Dr Claudia Abanco

Abstract. The objective of this work is to propose a standard classification of seismic
signals generated by gravitational processes and detected at close distances
(<1 km). We review the studies where seismic instruments have been
installed on unstable slopes and discuss the choice of the seismic
instruments and the network geometries. Seismic observations acquired at 13
unstable slopes are analyzed in order to construct the proposed typology. The
selected slopes are affected by various landslide types (slide, fall, topple
and flow) triggered in various material (from unconsolidated soils to
consolidated rocks). We investigate high-frequency bands (>1 Hz) where
most of the seismic energy is recorded at the 1 km sensor to source
distances. Several signal properties (duration, spectral content and
spectrogram shape) are used to describe the sources. We observe that similar
gravitational processes generate similar signals at different slopes. Three
main classes can be differentiated mainly from the length of the signals, the
number of peaks and the duration of the autocorrelation. The classes are the
“slopequake” class, which corresponds to sources potentially occurring
within the landslide body; the “rockfall” class, which corresponds to
signals generated by rock block impacts; and the “granular flow” class,
which corresponds to signals generated by wet or dry debris/rock flows.
Subclasses are further proposed to differentiate specific signal properties
(frequency content, resonance, precursory signal). The signal properties of
each class and subclass are described and several signals of the same class
recorded at different slopes are presented. Their potential origins are
discussed. The typology aims to serve as a standard for further comparisons
of the endogenous microseismicity recorded on landslides.
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Abstract. Monitoring of debris flows using ground vibration sensors has increased in the last two decades. However, the correct interpretation of the signals still presents ambiguity. In the Rebaixader monitoring site (Central Pyrenees, Spain) two different ground vibration stations are installed. At the first station the ground velocity signal is transformed into an impulses-per-second signal (low frequency, 1 Hz). The analysis of the data recorded at this station show that the shape of the impulses signal is one of the key parameters to describe the evolution of the event. At the second station the ground velocity signal is directly recorded at high frequency (250 Hz). The results achieved at this station show that the differences in time series and spectral analysis are helpful to describe the temporal evolution of the events. In addition, some general outcomes were obtained: the attenuation of the signal with the distance has been identified as linear to exponential; and the assembly of the geophones to the terrain has an important effect on the amplification of the signal. All these results highlight that the definition of ground vibration thresholds for debris-flow detection or warning purposes is a difficult task; and that influence of site-specific conditions is notable.
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