| Popis: |
The general goal of this thesis has been to characterize the seismic source of an earthquake. A source can be characterized by macroscopic characteristics in the approximation of a point source, as the location of the hypocenter, moment magnitude of the event and the focal mechanism. Moreover, we can also provide characteristics of the extended source, as its spatial dimensions or the final slip map on the fault plane rather than an average value. Another punctual quantity that can be provided is rupture velocity. We have focused not only on the moderate-to-large events, but we have also tried to infer characteristics for micro-seismicity, believing that the latter is a key to understanding more large-scale mechanisms (De Matteis, 2012). In particular, we focused on the estimation of focal mechanism of microearthquakes (Mw< 3). Firstly, we have developed an algorithm in an evolutionary Bayesian framework to give a rapid estimation of the earthquake focal mechanism using the few seconds of P-wave since the origin time of a set of recording stations for moderate earthquakes (Mw 4.5 to 6.5). Then, opportunely modifying this procedure allowing for the inclusion of S/P amplitude and inclusion of polarity, we tried to infer the focal mechanism also of micro-earthquakes (local magnitude < Ml 3.0). We then investigated two different approaches for back-projection. We performed the back projection as in Marcklin et al,2012 but near source and applied to a moderate event, the 6.5 Mw 2016 Norcia earthquake, retrieving the dominant rupture propagation toward south. Moreover, the duration (~8s) is in agreement with references. This approach beamforms and stacks displacement amplitude directly on the fault plane to retrieve slip rate and slip on the plane. There is also a different strategy to perform the back-projection (Xie & Meng, 2020) which locates the seismic radiators, that are sub-events of an earthquake. We applied the Multi-array Back-projection to study 3 different earthquakes at local and regional scales, in different tectonic regimes, to determine the location of seismic radiators and we used the distance from seismic radiators as source-to-site distance metric to considering for the path effect in local Ground Motion Prediction Equations. This technique does not require the discretization of the source in sub-sources, neither to orient the fault plane in the space. It works to match the coherency of waveforms to locate seismic radiators, which distribution reflects the main direction of the rupture and also the rupture length. We used the distance of stations to radiators at local scale in an evolutionary approach to obtain estimation of Peak Ground Acceleration (PGA) in time, predicting in good agreement the observed PGA some seconds before. |
