Seismic monitoring of torrential and fluvial processes

Abstract

Abstract. In seismology, the signal is usually analysed for earthquake data, but earthquakes represent less than 1 % of continuous recording. The remaining data are considered as seismic noise and were for a long time ignored. Over the past decades, the analysis of seismic noise has constantly increased in popularity, and this has led to the development of new approaches and applications in geophysics. The study of continuous seismic records is now open to other disciplines, like geomorphology. The motion of mass at the Earth's surface generates seismic waves that are recorded by nearby seismometers and can be used to monitor mass transfer throughout the landscape. Surface processes vary in nature, mechanism, magnitude, space and time, and this variability can be observed in the seismic signals. This contribution gives an overview of the development and current opportunities for the seismic monitoring of geomorphic processes. We first describe the common principles of seismic signal monitoring and introduce time-frequency analysis for the purpose of identification and differentiation of surface processes. Second, we present techniques to detect, locate and quantify geomorphic events. Third, we review the diverse layout of seismic arrays and highlight their advantages and limitations for specific processes, like slope or channel activity. Finally, we illustrate all these characteristics with the analysis of seismic data acquired in a small debris-flow catchment where geomorphic events show interactions and feedbacks. Further developments must aim to fully understand the richness of the continuous seismic signals, to better quantify the geomorphic activity and to improve the performance of warning systems. Seismic monitoring may ultimately allow the continuous survey of erosion and transfer of sediments in the landscape on the scales of external forcing.

Seismic monitoring of small alpine rockfalls – validity, precision and limitations

“…However, that study had a larger network aperture and focused on event volumes of 10 3 -10 6 m 3 . Instead, rock mass volumes in the Lauterbrunnen Valley were generally well be-low 1 m 3 and our study had only four operating seismic stations, organised in a topology and station spacing that are comparable to those from other studies (Hibert et al, 2014;Burtin et al, 2016).…”

“…Two distinct, short seismic activity phases (yellow polygons 1 and 2) are followed by an emergent and prolonged period of activity (yellow polygon 3) after 7.5 s of calm. Levy et al, 2015;Burtin et al, 2016) with a finite velocity. Thus, in a homogeneous medium, the seismic signal arrives at different seismic stations at different times and with systematic, frequency-and distance-dependent changes of the signal properties.…”

“…Further options to reduce false detections can be setting thresholds for minimum and maximum event duration, signal amplitude variance throughout the network, comparison with existing catalogues (e.g. the Swiss earthquake catalogue), and signal-to-noise ratio (SNR) (Burtin et al, 2014(Burtin et al, , 2016, the latter being the ratio of the maximum and average value of the signal envelope of an event. However, all these thresholds must be adjusted to an existing data set of potential rockfall events and their effects should be inspected.…”

“…The latter is due to the preferential signal attenuation of higher-frequency waves, fragmentation of rocks during impact and changing amplitudes with time due to the moving source approaching or passing by a station (Burtin et al, 2013(Burtin et al, , 2016. Approaches that use the full waveform (Lacroix and Helmstetter, 2011) or its envelope (Burtin et al, 2013) are more appropriate to locate the source of seismic signals resulting from such processes.…”

“…Topography correction is necessary because rockfalls and other gravitational mass wasting processes generate surface waves that propagate following the topography (Dammeier et al, 2011;Hibert et al, 2014;Lin et al, 2015;Burtin et al, 2016). The results of this correction were stored in distance maps.…”