SURVEILLANCE OF COLLI ALBANI VOLCANOES

Average speeds of ground movement measured with GPS techniques (period 2006-2018) and InSAR (period 1992-2010).

Figure 2 – Above: the gaseous manifestation of Cava dei Selci; left: Carrying out a CO diffuse flux survey₂ from the ground in the target area; des.: the continuous CO flux monitoring station₂ from the soil and environmental parameters. Bottom: variation of CO diffuse flux₂ from the ground measured on the target area of ​​Cava dei Selci (6.000 m²) in the period April 2000-April 2012.

The Colli Albani volcanic complex is located in central Italy about 15 km southeast of the city of Rome, in an area belonging to the Roman potassic and ultrapotassic magmatic province. Recurring seismic activity, temperature variations and water composition, gas emissions, significant ground deformations measured with high-precision leveling techniques and detected through InSAR and GPS data (Riguzzi et al. 2009, Marra et al. 2016; Trasatti et al. al., 2018), indicate that the volcano cannot be considered completely extinct.

Geodetic Monitoring (GPS+SAR)

The Colli Albani area has been monitored since 1990 thanks to the creation of the first discrete GPS network (Anzidei et al., 1998). Starting from 2007, the development of an intense anthropic and industrial activity has suggested to switch from the discrete GPS network to a continuous acquisition network (CGPS). Currently, the network consists of 7 continuous stations (CGPS) of which 5 transformed from discrete stations and 2 (LARN and ALPA) installed from scratch (Tab. 1). The stations are connected remotely via LAN network or UMTS router and the data is collected in the servers of the RING network ((http://ring.gm.ingv.it).

Table 1 - GPS stations of the Colli Albani monitoring network

The implementation of the continuous GPS network of the Colli Albani and its inclusion within the RING network offers the opportunity to detect the local movements of the volcano and to insert them in the characteristic geodynamic context of the area.

The speed range obtained from the processing of 12 years of data collected first by the stations belonging to the discrete and then continuous network is shown in Figure 1. The arrows identify the annual mean horizontal speeds from GPS data in the time interval 2006-2018.

The data were compared with those from SAR images which, although involving a slightly different time window of observation (1992 - 2010) are well linked to the last 12 years of deformation history of the Colli Albani.

The colored dots in the image identify the average annual speeds of ground movement, calculated with respect to the line of sight (LoS) of the satellite (angle of inclination with respect to the vertical ≅ 32°). Positive values ​​identify a movement towards the satellite (uplift), while negative ones indicate a movement away from the satellite (subsidence). The SAR data used comes from the ERS 1-2 and ENVISAT ASAR satellite constellations, acquired in descending orbit. The images were processed using the PSI technique. 

Geochemical monitoring 

The Colli Albani area is characterized by the presence of areas with anomalous release of endogenous gases whose chemical and isotopic composition suggests an origin from the degassing of magmatic bodies or from the mantle (Carapezza and Tarchini, 2007). The main degassing area is Cava dei Selci (Fig. 2), but anomalous gas emissions also occur from the bottom of Lake Albano, and in the peripheral areas of Solforata, Ardea and Tor Caldara (Carapezza et al., 2012).

Geochemical monitoring of the Colli Albani began in 2000 following a death of cows and sheep in the Cava dei Selci area (Carapezza et al., 2003). In the following years it was also extended to the control of the gaseous emissions of Lake Albano (Carapezza et al. 2008). At the same time, the study of the aquifer and the monitoring of water points in the area which were found to be rich in CO was carried out₂ dissolved and Rn (Pizzino et al., 2002).

