The first typology of gravity seismic sources for ground motion analysis

Landslides generate seismic waves, as previous measurements from seismometers placed near unstable slopes have shown. However, the analysis of these waves remains a challenge, partly due to the diversity of seismic signals and nomenclatures adopted. Researchers from various laboratories [1] have compiled and analysed observations of several landslides around the world using the mobile park SISMOB of RESIF, in order to propose the first general classification of these sources and their seismic signals.


Gravitational instabilities are natural hazards that have a strong impact on human societies. To limit the associated risks, a better understanding of the physics that controls the failure of unstable slopes is needed. Seismic monitoring has shown that it is possible to obtain information on the processes that take place within them. This approach can help to better understand the occurrence of abrupt accelerations in relation to climatic, tectonic, or anthropogenic forcings. In the long term, combined with other means of observation, seismic monitoring could make it possible to monitor the stability of a slope in real time. However, the processes behind the seismic signals recorded by the networks deployed on these unstable slopes are still poorly understood. This is partly due to the heterogeneity of the instrumentation networks deployed, the complexity of the areas studied (complex topography, rheology difference, state of deterioration of the environment) and the variety of physical processes that can occur within an unstable slope.

Through the analysis of seismological records acquired on 13 unstable slopes around the Globe, the research team proposes the first standard typology of seismic source signals induced by the destabilization of unstable slopes (fracture opening, shear, flow, collapse - Figure 1). To produce this typology, the approach adopted was to define attributes specific to the seismic signals generated by each type of source, taking into account in particular the nature of the seismic waves (strongly influenced by the environment in which they propagate) and the disparity in the characteristics of the seismic sensors. To allow the comparison of seismic signals, systematic data pre-processing was implemented (instrumental correction, filtering) and nine attributes on the waveform and frequency content of seismic signals were selected (signal duration, signal asymmetry coefficient, number of peaks in the signal envelope, signal autocorrelation duration, mean frequency, maximum energy peak frequency, bandwidth, and minimum and maximum signal frequencies).

The comparative analysis of the signals made it possible to define three main classes of seismic sources common to all the gravitational instabilities studied: "slopequake" (SQ), "rockfall" (RF) and "granular flow" (GF). For the "slopequake" (SQ) class, subcategories are proposed in relation to the frequency content of the signal: "low-frequency slopequake" (LF-SQ), "high-frequency slopequake" (HF-SQ), "hybrid slopequake" (Hybrid-SQ), "slopequake with precursors" and "tremor-like slopequake".

Figure 1
Conceptual model of seismic sources of gravity origin on an unstable slope with (a) a water-saturated flow (e.g. torrential lava, mudflow), (b) a dry granular flow (e.g. debris avalanche), (c) a block fall, (d) a tension fracture opening, (e) a tension crack opening, (f) a friction process and (g) fluid migration in a fracture
Figure 2
Synthèse de la classification avec les valeurs pour chaque attribut et un exemple de forme d’ondes pour chaque classe

 

The authors of the study indicate that this first typology and the proposed analytical approach are a first step in comparing seismological observations and microseismicity catalogues created on many unstable slopes. The typology will allow:

  • explain the variability of the so-called slopequakes sources observed on the instrumented slopes;
  • to better understand the physics of the different seismogenic sources;
  • to better understand the spatial and temporal variations of seismic activity in relation to surface deformations and different forcings.

This approach, aimed at achieving a standard typology, is also an important step in the deployment of automatic solutions for the classification of seismic sources specific to slow landslides but also to other geological objects such as volcanoes, glaciers or reservoirs.

***For more information

The library of seismic signals used to construct the classification is available online on the website of the OMIV National Observation Service (Multi-disciplinary Observatory of Versant Instability)

***Source

Towards a standard typology of endogenous landslide seismic sources, Provost, F., Malet, J.-P., Hibert, C., Helmstetter, A., Radiguet, M., Amitrano, D., Langet, N., Larose, E., Abanco, C., Hürlimann, M., Lebourg, T., Lévy, C., Le Roy, G., Ulrich, P., Vidal, M., Vial, B., Earth Surface Dynamics (2018), doi:10.5194/esurf-6-1059-2018

***Contacts

– Jean-Philippe Malet, IPGS/EOST : 03 68 85 00 47
– Floriane Provost, IPGS/EOST : 03 68 85 00 47
– Mathilde Radiguet, ISTerre : 04 76 63 51 19

***This news was also relayed by

– The CNRS Institute of Universe Sciences (INSU)
– The Strasbourg School and Earth Sciences Observatory (EOST)
– The French Seismological and Geodetic Network (RESIF)

[1] including, among others, the French laboratories, the Institute of Earth Physics in Strasbourg and the Institute of Earth Sciences