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A parametric analysis has been performed also with respect to the source focal mechanism, magnitude and hypocentral depth.

Two different focal mechanisms have been taken into account: the focal mechanism associated to the seismogenic zone ZS906 and the focal mechanism of the 24.11.2004 Salò earthquake (MW=5.2) that is the strongest earthquake occurred in the Garda lake area for which an estimate of the focal mechanism is available. The parameters of this focal mechanism (strike=231°, dip=47°, rake=64°) are those proposed by Guidarelli and Panza (2006) by means of INPAR, a waveform inversion method that is particularly suitable for shallow sources.

Three different magnitudes have been considered: M=6.5, which is the magnitude reported in the CPTI04 earthquake catalogue (Gasperini et al., 2004), M=6.8, which is the seismic potential of the fault system characterizing the Pedealpine margin (Serva, 1990), M=7.0, which is the magnitude proposed by Guidoboni et al. (2005).

The hypocentral depth has been selected, accordingly with Panza et al. (2001) depending on the magnitude value, as explained in chapter 2: h=10 km for M=6.5 and 6.8 and h=15 km for M=7.0.

The ground velocity scenarios obtained for the different combinations of the source parameters (focal mechanism, magnitude and hypocentral depth) and for the epicentral location considered by Boschi et al. (1995b, 1997, 2000) and reproposed by Guidoboni et al.

(2005) are shown in Figure 4.6, Figure 4.7 and Figure 4.8, respectively.

a) b)

Figure 4.6 - Velocity ground shaking scenarios computed for a source with the epicentral location proposed by Guidoboni et al. (2005), M=6.5 and h=10 km. The adopted focal mechanisms are: a) the focal mechanism of the seismogenic zone ZS906 (Meletti et al., 2008); b) the focal mechanism of the 24.11.2004 Salò earthquake (Guidarelli and Panza, 2006).

a) b)

Figure 4.7 - Velocity ground shaking scenarios computed for a source with the epicentral location proposed by Guidoboni et al. (2005), M=6.8 and h=10 km. The adopted focal mechanisms are: a) the focal mechanism of the seismogenic zone ZS906 (Meletti et al., 2008); b) the focal mechanism of the 24.11.2004 Salò earthquake (Guidarelli and Panza, 2006).

a) b)

Figure 4.8 - Velocity ground shaking scenarios computed for a source with the epicentral location proposed by Guidoboni et al. (2005), M=7.0 and h=15 km. The adopted focal mechanisms are: a) the focal mechanism of the seismogenic zone ZS906 (Meletti et al., 2008); b) the focal mechanism of the 24.11.2004 Salò earthquake (Guidarelli and Panza, 2006).

Again, these scenarios have been checked against the available observations of Figure 4.4 through the Spearman test. The results are shown in Table 4.3. In all the cases, the values calculated considering the maximized database are higher than those calculated considering the minimized database. The source configuration that better reproduces the macroseismic observations is the one for which the Spearman’s rank correlation coefficient is maximum.

The highest value is 0.72, which is a sufficiently good value considering the scarcity and the quality of the starting data. It is computed considering the relation peak velocities–intensities based on the ISG data and corresponds to the source configuration with M=6.8 and the Salò earthquake focal mechanism (Guidarelli and Panza, 2006).

Table 4.3 - Spearman’s rank correlation coefficients obtained comparing the MCS observed intensities (Figure 4.4) and the ones computed for the different analyzed focal mechanism-magnitude-hypocentral depth combinations. Concerning the observed intensities, both the maximized (“MAX”) and minimized (“MIN”) database have been considered. The computed intensities have been obtained through the empirical relationships (Panza et al., 1997a) between the velocities and the macroseismic intensities reported in the data base “ING” and “ISG”. The highest Spearman’s correlation coefficient in each column is indicated by an asterisk.

MAX MIN

Conversion PGV-I (ING data)

Conversion PGV-I (ISG data)

Conversion PGV-I (ING data)

Conversion PGV-I (ISG data) M=6.5,zs9 0.62 (0.0005) 0.57 (0.0017) 0.57 (0.0016) 0.45 (0.0166) M=6.5,Salò 0.67 (0.0001)* 0.65 (0.0002) 0.52 (0.0050) 0.60 (0.0008) M=6.8,zs9 0.64 (0.0003) 0.48 (0.0105) 0.55 (0.0024) 0.43 (0.0230) M=6.8,Salò 0.66 (0.0001) 0.72 (0.00002)* 0.59 (0.0009)* 0.63 (0.0004)*

M=7.0,zs9 0.56 (0.0022) 0.58 (0.0014) 0.53 (0.0038) 0.52 (0.0045) M=7.0,Salò 0.58 (0.0011) 0.42 (0.0279) 0.55 (0.0025) 0.39 (0.0426)

The misfit between the observed and computed MCS intensities has been also estimated computing the total misfit dtot=di, where di=|IOBS-ICALC| is the absolute value of the difference between observed and computed intensities, and the number of sites in which the computed intensities are equal to the observed ones (di=0). The total misfit dtot=di and the number of points in which di=0 for the maximized database are shown in Table 4.4. For the intensities computed considering the relation peak velocities–intensities based on the ISG data, the source configuration with M=6.8 and the Salò earthquake focal mechanism (Guidarelli and Panza, 2006) is the one that satisfies both the criteria of minimum total misfit (dtot=17) and maximum number of sites in which the computed intensities are equal to the observed ones (di=0 in 13 sites). The maximum number of sites in which di=0 (di=14) is reached when the regression relationship based on the ING data, M=7.0 and the Salò earthquake focal mechanism (Guidarelli and Panza, 2006) are considered. However the corresponding total misfit (dtot=20) and the Spearman’s rank correlation coefficient (0.58) are not likewise good.

Therefore we are in favour of:

- the epicentral location proposed by Guidoboni et al. (2005);

- a magnitude close to the seismic potential of the fault system characterizing the Pedealpine margin in the Veronese area, estimated to be about M=6.8 (Serva, 1990);

Table 4.4 - Total misfit dtot=di and number of points in which di=0 for the maximized database. The computed intensities have been obtained through the empirical relationships (Panza et al., 1997a) between the velocities and the macroseismic intensities reported in the data base “ING” and “ISG”. The minimum total misfit and the maximum number of sites in which di=0 in each column are indicated by an asterisk.

dtot di=0

Conversion PGV-I (ING data)

Conversion PGV-I (ISG data)

Conversion PGV-I (ING data)

Conversion PGV-I (ISG data)

M=6.5,zs9 23 28 9 7

M=6.5,Salò 24 32 9 5

M=6.8,zs9 22 21 9 11

M=6.8,Salò 20* 17* 12 13*

M=7.0,zs9 21 21 13 10

M=7.0,Salò 20* 22 14* 11

The agreement between the scenario corresponding to the “best” source parameters (Figure 4.7b) and the isoseismals of intensity VIII and IX computed with the MPF method (Figure 4.3) is shown in Figure 4.9.

Figure 4.9 - Agreement between the “best” scenario (Figure 4.7b) and the isoseismals of intensity VIII and IX computed with the MPF method (Figure 4.3).

The picture shows that the area characterized by intensity IX is well reproduced by the model.

The agreement is not so good with the area of intensity VIII, but it must be considered that in the SE quadrant of the scenario only two observations are available (Figure 4.4).