PSI - Issue 78

Daniele Storni et al. / Procedia Structural Integrity 78 (2026) 237–244

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3. The seismicity of the area The 1997-1998 earthquake sequence affected central Italy between September 1997 and April 1998, primarily impacting the regions of Umbria and Marche. It included eight mainshocks with Mw > 5.0, notably the Mw 5.7 event at 02:33 CEST on 26 September, followed nine hours later by a stronger Mw 6.0 shock. Both occurred near Annifo, producing intensities of VIII and X on the Mercalli scale. The sequence was preceded by foreshocks (e.g., Mw 4.5 on 3 September) and followed by thousands of aftershocks, over thirt y exceeding Mw 3.5. In the first 40 days, several strong earthquakes (Mw 5.0 – 6.0) triggered a fault system about 45 km long along the Apennines, with breaks going down to 8 km deep and showing a pattern of normal faulting that dips to the southwest and moves in the opposite direction of the rupture. A notable feature was the progressive migration of seismicity from NW to SE, activating adjacent fault segments. The most affected municipalities included Foligno, Nocera Umbra, Preci, Sellano, Assisi, and Spello in Umbria, and Fabriano, Serravalle di Chienti, and Camerino in Marche. The overnight shock caused widespread structural damage, and the later Mw 6.0 event led to collapses of already weakened buildings — including the partial destruction of Giotto’s vault in the upper Basilica of St. Francis in Assisi. Additional damage was rep orted across the border in Marche, notably in Visso, Fabriano, Matelica, Pioraco, and Fiuminata. Interestingly, the 26 September 1997 event caused the most severe damage to the Basilica of St. Francis, despite not being the strongest earthquake historically to affect the site. Earlier events in 1279, 1747, 1751, and 1832 were of equal or greater magnitude and had epicenters even closer to the Basilica. than the 1997 earthquake. Figure 2.a shows homogeneous parametric data, both instrumental and macroseismic, related to earthquakes with maximum intensity MCS Io ≥ 5 or magnitude Mw ≥ 4.0 in the area surrounding Assisi during the period 1000 – 2020 (Rovida et al., 2022; CPTI15, version 4.0). In addition, the Italian Macroseismic Database (Locati et al., 2022; DBMI15, version 4.0) reports a total of 117 seismic events (Figure 2.b) historically documented for Assisi, with a minimum intensity Io of 3.0, including the earthquakes mentioned above.

Fig. 2. A composition of the: a) Map of instrumental and macroseismic earthquake epicenters with maximum intensity MCS Io ≥ 5 or magnitude Mw ≥ 4.0 , from CPTI15; b) Earthquakes historically documented for Assisi, from DBMI15; c) Map showing the epicenters of 13 earthquakes with magnitude M L > 3.0 recorded by the BAS01 station (red triangle) since its installation in October 2023. The plot also includes the location of an M L 2.0 earthquake. For this event, along with an ML 3.5 earthquake (orange circles), Figure 3 shows the corresponding seismogra ms. The table in the bottom right reports the hypocentral locations of earthquakes with a magnitude ≥ 3.0 recorded since the installation of BAS01.