Marussi horizontal pendulums of Grotta Gigante

The Grotta Gigante pendulums are horizontal pendulums with Zöllner type suspension, as are the Friuli tiltmeters. A mass (about 18 kg) is fixed at one extremity of the arm of each pendulum. The arm is suspended by two wires(Steel Nickel Chrome, diameter: 0.6 mm, property: inoxidizable, breaking load: elevated), an upper and a lower one, so that the pendulum arm can rotate in a subhorizontal plane. The movement of the pendulum arm about a virtual rotation axis, the axis being defined by the upper and lower suspensions of the wires, is recorded by an optical system. The tilt of the virtual rotation axis due to crustal movements is recorded by the rotation of the pendulum arm, which is orders of magnitude greater than the tilt of the axis. The crustal tilts are recorded by each of the two pendulums along one direction, EW and NS respectively.

Due to their exceptional dimensions, made possible by the sepctacular size of the cave (112 m height), the Grotta Gigante pendulums are extremely stable with a background noise which is several orders of magnitude less than traditional instruments of smaller size. This fact allows to detect extremely weak signals, that otherwise are masked by the noise.

In December 2003 a new recording system was installed, based on a solid-state acquisition system intercepting a laser light reflected from a mirror mounted on the horizontal pendulum beam. The sampling rate is 30 Hz, which turns the long- base instrument to a very-broad-band tiltmeter, apt to record the tilt signal on a broad-band of frequencies, ranging from secular deformation rate through the earth tides to seismic waves (Braitenberg etal, 2006).

Schematic view of pendulums
The cave and the pendulums

The observations

The GG-station was originally built as an earth tides observatory. A series of studies were carried out concerned with the earth tides and the loading effects of the Adriatic sea (Bozzi Zadro (1972) and references therein). The instruments though proved useful in the subsequent years for the detection of earth signals at increasingly shorter periods, down to those typical of surface waves. The Chilean earthquake of 1960, May 22 activated free oscillations which were recorded by the pendulums for 82 hours on both components. In the frequency range between 0.02 cycle/min and 0.2 cycle/min a complete sequence of torsional eigenfrequencies (2 = l = 22) could be identified for the first time and also the lowest fundamental spheroidal modes were recorded (Bolt and Marussi, 1962; Bozzi Zadro and Caputo, 1968). A subsequent spectral analysis of the same data showed the presence of some low frequency components, not belonging to the set of free oscillations, but equal to the sums or differences of frequencies of the normal modes. These could be explained with departures from linearity of the elasticity of the earth's body (Bozzi Zadro, 1971). Starting with 1973 a new kind of observation was made, which initiated with a sudden permanent deflection of some msec in both components. After this deflection the pendulums started to record perturbations which lasted for several hours. The number and duration of the perturbations increased between 1973 and 1976, when they suddenly disappeared with the M=6.4 May 6, 1976 Friuli earthquake (Zadro, 1978). The observations were interpreted as very long period elastic preseismic waves generated by aseismic slip on a fault neighbouring the main fault of the 1976 event (Bonafede et al., 1983). In the subsequent years up to the present the GG-station was kept as reference station to the tilt-strainmeter network installed in Friuli in 1977. The instruments have shown long term stability over the several decades of functioning. In order to better evidence the long term behavior of the records, the data have been interpolated with a polynomial of order 6. The long period continuous tilt records of two Friuli and the GG-station were jointly studied, revealing that the tilt signals are correlated at pluriannual periods (Rossi and Zadro, 1996). The correlation has been interpreted in terms of a deformation of the Northern Adriatic plate with principal directionalities aligned with the alpine (EW) and dinaric (NW-SE) orientations.

The long-base tiltmeters of the Grotta Gigante are ideal for the detection of the free oscillation modes, due to their long-period eigenfrequency (6–9 min). A comperative analysis has been made of the free oscillations generated by the Chile 1960 and Sumatra–Andaman Islands 2004 earthquakes (Braitenberg and Zadro, 2007; Zadro et al., 2010).

The comparative analysis of the tide gauge observations of sea level along the north-eastern Adriatic coast confirm the tilting indicated by the long-base tiltmeter data (Antonioli etal, 2009).

Tenze et al. (2011) have investigated whether the inflow volume of the Reka/Timavo river has an impact on the crustal deformation and the Karst system. This river along its pathway from the Mountains to the Adriatic Sea, flows underground at the base of the karst, after disappearing in the Skocian Cave. The tilt data from Grotta Gigante cave were analyzed and compared with the hydrologic measurement from the Trebiciano Abyss, where the water level of the underground river is measured. Analyzing the daily sampled data a clear correlation between the hydrologically induced tilt signal and the high levels in the Trebiciano Abyss was found, with a maximum phase shift of a day. The tilt signal has a characteristic orientation (N150W-N30E). The induced tilt is a transient, as the deformation returns to its state it had before the hydrologic event. The relation between tilt amplitude and water level in Trebiciano Abyss is linear (100 nrad of inclination each 9.7 meters of level variation). There is a threshold value of 23.2 meters for the water level increase in the Trebiciano Abyss, below which no tilt signal is observed.

Tenze et al. (2012) have quantified the deformation of this natural cave induced by the environmental factors of temperature, rainfall and water flow.

Devoti et al. (2015) also demonstrate the direct link between the aquifer system cycles and the induced surface deformation, providing interesting insights of 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.

