Following the same procedure proposed by Panza et al. (1997a), updated correlation relationships between the macroseismic intensities felt in Italy and the peak values of displacement, velocity and DGA have been computed to account for the currently adopted earthquake catalogue (CPTI04), seismogenic zones (ZS9), focal mechanisms (Meletti et al., 2008) and elastic design response spectrum (Norme Tecniche per le costruzioni D.M.
14/09/2005).
These correlation relationships are particularly relevant for countries with a long seismological history since it facilitates the engineering use of historical events that are quantified only in terms of macroseismic intensity. An example of the application of the correlation relationships to the analysis of an historical earthquake will be provided in chapter 4.
The peak values of displacement, velocity ad DGA have been calculated following the neo-deterministic procedure described in the previous paragraphs, but adopting a different window size for the smoothing of seismicity, since the purpose of this analysis is the comparison with past earthquakes: a smoothing radius equal to 1 cell is used for events with magnitude larger than 6.75 (Panza et al., 2001), while no smoothing is applied to smaller events. In this way no error is associated to the location of the earthquakes and only the source dimension is accounted for (3 cells~ 50 km, comparable with the rupture dimension for an earthquake with magnitude 6.75).
Two sources of Intensity data have been used: (1) the map of maximum macroseismic intensities felt in Italy, made by Istituto Nazionale di Geofisica (ING) (Boschi et al., 1995a), where the intensity values range between the V and the XI grade of MCS scale and (2) the set of maximum intensities felt in every municipal land, compiled jointly by ING, SSN and GNDT (ISG) (Molin et al., 1996), where the intensity values range between the VI and the X grade of MCS scale. The analysis has been performed in the period common to both intensities and earthquake catalogues (i.e. up to 1995).
Since peak values of ground motion and intensities are poorly correlated and their scatter is considerable (Ambraseys, 1974; Decanini et al., 1995), the correlation hypothesis:
I b b y) 0 1
log( (2.2)
(where y is a peak value and I is the intensity) is applied considering the average data. The
mean values of displacement, velocity and DGA have been computed for every value of intensity and they are reported in Table 2.2 and Table 2.3.
Table 2.2 - Mean values of displacement, velocity and DGA versus ING intensities.
I D (cm) V(cm/s) DGA(g)
V 0.1 0.3 0.005
VI 0.9 1.6 0.02
VII 1.8 3.3 0.04
VIII 3.3 6.8 0.09
IX 5.9 14.4 0.17
X 8.4 23.5 0.27
XI 8.2 23.6 0.28
Table 2.3 - Mean values of displacement, velocity and DGA versus ISG intensities.
I D (cm) V(cm/s) DGA(g)
VI 0.4 0.8 0.01
VII 1.1 1.7 0.03
VIII 2.2 4.0 0.05
IX 4.1 9.3 0.11
X 7.4 19.7 0.22
From intensity VI to intensity X the ISG mean values are lower than ING mean values. The application of Equation (2.2) to the data of Table 2.2 and Table 2.3 has given the results reported in Table 2.4 and Table 2.5, respectively, where 2 is determined assigning to the values obtained from the regression coefficients an error of 2.
Table 2.4 - Results of regression (2.2) for ING data.
Displacement (cm) Velocity (cm/s) DGA (g) b0 = -2.1 ± 0.5 b0 = -1.8 ± 0.3 b0 = -3.5 ± 0.3 b1 = 0.30 ± 0.06 b1 = 0.31 ± 0.04 b1 = 0.29 ± 0.04
Table 2.5 - Results of regression (2.2) for ISG data.
Displacement (cm) Velocity (cm/s) DGA (g)
b0 = -2.2 ± 0.2 b0 = -2.27 ± 0.03 b0 = -3.81 ± 0.03 b1 = 0.31 ± 0.02 b1 = 0.358 ± 0.004 b1 = 0.317 ± 0.003
23= 2.0 23 = 2.2 23 = 2.1
For each intensity data set (ING and ISG) the slopes (b1 parameter) of regression (2.2) are, within the errors, comparable between themselves, but the slopes obtained with ING data are smaller than the slopes obtained with ISG data.
