Seismic monitoring of precursory fracture signals from a destructive rockfall in the Vorarlberg Alps, Austria
In this study we describe the seismic analysis of precursory patterns of a rockfall in the "Rappenlochschlucht", a gorge located in the Vorarlberg Alps, Austria. The rockfall with an estimated volume of 15 000 m
3 occurred on 10 May 2011 (10:48:43 UTC) and destroyed a massive bridge construction. Fortunately, the rockfall did not cause any casualties.
A permanent seismic network consisting of three seismic small arrays was installed in July 2009 in 5 km distance to the gorge, at the Heumoes slope, in order to detect and locate slope-related fracture processes within a radius of a few hundred meters. By chance, the rockfall with an estimated equivalent local magnitude of ML,eq = 2.3 was recorded by the seismic network. We observed several smaller rockfall events up to three hours, and 12 fracture signals up to five hours prior to the rockfall. The smaller rockfalls and the fractures were both located in the vicinity of the source area where the main event emerged, applying absolute and relative localization methods.
These specific types of fracture signals located near by the gorge "Rappenlochschlucht" have never been observed in almost two years of permanent seismic monitoring. We interpret these fractures with magnitudes between ML = 0.4 and −0.5 as precursory signals of the main rockfall event. The observed fractures and the weaker rockfalls are sequences of initial stress relief within the rock mass and mass transferring processes, respectively, finally causing the destructive main rockfall event.
To investigate possible triggers of the destructive rockfall event, several meteorological and hydrological data as well as the local seismicity during that period of time were analyzed and discussed in detail in this study. Unfortunately, no triggering factor of the rockfall event was identified, and remains therefore unknown.