Braitenberg and Nagy (2014) show the observation sequence of tilt for the time interval 1966-2012, and discuss the deformation due to the following causes: temperature, underground water-level, sea level of the Adriatic sea, position of sun and moon and vibration of the Earth due to some of the greatest mega-earthquakes ever recorded. To illustrate the association of different frequency ranges to different causes of deformation to the general public of the cave, the analogues to music are used. The results obtained from the decade long records of the Grotta Gigante pendulums are excellent example of scientifically sound and important instrumentation installed in a show-cave visited regularly by the public.

References:

Bozzi Zadro M., Caputo M. (1968). Spectral, bispectral analysis and Q of the free oscillations of the earth. Supplemento al Nuovo Cimento, Serie I, Vol. 6, 67-81.

Bozzi Zadro M. (1971). Non-linear effects in the free oscillations of the earth. Boll. Geof. Teor. Appl., XIII, N.51-52, 187-195.

Bozzi Zadro M. (1972). Earth tides and ocean load effects recorded at Trieste. Boll. Geof. Teor. Appl., XIV, N.55, 192-202.

Bolt B.A., Marussi, A. (1962). Eigenvibrations of the earth observed at Trieste. Geophys. J. R. Astr. Soc., 6: 299-311.

Bonafede M., Boschi E., Dragoni M. (1983). Viscoelastic stress relaxation on deep fault sections as a possible source of very long period elastic waves. J. Geophys. Res., 88: 2251-2260.

Zadro M. (1978). Use of tiltmeters for the detection of forerunning events in seismic areas. Boll. di Geod. e Sc. Affini. XXXVII, 597-618.

Rossi G., Zadro M. (1996). Long-term crustal deformations in NE Italy revealed by tilt-strain gauges. (PDF file, 1.8 MB) Physics of the Earth and Planetary Interiors, 97, 55-70.

Zadro M., Braitenberg C. (1999). Measurements and interpretations of tilt-strain gauges in seismically active areas. (PDF file, 3.9 MB) Earth Science Reviews, 47, 151-187.

Braitenberg C. (1999). The Friuli (NE Italy) tilt/strain gauges and short term observations. Annali di Geofisica, 42, 1-28.

Braitenberg C., Zadro M. (1999). The Grotta Gigante horizontal pendulums - instrumentation and observations. Boll. Geof. Teor. Appl., Vol.40, N°3/4, 577-582.

Braitenberg, C., Nagy I., Romeo G., Taccetti Q. (2004) The very broad-band data acquisition of the long-base tiltmeters of Grotta Gigante (Trieste, Italy) in: Progress in Geodesy and Geodynamics, Zhu Yaozhong and Sun Heping (Eds.), 457-462, Hubei Science and Technology Press, Wuhan.

Braitenberg C., Nagy I., Papacchioli S., (2004) Pendoli geodetici per registrare le deformazioni della Grotta Gigante, Rassegna tenica del Friuli Venezia Giulia, Anno LV, Luglio/Agosto 2004, 16-20.

Braitenberg C., Romeo G., Taccetti Q., Nagy I. (2006) The very broad-band long-base tiltmeters of Grotta Gigante (Trieste, Italy): secular term tilting and the great Sumatra-Andaman Islands earthquake of December 26, 2004. J. of Geodynamics, 41, 164-174.( PDF file 738 KB).

Braitenberg C., Zadro M. (2007). Comparative analysis of the free oscillations generated by the Sumatra-Andamans Islands 2004 and the Chile 1960 earthquakes, Bulletin of the Seismological Society of America, Vol. 97, No. 1A, pp. S6-S17, January 2007, doi: 10.1785/0120050624. (PDF file 144 KB).

Antonioli F., Ferranti L., Fontana A., Amorosi, A. M., Bondesan A., Braitenberg C., Dutton A., Fontolan G., Furlani S., Lambeck K., Mastronuzzi G., Monaco C., Spada G., Stocchi P. (2009) Holocene relative sea-level changes and vertical movements along the Italian and Istrian coastlines, Quaternary International, 206, 102-133, ISSN 1040-6182, dOI:10.1016/j.quaint.2008.11.008., (PDF file 2,6 MB).

Zadro M, Braitenberg C., Nagy I. (2010). The free oscillations modes of Chile 2010 and 1960 events observed with the Grotta Gigante horizontal pendulums. Eos Trans. AGU, 91(26), Meet. Am. Suppl., Abstract U41A-01 (The Meeting of the Americas - AGU, Foz do Iguassu, Brazil, 8-12 August 2010), (PDF file 5,48 MB).

Tenze D., Braitenberg C., Nagy I., Cucchi F. (2011). Deformazioni indotte da flussi idrici sotterranei nel Carso Triestino. Atti e Memorie della Commissione Grotte "E. Boegan", Vol.43, 41-55, (PDF file 1,92 MB).

Tenze D., Braitenberg C., Nagy I. (2012). Karst deformations due to environmental factors: evidences from the horizontal pendulums of Grotta Gigante, Italy. Bollettino di Geofisica Teorica ed Applicata, 53, 331-345, doi:10.4430/bgta0049, (PDF file 3,00 MB).

Braitenberg C., Nagy I. (2014). Illustrating the superposition of signals recorded by the Grotta Gigante pendulums with musical analogues. Acta Carsologica, 43/1, 139-147, (PDF file 847 KB).

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, doi:10.1016/j.epsl.2015.03.023, (PDF file 2,23 MB).
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