GNGTS 2023 - Atti del 41° Convegno Nazionale

Session 2.3 GNGTS 2023 On December 29, 2020, at 11:20 UTC, an earthquake of magnitude Mw 6.4 occurred near Petrinja, about 50 km from Zagreb in central Croatia (Marku š ic, et al., 2021). The earthquake was felt very clearly in northeastern Italy and in the city of Trieste. The social media of the OGS Seismological Research Centre, which monitors the seismicity of NE Italy (Bragato et al., 2021), recorded an unusual increase in traffic and posts by people from Trieste. People were frightened and ready to share their experiences with their fellow citizens. We immediately seized the opportunity to gather as much information as possible about the perception of the earthquake in the city of Trieste, in an organised way. For this purpose, through social media (Facebook, Twitter) we created a pool similar to "hai sentito il terremoto" (Sbarra et al., 2009, www.hsit.it ) and asked the citizens of Trieste to fill it in and forward our request to all their contacts in the city. Whatsapp was also used to disseminate the questionnaire. Within a few hours, we collected 6582 questionnaires from Trieste and almost 3000 questionnaires from different places outside Trieste. For comparison, we refer to the results we obtained for the 2012, 20 May Emilia earthquake (Fig. 1) when we carried out a similar experiment and collected only 587 questionnaires. At that time we advertised the questionnaires through local newspapers and mailing lists. Social media was not yet as popular, and our Facebook and Twitter channel did not open until June 2013 (https://www.facebook.com/ogscrs) . In addition to social media, the speed with which the questionnaire was distributed certainly had an impact on 2020. The collected questionnaires were visually inspected and questionnaires of poor quality were rejected according to the criteria mentioned in Tosi et al. (2015): duplicate entries, contradictory answers, lack of information, and discrepancy between calculated and theoretical intensity. Correspondence between responses and intensity degrees (MCS and EMS scales) was determined using a scoring matrix according to the method of Tosi et al. (2015). From the results collected (Fig. 2), it appears that 70% of the population of Trieste reported feeling the earthquake moderately strongly, and only 21% that they did not feel it. Most people who felt the earthquake were indoors, sitting or lying down, or at least not moving. Most people were between the first floor and the first 5 floors of the buildings. Of the people who were moving outside, most said they did not feel the quake, but some did. The theoretical intensity calculated for this earthquake was IV (Gómez Capera et al., 2017), which is in good agreement with the intensity estimated by the questionnaires. After georeferencing each questionnaire and assigning an intensity value, we plotted a map of the intensities obtained for Trieste. The spatial data were imported into the environment GIS. A hexagonal grid with a size of 200 meters was used, and for each cell the average of the Mercalli-Cancani-Sieberg (Sieberg 1930, MCS) intensity residuals was calculated (the difference between the observed intensity and the theoretical intensity). We used a hexagonal grid because it provides better readability than a standard rectangular grid and improves the visual clarity of spatial distributions and homogeneity of cell neighborhoods, as the pattern is completely symmetric with respect to distance (Iurcev et al., 2021). Spatial data were then filtered in order to obtain a residual smoothed map, to be compared with geological maps (Fig. 3). To obtain this map,