The alluvial events in the last two centuries at Sarno, southern Italy: their classification and power-law time-occurrence

Summary

In past and recent years, the town of Sarno, Southern Italy, about 17 km E from the Vesuvio volcano, has been devastated by alluvial events with abrupt slippings of the volcanoclastic mantle covering the surrounding mountains. The succession of these calamities has been accurately reconstructed since 1794 on the basis of old documents and tentatively classified according to an index of strength that takes into account both the rainfall infiltrated into the fractured rock in the 30 days before the event and the rain falling during the event, apart from other descriptive terms characterizing the event. The catalogue has been verified to be complete only for events classified at least as weak. The application of the Cantor dust method to such a catalogue has allowed the identification of a scale-invariance in the time clustering of the events, even if limited to the interval between 8 and 64 months. An important result is the evidence of a significant decrease in the time clustering of the events after 1854, immediately after the Little Ice Age (ca 1430–1850), characterized by a significant decrease of the time clustering of rainy days in Italy, related to the coherent decrease of zonal atmospheric circulation over the Mediterranean area. Moreover, the application of the rank-ordering statistics to the repose-times between successive events, classified at least as moderate and strong, has provided further significant evidence for time-scale-invariances. The reality of different scale-invariance laws in the time-occurrence of alluvial events, even if indicative of the critical state of the pyroclastic cover of the mountains surrounding Sarno, might be very helpful for the assessment and reduction of the hazard of future disasters. For example, on the basis of the results of the rank-ordering statistics, the most probable occurrence of an alluvial event at Sarno, classified at least as strong is predicted to occur within the 2007–2017 interval.