Cansiglio Karst Plateau: 10 Years of Geodetic–Hydrological Observations in Seismically Active Northeast Italy
Ten years’ geodetic observations (2006–2016) in a natural cave of the Cansiglio Plateau (Bus de la Genziana), a limestone karstic area in northeastern Italy, are discussed. The area is of medium–high seismic risk: a strong earthquake in 1936 below the plateau (Mm = 6.2) and the 1976 disastrous Friuli earthquake (Mm = 6.5) are recent events. At the foothills of the karstic massif, three springs emerge, with average flow from 5 to 10 m3/s, and which are the sources of a river. The tiltmeter station is set in a natural cavity that is part of a karstic system. From March 2013, a multiparametric logger (temperature, stage, electrical conductivity) was installed in the siphon at the bottom of the cave to discover the underground hydrodynamics. The tilt records include signals induced by hydrologic and tectonic effects. The tiltmeter signals have a clear correlation to the rainfall, the discharge series of the river and the data recorded by multiparametric loggers. Additionally, the data of a permanent GPS station located on the southern slopes of the Cansiglio Massif (CANV) show also a clear correspondence with the river level. The fast water infiltration into the epikarst, closely related to daily rainfall, is distinguished in the tilt records from the characteristic time evolution of the karstic springs, which have an impulsive level increase with successive exponential decay. It demonstrates the usefulness of geodetic measurements to reveal the hydrological response of the karst. One outcome of the work is that the tiltmeters can be used as proxies for the presence of flow channels and the pressure that builds up due to the water flow. With 10 years of data, a new multidisciplinary frontier was opened between the geodetic studies and the karstic hydrogeology to obtain a more complete geologic description of the karst plateau.
KeywordsGeodesy karst hydrogeology GPS tilt measurements
The study region is located in northeastern Italy, in the seismically active area of the karstic Cansiglio Plateau. The present seismicity of NE Italy is well manifested toward the Friuli region, whereas toward the western sector a relative calmness is found. This picture emerges when considering the local seismicity recorded since the 1976 disastrous Friuli earthquake, certainly biased by the post-seismic sequence of this event. The western sector was hit in 1936 by the destructive Cansiglio earthquake, showing that the seismic potential is high in the entire region, reaching also farther west to the eastern Venetian sector. For this reason, 10 years ago it was decided to monitor the deformation of the area, installing two Zöllner-type Marussi tiltmeters in a natural cavity at 25 m depth (Bus de la Genziana). They are operating continuously since 2005 (Braitenberg et al. 2007).
During this period, we proposed an interdisciplinary study of karstic aquifers using hydrogeological data, tiltmeters and GPS observations (Grillo et al. 2011). During the year 2010, two data acquisition campaigns have been carried out to integrate the research started with the tiltmeter recordings: hydrogeological flow measurements (level, conductivity and temperature) in two principal springs and the installation of a small geodetic GPS network. The geodetic campaign extended from May to October 2010 measuring two GPS benchmarks in the neighborhood of the FReDNet permanent station CANV (950 m a.s.l; Zuliani 2003; Zuliani et al. 2009), the results of which demonstrated the presence of an observable hydrologic signal in the GPS time series (Devoti et al. 2015).
To monitor the underground hydrodynamics of Bus de la Genziana and to correlate it with the geodetic recordings, for the first time a multiparametric logger (temperature, stage, electrical conductivity) was installed in the siphon located at the bottom of the Genziana cave (587 m deep) from March 2013 to December 2016 (Grillo and Braitenberg 2015).
The decade-long tilt and GPS observations in the karstic area allows us to univocally characterize the expected deformation signal in relation to the rainfall and spring discharge from the karstic plateau. The results can be of relevance for other karstic areas worldwide (see, e.g., Longuevergne et al. 2009; Tenze et al. 2012; Braitenberg 1999b, c), because hydrologic karst systems have often common characteristics, such as prominent epikarst, a well-developed network of deep channels in which water is efficiently drained toward the base level, and a less important matrix-flow component. The knowledge of the geometry of the cave network is important, because boreholes to the channels produce drinking water provision. An improved knowledge of the relation between the karstic water flows and the geodetic signal will allow the use of the geodetic measurement as a hydrologic investigation tool in the future. The main features of the geodetic measurements are furthermore generally comparable to the signals of analogous instrumentation operating in non-karstic environs affected by fissures and faults. The understanding of the physical relations between extensometer, tilt and gravity observations and the hydrology is fulfilled also elsewhere, e.g., in the station Moxa, Germany (Jahr 2017). Here, induced pore pressure variations through pumping and injection experiments were made to test the poroelastic deformation models. The aim is to use the physical relations to plan hydro-deformation experiments to recover in situ rock physical properties (Wang and Kümpel 2003; Jahr et al. 2008).
