A revised image of the instrumental seismicity in the Lodi area (Po Plain, Italy)

We analysed the instrumental seismicity in a sec- tor of the Po Plain (Italy) to define the baseline for seis- mic monitoring of a new underground gas storage plant that will use the depleted gas reservoir of Cornegliano Lau- dense, near Lodi. The target area – a square approximately 80km×80km wide – is commonly considered aseismic. The analysed period, 1951–2019, includes all available in- strumental data. We gathered the P- and S-phase readings collected by various agencies for more than 300 events, ap- proximately located inside the target area. We processed the earthquakes uniformly, using absolute location algorithms and velocity models adopted by the regional and national monitoring networks. The relocated earthquake dataset de- picts an image of weak and deep seismicity for this central sector of the Po Plain, which is quite different from the initial one derived from the existing earthquake catalogues. Within a distance of approximately 30 km from Lodi, earthquakes are extremely rare (on average 0.5 earthquakes per year, as- suming a completeness magnitude Mc = 2.7 from the 1980s); only two weak events fall at less than 15 km distance from the reservoir in the whole period 1951–2019. The strongest events instrumentally recorded are related to the seismic se- quence of Caviaga in 1951 that represent the first instru- mental recordings for that area. Confirming the hypocentral depths recently proposed by Caciagli et al. (2015), the events are far from the gas reservoir; we suggest common tectonic stress of the main shock of 1951 and the M4.2 earthquake of 17 December 2020, based on the similarities in depth, lo- cation, and focal mechanism. While it is clear that the deep seismicity corresponds to the collision between the North- ern Apennines and the Southern Alps, the characterization of the geological structures that generate earthquakes appears uncertain. Our results are a preliminary benchmark for the definition of seismogenic zones in the Lodi area, whose defi- nition can be improved with the existing observational capa- bilities now available in the surroundings.