Multidisciplinary analysis of the deadly snow avalanche of 18 January 2017 at Rigopiano (Central Italy)

Abstract

On January 2017, a snow avalanche devastated a Resort-hotel in the municipality of Rigopiano in Abruzzo (Central Italy), unfortunately killing 29 people. Due to the bad weather conditions, no visual observation was made; many are the questions and hypotheses around this tragic event. On-site inspections revealed that the hotel was horizontally cut by shear forces and dislocated by 48 m in 70°N direction, once the increasing avalanche pressure exceeded the structural shear strength of the building. Analyses of phone calls and inspections of the victims’ mobile phones revealed that the avalanche struck sometime between 15:40 and 15:54 (UTC). Within this eligible 24 min time window, we scanned regional seismograms for any “suspicious” signal that could have been generated by the avalanche and found three weak seismic transients, starting at 15:42:38 UTC, recorded by the nearest operating station GIGS located in the Gran Sasso underground laboratory at a distance of approximately 17 km from Rigopiano. Directional analysis of the strongest seismic avalanche signal, as well as of the synthetic seismograms, match best when assuming a single force seismic source, attacking in direction of 120°N. Hundreds of simulations of the avalanche dynamics – calculated by using a 2D rapid mass movement simulator – indicate that the seismic signals were rather generated as the avalanche flowed through a narrow and twisting canyon directly above the hotel. Once the avalanche enters the canyon it is travelling at maximum velocity (37 m/s) and is twice strongly deflected by the rock sidewalls. These impacts created a distinct linearly polarized seismic “avalanche transient” that had be used to time the destruction of the hotel. Concluding, the avalanche of Rigopiano detached at 15:41:59 (UTC), entered in a canyon (15:42:35), entrained trees and boulders - increasing the mass significantly - an hit the hotel on 15:43:20 at a speed of 100 km/h.Our results demonstrate that seismic recordings combined with simulations of mass movements are indispensable to remotely monitor snow avalanches.