2 Geographical and Geological Setting
A geological description of Cansiglio Plateau is discussed in Cancian and Ghetti (1989). The outcropping rocks range in age from Upper Jurassic to Paleocene and are mainly composed of carbonates (Fig. 1). The eastern area is characterized by a thick succession of Cretaceous peritidal carbonates (Cellina limestones), while the central western part is characterized by slope breccia deposits (Fadalto Formation and Mount Cavallo Formation), all capped by basinal marly carbonates (Scaglia Formation).
The River Livenza flows down from the southeastern slope of the carbonatic Massif of Cansiglio–Cavallo. It is supplied by three main springs: the Gorgazzo, which has a recharge basin of 170 km2, the Santissima of 500 km2 and the Molinetto of 230 km2. All three have an average flow from 5 to 10 m3/s (Cucchi et al. 1999).
The massif is characterized by a markedly deep karst, with about 200 caves and clear karst surface morphology. Although the annual mean precipitation is about 1800 mm, the Cansiglio Plateau for the time being has no surface runoff, but acts like an endorheic basin with a pronounced system of underground drainage through caves. Essentially, two noteworthy caves are considered: the Bus de la Genziana (Fig. 1) with a maximum depth of 587 m and a development of 8 km, and the Abisso Col de la Rizza, the deepest cavity of the area, reaching 800 m below the surface. Morphologically, all caves have a complex tunnel system, including shafts, halls, canyons, meanders and sometimes are also well decorated (Grillo 2007).
The hydrologic connection between the Cansiglio Plateau and two of the three main Friulian sources, the Santissima and Molinetto origins of the Livenza River, have been demonstrated by recent tracing examinations (Filippini et al. 2016).
3 Geodetic and Hydrologic/Environmental Data
3.1 Climate Data
Rainfall, snow, atmospheric pressure and air–temperature data were obtained from the ARPA Veneto (regional Meteorological Service and Snow and Avalanches Service), considering the weather station Cansiglio–Tramedere (1028 m a.s.l.) for hourly sampling. It is placed at about 8 km north from the CANV station and 2.5 km from the tiltmeter location.
3.2 Hydrological Monitoring
Multiparametric loggers (CTD-Diver, Schlumberger Water Services) have been installed in 2010 in the springs Santissima and Gorgazzo located in Polcenigo at the foothills of the massif, while a barometer (Baro-Diver, Schlumberger Water Services) was placed close to one of the springs for barometric correction of the stage data. The pressure is used to reduce the stage observations for atmospheric pressure variations using the inverse barometric hydrologic response. These instruments record variations in water level (h), temperature (T) and specific electrical conductivity (EC) every hour.
The hydrometric station of the Livenza River is located at the foothills of the massif in the city of Sacile, about 10 km south from the springs area. The data consist in the river level measurement with hourly sampling and are available through the monitoring network of the Civil Protection of Friuli Venezia Giulia.
3.3 Marussi Tiltmeters
Since 2005, the University of Trieste runs a tiltmeter station in the natural cave “Bus de la Genziana” 25 m below the entrance (Grillo et al. 2011). The Genziana tiltmeters are horizontal pendulums with Zöllner-type suspension and described in detail (Braitenberg 1999a; Zadro and Braitenberg 1999). They are sturdy instruments due to their relatively big size (0.5 m tall) and stable mount, inside a cast iron bell resting on compact rock. The iron bell is sustained by three supports placed on the solid rock. The horizontal arm rotates in the horizontal plane around a subvertical axis, the rotation angle being picked up by a magnetic transducer. The pendulum is sensitive to a tilting of the ground at 90° with respect to the off-vertical angle of the pendulum’s rotation axis (see Fig. 3c) plus a mass attraction effect. The digital data acquisition has a sampling rate of 1 h and uses an inductive transducer. The resolution of the tiltmeter is near 5 nrad, the value corresponding to the unit value of the digitizing process. Sign convention is positive for east- and northward tilting of the pendulum (tilting down).
