Testing seismic amplitude source location for fast debris-flow detection at Illgraben, Switzerland

Abstract

Abstract. Heavy precipitation can mobilize tens to hundreds of thousands of cubic meters of sediment in steep Alpine torrents in a short time. The resulting debris flows (mixtures of water, sediment and boulders) move downstream with velocities of several meters per second and have a high destruction potential. Warning protocols for affected communities rely on raising awareness about the debris-flow threat, precipitation monitoring and rapid detection methods. The latter, in particular, is a challenge because debris-flow-prone torrents have their catchments in steep and inaccessible terrain, where instrumentation is difficult to install and maintain. Here we test amplitude source location (ASL) as a processing scheme for seismic network data for early warning purposes. We use debris-flow and noise seismograms from the Illgraben catchment, Switzerland, a torrent system which produces several debris-flow events per year. Automatic in situ detection is currently based on geophones mounted on concrete check dams and radar stage sensors suspended above the channel. The ASL approach has the advantage that it uses seismometers, which can be installed at more accessible locations where a stable connection to mobile phone networks is available for data communication. Our ASL processing uses time-averaged ground vibration amplitudes to estimate the location of the debris-flow front. Applied to continuous data streams, inversion of the seismic amplitude decay throughout the network is robust and efficient, requires no manual identification of seismic phase arrivals and eliminates the need for a local seismic velocity model. We apply the ASL technique to a small debris-flow event on 19 July 2011, which was captured with a temporary seismic monitoring network.The processing rapidly detects the debris-flow event half an hour before arrival at the outlet of the torrent and several minutes before detection by the in situ alarm system. An analysis of continuous seismic records furthermore indicates that detectability of Illgraben debris flows of this size is unaffected by changing environmental and anthropogenic seismic noise and that false detections can be greatly reduced with simple processing steps.

Towards a standard typology of endogenous landslide seismic sources

“…The seismic signal amplitude hence increases progressively as the front is passing in the vicinity of the sensor (Abancó et al, 2012;Hürlimann et al, 2014;Burtin et al, 2016;Walter et al, 2017) and decreases progressively, as the front is moving away from the sensor. Large spikes and low-frequencies may be observed in the seismic signal corresponding to the front of the debris flow generated by large boulders impacts.…”

“…They occur during rainfall episodes when fine material and boulders run down the stream over long distances 5 (> 500 m). The seismic sensors are often installed at very close distance to the flow path so high frequencies up to 100 Hz may be recorded (Abancó et al, 2014;Burtin et al, 2016;Walter et al, 2017). Little energy is present in the low-frequencies (< 10 Hz) depending on the amount of water and the size of the rock blocks involved in the flow (Burtin et al, 2016).…”

“…However, a certain number of recorded signals do not exhibit impulsive first arrivals and clear P and S-waves onsets. For this kind of signal, location methods based on the inter-trace correlation of the surface waves (Lacroix and Helmstetter, 2011) or on the amplitude (Burtin et al, 2016;Walter et al, 2017) are more suitable and easier to automatize. Other methods such as HypoLine (Joswig, 2008) aim at integrating different strategies (i.e.…”

“…This strategy showed accurate location of impulsive events while the error on the epicenter of emergent events is reduced by the use of ASL to constrain the location. Many studies approximate the media attenuation field and/or the ground velocity, or do not take into account the topography, leading to mis-location of the events that prevents for accurate 25 interpretation of certain sources and leads to false alarms (Walter et al, 2017).…”

“…Seismological observations from 14 sites have been used to propose the typology (Table 1) Walter et al (2017)). Other networks are deployed for campaign measurements in order to document specific activity periods (e.g.…”