constant stress drop and computing effective rupture areas from the Brune model, we draw the average slip corresponding to the early stages of the rupture against the final magnitude. Despite of the dispersion of data, we found that standard scaling relations still hold in the whole investigated magnitude range. Results of our analysis suggest that the initial slip is an indicator of the rupture size and the total moment in the explored magnitude range implying that the rise time has to be negligible at this time scale.