3.4 GPS Observations
Two permanent GPS stations are available on the Cansiglio Massif, and five temporary stations had been installed to outline the area affected by the hydrologic deformation. The permanent stations are the following: the Caneva station (CANV), belonging to the FReDNet (http://frednet.crs.inogs.it; Battaglia et al. 2003), and the Tambre station (TAMB), owned by the Regione Veneto authority (http://18.104.22.168/Web). The CANV station is located at 800 m a.s.l. at the southern margin of the Cansiglio–Cavallo Plateau at a distance of about 8 km from the Genziana cave and is established on a reinforced concrete pillar anchored on bedrock. The TAMB station is located on the plateau at about 400 m from the entrance of the Genziana cave, settled on the roof of a stone-made house. Five monitoring stations were set up along the southern margin of the plateau and on top of Mt. Pizzoc in the southwestern edge of the plateau and were measured occasionally in the last years (CN01, CN03 and CN04 in Fig. 1).
4 Description of Geodetic Observations in Relation to Hydrologic Data
4.1 Characteristic Signals of Tiltmeter Observations
The contribution of the barometric effect is negligible in comparison to the hydrologic effect, and correlation to atmospheric pressure gradient changes could not be found (Kroner et al. 2005; Boy et al. 2009), probably masked by the hydrologic effect.
4.2 Discussion of Hydrogeological Campaign Compared to the Tiltmeter Observations
The Cansiglio karst aquifer is a mature pre-alpine deep karst, characterized by high permeability and rapid runoff through enlarged fractures and caves, although mitigated by the existence of a base flow component through a network of smaller channels (Grillo 2007). The underground karst phenomenon is mainly developed in Monte Cavallo limestone with a complex of caves 600–800 m deep, controlled by the geological–structural setting. The aquifer has a high conductivity and high vulnerability (Cucchi et al. 1999).
The electrical conductivity values are on average of 230 μS/cm, while the parameter reduces down to 150 μS/cm when the karstic system fills due to rainfall. The water temperature variations are well correlated to the conductivity and also have the characteristic impulsive increase with slow recovery. The medium temperature is 8 °C, with variations limited to − 0.15° to +0.30°. Increase of temperature correlates with reduction of electrical conductivity and with onset of rainfall (Grillo and Braitenberg 2015).
These parameters show that the hydrologic system is affected by mixing of new infiltration water, which is very fast in the inflow, but slower in the outflow, so the water level abruptly increases and then slowly decays.
It is to be expected that the tilting correlates to the filling of the siphon below the instruments. This is most convincingly seen in Fig. 7b, where in a time window of 2.5 months data for the tilt, siphon water height, level of River Livenza and rainfall are shown. The river level must be at least 3.3 m high to flood the siphon, and at all times that the river rises above this threshold level the siphon is filled. Every time the siphon is filled, there is a tilting event, with 2 µrad tilting for a rise of 5 m in the siphon. The tilting has a short duration and marks the filling stage of the siphon. The marking of the filling stage is demonstrated best by analyzing the time derivative of the siphon, as shown in the next figure. Only in rare cases in which the siphon is filled above the 5 m mark do the tiltmeters also have a slow recovery of deformation, as at the event A in Fig. 7b.
To further illustrate the tiltmeter response during the rising stage in the siphon, we calculate the first time derivative of the stage and compare it to the tilt records, calculating the cross-correlation function between the two. A negative lag corresponds to a delay in the siphon The cross-correlation function is maximum (cross-correlation coefficient 0.38) for zero delay between the diver stage rate and the tiltmeter records, and has a smaller but well-developed negative extreme at − 54 h delay of the diver change rate with respect to the tilting.
4.3 Model for Explaining the Diver and Tiltmeter Observations
The physical model that explains the tilt observations must comply with the observations. We have no direct observations of the shafts, so the physical model we propose here is a conjecture. To justify the model, we summarize the responses of the diver and the tilt to variations in rainfall and the Livenza River, and then formulate a structural model that explains the observations. The conceptual model can be used for a numerical simulation of the hydraulic flows and the induced deformation, which requires a separate study based on the present one.
