The term "seismic event" refers to any phenomenon that generates seismic waves (or sub-surface vibrations) that propagate within the Earth and along the ground surface. A seismic event is always characterized by a place of origin (source), a time of origin, and an energy released (magnitude). The sources that can generate a seismic event are earthquakes (tectonic or volcanic), explosions (underground, surficial, or atmospheric), vehicles (trains, buses, cars, etc.), industrial machinery in action (rotating engines, presses, etc.), other natural causes than earthquakes (meteorite impacts, ocean waves, atmospheric disturbances, wind, etc.), and other human activities (bursts in mines or quarries, underground activities, etc.), as well as scientific investigations with "active" sources such as seismic prospecting). The term “earthquake”, in particular, identifies a seismic event associated either with the release of elastic energy accumulated within the Earth's crust (tectonic origin) or with volcanic phenomena (volcanic origin).
The term “seismic hazard” commonly refers to ground shaking that can occur in a certain location due to recurring seismic events in the area (seismicity). As is true for meteorology, today in the world, the most commonly used and accredited evaluations of seismic hazard are obtained from probabilistic types of estimates; they combine the available observations on natural phenomena—earthquakes and the faults that generate them—with mathematical models that simulate their frequency and energy over time and calculate the movement of the ground that can result in the surrounding area. For this reason, seismic hazard maps, such as those used to develop seismic regulations, indicate the value of ground motion that is exceeded with a certain level of probability in a given period of time (e.g., acceleration that has a 10% probability of being exceeded in 50 years).
The size of a seismic event can be measured by means of magnitude or macroseismic intensity. The magnitude is a physical quantity and is expressed through a pure decimal number. It is calculated from the amplitude of seismic waves recorded by seismographs or by estimating the seismic moment, also derived from instrumental recordings. Each increase of one unit of magnitude corresponds to an increase of approximately thirty times the emitted energy. The best known magnitude scale is the one proposed by Richter in the 1930s. The strongest event recorded in Italy was the Messina earthquake of 1908, with M~7.1 and more than 80,000 estimated victims.
The macroseismic intensity (or intensity) of a seismic event classifies the effects caused on the environment, things and people and is therefore assigned to specific locations affected by the event (site intensity). The epicentral intensity represents the extrapolation of the effects that would have occurred at the epicentre, i.e., theoretically at the point on the Earth's surface closest to the source of the earthquake; in this way, intensity can be considered a measure of the energy released.
In Italy, the intensity of a seismic event is evaluated according to the MCS scale (Mercalli-Cancani-Sieberg, better known as the Mercalli scale), which is composed of twelve level. In the proposed classification, events are perceivable starting from level III; from VI to VIII, there is damage to buildings; starting from level IX, the effects are destructive, with frequent total collapses and effects on the surrounding environment.
Cornegliano Laudense as of 2003 was classified in Seismic Zone 4, in which seismic danger of one or less is foreseen for Italy. This classification for Cornegliano was modified to Zone 3 following the resolution of the Lombardy Region n. 2129 of July 11, 2014; according to this resolution and following the earthquakes in Emilia in 2012, the level of control was increased for most of the municipalities in Lombardy that had previously been classified in Zone 4.
The area of Cornegliano Laudense has been affected in the past by moderate, infrequent, and deep seismic events. Except for the earthquakes of 1786 and 1951 in the Lodigiano area, all the strongest earthquakes occurred more than 15-20 km from Cornegliano Laudense. The event of greatest interest for the Lodigiano area is the one on 15/5/1951, also known as the Caviaga earthquake, which until very few years ago, was believed to have been caused by the gas extraction activities practised in the area (Caloi et al, 1956), was more recently attributed to a natural tectonic origin (Caciagli et al., 2015 and Vannoli et al., 2014). It is also important to note that the macroseismic catalogue does not report any damage in the locality of Cornegliano Laudense, while the earthquake generated damage in all nearby locations and was felt in a large surrounding area.
For further details, see the page
Historical and instrumental seismicity in the “Seismic Monitoring” section.
To answer this question, it is necessary to distinguish the seismic events to which we refer.
The seismic network detects numerous signals that can be associated with seismic events related to human activities or natural events such as weather disturbances. The frequency of these signals is related to the frequency with which these events occur. With regard to weather disturbances, these are more prevalent in late spring and summer; signals related to these events are recognized and excluded from the catalogue of seismic events. As far as signals related to human activities are concerned, trains are undoubtedly the most frequent and strongest source of seismic signals detected by the stations of the network. More specifically, the OL05 and OL06 stations are affected by the passage of trains along the line through Lodi (with an average frequency of 15-30') and by the passage of vehicles (e.g., the OL05 station is located near state highway SS9). The OL02, OL03 and OL04 stations are affected by the passage of trains along the high-speed line (with an average frequency of 10') and the traffic on the A1 freeway.
For earthquakes, the answer to this question presents some aspects of greater complexity. From the available data, it is possible to estimate an event frequency equal to 50-70 years with an intensity close to the damage threshold.
In fact, the Lodigiano area is affected not only by moderate, infrequent and deep events in this sector of the Po Valley but also by more energetic and distant events, mainly those that occur along the active tectonic structures of the Southern Alps and Northern Apennines.
Lodi is the inhabited centre for which the most information is available, and the seismic history of Lodi, reported in the Macroseismic Database DBMI15 (Locati et al., 2016), consists of approximately 40 observations related to earthquakes since the Middle Ages. Since 1500, 8 events with intensities greater than or equal to 5 have been reported, without ever reaching the threshold for the first damage at the sixth level on the macroseismic scale.
For further details, see the page Historical and instrumental seismicity in the section “Seismic Monitoring”.
The detection of seismicity before the start of storage activities helps in assessing the natural background seismicity under "undisturbed" conditions. In the period 1/1/2017-31/11/2018 (before the start of storage activities), the new Cornegliano Laudense Seismic Network (RSCL), which has a much higher sensitivity than the national grid, detected a total of 11 events with very low magnitude, all in the External Area of detection and all attributable to tectonic causes. These are neither events due to well drilling, which had already been completed, nor microevents induced by storage, which had not yet started.
Superficial events can be felt by humans already starting from very low magnitude (approximately magnitude 2.0). However, this perception depends on many factors: the distance from the epicentre, the depth of the hypocentre, local ground conditions (amplifying soils can increase the perception), and the time when an earthquake occurs (at night, a quake more easily noticed).
The events that can cause damage in Italy generally have magnitudes greater than 5.5. However, the seismic motion is largely influenced by the local seismic response, as well as the depth of the source, and the damage depends on the type and state of conservation of the structure.