Which inhabited centers are most vulnerable to strong seismic tremors in the Apennines? A group of researchers from INGV has created a ranking of priorities, useful for guiding information campaigns and preventive interventions. The study was published in International Journal of Disaster Risk Reduction
Less than 10% of the Italian population is exposed to potentially destructive seismic shaking. It is a relatively modest share of territory, which makes yours less difficult securing provided, however, that you choose well where to invest. According to a study by the National Institute of Geophysics and Volcanology (INGV), inspired by the strong difference in the seismic response of Amatrice and Norcia following the earthquake of August 24, 2016.
The work, The forgotten vulnerability: A geology -and history- based approach for ranking the seismic risk of earthquake-prone communities of the Italian Apennines, was published on International Journal of Disaster Risk Reduction (http://storing.ingv.it/cfti/cftilab/forgotten_vulnerability/).
"To identify the municipalities that fall on the surface projection of the great seismogenic faults of the Apennines susceptible, in their history, to strong shaking", explains Gianluca Valensise, research manager of INGV, "two large databases of the INGV, the Database of Individual Seismogenic Sources (http://diss.rm.ingv.it/diss/) and the Catalog of Strong Earthquakes in Italy (http://storing.ingv.it/cfti4med/)".
For each of the 716 selected municipalities (municipal areas with all their fractions) the seismic history was analysed, verifying the date of the last destructive earthquake. “The analysis concerned the Apennine ridge, about 1000 km from Liguria to Calabria, which alone releases about 70% of the total seismic moment of our peninsula. The method can be extended to all other seismic areas of Italy. Finally, the data on the population and the incidence and typology of buildings built before 1918, or at least centenarians, were selected with the ISTAT database”.
In the final ranking, the 716 municipalities are ordered starting from those for which there is no information of seismic damage (they have not yet suffered it or are not known) and which, therefore, may be vulnerable and unprepared, up to those municipalities which recent times they have suffered strong earthquakes and therefore more "prepared" for future strong earthquakes (see attached map).
"As regards Amatrice and Norcia, which are located almost at the same distance from the fault that generated the earthquake", adds Valensise, "the accelerograms of the recent shock, recorded by two stations located in the two inhabited centres, show that the level of the shaking suffered it was comparable, even if slightly more severe than Amatrice”.
From preliminary investigations, Amatrice has not suffered from significant local amplifications. "But if in Amatrice for the earthquake of August 24 (M 6.0) the effects were of the X-XI degree (Mercalli-Cancani-Sieberg scale), consistently with the almost total devastation of the town, in Norcia they were of the VI degree”, continues Valensise.
With the earthquake of 30 October (M 6.5), located very close to Norcia, the effects then rose to degree XI for Amatrice and to degree VIII-IX (with over 220 victims in Amatrice and none in Norcia).
“The difference in the seismic response of these two symbolic localities of the 2016 earthquakes”, adds Valensise, “is due to a high vulnerability of the buildings in Amatrice, which contrasts with a very low vulnerability for the buildings in Norcia, including those historical (with the exception of the churches and the Basilica of San Benedetto). To save the houses of Norcia would therefore have been the familiarity with strong shaking. Experience that has failed, however, in Amatrice.
“After the devastating earthquake of 1703 (which represents year zero for both locations), Norcia underwent several destructive earthquakes, up to that of 1979, each of which made it necessary to rebuild or strengthen the buildings. This did not happen in Amatrice, which has only experienced minor earthquakes since 1703”, continues Valensise.
The vulnerability of historic population centers increases as the time elapsed since the last seismic reconstruction increases, as a result accumulated both of the aging of the housing stock, and of the lack of seismic improvement interventions, as a sort of forgetting. To this it must be added that if a seismogenic fault has been quiescent for centuries, its probability of causing a destructive earthquake increases greatly compared to a fault that has given a strong earthquake in relatively recent times.
“An operational tool, therefore, capable of giving a ranking of priorities in the allocation of resources for the securing or preventive actions in the territories identified as most vulnerable”, concludes Valensise.
Extended
The forgotten vulnerability: a geology-and history-based approach for ranking the seismic risk of earthquake-prone communities of the Italian Apennines
G. Valensise, G. Tarabusi, E. Guidoboni, G. Ferrari
The 2016–2017 Central Italy earthquakes have shown that the local seismic risk is dominated by the extreme vulnerability of the building stock. We attempt to rank the vulnerability of Apennines' settlements based on a combined geological-historical approach. We first discuss the reasons of the apparent paradox caused by the very different seismic response of Amatrice and Norcia, both strongly hit by the 24 August 2016 earthquake (Mw 6.0). Based on the awareness that strong earthquakes force building reconstructions and changes in the individual and societal perception of seismic risk, we assume that the global vulnerability of Italian settlements increases with time since the last significant earthquake. We focus on the very active seismogenic areas straddling Italy's Apennines. We then use data on the local seismogenic sources and earthquake history to (1) select the municipalities that are more likely to suffer from destructive ground shaking, and (2) rank them as a function of the time elapsed since the latest earthquake, ie in terms of increasing vulnerability. We hence identified 716 municipalities, totaling about 5% of the Italian population, over 50% of which have not experienced destructive shaking since 1861, when the Kingdom of Italy reunited a number of smaller states. As such they are primary candidates to a poor performance in future significant earthquakes (Mw > 5.5) and should be given priority in any statewide vulnerability reduction plan. All results and elaborations, including the seismic histories of each of the selected localities, are also supplied in a specifically designed web-GIS.
Link to the study https://www.researchgate.net/publication/319595679_The_forgotten_vulnerability_a_geology-_and_history-based_approach_for_ranking_the_seismic_risk_of_earthquake-prone_communities_of_the_Italian_Apennines
Distribution of 716 capitals of the municipalities (representative of the entire municipal areas) selected with the procedure described in the text (from G.Valensise, G.Tarabusi, E. Guidoboni, G. Ferrari, 2017). The areas bordered in yellow represent the surface projection of the large seismogenic sources that run on top of the Apennines.
In violet: 38 municipalities for which there is no historical news of seismic destruction;
in red: 315 municipalities which in our ranking correspond to the municipal areas which have not suffered destructive earthquakes since 1861 (unification of Italy);
in black: 363 municipalities ordered according to the distance in time since the last destructive earthquake, after 1861.
Aerial photos of Amatrice (left) and Norcia (right), taken in early November 2016.
In Amatrice the earthquake of 24 August (M 6.0) had effects of the X-XI degree (Mercalli-Càncani-Sieberg scale), consistently with the almost total devastation of the town, while in Norcia a VI degree was reported. With the earthquake of 30 October (M 6.5), located very close to Norcia, the effects rose to degree XI for Amatrice and to degree VIII-IX for Norcia: Amatrice was almost totally destroyed - with a toll of over 220 victims, while Norcia, who didn't have to complain of any casualties, is slowly returning to normal
Rome, October 30 2017
Website: http://comunicazione.ingv.it
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