4.4 Discussion of GPS Observations in Relation to Subsurface Hydrologic Flows
The hydrologic-induced deformation has also been detected with GPS stations installed on the karstic plateau. The results of a dedicated campaign were discussed in Devoti et al. (2015) and it was demonstrated that the horizontal movement of the stations after rainfall is outward toward the margins of the plateau and orthogonal to the prevailing fracture direction (Devoti et al. 2016). The signals were interpreted as due to filling and subsequent pressure exerted by vertical fractures.
On comparing the tiltmeter’s signal recorded at the Genziana station with the local rainfall series, the Livenza River gauge height, and the time series of the GPS stations located on the southern slopes of the Cansiglio Massif, a clear correspondence with the water runoff is seen (Fig. 10).
The hydrologic-induced deformation was particularly evident during the flood of the Livenza River between October 31 and November 3, 2010 (Fig. 10) (Grillo 2010). In those days, a total amount of 520 mm of rain fell with a peak 29 mm/h. The GPS station and the tiltmeters recorded the displacement and deformation induced by the water influx into the system instantly, and also due to the underground water runoff during the flood. The flood resulted in a GPS maximum displacement of 10 mm to the east and 15 mm to the south. The tiltmeters show a 1 µrad variation toward east and then a continuous westward drift of about 3 µrad during the rainy days, recovering the initial easterly position in the following week. The NS component provided initial complex variations during the rainy period, drifting first to the north and then slowly to the south and recovering the initial position in the following week (see inset in Fig. 10).
We observed a variety of signals in and around the caves of the Cansiglio Plateau, ranging from standard hydrological signals such as rainfall, stage data, temperature and electrical conductivity, to GPS observations and measurements of tilt in the cave Bus de la Genziana. From this variety of observations, we show how the tiltmeters respond to the fast infiltration, related to the amount and duration of the rainfall, and to a lesser extent to the phreatic discharge, because of lower pressure change. In contrast, the horizontal displacement from GPS does not show any reaction from the fast infiltration, but records the slower phreatic discharge. The amplitude relation between the GPS movement and the amount of rainfall is approximately linear, accounting for 3 mm displacement every 100 mm cumulative rainfall for strong events. The prevailing movement is horizontal along a stable direction 130–160° north azimuth for stations at the eastern part of the plateau. The mutual correlation is significant, with a high correlation coefficient of 0.92 (Devoti et al. 2015). The direction of movement points to the area of the two springs mentioned above (Fig. 7) and is directed orthogonal to the frontal thrust systems of the Cansiglio Massif. The high correlation with the rainfall suggests that the deformation of the GPS, like the tiltmeters, is caused by subsurface hydrologic processes in the karstic vadose zone rather than with deep-rooted water table variations in the phreatic zone, as pointed out by Devoti et al. (2015).
The region to which the Cansiglio carbonatic massif belongs is subjected to compressional stress-oriented SSE–NNW with subhorizontal angle, as has been deduced from focal mechanisms (Bressan et al. 2003). This direction is nearly compatible with southward tilting that has been observed over the past 10 years. Different studies (Davis et al. 1983; Yeats 1986; Huang and Johnson 2016) have proposed the model in which an anticline on an active thrust fault shows growth and increase of the anticline topographic slopes. A southward tilting, such as the one documented by the tiltmeters, could be interpreted by this model as being the expression of the southward movement of the massif along the two thrust faults that mark the margin of the southern border of the massif (Fig. 1) and are responsible of the southward tilting of the slope facing the thrust fault.
We describe an active slope deformation monitored with GPS and tiltmeter stations in a karstic limestone plateau in southeastern Alps (Cansiglio Plateau), one of the most interesting karstic areas of northeastern Italy. Considering the geophysical and hydrogeologic setting, the geodetic tilt station located in Bus de la Genziana is situated in a strategic position. The tiltmeters record crustal movements: long-term tectonic movement and hydrogeological information (epikarst fast infiltration and phreatic slow unloading).
The long-term tilt has a definite southward direction, with episodes of stability and northward movement. The tilting reflects the actual tectonic situation in northeast Italy, which shows the convergence of the Adriatic and Euroasiatic plates.
