Frequently asked questions

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 levels. 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 1/1/2017-30/11/2018 (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 is 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.

The term “induced seismicity” is defined as a set of seismic events caused by human activities. These activities include oil and gas extraction, CO2 storage, clay gas extraction, fracking, geothermal energy production, mining activities, open pit excavations, artificial lakes and dams, and explosions.

Induced seismicity is detected by the same techniques with which natural seismicity of low magnitude is detected. Therefore, dedicated seismic monitoring networks are required to detect very weak events, providing accurate location and magnitude estimates. In some cases, these networks are complemented by instruments installed in deep wells to provide measurements closer to the point where the activities occur. However, no instrumental measurement allows distinguishing between natural and induced events in a certain way.

Induced seismicity is due to variations in the state of stress within a rock mass, which are generated by human activities and not by natural causes. If the resulting stress state exceeds the resistance threshold of the material or reduces the cohesion force (i.e., friction) that keeps the faults locked, ruptures or seismic events are produced. The injection of fluids into the sub-surface or their extraction and increases or decreases in the hydrostatic load are the most frequent and documented causes of induced seismicity.

If the seismic network detects an increase in microseismicity, a more in-depth scientific analysis is carried out to establish a possible link with ongoing activities and the possible evolution of the detected phenomena. In addition to the magnitude and peak values of velocity and acceleration, the analysis considers other parameters, such as the number of events detected, correlation values, and perturbation values of the stress field calculated via numerical modelling.

The HiQuake database (https://inducedearthquakes.org/) lists cases of earthquake-induced seismicity around the world. For gas production and/or storage activities, the database lists the following 7 cases in 5 different countries:

• Gazli, Uzbekistan (Plotnikova et al., 1996) — Storage uses a natural gas production depot that was converted to storage in 1988. Plotnikova et al. (1996) reported seismicity with magnitudes between 4 and 5 during the storage period, especially during the first three years of reservoir filling. Since 1988, storage has been used for gas. It should be noted that the Gazli site is known for two strong earthquakes, M 6.8 in 1976 and M 7.2 in 1984, that occurred during the period of exploitation of the reservoir for gas production, which caused extensive damage and many deaths in the city of Gazli. Therefore, the seismicity detected during the subsequent period of storage may have been related to the two major events that occurred previously. However, the scientific information is rather uncertain because the available literature is mainly in the Russian language.
• Castor gas storage project, offshore Spain (Cesca et al., 2014; Gaite et al., 2016) — This project aimed to use a depleted oil field (Amposta oil field) consisting of fractured carbonate rocks located offshore in the Gulf of Valencia (northern Spain), approximately 20 km from the coast. A storage capacity of approximately 1.3 billion standard cubic metres of natural gas was planned, sufficient to meet 25% of Spain's storage needs. The seismic sequence began three days after the start of the injections, with increasing events up to ML 2.6. The injection was paused after 12 days, but this was not enough to stop the earthquakes; on the contrary, the strongest Mw 4.3 event occurred two weeks after the injection was interrupted. In total, more than 1000 earthquakes were detected, of which more than 420 had ML ≥ 2. Weak seismicity was still ongoing in 2016 (Gaite et al., 2016). The seismic sequence induced by the storage at Castor led to a strong negative public reaction because the population was very sensitive to the problem of seismic risk after the Mw 5.1 earthquake in Lorca (approximately 250 km farther south along the coast) in 2011. Following the earthquakes that occurred, the project was suspended. In retrospect, it was recognized that the gas injection activities reactivated a fault in the Amposta fault system. In addition, both the seismic monitoring infrastructure and especially the ability to analyse the seismic data collected in an effective and timely fashion were found to be inadequate.
• Bergermeer, Norg and Grijpskerk storage camps in the Netherlands (Anonymous, 2014) — For these cases, only very limited numbers of very weak events are reported, for which no scientific study has yet been published. The only documented evidence concerns some microevents with a maximum magnitude of 0.7 that occurred in 2013 at the Bergemeer repository during the cushion gas creation phase and were detected by microseismic arrays dropped into the well. Additionally, in this case, it is important to distinguish these activities from the better known gas production activity in Groeningen that has caused some known earthquakes reported in the literature.
• Háje, Czech Republic (Benetatos et al., 2013; Zedník et al., 2001) — Storage used underground caverns, and two earthquakes of magnitude 0.2 and 0.4 were recorded near the area in 2009.
• Hutubi, China (Tang et al., 2015; Zhou et al., 2019) — Storage used a natural gas production depot that was converted to storage in 2013. Approximately 200 events were detected during the first gas injection cycles, with two major events of M 2.7 and M 3.0 in 2013 and some minor events in 2014, approximately 3 km and 1 km from the reservoir, respectively. The most recent study (Zhou et al., 2019) consider that these events were induced and caused by the reactivation of pre-existing faults due to the spread of poro-elastic stress.

