PSI - Issue 44

Angela Chiecchio et al. / Procedia Structural Integrity 44 (2023) 11–18 Angela Chiecchio/ Structural Integrity Procedia 00 (2022) 000–000

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Table 1 shows a summary of the best matching scenarios identified for the three considered sites, together with the corresponding scenario seismic source (ZS), the associated ( M w - R epi ) pair and the calculated error index ( err ). For instance, for the site of Firenze, two different scenarios dominate the hazard in 475 years: a first scenario associated to source ZS915, with ( M w - R epi ) = (6.1, 19 km), and a second scenario from source ZS916 with ( M w - R epi ) = (5.7, 11 km). Note that, for most cases, the BMSE scenarios are compatible with the historical observations. Table 1 Best matching scenarios calculated for three reference sites: Firenze, L’Aquila and Catania. For each scenario, err is the corresponding error index, ZS is the name of the source, while M w - R epi represents the magnitude-epicentral distance of the computed scenario. The scenarios are identified as the ones having values of err <= 0.3, and they are listed for increasing values of err .

BMSE scenario 1

BMSE scenario 2

BMSE scenario 3

Site

ZS

M w 6.1 6.5 6.8

R epi [km]

ZS

M w 5.7

R epi [km]

ZS

M w

R epi [km]

err

err

err

Firenze

915 923 935

19 18 29

0.06 0.09

916

11

0.13

- -

- -

- -

- -

L’Aquila Catania

-

-

-

-

0.1

929

7

37

0.12

936

5.5

6

0.42

4. Conclusions This work sketches an alternative paradigm to the well-known probabilistic disaggregation to identify a design earthquake or a scenario earthquake compatible with a UHS. This alternative, denoted by Best Matching Scenario Earthquake (BMSE), aims at identifying one or more earthquake scenarios that minimise the difference between a target UHS (associated with a certain return period) and the simulated one using a proper GMM, for an arbitrarily broad range of structural periods. Therefore, in the case of  =0, the BMSE should be considered as the earthquake the median spectral ordinates of which best approach the UHS. In case different seismic sources affect the hazard at the site, different BMSE can be obtained for each source. A few examples of the application of the BMSE with reference to the current seismic hazard model of Italy are shown. The results, herein presented for Firenze, L’Aquila and Catania, were found to be consistent both with the seismotectonic framework and the historical seismicity of the respective areas. Although these results need some further extensions (e.g., to longer return periods), at the present stage the BMSE approach has several advantages: (i) contrary to disaggregation, which requires the selection of a specific structural period, scenarios are related to a broad period range, (ii) the number of standard deviations from the median value of the GMM can be arbitrarily set and in most cases of practical relevance for Italy excellent results may be obtained by setting  =0, (iii) the method may be applied to whatever seismogenic model and GMM (or set of GMMs), with reference to the PSHA of the target spectrum, (iv) it is of immediate and non-ambiguous use. In the framework of seismic codes, the BMSE could be also useful to find the so-called “design” earthquake, i.e., the dominating earthquake scenario for a certain site, within a prescribed time frame. Therefore, while disaggregation is suitable to extract, from the plurality of earthquakes contributing to a UHS, those dominating the hazard for specific structural types, therefore highlighting a plurality of “dominating earthquakes” for as many structural types as required, the BMSE aims instead at the objective stated almost 30 years ago by McGuire (1995), i.e., “ closing the loop between the multitude of earthquakes considered by the hazard analysis and the requirement for one or few design earthquakes by users of hazard studios. The result should be better understood seismic hazard and better decisions on seismic design, analysis and retrofit ”. Acknowledgements This work has been supported by the Department of Civil Protection within the DPC-ReLUIS project WP18 “Normative contributions related to seismic action (CONpAS)”, Task 5 “Definition of the design earthquake