on the July 10, 2024
Researchers from the GEOAZUR laboratory (Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, IRD) and the GEGENAA laboratory at the University of Reims Champagne-Ardenne published a study in the American Geophysical Union's journal Earth and Space Science in June 2024, highlighting an interaction between fluids and asismic slip in the triggering and maintenance of seismic activity following extreme weather events.
On October 2, 2020, the Alpes Maritimes department in southeastern France was hit by storm Alex. Rainfall of unprecedented intensity reached almost 600 mm in less than 24 hours, causing numerous floods and landslides, resulting in significant human and material damage. The valleys north of Nice (Vésubie, Roya, Tinée) were particularly hard hit.
Significant increase in seismic activity
Unusually, in the Tinée valley, the stations of the French seismological network Epos-France (a consortium coordinated by CNRS), recording an average of three earthquakes per month of magnitude greater than 0.4, detected a significant increase in seismic activity in the days following Storm Alex (23 earthquakes in October). An in-depth study of continuous signals from nearby seismological stations identified 188 earthquakes of magnitudes ranging from -1 to 2 between October and December 2020. These earthquakes are organized in swarms between 3 and 4 km below the surface. One of the seismic swarms observed shows a bidirectional double-speed migration, an indicator of the multiplicity of physical processes at work.
To identify the mechanisms involved in triggering and maintaining seismicity, hydromechanical modelling was carried out. Given the short time lag between the extreme meteorological phenomenon and the occurrence of the earthquakes, the team explored theimpact of surface mass additions and withdrawals, caused respectively by massive flooding and multiple landslides, as well as the contribution of a fluid pressure front in the crust. A final transient mechanism, asismic slip, recently revealed by seismic swarm analysis, was also introduced.
A new perspective on earthquake triggering
The results show that stress perturbations caused by landslides or surface flooding are negligible at the depth at which seismicity occurs. Furthermore, the analysis reveals that stress variations generated by the diffusion of fluids from the surface alone explain only around 2% of the seismicity observed (i.e. 4 of the 188 earthquakes recorded). A third model, based on sequences of asismic shifts in the form of pulses of varying duration and amplitude, perfectly reproduced the temporal evolution of seismicity observed in the Tinée during this period. Their results show that the increase in fluid pressure in crustal faults following Storm Alex initially led to asismic deformation. The intermittent stress perturbations associated with these asismic shifts on the faults are then sufficient to trigger seismic swarms.
- To find out more :
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Jacquemond, L., Godano, M., Cappa, F., Larroque, C. (2024), Interplay between fluid intrusion and aseismic stress perturbations in the onset of earthquake swarms following the 2020 Alex extreme rainstorm. Earth and Space Science, 11, e2024EA003528. https://doi.org/10.1029/2024EA003528
- Research contacts
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- Laeticia Jacquemond - laeticia.jacquemond@univ-cotedazur.fr
- Maxime Godano - maxime.godano@univ-cotedazur.fr
- Frédéric Cappa - frederic.cappa@univ-cotedazur.fr
- Christophe Larroque, UMR Géoazur, University of Reims Champagne-Ardenne - christophe.larroque@univ-reims.fr
