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
Introduction
The Grotta Gigante (giant cave) situated in the Trieste Karst (latitude 45.7083°N and longitude 13.7633°E) bears the Guinness Award of greatest cave in the world, as it has an ellipsoidal shape of 130 m length, 65 m width and 107 m height. The geographical location of the station is seen in Fig. 1 (Trieste), where the topography (SRTM, 2004) of the studied area is shown. The other deformation station (Villanova) refers to a tilt-strain meter observatory we operate north of Trieste (Braitenberg, 1999b). Prof. Marussi (1959) had the brilliant idea to use the height of the cave to build a couple of long-base tiltmeters of the horizontal pendulum type with Zöllner suspension. After this date, the design of the mechanical part of the instruments was completely overhauled and the present instrumentation was installed in 1966. The horizontal pendulums consist of a sub-horizontal pendulum arm suspended by an upper wire fixed at the vault of the cave and a lower wire fixed to the ground of the cave. The distance between upper and lower mountings is 95 m. The total weight of the pendulum (including wires) is 18.7 kg, the horizontal beam has a length of 1.4 m and the period of oscillation of the pendulum in the horizontal plane is of 6 min (Marussi, 1959, Braitenberg, 1999b, Braitenberg and Zadro, 1999). A horizontal shift of the upper relative to the lower mounting of the pendulum (shear), a tilt of the cave or the inclination of the vertical are recorded as a rotation of the beam in the horizontal plane about the rotation axis, which lies on the line connecting the upper and lower mounting points of the pendulum. The static amplification factor for tilt (ratio of the angle of rotation of the beam in the horizontal plane with the tilting-angle of the line connecting upper and lower mountings) is about 24,000. The original recording system was optical on photographic paper, with an amplification of 4.4 nrad/mm. This system is very reliable and has been recording without greater problems since the time of the installation. The pendulums have been overhauled in 1982/1983 and in 1997, and some parts as the polyethylene tubes protecting the wires have been exchanged. Recently, in December 2003, a new digital acquisition system was installed, which is supposed to replace the photographic recording in the future, once its reliability has been ascertained. The advantages given by the digital acquisition system are the automatic readout, a drastically increased time and signal resolution, wherefore the instruments acquire the characteristics of a very broad-band tiltmeter.
In the sequel we give a short description of the acqusition system and show the secular tilt over the full observation period and the observation of the Sumatra-Andaman Islands M = 9 seismic event of December 26, 2004.
Section snippets
The new acquisition system
The new acqusition system records the position of a laser light reflected by a mirror mounted on the horizontal pendulum beam in correspondence of the rotation axis. The sensor is an analogical position sensitive detector (PSD) made of a long P–N junction, which can be illuminated across a transparent metallization surface (Fig. 2b). The laser-light generates charge carriers that form the current i. The current that flows in the P-crystal is collected by the metallization, the current that
Secular crustal deformation for the years 1966–2004
The long period continuous observations by means of tiltmeters, which cover the remarkable time interval of nearly 40 years, are shown in Fig. 3a. The original data-sampling rate of the photographic readout is 1 h, which has been reduced to the rate of 1 day after application of a low-pass anti-alias filter. One evident feature of the observations is the regular yearly oscillation, seen in all crustal deformation stations of good quality, and mainly due to the thermal influence of 1 year period
Observations at seismic frequencies—the Sumatra-Andaman Islands event 2004
The increase in the signal and time resolution of the new digital acquisition system makes it possible to record the horizontal ground movement due to the passage of seismic waves with the long-base pendulums. The instrumental response function of the pendulums is that of a damped oscillator with damping factor of 0.85 and a reduced pendulum length of 134.32 cm. The eigenfrequency of the pendulums is of 360 s. For illustration we have selected the recent seismic event off the western coast of
Conclusions
The Grotta Gigante (Giant Cave) in the Trieste Karst (Italy) has housed for almost 40 years a couple of long-base tiltmeters, which have given a unique continuous record of long-term crustal deformation. As explained in previous studies (Braitenberg and Zadro, 1999, Zadro and Braitenberg, 1999, Braitenberg et al., 2001), the comparison with the noise-spectra of traditional short-base tiltmeters operative in the same cave since 1999, showed that the long-base pendulums have lower noise level and
Acknowledgements
The financial contribution of Istituto Nazionale di Geofisica e Vulcanologia and of the Department of Civil Protection is thankfully acknowledged. Franco Stravisi (University of Trieste) is thanked for the access to his database of meteorologic data. The Commissione Grotte “Eugenio Boegan”, Soc. Alpina delle Giulie, is thanked for access to the meteorological data of the station Borgo Grotta Gigante. Stefano Furlani (University Trieste) is thanked for discussions on geomorphological
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