The obtained relationships nicely confirms the earlier results of Cancani (1904), who modified the original Mercalli scale into MCS and assigned the maximum values of ground movement's acceleration to each grade, so that an increment of one intensity degree roughly corresponds to a doubling of the PGA, and do not differ significantly from the previous results obtained by Panza et al. (1997a).
The results tabulated for different intensities are shown in Table 2.6 and Table 2.7.
Table 2.6 - Conversion between the macroseismic observed MCS intensities (ING) and the interval values of displacement, velocity and DGA.
Intensity Displacement (cm) Velocity (cm/s) DGA (g)
V 0.2 - 0.4 0.4 - 0.8 0.007 - 0.013
VI 0.4 - 0.7 0.8 - 1.7 0.013 - 0.025
VII 0.7 - 1.5 1.7 - 3.5 0.025 - 0.05
VIII 1.5 - 3.0 3.5 - 7.0 0.05 - 0.10
IX 3.0 - 6.0 7.0 - 15.0 0.10 - 0.20
X 6.0 - 12.0 15.0 - 30.0 0.20 - 0.35
XI 12.0 - 24.0 30.0 - 62.0 0.35 - 0.70
Table 2.7 - Conversion between the macroseismic observed MCS intensities (ISG) and the interval values of displacement, velocity and DGA.
Intensity Displacement (cm) Velocity (cm/s) DGA (g)
VI 0.5 - 1.0 0.8 - 1.7 0.01 - 0.025
VII 1.0 - 2.0 1.7 - 4.0 0.025 - 0.05
VIII 2.0 - 4.0 4.0 - 9.0 0.05 - 0.10
IX 4.0 - 8.0 9.0 - 20.0 0.10 - 0.20
X 8.0 - 17.0 20.0 - 46.0 0.20 - 0.50
Using these results it is possible to compute ground shaking scenarios in terms of macroseismic intensity, which is the quantity that characterizes the impact of the earthquakes on people, buildings and the environment and appears to be an appropriate parameter to express the seismic hazard (Gómez Capera, 2006), since until 1900 most of the data contained in the Italian earthquake catalogue, as well as in other countries, is based mainly on macroseismic intensity.
2.3 Stability analysis for the neo-deterministic method
A stability analysis has been performed in order to assess the influence of the temporal length of the input earthquake catalogue on the results of the neo-deterministic method. Two time windows, both 500-years long, are considered, namely [1000,1500) and [1500,2000). In doing the stability test, among the parameters representative of earthquake ground motion, we have focused our attention on the DGA. This choice gives the possibility to make a comparison between the neo-deterministic and the probabilistic maps (see chapter 3).
DGA maps corresponding to these limited periods of time have been computed and compared with the map shown in Figure 2.6c. The comparison has been performed in terms of macroseismic intensity converting the DGA values into macroseismic intensities (MCS), using the empirical relationship based on ING data reported in paragraph 2.2. The differences between the map for the whole catalogue and the maps for the 500 years length catalogue are
a) b)
Figure 2.7 - Differences in intensity between the neo-deterministic map computed considering the whole catalogue (I1000) and the I500map computed for the period a) [1000,1499], b) [1500,1999]. The upward triangles indicate a positive difference, while the downward triangles indicate a negative difference.
In both maps, the differences are not negligible; therefore we can assess that 500 years of catalogue are not sufficiently representative of the seismicity of the Italian area. As a consequence the results are strongly dependent on the 500 years period considered.
Differences are larger in Figure 2.7a than in Figure 2.7b. This means that the seismicity level, defined by earthquakes with M5.0, increased in the last 500 years, providing a greater contribution to the map in Figure 2.6c, with respect to that given by the earthquakes occurred in the period [1000,1499]. A similar conclusion has been obtained by Vorobieva and Panza (1993).
This observation suggests that the available information from past events may well not be representative of future earthquakes and that the use of independent indicators of the seismogenic potential of a given area is needed.
2.4 Incorporating the input from seismogenic nodes into the