Hydrotectonics and fault criticality analysis in karstic regions from microseismic and hydrogeological observations
Authors:   , Perrochet Léa 
Published: April 2024

This study aims at assessing fault criticality in karstic regions through the analysis of hydrogeological and microseismic data, at enhancing the understanding of interactions between seismic activity and groundwater circulation and using this insight to estimate the present-day stress state of the shallow crust. These objectives are prompted by the present concerns regarding induced seismicity and its associated risks in projects involving for example deep geothermal energy exploitation, CO2 sequestration or radioactive waste disposal. In the context of induced seismicity, fluids play a major role by influencing the effective stresses and thus fault stability. While human-induced seismicity is often observed in cases of waste water injection or reservoir stimulation, naturally triggered seismicity following seasonal recharge can also be observed, especially in karstic areas where the channeling effect of karst conduits has a major influence on pore pressure. Thus, combining natural seismicity and hydrogeological data can inform on the interaction between fluid flow and fault rupture. To fulfill these goals, the initial phase involves the identification and the quantification of what mechanisms may trigger seismicity following an important recharge period (heavy precipitation or snow melt) in karstic regions. The identified mechanisms are 1) an increasing vertical load due to the additional water within the karst conduits, 2) poro-elastic deformation resulting from the additional load, 3) a massive pore pressure increase resulting from a direct hydrogeological connection between the infiltrated water and focal depth and 4) pressure diffusion process. These mechanisms, whether they act alone or together, influence the pore pressure, however the timing of their effectiveness varies. The response of the pore pressure to an increasing hydraulic head is instantaneous for mechanism 1-3 but delayed for 4. Using data from 3 test sites and 1D analytical solutions to calculate pore pressure response to an increasing hydraulic head allows to determine, which process is more likely to trigger seismicity following heavy rainfalls. In parallel to this initial phase, a seismic and hydrogeological monitoring network is deployed in the Neuchˆatel Jura to create a seismic catalog and collect continuous flow rates from major karstic springs, which is used as proxy for groundwater level variations. These two data sets are then combined and a statistically significant link is found between high groundwater conditions and seismic activity. A detailed analysis of the seismic activity of the analyzed fault zones and their associated rain-triggered events, coupled with the afore-mentioned 1D analytical solutions is then performed. For each fault zone a triggering pore pressure increase is defined and used to determine the sensitivity of the faults to hydraulic head variations. Although the majority of the investigated faults show some activity following important precipitations, events do not occur on a regular basis. The identification of rain-triggered events, the detection threshold of seismic events as well as hydromechanical properties of the faults may explain this lack of consistency. Integrating this knowledge, together with the overall seismic activity of the region and the geological and hydrogeological context, a conceptual hydromechanical model for the study area is presented. Finally, the state of stress of the shallow crust is estimated using two distinct methods and a range of values and orientations is presented. As a conclusion, this study mainly stresses the importance of considering the hydrogeological context when discussing rain-triggered seismicity, as the channeling effect of karst conduits drastically increases the pore pressure at depth, and consequently has a major influence on the magnitude of the triggering pore pressure. This study also contributes to a higher understanding of the seismic activity near Lake Neuchˆatel and how it is influenced by the geological and hydrogeological context.