We analysed the instrumental seismicity in a sector of the Po Plain (Italy) to define the baseline for seismic monitoring of a new underground gas storage plant that will use the depleted gas reservoir of Cornegliano Laudense, near Lodi. The target area – a square approximately 80 km × 80 km wide – is commonly considered aseismic. The analysed period, 1951–2019, includes all available instrumental data. We gathered the P- and S-phase readings collected by various agencies for more than 300 events, approximately 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 depicts 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, assuming a completeness magnitude M_c = 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 sequence of Caviaga in 1951 that represent the first instrumental 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 M_4.2 earthquake of 17 December 2020, based on the similarities in depth, location, and focal mechanism. While it is clear that the deep seismicity corresponds to the collision between the Northern 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 definition can be improved with the existing observational capabilities now available in the surroundings.

中文翻译：

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Lodi 地区仪器地震活动的修订图像（意大利波平原）

我们分析了 Po 平原（意大利）一个地区的仪器地震活动，以定义一个新的地下储气库的地震监测基线，该储气库将使用 Lodi 附近的 Cornegliano Laudense 枯竭气藏。目标区域——一个约 80 公里的正方形 × 80 公里宽 - 通常被认为是抗震的。分析的 1951-2019 年期间包括所有可用的仪器数据。我们收集了各个机构为大约位于目标区域内的 300 多个事件收集的 P 和 S 相读数。我们使用区域和国家监测网络采用的绝对定位算法和速度模型统一处理地震。重新定位的地震数据集描绘了波平原中部地区的弱深地震活动图像，这与从现有地震目录中得出的初始图像大不相同。在距洛迪约 30 公里的距离内，地震极为罕见（假设完整震级M _c  =平均每年 0.5 次地震） 2.7 来自 1980 年代）；在整个 1951-2019 年期间，只有两个弱事件落在距水库不到 15 公里的距离。仪器记录的最强事件与 1951 年 Caviaga 的地震序列有关，它代表了该地区的第一个仪器记录。确认 Caciagli 等人最近提出的震源深度。(2015), 事件远离气藏；我们建议 1951 年主震和M _{4.2 的}共同构造应力2020 年 12 月 17 日的地震，基于深度、位置和震源机制的相似性。虽然很明显深地震活动对应于北亚平宁山脉和南阿尔卑斯山之间的碰撞，但产生地震的地质结构的特征似乎不确定。我们的结果是 Lodi 地区地震带定义的初步基准，可以利用周围现有的观测能力改进其定义。