Spatiotemporal patterns and triggers of seismically detected rockfalls

Abstract. Rockfalls are an essential geomorphic process and an important natural hazard in steep landscapes across the globe. Seismic monitoring can provide precise information on the timing, location and event anatomy of rockfalls, parameters that are otherwise hard to constrain. By pairing data from 49 seismically detected rockfalls in the Lauterbrunnen Valley, Swiss Alps, with independent information about 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 seasonally changing activity hotspots, (iii) and explore temporal activity patterns at different scales, ranging from months to minutes, to quantify relevant trigger mechanisms. Seismic data allows the classification of rockfall activity into three distinct phenomenological types and 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 methods with longer integration times, rockfall in the monitored limestone cliff is not spatially uniform but shows a downward shift of rock mass release spots by 33 m per month over the year, most likely driven by a continuously lowering water table. Freeze-thaw-transitions 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 as well as during the highest temperature change rates. This study is thus the first one to show direct links between proposed rockfall triggers and the spatio-temporal distribution of rockfalls under natural conditions, and extends existing models by providing seismic observations of the rockfall process prior to the first rock mass impacts.