This seismicity cluster is well dissociated from the natural seismicity of the Pyrenees chain, which extends further south: it has long been considered induced, that is to say generated by the exploitation activities of the sub-surface. ground pipes in this region.
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However, the exact cause of this cluster of earthquakes was still debated. “It has long been attributed to past activities of intense gas extraction from the deep reservoir, which would have led to its contraction and progressive collapse, leading the rocks to a state close to rupture,” recalls the Toulouse III University. Paul Sabatier in a press release.
But another hypothesis began to emerge three years ago: the major role of wastewater from industrial activities, injected into the reservoir.
To explore this hypothesis, a Franco-German research team (including members from the University of Grenoble-Alpes, the University of Pau and Pays de l’Adour and the German laboratory in Potsdam, German research center for geosciences) then sought to better understand these links between human activities and seismicity.
A “dominant” role
“A network of around fifteen seismological sensors has been deployed around Lacq in order to better locate these induced earthquakes. They are triggered in and below the deep reservoir, most of them between four and five kilometers deep,” explains Laeticia Jacquemond, lead author and Master 2 student in Earth Sciences, Planets and Environment at the university during the development. of this study.
Ground movements were recorded continuously for three years. This made it possible to detect several hundred microseisms which could be precisely located and analyzed by scientists.
By compiling the analysis of recent earthquakes, but also those occurring over the last 50 years, the Franco-German team highlighted “the preponderant role of injection in the generation of seismicity in the Lacq region. “. “The majority of earthquakes are very close to the limits of the reservoir and the seismic energy released is directly linked to variations in the volume injected into the reservoir,” indicates the University of Toulouse, which believes that the study “provides crucial insight into the interactions between injection activities and seismicity in the Lacq region.
Other studies to be carried out
Jean Letort, teacher-researcher at IRAP (Institute for Research in Astrophysics and Planetology), underlines “the lack of instrumentation since the 2000s”, “the importance of continuous monitoring” and “the need for more in-depth studies on the prediction of seismic hazard and risk”. According to him, the challenge must be to develop models for predicting seismicity according to injection rates and volumes, “which requires access to detailed information on injection operations and on the properties of the reservoir”.
The Toulouse university study should therefore lead to other series of research on the processes generating earthquakes during injection operations in reservoirs. “This is an essential issue for the management of wastewater injections but also for deep geothermal energy and carbon dioxide sequestration projects, in order to best assess the hazard and risk associated with these projects,” believes Toulouse III-Paul Sabatier University.
