La storia sismica di Siena: stato delle conoscenze (aggiornato a gennaio 2022)

Abstract

To this day it remains impossible to determine exactly ‘when’ the next large earthquakes will take place. However, it is possible to say ‘where’ they are most likely to happen and ‘how’ they might interact with a given stretch of country and the buildings, infrastructures and people therein. In order to achieve such a probabilistic forecasting several kinds of studies are needed. Geological, geophysical and seismotectonic studies help to identify the structures (if any) capable of generating earthquakes and to understand how they function. The historical study of past earthquakes (historical seismology) is also very important, particularly in countries such as Italy, where the most destructive earthquakes can have very long return periods (centuries or even millennia) and one needs to go as far back in time as possible in order to draw a reliable picture of the trends of seismicity. Historical seismologists search for historical evidence of the effects of past earthquakes on the studied areas, translate these raw data into macroseismic intensity grades and derive from them the parameters identifying each earthquake. The Italian seismic catalogue is one of the richest and longest-reaching in the world. The current Parametric Catalogue of Italian Earthquakes CPTI15 version 4.0 (released in January 2022) spans the period from 1000 AD to 2020, in which period it lists 4860 earthquakes with epicentral location within the current boundaries of Italy or just outside them, with a set threshold of intensity (I ≤ V MCS) or instrumental magnitude (I ≤ Mw 4.0 or higher). The Italian Macroseismic Database DBMI15, version 4.0 (January 2022), contains 123981 macroseismic intensity data based on observations of effects related to 15343 Italian locations and 4894 earthquakes occurred in Italy and its immediate surroundings between 1000 AD and 2020. From these data the “seismic history” of each locality can be derived, i.e. a list of all the macroseismic effects observed in that locality within the time span covered by the reference catalogue. The maximum intensity attested at a place can be considered (with due caution) as an empirical evaluation of the highest intensity threshold that – as far as current knowledge goes – it should be reasonably expected in the future. The level of relative completeness of seismic histories is very variable, for many reasons. For instance, understandably enough, historically relevant towns are most likely to have a longer and more detailed seismic history than small, marginal villages. Siena, whose seismic history is discussed here, is the second Tuscan locality for the quantity of available macroseismic observations (147), after Florence (234) and before Lucca (108). The current seismic history of Siena is therefore comparatively rich and well documented. Not all of the 147 intensity data constituting the seismic history of Siena reflect the effects of ‘local’ earthquakes, i.e., those with epicentral location in the Sienese or Tuscan Provinces but all the earthquakes responsible for the major seismic effects observed in Siena are of local origin. On the basis of currently available knowledge, the territory of the Province of Siena is historically characterised (or rather in the last millennium or so) by fairly frequent seismicity, but much less relevant than that localised along the central-northern Apennine belt (from Garfagnana-Lunigiana to Mugello and the Upper Valtiberina), where Mw 6.0 threshold has been reached several times in this period, and sometimes exceeded. In Siena there are 25 seismic resonances evaluated as having an intensity ≤VI MCS. This should correspond to a scenario of effects in which approximately half of the buildings undergoes slight damage (slight cracks in plasterwork, possible fall of a few tiles or stones from chimneys); 25% of the buildings is affected by moderate damage (slight cracks in walls, considerable fall of plasterwork, tiles and many chimneys); no more than 5% of buildings is so damaged as to become temporarily uninhabitable. This intensity was assigned to Siena in relation to several earthquakes whose epicentral location occurs within a radius of a few kilometres from the town. No matter how rich the historical macroseismic data base available for Siena, there are still good margins for improvement. This is demonstrated by ongoing research initiatives, including the systematic perusal of serial historical sources (diaries, newspapers and historical seismic questionnaires). It can be hoped that forthcoming updates of the seismic history of Siena may lead to a conspicuous increase in the quantity of available data, though probably not to significant changes in the information framework briefly outlined in this paper.