Application of methodologies for the analysis of microseismicity in industrial areas: a case study from underground gas storage in Cornegliano Laudense

Underground gas storage (UGS) systems are commonly used to balance seasonal fluctuations in demand and to secure strategic reserves by storing gas in geological trap formations. All underground industrial activities, including UGS, can affect the pore pressure and pre-existing stress state in seismogenic layers, potentially triggering earthquakes. Monitoring microseismicity in such a context is crucial, especially in urban areas. The study area of this work focuses on the Cornegliano Laudense UGS site, near Milan, one of 15 such sites in Italy, where the National Institute of Oceanography and Applied Geophysics (OGS) conducts seismic monitoring. In 2017, nine seismic stations were installed in accordance with national guidelines to establish a baseline of natural seismicity before the start of gas storage activities in December 2018.

Previous studies have shown that the central sector of the Po Plain has weak and deep seismicity due to crustal shortening between the Alpine and Apennine fronts. However, shallow seismicity has occasionally been recorded since monitoring began. Shallow seismicity was recorded both before and after the onset of storage activities, suggesting that it may be related to shallow tectonic structures. At the end of September 2024, the local seismic network detected its first seismic sequence, consisting of nine shallow microearthquakes with magnitudes between 0.9 and 1.3 ML and a depth of ~ 2.5 km. These seismic events occurred near a known thrust fault just outside the storage area.

We present preliminary results of a seismicity analysis performed to understand the origin of these shallow microearthquakes. Detection and location of such small earthquakes is challenging due to their low magnitude and low signal-to-noise ratio in this area. To improve detection, we applied a template matching technique based on the cross-correlation of continuous seismic data with well-located events, known as templates. This process revealed more than 150 seismic events throughout the entire monitoring period, with magnitudes ranging from -1 to 1.6 ML. Initially assigned to the locations of their templates, the hypocenter locations were refined by identifying P and S wave arrival times, where possible, applying both absolute (NonLinLoc) and relative (HypoDD) location methods. Our analysis also identified small clusters of past seismic events like those in the September 2024 sequence using a template matching method. For each sequence, we calculated composite focal mechanisms using the SKHASH code, combining polarities and S/P amplitude ratios for more reliable results. Finally, we examined seismicity diffusion patterns to assess potential fluid movement influences, as seismic events triggered by fluid intrusion often show characteristic spatial and temporal migration patterns.