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
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.
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.
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.