Analysis of Pre-collapse Failure Signals from a Destructive Rockfall by Nanoseismic Monitoring

Abstract

On May 10th 2011 a huge, destructive rockfall with an estimated rock mass of 15,000 m3 occurred in scenic “Rappenlochschlucht” gorge in the Vorarlberg Alps, Austria. At creeping Heumoes slope, in a distance of some 5 km to Rappenlochschlucht, a permanent seismic network was installed in July 2009 to analyze the spatio-temporal occurrence of slidequakes related to slope movement of a few cm/a. The rockfall happened outside the monitoring region of Heumoes slope but could be recorded with an estimated equivalent local magnitude ML,eq 2.3 by the seismic network. Two preceeding, much weaker rockfalls could also be identified and located. Beside these rockfall events, we observed twelve fracture signals with impulsive signatures which were at first glance very similar to the slidequakes of Heumoes slope. All twelve fracture signals with magnitudes −0.5 < ML < 0.4 recorded on May 10th 2011 are originated in the rockfall area of Rappenlochschlucht, and occurred some 5 h to15 min before the final, destructive rockfall took place. We assume these fracture signals indicate the progressive detachment of the rockfall volume from the rock massif. To investigate the effect of potential external triggers, several meteorological and hydrological data were analyzed. No triggering effect of the rockfall at Rappenlochschlucht on May 10th 2011 could be identified yet, and the seismically observed precursory fractures and small rockfalls were the only indicators of the pending hazard.