GNGTS 2022 - Atti del 40° Convegno Nazionale

GNGTS 2022 Sessione 2.2 379 (i.e. few earthquakes recorded at long distance). Similarly, the cumulative seismic moment M 0 with increasing distance is computed around each node of the strong-motion networks (Fig. 2, bottom row). The seismic moment M 0 for historical earthquakes and instrumental earthquake with M w >5 is calculated by converting the provided values of the moment magnitude by means of the relation proposed by Hanks and Kanamori (1979): M 0 = 10 (Mw+10.7)±1.5 . In this diagram it is possible to distinguish which stations are optimally placed (i.e. high M 0 at short distance) from the bad placed stations (i.e. low M 0 at long distance) with respect to the largest events. For each country, some strong-motion stations follow the same trend, while others are scattered. Of course, the number of earthquake and the released seismic moment are different between the various countries (Tab. 1), so that to compare the three networks we normalized the two series of plots considering the maximum values at a distance of 100 km (Fig. 3). In this way the plots indicate how the earthquake network of each country is optimized for its type and distribution of seismicity, with respect the other countries’ networks. For the seismic network (Fig. 3, left), Italy seems to have a network which better fits the seismicity within the whole range of distance considered; conversely, the seismic stations in Taiwan appears to be generally worse placed. For the strong motion network, Italy and Taiwan record relatively higher values of M 0 at short distance (< 20 km); at larger distance the networks of Italy and New Zealand record the relatively higher M 0 . A limitation to this comparison is represented by the different time span of the historical catalogues: the Italian one extends for more than 1,000 years, while the other are definitely shorter. In the comparisons, it should be also taken into account that the density of both seismic stations and strong-motion stations is similar for Italy and New Zealand but it is greatly higher in Taiwan (Tab. 1). For instance, Italy and New Zealand count one seismic station every 500 km 2 while Taiwan counts one seismic station every 130 km 2 . The same occur for the strong- motion stations (460, 740 and 50 km 2 , the density for the three countries, respectively) and can be related also to the different population density of the countries (Siino et al. , 2020). The different density of the earthquake monitoring network can account for some of the differences highlighted by the plots in Fig. 3. Fig. 3 - Cumulative number of instrumental earthquakes located at increasing distance from each seismic station, and cumulative seismic moment at increasing distance from each node of the strong-motion network. The curves have been normalized considering the maximum number of earthquakes and the maximum seismic moment recorded at a distance of 100 km Code color: Italy (blue lines), New Zealand (black lines), and Taiwan (red lines).