Articles | Volume 5, issue 4
Research article
29 Nov 2017
Research article |  | 29 Nov 2017

Spatiotemporal patterns, triggers and anatomies of seismically detected rockfalls

Michael Dietze, Jens M. Turowski, Kristen L. Cook, and Niels Hovius

Abstract. Rockfalls are a ubiquitous geomorphic process and a natural hazard in steep landscapes across the globe. Seismic monitoring can provide precise information on the timing, location and event anatomy of rockfalls, which are parameters that are otherwise hard to constrain. By pairing data from 49 seismically detected rockfalls in the Lauterbrunnen Valley in the Swiss Alps with auxiliary meteorologic and seismic data of potential triggers during autumn 2014 and spring 2015, we are able to (i) analyse the evolution of single rockfalls and their common properties, (ii) identify spatial changes in activity hotspots (iii) and explore temporal activity patterns on different scales ranging from months to minutes to quantify relevant trigger mechanisms. Seismic data allow for the classification of rockfall activity into two distinct phenomenological types. The signals can be used to discern multiple rock mass releases from the same spot, identify rockfalls that trigger further rockfalls and resolve modes of subsequent talus slope activity. In contrast to findings based on discontinuous methods with integration times of several months, rockfall in the monitored limestone cliff is not spatially uniform but shows a systematic downward shift of a rock mass release zone following an exponential law, most likely driven by a continuously lowering water table. Freeze–thaw transitions, approximated at first order from air temperature time series, account for only 5 out of the 49 rockfalls, whereas 19 rockfalls were triggered by rainfall events with a peak lag time of 1 h. Another 17 rockfalls were triggered by diurnal temperature changes and occurred during the coldest hours of the day and during the highest temperature change rates. This study is thus the first to show direct links between proposed rockfall triggers and the spatiotemporal distribution of rockfalls under natural conditions; it extends existing models by providing seismic observations of the rockfall process prior to the first rock mass impacts.

Short summary
Rockfall is an essential geomorphic process and a hazard in steep landscapes which is hard to constrain with traditional approaches. Seismic methods allow for the detection, location, characterisation and linking of events to triggers by lag times. This new technique reveals 49 rockfalls in 6 months with seasonally varying locations. Freeze–thaw action accounts for only 5 events, whereas 19 rockfalls were caused by rain with a 1 h peak lag time, and 17 events were due to diurnal thermal forcing.