GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 1.1 GNGTS 2023 The challenging task of estimating rupture directivity of moderate earthquakes in near real-time A. Cuius 1 , H. Meng 2 , A. Saraò 3 , G.Costa 1 1 Università degli Studi di Trieste, Italy 2 Southern University of Science and Technology, Shenzhen, Guangdong Province, P.R. China 3 Istituto Nazionale di Oceanografia e di Geofisica Sperimentale - OGS, Italy Radiation from an extended seismic source when a rupture propagates in preferential directions is called the directivity effect. It is manifested by seismic spectral variations depending on the observation location. Directivity produces azimuthal and spectral variations in ground motion that can be used to infer information about the orientation of the fault plane or to investigate a predominant orientation of rupture propagation in a particular region or during a seismic sequence. In addition, directivity at low frequencies and during moderate-to-strong events can be responsible for potentially destructive pulses with large ground motions, while at high frequencies and during small-to-moderate events, the most pronounced effect is given by the shift in corner frequencies, which can lead to high-frequency energy arrivals in short time intervals (Abercrombie et al. 2017) . Rupture processes of large earthquakes have been extensively examined by seismic waveform analysis  (e.g., Hartzell and Heaton, 1983; Fukuyama and Irikura, 1986; Ye et al., 2016) and directivity effects have also been observed in moderate and small earthquakes (e.g., Boatwright, 2007; Chen et al., 2010; Abercrombie et al. 2017; Meng et al., 2020; Meng and Fan, 2021) The importance of directivity in small to moderate earthquakes is widely recognised for both seismological studies on earthquake sources and engineering applications (e.g. Colavitti et al., 2022), and to estimate it in near real time would provide useful information for post-earthquake management. However, the determination of directivity effects and source parameters for small-to -moderate magnitude earthquakes remains a challenge. The accuracy of the results depends on the quality of the data, the coverage of the seismic network, the computational method used, as well as on the complexity of the rupture. One of the most common techniques is to measure the duration of the source pulse (called Apparent Source Time Function) at each location and then model it by a line source (Fig 1). To overcome the problems associated with the presence of path and site effects some approaches rely on the deconvolution of waveforms by an empirical Green function (eGf), (Calderoni et al. 2015; Calderoni, Rovelli, and Di Giovambattista 2017; McGuire 2017; Meng et al. 2020) but finding the right eGf can sometimes be difficult.