Application of Nanoseismic Monitoring: Seismic characterization of slope dynamics caused by softrock-landslides: The Super-Sauze case study

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1 Application of Nanoseismic Monitoring: Seismic characterization of slope dynamics caused by softrock-landslides: The Super-Sauze case study Marco Walter & Manfred Joswig Conference Landslide Processes 6-7 February 2009 Strasbourg, France

2 Outline Nanoseismic Monitoring the method and applications Short introduction to our work within the DFG project Natural Slopes General setting Observed seismic signals Comparison with prior observations Possible source mechanisms Summary and Outlook

3 Nanoseismic Monitoring the method and applications Nanoseismic Monitoring allows a tenfold higher resolution in detection and localization of fractures compared to common seismic techniques Method is based on seismic mini-array techniques: SNS (Seismic Navigating System) one 3c central station, three 1c outter stations Signal Processing with HypoLine: graphical jackknife technique influence of every parameter on event location is visualized in real time displays the most plausible solution Applications: e.g. sinkholes (Israel), landslides, active fault mapping, CTBTO

4 Short introduction to our work within the Research Unit Natural Slopes Subproject Geophysical investigations of the Heumoes slope Focus on the seismic monitoring of fracture processes

5 Short introduction to our work within the Research Unit Natural Slopes Detection and localization of discrete fractures -2.2 < M L < -0.7 Correlation to parts of the slope showing higher movement rates

6 Short introduction to our work within the Research Unit Natural Slopes water saturation varies with the season water saturated area Temporal occurrence of the fractures up to ~26 h after rain events rainfall subsurface water dynamics stress relief? Super Sauze

volume of 750.000 m 3 movement velocities between 0.01 and 0.

7 Super-Sauze triggered in the 1960 s altitude ranges between 2105m and 1740m length of 825m, average width of 135m estimated volume of m 3 movement velocities between 0.01 and 0.40 m per day mudslide consists of jurassic marls with moraine components

8 10 days field campaign from 14 th 24 th of July 2008: Installation of 4 seismic small arrays (SNS) Samplingrate of 400 (500) Hz Continuous recording 8 calibration shots: Slope: 1740 m v P ~ 600 m / s v S ~ 310 m / s v P / v S ~ 1,95 Bedrock below: v P ~ 2000 m / s Layer above homogeneous halfspace as underground model 2105 m

9 During the fieldcampaign in July 2008, we recorded, processed and located different types of signals caused by varying slope dynamics: Rockfalls Material failure Local earthquakes event type A event type B and C The signals of the different event types vary in: type A type B type C Signal duration sec.-min. 2-5 sec sec. Frequency content Hz Hz Hz Stations 2-4 SNSs 1-3 SNSs 1 SNS Amplitude [nm/s]

10 Event type A rockfalls High frequent noise comparable to avalanches spikes caused by falling blocks duration of a few seconds (single event) up to 20 minutes (multiple event) ~80 % with backazimuth of ~ 160 to S1C ~20 % with backazimuth of ~ 220 to S1C 1740 m 2105 m

Event type B material failure 34 events generated by material failure Clear P- and S- onsets Signals recorded with at least 2 SNS for localization Magnitude: -3.

11 Event type B material failure 34 events generated by material failure Clear P- and S- onsets Signals recorded with at least 2 SNS for localization Magnitude: -3.2 < M L < -1.3 Typical waveforms and sonogram patterns of one event of type B, M L = -2.2 in ~120 m distance, recorded with a 3cstation, and the determined phase onsets (red).

Event type B material failure events located in the upper and the mid-part of the mudslide the three events outside probably generated by cracking hardrock

12 Event type B material failure events located in the upper and the mid-part of the mudslide the three events outside probably generated by cracking hardrock spatial distribution of events correlates with higher movement velocities of the slope many of these events have been located close to the emerging in-situ crests

13 Event type B material failure Epicentres mapped on aerial picture from 1956: many of these events have been located close to the in-situ crests hidden by the mudslide today

Event type B material failure temporal occurrence of the events is more or less statistically distributed BUT: a cluster of events with the highest magnitudes occurred right after a

14 Event type B material failure temporal occurrence of the events is more or less statistically distributed BUT: a cluster of events with the highest magnitudes occurred right after a rain event on 21 st of July 2008 Rain intensity (blue) and temporal occurrence of the type B events with their magnitudes ML (green) during the field campaign 14th -24th of July 2008.

$Event type C material failure 44 fractures Type C generated by material failure detected with one single SNS no clear phase onsets enormous attenuation effects and amplitude$

15 Event type C material failure 44 fractures Type C generated by material failure detected with one single SNS no clear phase onsets enormous attenuation effects and amplitude decrease Typical waveforms and sonogram patterns of one event of type C, recorded with a 1c-station close to the source location (left) and with a 1c-station in a distance of ~25m (right).

16 Event type C material failure not located, but source area estimated, close to the station which recorded the highest amplitudes Location of the installed seismometer stations (black dots) with the amount of the events of type C generated close to them (blue).

17 Event type C material failure again, most of these events has been generated close to the emerging in-situ crests Location of the installed seismometer stations (black dots) with the amount of the events of type C generated close to them (blue).

18 Event type C* material failure hundreds (!) of downscaled type C events, which lie barely above the natural noise-level only detected with one single station similar frequency content but: lower amplitudes and shorter signal duration type C type C* Signal duration 2-20 sec. < 2 sec. Frequency content Hz Hz Stations 1 SNS 1 station Amplitude [nm/s]

19 Event type C* material failure again, ~80% of these events has been generated close to the emerging in-situ crests 1740 m 2105 m

20 Conclusions General observations: Most of the observed signals have been generated near by the (emerging) in-situ crests indicating the strongest deformation processes close to them Different Types of events indicate different source mechanisms Possible source mechanisms (current hypotheses): Events of type B caused by impulsive fractures within the slope material Events of type C /,C* caused by scratching and grinding of the slope against the in-situ crests / hardrock basement

21 Summary and Outlook Slope dynamics have been successfully monitored by applying the method Nanoseismic Monitoring at the Super-Sauze mudslide Differentiation of diverse types of events The spatial distribution of events correlates with higher movement velocities Most of the events are localized at the boundary between the sliding material and the (emerging) in-situ crests Determination of source processes by further research: data processing of 10 days field-campaign in october 2008 fractures vs. Piezometers / Inclinometers shear strength analysis

22 Many thanks to: the DFG for funding our work within the research unit Natural Slopes Jean-Phillipe Malet (University of Strasbourg) All the colleagues who supported us during the field campaign you for your attention! Nanoseismic Monitoring: Research Unit Natural Slopes :

Joint analysis of the Super-Sauze (French Alps) mudslide by nanoseismic monitoring and UAV-based remote sensing

first break volume 27, August 2009 special topic Joint analysis of the Super-Sauze (French Alps) mudslide by nanoseismic monitoring and UAV-based remote sensing Marco Walter, 1* Uwe Niethammer, 1 Sabrina