The interplay of permeability dynamics, fault weakening, and stress conditions in the seismogenesis around the city of Novy-Kostel from 2000 to 2018

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The triggering and driving role of high-pressure fluids in the seismogenesis of earthquake swarms, and especially in NW Bohemia, has been suspected since decades for various reasons: the spatial relation with mofettes and thermal springs, which show a response to the seismicity and the match of pore pressure diffusion models with the seismic migration pattern are the two most prominent examples. Further, there are several indications of a dynamic hydraulic system: Variations in CO2 concentrations degassing at the surface match GPS measurements of horizontal compression closing the fault; Changes in seismic wave velocity indicate changes in porosity during fracturing; Reactivation of low permeable layers provide pathways for fluid migration. In this project, we will investigate whether (i) a dynamic permeability of the different fault segments in dependence of stress, pressure and temperature; (ii) a reduced fault strength due to mineral dissolution; and (iii) the fault geometry within the local stress field, are either triggers, driving forces or facilitating conditions for the characteristic seismic migration pattern in the region. To address these research question, we propose the design of a three-dimensional, physics-based, thermo-hydro-mechanical model including multi-phase, multi-component flow of uprising supercritical carbon dioxide and deep groundwater. The features of such a model exceed existing approaches by far and allow a systematic testing of the different hypotheses. With its three-dimensional design, the model will be the first to reproduce the stress conditions and model the fluid flow along the complex fault geometry deduced from the moment tensor analysis. Especially for the swarms of 2011, 2014, and 2018, the 3D geometry is crucial to reproduce the spatio-temporal evolution of the seismic activity in these years. Incorporating the equations of state for CO2, considering phase changes, dissolution, and degassing of CO2 this model avoids potentially over-simplifying assumptions of single-phase fluid flow. Further, by considering a porosity/permeability evolution coupled with pore pressure, stress state, and temperature, this model will allow the simulation of multiple earthquake swarms and potentially enhance our understanding of the periodicity and spatio-temporal evolution of earthquake swarms. The model outcome can be compared with the long history of measurements at the ICDP Eger Rift observatory, which poses a unique opportunity for model calibration and validation. In general, the ICDP Eger Rift observatory provides a long history of observations crucial for the success of the project, with stratigraphy, flow measurements and high-quality seismic catalogs. The projects results will reveal the influence of permeability dynamics and shed light on the role of various favoring, triggering, and requisite factors for the different occurrences of seismic activity in the West-Bohemia/Vogtland region.