GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 1.1 31 Intensity attenuation with distance and estimation of epicentral intensity . We calculated epicentral distance for each MDP (half degrees were not considered and intensities lower than 4 were filtered out) and we computed regression equations using the intensity-level binning proposed by Bakun andWentworth (1997). The average epicentral distance for MCS data points is 61.28 ± 28.63 km, and the maximum value is 636 km. Environmental effects are indeed documented mainly in the near-field, at average distances of 27.76 ± 18.93 km (maximum value: 210 km). Attenuation relations were calculated as regressions through a least squares method, in the linear form: I = a * D + b (1) Where I is the intensity (in MCS or ESI scale), D the epicentral distance (in km), a and b are free parameters. Despite several forms have been proposed for traditional intensity scales (i.e., logarithmic, bi-linear, square and cubic root, log-linear; see Pasolini et al. , 2008 for a comprehensive review), we here adopted the linear form. The slope [coefficient a in Eq. (1)] is higher for ESI-derived equations in respect of MCS- derived ones: values are of -0.070 ± 0.047 for MCS and -0.120 ± 0.072 for ESI, corresponding to a decrease of one degree of intensity every 14 and 8 km, respectively. It is worth noting that I 0 and I max values for the MCS scale are always equal or higher than ESI ones (Tab. 1). On the contrary, the b parameter of Eq. (1), which represents the intensity predicted at the epicenter, is higher for ESI in 9 out of 14 earthquakes. The comparison between I 0 and b furnishes some insight, but should be treated only in a qualitative way, due to the different nature of I 0 (which can assume only discrete values, i.e., integer or half degrees) and the b coefficient (which instead is a continuous variable). For the MCS scale, the predicted Tab. 1 - Summary characteristics of the investigated earthquakes. Epicentral intensity ( I 0 ), maximum intensity ( I max ) and the intensity predicted at the epicenter (i.e., b coefficient of the derived equations) are shown for both the MCS and ESI scales. I 0 for the ESI scale was computed with the same criteria adopted in the CPTI15 for the MCS scale (Rovida et al. , 2016). Epicentral intensity ( I 0 ) Maximum intensity ( I max ) Intensity predicted at Difference epicenter (b coefficient) ( I 0 – b ) Date Locality MCS ESI (MCS-ESI) MCS ESI (MCS-ESI) MCS ESI (MCS-ESI) MCS ESI 24/08/2016 Amatrice 10,5 9,0 1,5 10,5 9,0 1,5 9,29 9,44 -0,14 1,21 -0,44 06/04/2009 L’Aquila 9,5 9,0 0,5 9,5 9,0 0,5 8,75 8,84 -0,09 0,75 0,16 26/09/1997 Colfiorito 8,5 8,0 0,5 9,0 9,0 0,0 8,33 9,54 -1,22 0,17 -1,54 23/11/1980 Irpinia 10,0 10,0 0,0 10,0 10,0 0,0 9,38 8,48 0,91 0,62 1,52 23/07/1930 Irpinia 10,0 10,0 0,0 10,0 10,0 0,0 10,12 10,41 -0,28 -0,12 -0,41 13/01/1915 Fucino 11,0 10,0 1,0 11,0 10,0 1,0 10,57 11,17 -0,60 0,43 -1,17 28/12/1908 Messina 11,0 10,0 1,0 11,0 11,0 0,0 10,04 11,08 -1,04 0,96 -1,08 16/12/1857 Basilicata 11,0 10,0 1,0 11,0 10,0 1,0 10,23 6,88 3,35 0,77 3,12 26/07/1805 Molise 10,0 10,0 0,0 10,0 10,0 0,0 9,93 9,14 0,79 0,07 0,86 05/02/1783 Calabria 11,0 11,0 0,0 11,0 11,0 0,0 10,85 10,61 0,24 0,15 0,39 02/02/1703 L’Aquila 10,0 10,0 0,0 10,0 10,0 0,0 9,74 12,58 -2,84 0,26 -2,58 14/01/1703 Norcia 11,0 11,0 0,0 11,0 11,0 0,0 10,22 12,60 -2,38 0,78 -1,60 05/09/1694 Irpinia 10,0 9,0 1,0 10,0 10,0 0,0 9,39 9,98 -0,60 0,61 -0,98 05/06/1688 Sannio 11,0 8,0 3,0 11,0 8,0 3,0 11,96 9,38 2,58 -0,96 -1,38 Mean 9,22 9,86 0,41 -0,37 Std dev 1,00 1,14 0,54 1,48