The long-term movement is overprinted by fast tilting induced by rainfall and a slower movement due to the phreatic water discharge. GPS observations record the slower movement in the horizontal components as well, and movement is principally horizontal in the SSW direction and back, and can reach the order of 1 cm during the hydrologic-induced movement. The recovery of the deformation following the rain can last up to 2 weeks. The interpretation of the different time constants is a fast water pressure change in the epikarst and a slower water pressure buildup at deeper karst levels, where water fills the karstic channels, with local level increments of up to 50 m. We can approximate the behavior of the Cansiglio Massif as a network of drainage channels with dominant preferential directions, flowing in the karst vadose reticulum with different trigger times depending on the amount and duration of rainfall. The long-term deformation pattern revealed by geodetic instruments probably reflects the discharge of the karst aquifer, a first impulsive reaction due to rapid and turbulent flow in the conduit network, followed by a slow discharge in the porous matrix (pores and fissures).
The direct link between the aquifer system cycles and the induced surface deformation provides interesting insights into karst-style hydrological processes that could also be relevant in the assessment of hydrologic hazards. The GPS and the tilt observations are complementary and sensitive enough to study and monitor the effects of water infiltration in karst systems.
Considering that the southern Alpine front accommodates a compression of a few millimeters per year and that the area is known to be in Zone 2 (defined as: Zone 2: Municipalities in this area may be affected by quite strong earthquakes, Protezione Civile 2015) at medium to high seismic risk, we are asking how and if these sudden shifts due to hydraulic load may affect the geophysical and geodynamic context of the Cansiglio area. The analysis of time series of the permanent GPS at Caneva suggests an elastic response of a hydro-structure with a drainage system directed along NW–SE, parallel to the direction of the complex headwater of Polcenigo–Caneva. Furthermore, an improved knowledge of the relation between the karstic water flows and the geodetic signal will allow the use of the geodetic measurement as a hydrologic investigation tool in the future.
We thank Alberto Casagrande for the constant engagement and for the precious collaboration; A.R.P.A. Veneto Centro Meteorologico of Teolo for providing the meteorological data; Ivano di Fant (Ufficio Idrografico del Servizio Gestione delle Risorse Idriche—Friuli Venezia Giulia) for the hydrometrical data of Livenza River; Dr. Alberto Riva for providing the geological map; OGS-RSC Working Group for publishing the local seismicity on their homepage; Dr. Paola Favero and the Comando Forestale of Pian Cansiglio for the hospitality and the collaboration; the local cavers (in particular, the speleologists of Unione Speleologica Pordenonese CAI Pordenone) for the support and collaboration in the installation and sampling of the diver in the bottom of Bus de la Genziana. We acknowledge the use of the GMT software (Wessel et al. 2013). We thank Georg Kaufmann and an anonymous reviewer for meticulous reviews.
- A.R.P.A. F.V.G. (2006). Rilevamento dello stato dei corpi idrici sotterranei della Regione Friuli Venezia Giulia (Survey on the state of underground hydrologic units of the Friuli Venezia Giulia Region). Final Report, pp. 68–71. Regione Autonoma Friuli Venezia Giulia.Google Scholar
- Braitenberg, C. (1999c). The hydrologic induced strain—A review. Marees Terrestres Bulletin D’Informations, 131, 1071–1081.Google Scholar
- Braitenberg, C., Grillo, B., Nagy, I., Zidarich, S., & Piccin, A. (2007). La stazione geodetico—geofisica ipogea del Bus de la Genziana (1000VTV)—Pian Cansiglio. Atti e Memorie della C.G.E.B., S.A.G. CAI, Trieste, Italia, 41, 105–120.Google Scholar
