Seismic signature of the deadly snow avalanche of January 18, 2017, at Rigopiano (Italy)

Most snow avalanches occur unobserved, which becomes particularly dramatic when human lives are involved. Seismological observations can be helpful to unravel time and dynamics of unseen events, like the deadly avalanche of January 18, 2017, that hit a Resort-hotel at Rigopiano in the Abruzzi (Italy). Particle motion analysis and spectrograms from data recorded by a close seismic broadband station, calculation of synthetic seismograms, as well as simulation of the flow, allowed us to construct the dynamics of the snow avalanche that buried alive 40 people, killing 29. Due to the bad weather conditions, no visual observation was made, thus making it impossible to determine the exact moment of the avalanche and to report necessary observations of the dramatic 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. Within an eligible 24 min time range of the avalanche, we 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 approximately 17 km away. Particle motion 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. Simulation of the avalanche dynamics-calculated by using a 2D rapid mass movement simulator-indicates 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"s that can be used to time the destruction of the hotel. Our results demonstrate that seismic recordings combined with simulations of mass movements are indispensable to remotely monitor snow avalanches. Seismology provides useful tools that can help to better understand the dynamics of seismic events, different from earthquakes, as e.g. volcanic eruptions, rock falls or huge landslides 1-5. There are only a few examples in literature where seismology was successfully used to study avalanches 6-10 , probably because the density of snow is up to ten times smaller, compared to debris, which results in a reduced ground coupling and in a smaller seismic signal amplitude. On January 18, 2017, in a remote location in the Abruzzo region (Central Italy), a deadly avalanche buried 40 people under the Resort-hotel "Rigopiano". In a dramatic rescue operation 11 people could be recovered, while for another 29 persons there was no way to escape. The bad weather conditions with heavy snowfall closed the access road, isolating the Rigopiano location from the outside world. The reduced visibility prevented any eyewitness report of the avalanche, thus the exact moment, as well as the dynamics of this catastrophic event, are still not confirmed. We use seismic recordings and on-site inspection, combined with numerical modelling, to reconstruct the dynamics and to determine the exact moment of the deadly avalanche.