Finally, Evans et al. (2015) reported a case of microseismicity (even better classified as nano-seismicity) detected at the storage of Germigny (northern France, aquifer storage), which has been operational since 1982. In the period between 1991 and 1992, microseismicity was detected at the instrumental level (less than 30 events), detected only by sensors dropped into deep wells and located in the production layer near the injection wells (Deflandre et al., 1993; Fabriol, 1993).

In Italy, there is no evidence of events related to gas storage activities.

It should be considered on the one hand that gas storage in Italy is carried out exclusively in depleted gas depots, and this type of storage is considered particularly safe. In many cases, the seismic monitoring of gas storage is carried out by the same (private) entity that performs the storage. However, for the storage of Collalto (TV), which is monitored with a dedicated seismic network by a public research organization (OGS) and whose data and analysis are public, there is also an absence of seismicity related to the storage activity.

The monitoring of induced seismicity is extremely important for understanding the physical processes at the origin of a seismic phenomenon. Monitoring also allows the identification and mapping of potential active faults near industrial sites. Real-time seismic monitoring is the most widely used control tool in many areas of resource use for energy production.

A seismic monitoring network is a set of seismic stations distributed across a given area and connected in real time through data transmission systems to a collection centre. Seismic monitoring networks are implemented for scientific purposes and civil protection actions. The seismic monitoring network records the ground vibrations caused by the passage of seismic waves. The network is very sensitive and detects all ground vibrations, both those of earthquakes and those generated by other sources such as the transit of vehicles (trains, trucks, etc.), the activities of industrial machinery, weather disturbances or explosions at quarries or of war ordnance.

The seismic monitoring network records the ground vibrations caused by the passage of seismic waves. The network is very sensitive and detects all ground vibrations, both those of earthquakes and those generated by other sources such as the transit of vehicles (trains, trucks, etc.), the activities of industrial machinery, weather disturbances or explosions at quarries or of war ordnance.

The seismograph is a tool that transforms the movement of the ground caused by a seismic event into a permanent record. It allows proper analysis and review of all seismic waves at any time.

The monitoring system continuously acquires signals from the stations of the network, analyses the signals to recognize any variations and, if these variations are found at more than one station, locates their origin and intensity. If the conditions are met, a so-called "seismic event" is declared. This entire process occurs in a few seconds from the moment the signal is recorded.

It may also happen that different events or false events due to random correspondence of signal changes are identified. Since the system is calibrated to be very sensitive (remember that the main purpose is to record microseismicity), there can be many false events. For this reason, all automatic determinations are reviewed by an experienced seismologist who analyses the detected events and confirms that they correspond to local earthquakes or events potentially related to the activity of the storage plant.

The motion of the ground recorded by the instruments is usually represented with graphs, called seismograms, which describe the movement of the ground over time. From the seismograms, it is possible to evaluate (both visually and through computer analysis) where a seismic event occurred, how strong it was and what are some of its other characteristics.

For more details, see the homonymous page.

In the seismogram, we can recognize the phases (i.e., particular traits of waveforms) related to specific wave fronts that are generated at the source or have traversed the geological structure of the Earth, which is complex, and the "seismic noise", i.e., the vibrations resulting from many processes with different origins (from atmospheric perturbations to oceanic motion, from human activities to earthquakes themselves); this noise continuously runs along the Earth's surface similar to the continuous wave motion of varying strength seen on the sea.

From the specific phases recognizable in the seismograms generated by a seismic event, it is possible to estimate (both visually and through computer analysis) where it happened, how strong it was and what are some of its other characteristics.

The structural works of the IGS Cornegliano Stoccaggio plant have been designed according to the technical standards for construction by adopting particularly strict and conservative parameters; the structures are therefore robust and certainly able to withstand the expected seismic events without sustaining damage.

In the case of hypothetical catastrophic events, much more violent and unlikely, the structures could experience localized damage but no collapse; in this case, the plant is equipped with intrinsic safety systems that, in the case of anomalies, interrupt the operations and place the plant under non-operational conditions.

Yes, the deposit can be safe in case of an earthquake. In fact, it does not consist of a large empty cavity filled with gas but porous rocks into which the gas infiltrates, a kind of large sponge located at a depth of approximately 1.5 kilometres. The rock that constitutes the reservoir, inside which the gas is stored, and the layer that seals it above (the so-called cap rock) behave elastically with the passage of seismic waves generated by an event. It must be remembered that the depleted gas reservoirs used in Italy for storage had contained gas formed naturally for millions of years and experienced and resisted all the seismic events that occurred in the area.

