GNGTS 2021 - Atti del 39° Convegno Nazionale

19 GNGTS 2021 S essione 1.1 EMPIRICAL EVIDENCE OF DIRECTIVITY EFFECTS FOR NORMAL FAULT EARTHQUAKES: THE STUDY CASE OF CENTRAL ITALY L. Colavitti 1 , G. Lanzano 1 , S. Sgobba 1 , F. Pacor 1 , F. Gallovič 2 1 National Institute of Geophysics and Volcanology, Section of Seismology applied to Engineering, Milano, Italy 2 Charles University, Faculty of Mathematics and Physics, Department of Geophysics, Prague, Czech Republic Corresponding author: Leonardo Colavitti, leonardo.colavitti@ingv.it INTRODUCTION The directivity effect of an earthquake is the focusing of the seismic wave energy along the fault in the direction of rupture propagation. This phenomenon, which represents a key factor in the ground shaking, produces azimuthal variation in the seismic radiation and can be used to infer both the orientation of the fault plane and the rupture scenario (Abercrombie et al ., 2017). In the 90s, directivity was initially observed and modelled for large events (e.g. Mw 7.3 1992 Landers earthquake) while over the years, thanks to the increasing data quantity and quality, directivity effects have been recognized even for moderate and small earthquakes (Pacor et al ., 2016; Convertito et al ., 2016). In this work, the empirical evidence of directivity effects on the ground motion are inferred from the calibration of a non-ergodic ground motion model for Fourier Amplitude Spectra (FAS- GMM). On this model, we investigated the frequency- and azimuth- dependence of the residuals corrected for source, path and site contributions. As a case study, we perform our analysis in Central Italy, which represents an excellent and almost unique natural laboratory for earthquakes occurring on normal faults. The approach is completely based on empirical observations, thanks to the large amounts of high-quality recordings following the seismic sequences of L’Aquila in 2009 (Chiarabba et al ., 2009) and Amatrice-Visso-Norcia in 2016 and 2017 (Chiaraluce et al ., 2017). DATASET The proposed FAS-GMM is calibrated for 69 ordinates of the Fourier spectrum in the frequency range from 0.5 to 25 Hz. The dataset is the same used in the work of Sgobba et al . (2021) and consists of more than 30’000 waveforms of 456 earthquakes in the magnitude range between 3.4 and 6.5, and more than 460 stations within 120 km from the epicenter. The high density of events and stations in such a relatively small region allows sampling a significant number of source-to-site ray-paths ( Fig. 1a ), thus making this dataset suitable to derive a fully non-ergodic GMM. MODELLING THE DIRECTIVITY EFFECTS ON NON-ERGODIC GMM The model is calibrated via a mixed-effect regression (Bates, 2015), providing the estimation of different repeatable effects on seismic motion (i.e. source, site and path) and reducing the random variability of the model. We adopt the same functional form of Sgobba et al (2021), to model the FAS amplitudes at each frequency: DATASET The proposed FAS-GMM is calibrated for 69 ordinates of the Fourier spectrum in the frequency range f om 0.5 to 25 Hz. The dataset is the same used in the work of Sgobba et al. (2021) and consists of more than 30’000 waveforms of 456 earthquakes in the magnitude range between 3.4 and 6.5, and more than 460 stations within 120 km from the epicenter. The high density of events and stations in such a relatively s all region allows sampling a significant number of source-to-site ray-paths ( Fig. 1a ), thus making this dataset suitable to derive a fully non-ergodic GMM. MODELLING THE DIRECTIVITY EFFECTS ON NON-ERGODIC The model is calibrated via a mixed-effect regression (Bates, 2015), providing the estimation of different repeatable effects on seismic motion (i.e. source, site and path) and reducing the random variability of the model. We adopt the same functional form of Sgobba et al (2021), to model the FAS amplitudes at each frequency: �� = + � ( ) + � ( , ) + � + 2 ���,� + 2 ������ + 2 � + � [1] Where is the offset and describes the scaling with moment magnitude Mw.