Small and large earthquakes: evidence for a different rupture beginning

Abstract:

The process of earthquake rupture nucleation and propagation has been investigated through laboratory experiments and theoretical modelling, but a limited number of observations exist at the scale of earthquake fault zones. Distinct models have been proposed, and whether the magnitude can be predicted while the rupture is ongoing represents an unsolved question. The ability to correctly distinguish a small shock from a large event through the analysis of the first P-wave observation is crucial for risk mitigation actions triggered by earthquake early warning systems. Here we show that the evolution of P-wave peak displacement with time is informative regarding the early stage of the rupture process and can be used as a proxy for the final size of the rupture. In the present study, we measure the peak displacement amplitude of filtered P-wave signals over a progressively expanding P-wave time window, starting from the P-wave onset time, and expanding the time window until the expected arrival of the S-waves. We use a large, high-quality dataset of 43 moderate-to-strong Japanese events, in the magnitude range between 4 and 9. We analyzed more than 7000 three-component waveforms recorded at 1,208 stations, spanning a wide distance range (0-500 km). We study the relationship between the time evolution of the peak displacement and the earthquake magnitude itself to investigate a possible different scaling for small and large events. For the analyzed earthquake set, we found that the initial evolution of peak displacement is different between small and large earthquakes. In particular, we show a rapid initial increase of the peak displacement for small events and a slower growth for larger ones. The figure shows the average values of P-wave peak displacement for some representative events while the insert box shows the expected initial slope of the curves for different magnitudes. This result suggests that the evolution of P-wave peak displacement holds information regarding the early stage of the rupture process and may be a proxy for the final size of the event. Our results indicate that earthquakes breaking in a region with a large critical slip displacement value have a larger probability to grow into a large size rupture than those originating in a region with a smaller critical displacement value.