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An additional requirement to the verification of the correct functioning of the programs (see paragraph A.4), is the validation of the products.

The procedure to validate the ground shaking scenarios both at national and local scale is subdivided in two parts:

1) demonstration that the computation of the synthetic seismograms, at the base of the ground shaking maps, is correct from a numerical point of view;

2) demonstration that the values of soil motion shown on the ground shaking maps are consistent with the seismic recordings;

A.5.1 Part 1 - Validation of the method

The first part of the adopted validation procedure has the purpose to demonstrate that the computing of the synthetic seismograms is correct from a numerical point of view, given a determined source and a structural physical model. This has been achieved comparing a two-component seismogram (radial and transversal two-components) at the base of the ground shaking maps, computed by the modal summation (Panza et al., 2001, see chapter 1), with the

differences technique (Fäh, 1992). The considered seismograms are shown in Figure A.6. The picture shows that the seismograms are consistent since the duration, time of arrival and peak values are very similar. The comparison has been performed in terms of displacement, since velocities and accelerations are obtained by derivation, starting from displacements.

a) b)

Figure A.6 - Seismograms used to validate the method at the base of the ground shaking maps.

a) Seismograms computed by the modal summation technique (Panza et al., 2001), b) seismograms calculated by the finite differences technique (Fäh, 1992). From top to bottom: radial components and transversal components of motion.

A.5.2 Part 2 - Validation of the ground shaking maps values

The values given by the ground shaking maps represent the maximum expected values on the Italian territory, associated with a given alarm (see chapter 5). Every time an area (CN or M8S) is alerted, the synthetic seismograms are computed a) using the sub set of sources inside the alerted area (national scale), to which a magnitude consistent with that of the alarm is assigned, or b) using the sub set of sources inside the alerted nodes (local scale).

The peak values obtained in such way represent the maximum values expected for an

earthquake that might occur during the period of validity of the alarm, inside the whole alerted area or inside one of the alerted nodes, with the magnitude consistent with that of the alarm itself.

Therefore, the validation consists in the comparison of the computed values (DGA) with the peak values of the accelerometric recordings (PGA). The equivalence between DGA and PGA has been discussed in paragraph 3.4. According to their definition, within measurements and modelling errors, DGA values should be greater or equal to the PGA values.

Therefore the comparison between DGA and PGA must be performed taking into account that:

- NDSHA values are values computed at the bedrock and therefore they should be compared with accelerograms recorded at sites “on rock”, to avoid, as much as possible, local site effects that can amplify severely the seismic response (an example of amplification is shown in Figure 5.13). Moreover, some stations can present some problems related to the station itself that the modelling obviously cannot reproduce; therefore the comparison made considering only one recording of an earthquake is not significative.

- NDSHA values are shown on the ground shaking maps in terms of interval values, where each interval corresponds to a macroseismic intensity, since differences in peak values lower or equal to one macroseismic intensity are not relevant for reliable seismic hazard studies.

Therefore, as in chapter 3, the comparison between DGA and PGA values has been performed in terms of macroseismic intensities in order to account for the problems related with the recordings. Since ±1 degree of macroseismic intensity is the intrinsic uncertainty associated with the intensity estimates, the computed DGA values are validated if IDGA (IPGA-1), where IDGAand IPGA are the intensities associated to the DGA and PGA values, respectively.

Validation has been performed at national scale considering the CN and M8S predictions, both real time and retrospective, confirmed by the occurrence of the expected earthquakes (1976.05.06, 2004.07.12, 2003.09.14, 1998.09.09, 1980.11.23, 2003.03.29). Ground shaking scenarios associated to these areas have been computed and the comparison has been performed for those sites for which the Italian Accelerometric Archive (ITACA, Working Group ITACA, 2008) provides the value of PGA. For the comparison, the coordinates of the sites have been rounded to 0.2 degrees, i.e. consistent with the discretization step used in the modelling of DGA. Figure A.7 illustrates an example of the proposed validation procedure referred to the 1980 Irpinia earthquake, correctly predicted by the M6.5+ M8S algorithm

values are expressed as interval values, each of them associated with an intensity (Panza et al., 1997). We can easily verify that the observed PGAs are all in the same or in the lower intensity class associated with the correspondent DGA values.

a) b)

Figure A.7 - a) M8S alerted area (yellow) associated with the magnitude range M6.5+, which correctly predicted the 1980 Irpinia earthquake (black circle); b) associated DGA scenario for those points that have a correspondent PGA observation; PGA values (Working Group ITACA, 2008), expressed in unit of g, are written above the DGA circles.

The validation of the local ground shaking follows the same guide lines of the validation at national scale. In this case, the values compared with the observations are the DGA values associated with the nodes inside the alerted areas, in which the epicentres of the occurred earthquakes (1976.05.06, 2004.07.12, 1998.09.09, 1980.11.23) are contained. Since more than one alerted area has predicted some earthquakes and more than one seismogenic node can be associated to the occurred earthquakes, the comparison has been performed checking at each site, for each earthquake, that the maximum ground shaking between all the alerted nodes is consistent with the observed one.

The comparison between the computed seismograms and the recorded accelerograms (filtered to 1 Hz) is shown in Panza et al. (2001).