The current geochemical monitoring program includes the following activities:

Quarry of the Selci

- restoration of a continuous CO flow monitoring station₂ from the soil and environmental parameters (Fig. 2); 

- installation of a radon probe in the soil, contiguous to the CO flux station₂;

- restoration of the network of fixed points (target area) for the periodic monitoring of the diffuse degassing of CO₂ from the ground and comparison with previous data (Fig. 2);

- annual sampling (to be intensified in the event of a crisis) of the event gas to determine its chemical and isotopic composition;

- installation in two water wells of a multi-parameter probe for the continuous measurement of the piezometric level, temperature and conductivity of the groundwater;

- installation of a radon and CO station₂ in the air in a natural cave in Ciampino. 

Albano lake

- six-monthly periodic campaigns (frequency to be increased in the event of a seismic crisis) for the control of the geochemistry of the lake, with the measurement of vertical profiles of the chemical-physical parameters of the water (T, pH, conductivity, Eh, O₂ dissolved), from the surface to the bottom, using a multiparameter probe. Chemical and isotopic analysis of water and dissolved gases on samples taken near the bottom of the most recent crater and at lesser depths. 

The chemical and isotopic analyzes of water and gas will be performed at the laboratories of the INGV section of Palermo.

References

Anzidei, M., Baldi, P., Casula, G., Galvani, A., Riguzzi, F., & Zanutta, A. (1998). Evidence of active crustal deformation of the Colli Albani volcanic area (central Italy) by GPS surveys. Journal of volcanology and geothermal research, 80(1-2), 55-65.

Carapezza, ML, Badalamenti, B., Cavarra, L., & Scalzo, A. (2003). Gas hazard assessment in a densely inhabited area of ​​Colli Albani Volcano (Cava dei Selci, Rome). Journal of Volcanology and Geothermal Research, 123(1-2), 81-94.

Carapezza, ML, & Tarchini, L. (2007). Accidental gas emission from shallow pressurized aquifers at Alban Hills volcano (Rome, Italy): Geochemical evidence of magmatic degassing? Journal of Volcanology and Geothermal Research, 165(1-2), 5-16.

Carapezza, ML, Lelli, M., & Tarchini, L. (2008). Geochemistry of the Albano and Nemi crater lakes in the volcanic district of Alban Hills (Rome, Italy). Journal of Volcanology and Geothermal Research, 178(2), 297-304. Carapezza, ML, Barberi, F., Tarchini, L., Ranaldi, M., & Ricci, T. (2010). Volcanic hazards of the Colli Albani. The Colli Albani volcano. Special Publications of IAVCEI, 3, 279-297.

Carapezza, ML, Barberi, F., Ranaldi, M., Ricci, T., Tarchini, L., Barrancos, J., ... & Perez, N. (2012). Hazardous gas emissions from the flanks of the quiescent Colli Albani volcano (Rome, Italy). Applied geochemistry, 27

Marra, F., Gaeta, M., Giaccio, B., Jicha, BR, Palladino, DM, Polcari, M., ... & Stramondo, S. (2016). Assessing the volcanic hazard for Rome: 40Ar/39Ar and In‐SAR constraints on the most recent eruptive activity and present‐day uplift at Colli Albani Volcanic District. Geophysical Research Letters, 43

Pizzino, L., Galli, G., Mancini, C., Quattrocchi, F., & Scarlato, P. (2002). Natural Gas Hazards (CO₂, ²²²Rn) within a Quiescent Volcanic Region and Its Relations with Tectonics: The Case of the Ciampino-Marino Area, Alban Hills Volcano, Italy. Natural hazards, 27

Riguzzi, F., Pietrantonio, G., Devoti, R., Atzori, S., & Anzidei, M. (2009). Volcanic unrest of the Colli Albani (central Italy) detected by GPS monitoring test. Physics of the Earth and Planetary Interiors, 177(1-2), 79-87.

Trasatti, E., Marra, F., Polcari, M., Etiope, G., Ciotoli, G., Darrah, TH, Tedesco, D., Stramondo, S., Florindo, F., Ventura G. (2018) Coeval uplift and subsidence reveal magma recharging near Rome (Italy). Geochem. Geophys. Geosyst., 19 pp. 1484-1498, 10.1029/2017GC007303