Landslides generate seismic waves, as shown by previous measurements from seismometers placed near unstable slopes. However, the analysis of these waves remains a challenge, partly due to the diversity of seismic signals and nomenclatures adopted. Researchers from different laboratories  have compiled and analysed observations from several landslides around the world using Résif’s SisMob mobile fleet, in order to propose the first general classification of these sources and their seismic signals. These results have just been published in an article by Earth Surface Dynamics.
Gravity instabilities are natural hazards that have strong impacts on human societies. In order to limit the associated risks, a better understanding of the physics that controls the rupture of unstable slopes is necessary. Seismic listening has demonstrated that it is possible to obtain information on the processes taking place within them. This approach can provide a better understanding of the occurrence of abrupt accelerations related to climatic, tectonic, or anthropogenic forcings. In the long term, combined with other means of observation, seismological monitoring could enable real-time monitoring of the stability of a slope. However, the processes at the origin of 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, difference in rheology, state of alteration of the environment) and the variety of physical processes that can occur within an unstable slope.
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 boulder fall, (d) a tension fracture opening, (e) a tension crack opening, (f) a friction process, and (g) fluid migration in a fracture.
Thanks to the analysis of seismological recordings acquired on 13 unstable slopes spread 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, flows, collapses – Figure 1). In order 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 seismological sensors. To enable comparison of seismic signals, systematic data pre-processing has been implemented (instrumental correction, filtering) and nine attributes on the waveform and frequency content of seismic signals have been selected (signal duration, signal asymmetry coefficient, number of peaks in the signal envelope, signal autocorrelation time, mean frequency, maximum energy peak frequency, bandwidth, and minimum and maximum signal frequencies).
The comparative analysis of the signals allowed to define three main classes of seismic sources common to all the gravity instabilities studied: “slopequake” (SQ), “rockfall” (RF) and “granular flow” (GF). For the “slopequake” (SQ) class, sub-categories 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”.
The authors of the study indicate that this first typology as well as the proposed analysis approach is a first step to compare seismological observations and microseismicity catalogues created on many unstable slopes. The typology will allow :
- to explain the variability of slopequakes observed on instrumented slopes,
- to better understand the physics of the different seismogenic sources,
- to better understand the spatio-temporal variations of seismic activity in relation to surface deformations and the different forcings.
This move towards a standard typology is also an important step towards 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 build the classification is available online on the website of the National Observatory Service OMIV (Observatoire Multi-disciplinaire des Instabilités de Versants).
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. (2018) Towards a standard typology of endogenous landslide seismic sources. Earth Surface Dynamics doi :10.5194/esurf-6-1059-2018
Jean-Philippe Malet, IPGS/EOST : email@example.com, 03 68 85 00 47
Floriane Provost, IPGS/EOST : firstname.lastname@example.org, 03 68 85 00 47
Mathilde Radiguet, ISTerre : email@example.com, 04 76 63 51 19
This news was also relayed by the Ecole et Observatoire des Sciences de la Terre de Strasbourg. (Eost)