The activities of extraction/storage of hydrocarbons and re-injection of fluids underground can induce surface deformation. Through geodetic satellite monitoring (DInSAR), it is possible to measure the trends over time of ground displacements with an accuracy of one centimetre, and in some cases even a few millimetres, on spatially extensive areas (from tens to tens of thousands of square kilometres). The ability to measure the deformations of the ground over large areas compared with the size of the deposit and above all to follow their temporal evolution allows highlighting any variations associated with the storage activities with respect to the pre-existing deformation scenario.

The ground can be deformed either by the deformation of the Earth's surface crust or by local phenomena of various kinds. The deformation of the Earth's crust occurs mainly due to tectonic movements, i.e., the relative displacements of continents that create zones of deformation with varying intensity. However, the ground also deforms due to other natural causes, such as seismic events, volcanic eruptions, faults, and landslides, and as a consequence of human activities (e.g., extraction of water from groundwater, of oil or gas from hydrocarbon deposits, and of mining material and gas storage).

The Global Navigation Satellite System (English acronym GNSS) is a system for geo-radiolocation and navigation on land, sea or air, using a network of artificial satellites in orbit and pseudolites. The geolocation systems provide a geo-spatial positioning service with global coverage that allows small, purpose-built, electronic receivers to determine their geographical coordinates (longitude, latitude and elevation or altitude) at any point on the Earth's surface or in the atmosphere with an error of a few metres (and up to millimetres in precise realizations such as permanent stations for geodetic use), by processing radiofrequency signals transmitted in the line of sight by such satellites.

DInSAR data are measurements of the deformation of the Earth's surface (expressed in centimetres or millimetres) obtained using radar data acquired by satellite. In particular, we use image sequences obtained using systems called SAR (synthetic aperture radar) acquired in a certain time interval by one or more satellites in an area of interest; through processing techniques called DInSAR (differential interferometry SAR), we obtain maps of the ground deformation in the area of interest, and for each point on these maps, we determine the evolution of the deformation in the time interval considered (i.e., how the deformation varies over time). The DInSAR analysis natively generates measurements of the ground deformation projected along the line of sight (LOS) of the sensor, which, in fact, "looks" at the globe not perpendicularly but at a certain angle. Taking advantage, however, of the fact that the sensors acquire data by covering both ascending orbits (i.e., moving from south to north) and descending orbits (moving from north to south), it is possible to combine the DInSAR results of the analysis performed for the different orbits and obtain the measurements of the vertical and east-west components of the ground movements.

The effects of elevation changes depend primarily on the location of the site where they occur. In many situations, the most important factor to consider is the differential elevation variation, i.e., the relative elevation variation between two "neighbouring" points (the term "neighbours" means a distance of interest with respect to the object whose effect is to be estimated). For masonry buildings with more than one floor, it is estimated that the maximum acceptable deformation is within 5 × 10^-4 mm, i.e., 5 mm for every 10 m [Skempton and McDonald, 1956]. For reinforced concrete buildings and steel structures, this value can be 10-15 times larger.

Gas storage can generate deformations of the order of magnitude of a few millimetres both horizontally and vertically. The deformations detected or estimated are much lower than those that can cause consequences for people and, in general, for the environment. However, it is important to monitor surface displacements both to have objective evidence of the effects caused by storage and to improve the interpretation of events.

Yes, it is natural for this phenomenon to possibly occur with shifts on the order of a few millimetres in an area that includes the storage volume. In fact, it is estimated that the induced deformations may be very limited and far below the values that can be perceived or cause any consequence for both humans and the environment. Continuous monitoring will, however, allow constant verification of the situation.

To date, monitoring does not show any position changes or deformations that exceed the natural background values.

The main results of DInSAR analysis are the average ground deformation velocity maps, expressed in cm/year or mm/year, and, for each point displayed in these maps, the time series of the deformation calculated in centimetres, i.e., the deformation trend during the time interval in which SAR data were acquired. In particular, the maps of average terrain deformation velocity generated by the DInSAR analysis of the Cornegliano Laudense area are geocoded maps; therefore, they can be superimposed on existing maps of the area and are represented in false colours, which correspond to the displacements of the ground. Usually, green represents stable zones (which do not deform over time), yellow and red indicate subsidence zones, and light blue and blue mark uplifted zones.

The reference to be mentioned for the use of parameters and information contained in this website is:

Monitoring of “Cornegliano Stoccaggio” Working Group, 2019. Monitoring of “Cornegliano Stoccaggio” website - http://rete-cornegliano.crs.inogs.it

If seismic data are used, please quote:

RSCL Working Group (2019). Cornegliano Laudense Seismic Network - http://rete-cornegliano.crs.inogs.it - https://doi.org/10.7914/SN/OL