Validity, precision and limitations of seismic rockfall monitoring
Michael Dietze1, Solmaz Mohadjer2, Jens M. Turowski1, Todd A. Ehlers2, and Niels Hovius11GFZ German Research Centre for Geosciences, Section 5.1 Geomorphology, Potsdam, Germany 2University of Tübingen, Department of Geosciences, Tübingen, Germany
Received: 28 Feb 2017 – Accepted for review: 14 Mar 2017 – Discussion started: 16 Mar 2017
Abstract. Rockfall in deglaciated mountain valleys is perhaps the most important post-glacial geomorphic process for determining the rates and patterns of valley wall erosion. Furthermore, rockfall poses a significant hazard to inhabitants and motivates the monitoring for rockfall occurrence in populated areas. Traditional rockfall detection methods, such as aerial photography and Terrestrial Laser Scanning (TLS) data evaluation provide constraints on the location and released volume of rock, but have limitations due to significant time lags or integration times between surveys, and deliver limited information on rockfall triggering mechanisms and the dynamics of individual events. Environmental seismology, the study of seismic signals emitted by processes at the Earth's surface, provides a complementary solution to these shortcomings. This approach is limited amongst others by the strength of the signals emitted by a source and their transformation and attenuation towards receivers. To test the ability of seismic methods to identify and locate small rockfalls, and to characterise their dynamics, we surveyed a 2.16 km2 large, near vertical cliff section of the Lauterbrunnen Valley in the Swiss Alps with a TLS and six broadband seismometers. During 37 days in autumn 2014, ten TLS-detected rockfalls with volumes ranging from 0.053 ± 0.004 to 2.338 ± 0.085 m3 were independently detected and located by the seismic approach, with a deviation of 81−29+59 m (about 7 % of the average inter-station distance of the seismometer network). Further potential rockfalls were detected outside the TLS-surveyed cliff area. The onset of individual events can be determined within a few milliseconds, and their dynamics can be resolved into distinct phases, such as detachment, free fall, intermittent impact, fragmentation, arrival at the talus slope and subsequent slope activity. The small rockfall volumes in this area require significant supervision during data processing: 2175 initially picked potential events reduced to 511 potential events after applying automatic rejection criteria. The 511 events needed to be inspected manually to reveal 19 short earthquakes and 37 potential rockfalls, including the ten TLS-detected events. Rockfalls do not show a relationship to released seismic energy or peak amplitude at this spatial scale due to the dominance of process-inherent factors, such as fall height, degree of fragmentation and distribution, and subsequent talus slope activity. The combination of TLS and environmental seismology provides a detailed validation of seismic detection of small volume rockfalls, and revealed unprecedented temporal, spatial and geometric details about rockfalls in steep mountainous terrain.
Dietze, M., Mohadjer, S., Turowski, J. M., Ehlers, T. A., and Hovius, N.: Validity, precision and limitations of seismic rockfall monitoring, Earth Surf. Dynam. Discuss., doi:10.5194/esurf-2017-12, in review, 2017.