I terremoti a Sulmona: indagini di sismologia storica per la microzonazione sismica

Abstract

He rich seismic history of Sulmona includes eleven earthquakes with Intensity Is≥6 MCS. Considering the different earthquakes with effects beyond the damage threshold, the comparison between the intensities attributed to Sulmona with those of the closer localities indicates that in many cases the town has suffered more severe damage. This suggests that the seismic history of Sulmona may have been conditioned by site effects, thus making more urgent the processing of a modern seismic microzonation. Within this light, an attempt has been made to reconstruct the damage distribution due to historical earthquakes in the old town centre. This practice was aimed at defining damage patterns to be compared with the results of the traditional geological and seismological investigations necessary to draw the “homogeneous microzones in the seismic perspective” and to define the possible site effects. In practical terms, we explored the possibility to identify trends in the repetition of the damage across the significant seismic history of the town. The trends may in turn suggest the occurrence of site effects at the scale of the town to be investigated with subsequent geological and seismological surveys. For this purpose, we have estimated the damage distribution due to the earthquakes occurred in 1706 (I 9-10 MCS at Sulmona), 1915 (I 8 MCS), 1933 (I 8 MCS), 1984 (I 6-7 MCS) and 2009 (I 5 MCS). The damage related to the oldest event cannot be defined in detail. However, historical information suggests that the entire town probably suffered strong effects. Therefore, in this case the presence of sectors in the old town centre more and less damaged cannot be hypothesised. As for the other mentioned earthquakes, the points of damage related to 1915 (181 points), 1933 (564), 1984 (141) and 2009 (91) have been defined. The obtained maps of the damage distribution do not indicate evident trends which may support the drawing of “homogeneous microzones in the seismic perspective”. However, sort of clusters of damage points have been observed close to the slopes bordering the terrace of the old town centre. This indication suggests that future geological and seismological surveys should be addressed to the characterisation of the site effects at the border of the old town. Moreover, this damage stratigraphy indicates that the vulnerability of the buildings has played a key role in the recent seismic history. In fact, 153 buildings, characterised by the repetition of the damage across the 1915, 1933, 1984 and 2009 earthquakes, have been identified. Most of the buildings have suffered damage due to two earthquakes among those mentioned above; more than twenty buildings have experienced damage due to three earthquakes; three buildings have suffered damage due to all mentioned earthquakes. This picture suggests a certain inaction and lack of care with respect to the post-earthquake restorations and for the maintenance of the historical buildings which are widespread throughout the entire old town centre of Sulmona.
Figura 1 Pianta del centro storico di Sulmona con I toponimi citati nel testo. In tratteggio, il limite approssimativo della Sulmo di età romana. Figure 1 Map of the old town centre of Sulmona with the place names mentioned in the text. The broken line defines the rough border of the Roman Sulmo.