A P-wave based methodology for the rapid source characterization

Abstract:

The modality in which seismic fractures nucleate and propagate is still an open issue. Some authors claim that small and large earthquakes are different from the early stage of the rupture. Others believe that there is no way to distinguish a small shock from a large event, until the rupture process has completed. Within this context, in a previous work we investigated the evolution of the early P-wave displacement amplitude with time for different magnitude events and observed differences among the curves. We found indeed a rapid initial increase for the small shocks and a slower growth for the largest events. We interpreted the observations in terms of a different rupture beginning for small and large earthquakes, related to the friction conditions during the nucleation phase of earthquake ruptures. In this work, we first investigate the effect of the distance attenuation on the observed time evolution of the P-wave peak displacement amplitude. We analyzed a few colocated events, belonging to the 2008 Iwate-Miyagi sequence, recorded in the same source-to-receiver geometry, to prove that geometrical/propagation factors are not responsible for the observed initial slope of the P-wave amplitude. Then, following the concept of expanding the P-wave time windows, we propose a straightforward methodology, based on the P-wave amplitude, to quickly characterize the extension of the seismic source. In particular, we investigate whether the progressive measurement of the P-wave displacement amplitude can provide a rapid estimate of the event magnitude and of the expected size of the rupture. Finally, the real- time measurement of the initial P-wave amplitude allows for a preliminary evaluation of kinematic parameters of the rupture process. The duration and extension of the nucleation phase, as well as the frictional properties associated to the nucleation zone together with the rupture velocity are inferred as long as the P-wave amplitude is observed.