- Bressan, G., Bragato, P. L., & Venturini, C. (2003). Stress and strain tensors based on focal mechanisms in the seismotectonic framework of the Friuli-Venezia Giulia Region (Northeastern Italy). Bulletin of the Seismological Society of America, 93, 1280–1297. https://doi.org/10.1785/0120020058.CrossRefGoogle Scholar
- Cancian, G., & Ghetti, S. (1989). Stratigrafia del Bus de la Genziana (Cansiglio, Prealpi Venete). Studi Trentini di Scienze Naturali—Acta Biologica, Trento, 65, 125–140.Google Scholar
- Cavallin, A. (1980). Assetto strutturale del Massiccio Cansiglio—Cavallo, Prealpi Carniche Occ. Atti del 2° Convegno di Studi sul Territorio della provincia di Pordenone (Piancavallo, 19–2 ottobre 1979).Google Scholar
- Cucchi, F., Forti, P., Giaconi, M., & Giorgetti, F. (1999). Note idrogeologiche sulle sorgenti del Fiume Livenza. Atti della Giornata Mondiale dell’Acqua “Acque Sotterranee: Risorsa Invisibile”, Roma, 23 marzo 1998 (Pubbl. n°1955 del GNDCI, LR49), pp. 51–60.Google Scholar
- Devoti, R., Falcucci, E., Gori, S., Poli, M.E., Zanferrari, A., et al.. (2016). Karstic slope “breathing”: Morpho-structural influence and hazard implication. Poster General Assembly EGU, Vienna.Google Scholar
- Devoti, R., Zuliani, D., Braitenberg, C., Fabris, P., & Grillo, B. (2015). Hydrologically induced slope deformations detected by GPS and clinometric surveys in the Cansiglio Plateau, southern Alps. Earth and Planetary Science Letters, 419, 134–142. https://doi.org/10.1016/j.epsl.2015.03.023.CrossRefGoogle Scholar
- Filippini, M., Casagrande, G., Fiorucci, A., Gargini, A., Grillo, B., Riva, A., et al. (2016). Geological and hydrogeological investigations for the design of a multitracer test in a major karst aquifer (Cansiglio-Cavallo, Italian Alps). Rendiconti online della Società Geologica Italiana, 39(suppl. 1). ISSN 2035-8008.Google Scholar
- Grillo, B. (2007). Contributo alle conoscenze idrogeologiche dell’Altopiano del Cansiglio. Atti e Memorie della C.G.E.B., S.A.G. CAI, Trieste, Italia, 41, 5–15.Google Scholar
- Grillo, B. (2010). Applicazioni geodetiche allo studio dell’idrogeologia del Cansiglio. AA. 2009–2010. Master Thesis, Environmental Sciences University of Trieste. Edizioni Accademiche Italiane. ISBN 978-3-639-66321-1.Google Scholar
- Grillo, B., & Braitenberg, C. (2015). Monitoraggio delle acque di fondo del Bus de la Genziana (Pian Cansiglio, Nord-Est Italia). Atti e Memorie della C.G.E.B., S.A.G. CAI, Trieste, Italia, 46, 3–14.Google Scholar
- Longuevergne, L., Florsch, N., Boudin, F., Oudin, L., & Camerlynck, C. (2009). Tilt and strain deformation induced by hydrologically active natural fractures: Application to the tiltmeters installed in Sainte-Croix-aux-Mines observatory (France). Geophysical Journal International, 178, 667–677. https://doi.org/10.1111/j.1365-246X.2009.04197.x.CrossRefGoogle Scholar
- Priolo E., Romanelli, M., Plasencia Linares, M. P., Garbin, M., Peruzza, L., Romano, M. A., et al. (2015). Seismic monitoring of an underground natural gas storage facility: The Collalto Seismic Network. Seismological Research Letters, 86(1), 109–123 + esupp. https://doi.org/10.1785/0220140087
- Protezione Civile. (2015). Retrieved October 10, 2017, from http://www.protezionecivile.gov.it/jcms/en/classificazione.wp.
- Yeats, R. S. (1986). Active faults related to folding. In R. E. Wallace (Ed.), Active tectonics (pp. 63–79). Washington, DC: National Academy Press.Google Scholar
- Zuliani, D. (2003). FReDNet: una rete di ricevitori GPS per la valutazione del potenziale sismico nelle Alpi sudorientali italiane. GNGTS, Atti del 22° Convegno Nazionale, Roma, 18–20 November 2003. https://doi.org/10.13140/rg.2.1.1350.1845.
- Zuliani, D., Priolo, E., Palmieri, F., & Fabris, P. (2009). Progetto GPS-RTK: una rete GPS per il posizionamento in tempo reale nel Friuli Venezia Giulia. Dimensione Geometra, 12, 30–36.Google Scholar