Hydro-Seismicity Triggered by Heavy Rainfall
Résumé
Seismic hazard assessment in stable continental regions (“intraplate domain”) involves specific difficulties, due to the lower recurrence of events compared to active zones, and to the fact that the triggering processes are particularly complex and still poorly understood. Several recent studies emphasize the role of local processes such as erosion, deglaciation, fluid circulation, etc. The concept of “hydro-seismicity” (Costain, Geol Soc Lond Spec Publ 432:91, 2017) explains a possible mechanism, the triggering of intraplate seismicity by the pressure increase in the interstitial fluid, decreasing the effective stress within the fault plane. Although climate change may not be perceptible at the level of the annual volumes of precipitation, the alternation of drought periods with intense rainy episodes may constitute a significant transient water supply, reaching shallow faults by infiltration into the subsoil and inducing the phenomenon of hydro-seismicity. In this context, we have shown that the local magnitude 5.4 earthquake of November 11, 2019, in Le Teil (France) could have been triggered by the transient increase in hydraulic pressure following heavy rainfall in the days preceding the event, while the trigger of the event was initially attributed to the activities of a quarry located near the earthquake zone. Using soil moisture data acquired by the SMOS satellite and modeling the height variations of the unsaturated zone by the ComPass code developed by the Bureau de Recherches Géologiques et Minières (BRGM), we show that water infiltration has induced hydraulic overpressures. The maximum value is located at the intersection of a system of faults at 1200 m depth; the hypocenter of the earthquake. The impact of this overpressure has been estimated to be several times greater than the cumulative effect of mechanical stress release due to the exploitation of the quarry over the two past centuries.