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OGSOGS – National Institute of Oceanography and Applied Geophysics
“Cornegliano Stoccaggio” Monitoring
Seismological
Research Centre

Seismic Monitoring

The seismic monitoring network

The Cornegliano Laudense Seismic Network (RSCL) consists of ten seismic stations and a permanent precision GNSS-GPS geodetic station focused on the Cornegliano Laudense gas storage reservoir. Seismic monitoring has been active since 1/1/2017.

The RSCL integrates some stations belonging to the regional seismic networks managed by OGS and the Centralized National Seismic Network (RSNC) managed by the National Institute of Geophysics and Volcanology (INGV); in this way, it is possible to detect seismic events, not perceivable by the population, which occur in all directions to a distance of a few tens of kilometres from the storage site.

All stations of the RSCL are equipped with a seismometer placed in a well to improve the quality of the detected signal and an accelerometer placed on the surface. The figure shows a map of the RSCL with highlighted domains of detection adopted by the monitoring system, according to the definitions in the MiSE Guidelines.

Map of the RSCL. The blue and green triangles indicate the stations of the RSCL and the Centralized National Seismic Network, respectively, used to integrate the surveys. The yellow area in the centre indicates the surface projection of the reservoir. The red lines indicate the perimeters of the Internal Domain (DI, thick line) and the Extended Domain (DE, thin line) of detection. The dotted line indicates the External Area, corresponding to a distance of 30 km from the storage site.

What is measured

The RSCL records ground vibrations caused by the passage of seismic waves. Since the instruments are very sensitive, all ground vibrations are detected, both those caused by earthquakes and those generated by other sources such as the transit of vehicles (trains, trucks, etc.), the activity of industrial machinery, weather events (wind, thunder, etc.) or explosions of quarry or war ordnance. A proper analysis of the signal generally allows discrimination among the different types of events.

The RSCL is able to detect seismic events of interest to the community (therefore perceivable with different intensities by the population) as well as events that are not perceived by local residents, whether these events are strong and distant earthquakes (for example, those that occur in Japan and Chile) or very weak tremors that occur near the network.

The monitoring system continuously acquires signals from the stations of the network and automatically analyses them to recognize any anomalies; if these variations are found simultaneously at multiple stations, the system locates the origin and quantifies the extent. If certain conditions are met, the phenomenon is declared a “seismic event”. All these steps occur within seconds of the signal being recorded.

The automatic system is calibrated mainly to recognize earthquakes, but it may happen that different phenomena or false events due to the random simultaneity of signal variations are also identified as seismic events. Since the main purpose of the RSCL is to record microseisms that are not identified by regional or national networks, the system has been calibrated to very high sensitivity thresholds, which means many false events may be detected. For this reason, all automatic determinations are reviewed by an expert seismologist who analyses the automatically identified events, confirming whether they actually correspond to local natural earthquakes or events potentially related to the activity of the storage facility.

What seismograms represent

The ground vibrations recorded by the instruments are usually represented with graphs, called seismograms, which describe the movement of the ground (in displacement, velocity or acceleration) as time passes. From the seismograms, it is possible to evaluate where the seismic event occurred, how strong it was and some of its other characteristics.

In the past, seismograms were plotted on paper and visually analysed. Today, digitized data are stored on electronic media and analysed through special programs to be interpreted or represented on video or printed on paper.

By way of illustration, some seismograms are represented below, as they appear in real time in the Seismograms in near real time section of this website or after they are processed by the staff in charge of their interpretation.

Daily recording from a station: Each line of the seismogram corresponds to one hour of recording. These representations are called “drumplots” or “helicorders” because they simulate what was formerly recorded by means of a trace imprinted on a rotating paper drum. The figure shows the recording of a single component of the ground motion (the position of a point in space is expressed by a Cartesian coordinate system) for an instrument that is acquiring data in “real time”; the trace then stops with the last data acquired by the instrument at the moment when the figure was produced (approximately 14:40). This type of visualization is also used to verify the correct functioning of the station.
Recording of the same event by several stations: Seismograms indicate an interval of time limited to a few minutes. Each station can be represented by several tracks representing the three recorded spatial components: two horizontal and one vertical. Signal processing involves the use of filters (this is the case represented here) to remove or emphasize some frequency components. This representation is used to control and manually review the signal of an automatically identified event.

