ISTerre seminar

Seismotectonic analysis of the Southern Alps and the Ligurian margin : contribution of aseismic forcing to the spatio-temporal dynamics of seismicity

Monday 24 November 2025 - 11h00
Laeticia Jacquemond - GEOAZUR
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Understanding the mechanisms driving seismicity in intraplate regions remains a major geophysical challenge, due to the superposition of low tectonic strain rates and various forcing (crustal fluids overpressure, aseismic slip on faults, rainfall, postglacial rebound). Microseismicity, by illuminating active fault structures at depth, offers a unique window into these complex dynamics. This thesis aimed to unravel the interactions between tectonic and transient forcings that trigger and sustain intraplate seismicity, and to quantify their respective contributions in space and time. Our dual objective was to identify active structures and reconstruct the forcing sequences underlying seismic activity in the southwestern Alps. By analyzing over 7 600 earthquakes recorded between 2014 and 2022 and gathered in the SismoAzur catalog, we identified multiple seismic clusters revealing intricate underlying processes. A detailed study of an unusual seismic episode, following extreme rainfall from Storm Alex in October 2020, led to the proposition of an original forcing sequence in which aseismic slip was initiated—or enhanced—by fluid overpressure in the upper crust (caused by rainfall), acting as a cascading driver of seismic activity. At the regional scale, high-resolution double-difference relocations allowed us to refine the geometry of seismically active structures. The detailed analysis of dense seismic clusters further highlighted the influence of transient forcings, particularly aseismic deformation in environments exhibiting low hydraulic diffusivity. Overall, our findings highlighted the influence of aseismic processes on the spatio-temporal dynamics of seismicity in the southwestern Alps, whether acting alone or in conjunction with other triggers. Discrete yet essential, these processes emerge as an underestimated driver of deformation.

Organizing team : Organisation labo

Salle Dolomieu, Maison des Géosciences, 38400 Saint Martin d'Hères