The free oscillations models of Chile 2010 and 1960 events observed with the Grotta Gigante horizontal pendulums

The Grotta Gigante is located on the Trieste Carst close to the Adriatic Sea in Italy. It is the greatest cave in the world in terms of single volume cave and has a diameter of 100 m. In 1959 the Marussi horizontal pendulums were installed for the first time. They are very stable tiltmeters due to the type of installation which uses the entire height of the cave (Marussi, 1959, Braitenberg et al., 2005). Though having been modernized and some maintenance having been applied, the instruments are perfectly controlled in their instrumental response during the 50 years of functioning. We have an absolute control on the correctness of the amplitudes by the tidal signal, which is observed very well on the instrument. At our knowledge it is the only station that uses the same instruments in the same location and has observed three of the five greatest events ever measured: Cile 1960 (M=9.5), 2010 (M=8.8), and Sumatra 2004 (M=9.3) (Bolt and Marussi, 1962; Bozzi Zadro and Marussi, 1967; Braitenberg and Zadro, 2007). We analyze the gravest modes of the free oscillations as 0S2, 0T2, 1S2 and determine frequencies and amplitudes of the modes for the events of Cile and Sumatra. Two minor events are added, as Cile 2007 and Sumatra 2005. For the spectral analysis we use the method described in Braitenberg and Zadro (2007), We find a few spectral amplitudes that are common to the different spectra and are intermediate to the theoretical free oscillation frequencies. We determine the Q values for the low degree free oscillation frequencies. At last we seek a direct comparison of the oscillation amplitudes for the different events. The observation of the 1960 event was made on photographic paper, the recent events are made with a digital system, which has some consequences on the resolution of the first part of the record. Bolt, B. A., and A. Marussi (1962). Eigenvibrations of the earth observed at Trieste, Geophys. J. R. Astr. Soc. 6, 299–311. Bozzi Zadro, M., and A. Marussi (1967). Polarization and total energy spectra of the eigenvibrations of the earth recorded at Trieste, Geophys. J. R. Astr. Soc. 12, 425–436. Braitenberg C., Zadro M. (2007). Comparative analysis of the free oscillations generated by the Sumatra- Andamans Islands 2004 and the Chile 1960 eatrhquakes, Bulletin of the Seismological Society of America, Vol. 97, No. 1A, pp. S6–S17. Braitenberg, C., G. Romeo, Q. Taccetti, and I. Nagy (2005). The verybroad-band long-base tiltmeters of Grotta Gigante (Trieste, Italy): secular term tilting and the great Sumatra–Andaman Islands earthquake ofDecember 26, 2004, J. Geodynamics 41, 164–174. Marussi, A. (1959). The University of Trieste station for the study of the tides of the vertical in the Grotta Gigante, in Proc. III International Symposium on Earth Tides, Trieste, Italy, 45–52.