Below are some examples of signals recorded by RSCL related to different types of events. In order, these are as follows:

  • seismograms related to a strong event that occurred in central Italy;
  • seismograms from a strong distant event (“teleseism”) that occurred in the Pacific Ocean;
  • signals due to trains in transit;
  • recordings of two local microearthquakes.
Registration at the OL01 station of the earthquakes with magnitude M>5 that occurred in central Italy on 18/1/2017. The different coloured tracks allow recognition of the overlapping signals; these events in central Italy tend not to be perceived by the population in the Lodi area. Note that by convention, the time of an earthquake is given by the Universal Co-ordinated Time (UTC), that is, the Greenwich meridian time (GMT). To correspond with Italian local time, it is necessary to add 1 hour (if solar time is in place) or 2 hours (if daylight saving time is in place) to the indicated value.
Registration at the OL04 station of a Mw 7.9 teleseism that occurred in the Pacific Ocean at 04:30:23.8 UTC on 22/1/2017 (source EMSC). These teleseisms are not perceived by the population.
Recurring signals due to the passage of trains at the OL09 station on 30/1/2017. The OL04 station is located in Lodivecchio (Loc. Comasina) a few hundred metres from a high-speed train line. Despite the installation of the sensors at a depth of 75 metres, the seismic waves are identifiable. The vibrations of the ground from passing trains are sometimes perceivable by the population in the immediate vicinity of the railway line.
Registration at the OL04 station of two local microquakes with magnitude M≤1.0 that occurred on 12/7/2017 at 00:17:53 and 00:18:30. Top: daily helicorder-type recording of the vertical component at the same station (each line is one hour of recording); the signals of the two microquakes are enclosed within the grey ellipse. Bottom: one minute of recording (three-component seismograms) of the two microquakes. These events were not perceived by the population.

What seismic monitoring is

Seismic monitoring is used to identify and measure seismic events in the vicinity of or potentially of interest for the Cornegliano Laudense gas storage facility called “Cornegliano Stoccaggio”.

If the vibration of the ground automatically identified within a few seconds by the system as a ‘seismic event’ corresponds to a local earthquake and has certain characteristics, the seismologist on call at OGS is alerted; within a few hours, the intervention and manual analysis of the data confirm or eliminate the event. The location and magnitude of the event is finally published on the page of the integrated monitoring system containing the “Recent Events”. In the case of “attention conditions”, the operator of the storage facility is notified promptly.

“Attention conditions” are situations in which there is an abnormal increase in microseismicity (in terms of magnitude and/or number of events detected) compared to conditions considered “normal” within the detection domains (the so-called seismic background). In this case, the seismologist in charge of monitoring intensifies the controls and analysis to obtain an accurate and updated picture of the detected microseismicity and verifies with the storage manager the activity carried out to establish or exclude any correlation between the storage facility and the detected microseismicity.

The Cornegliano Stoccaggio monitoring website provides complete information on the stations of the seismic network, graphs, technical and scientific documents and reports on the activity carried out. Further details can be found in the section with frequently asked questions (FAQ).

Further details

Seismo-tectonic framework

The area is characterized by weak to moderate seismicity. The only earthquake of real interest for the area of Lodi is that of 15/5/1951, also known as the Caviaga earthquake. Until very few years ago, it was believed that this event was caused by gas extraction activities in the area (Caloi et al., 1956). However, more recent studies have redetermined its location in a position compatible with a fault inherited in the rocks of the crystalline basement far from the Cornegliano reservoir. Therefore, today, this earthquake is attributed to a tectonic and not anthropogenic origin.

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The seismic network

The Cornegliano Laudense Seismic Network (RSCL) consists of ten seismic stations. A permanent GNSS1 geodetic station contributes to the Integrated Monitoring System.

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Network geometry

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Station Cards

All stations are equipped with an in-well seismometer, located at a depth of 75 m, and a surface accelerometer.

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Instrumentation

All stations are equipped with the same instrumentation, manufactured by Guralp Systems, Ltd. (see Table 1), consisting of a Radian digital broadband velocimeter (in the well, at a depth of approximately 75 m), a Fortis compact broadband accelerometer (on the surface) and a digitizer.

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Seismological analysis procedures

For the detection of seismicity of interest in the Cornegliano Laudense reservoir area, two reference areas have been identified, which correspond to the detection domains indicated in the Monitoring Guidelines of the Ministry for Economic Development.

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Near real-time seismograms

From this page, you can access the seismogram traces of all the stations in the Cornegliano Laudense Seismic Network in near-real time, as the images are updated every hour.

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Detected Seismicity

The area under examination is characterized by weak to moderate seismicity. Excluding the earthquakes of 1786 and 1951, which had non-anthropogenic origins, in the Lodigiano area, all the strongest earthquakes occurred more than 15-20 km from the storage plant.

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Historical and instrumental seismicity

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