Micro-seismicity associated with CO2 geological storage activities in Hontomín, Spain

We present the first local micro-seismicity catalogue including focal mechanisms for the Hontomín plant, the only site of its kind in Spain and one of the very scarce CO₂ storage facilities in Europe. The catalogue reveals a correlation between micro-seismicity occurrence and bottom hole pressure (BHP) at the injection well. A burst of micro-seismicity ranging from -1 to 0.4 M_L, thus imperceptible to human senses, took place coeval with the longest duration test of the period studied when the BHP was the highest. Events clustered at ∼ 0.4 km below the well bottom hole. Focal mechanisms could be well resolved for 3 events with similar results, showing a strong strike-slip component and a minor reverse component, with similarly oriented nodal planes. The local stress fields inverted from the focal mechanism solutions exhibit an orientation of the maximum horizontal stress axis (S_Hmax) that ranges from NE to ESE and differs from the principal stress orientations obtained in previous works for the same area. The micro-seismicity locations and tensors obtained are useful inputs to models aiming to better understand the deformation effects of fluids’ injection underground.