Variations of attenuation and VP/VS ratio in the vicinity of wastewater injection: A case study of Costa Molina 2 well (High Agri Valley, Italy)

By analyzing seismicity induced by the injection of wastewater into the Costa Molina 2 well, we characterized attenuation in the area. Attenuation quality factors QP and QS were calculated using the peak-frequency method, which is suitable for small-magnitude earthquakes registered in the region. Anomalously high attenuation with values of effective QP~48 and QP/QS ratio of less than 1 in the vicinity of the injection well was found, in comparison with QP~60 for the rest of the region. Furthermore, the VP/VS ratio was also found to be high in the close vicinity of the injection well. We determine how sedimentary rocks possibly change their geomechanical properties due to salt-water injection and find that observed changes of attenuation (increase) and VP/VS ratio (increase) are consistent with the geomechanical models and experiments. Our findings indicate that the observed anomaly in the vicinity of the Costa Molina 2 well is due to the (1) changes of the properties of the rock mass affected by injection and (2) 3D structure in the overburden rocks.