GNGTS 2013 - Atti del 32° Convegno Nazionale

Discussion and conclusions. In other parts of the world GPS networks recorded creep phenomena or silent or slow earthquakes (rupturing over a period of hours or days instead of seconds), in some cases accompanied by tremors, as observed, e.g., by Dragert et al. (2001), Lowry et al. (2001). In the region under study, after the tremors recorded by the horizontal pendulums of Grotta Gigante in the three years preceding the Friuli earthquake (Chiaruttini and Zadro, 1976), interpreted a-posteriori as slow earthquakes (Dragoni et al. , 1984/5), tiltme- ters and strainmeters evidenced the presence of quasi-periodic signals of longer-term (9 years and 33 years) with respect to the ones here analysed, and acting along directions about N70°E and N-S oriented respectively, hence, normal to the two main tectonic systems present in the region (the Alpine and the Dinaric ones) (Rossi and Zadro, 1996; Braitenberg and Zadro, 1999). The present study focused on short period variations, recognizable on all the time-series considered, and causing bending from the linear trend mainly in the Dinaric direction. The data were compared both with the seismic energy released in the region and with the distribution of the hydrological balance, as representing possible hydrological effect. The results of the comparison are slightly more in favour of a tectonic origin of the transient, but to come to a definitive conclusion requires more analyses in the next future. References Altamimi, Z., L. Metivier, and X. Collilieux; 2012): ITRF2008 plate motion model, J. Geophys. Res., 117, B07402, doi:10.1029/2011JB008930. Blewitt, G., and Lavallee D.; 2002: Effect of annual signals on geodetic velocity , J. Geophys. Res., 107(B7), 10.1029/2001JB000570. Braitenberg C. and Zadro M.; 1999: The Grotta Gigante horizontal pendulums – instrumentation and observations . Boll. Geof. Teor. Appl., 40, 3/4, 577-582. Bressan G., Bragato P.L. and 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, 3, 1280–1297. Bressan G., Kravanja S., Franceschina G.; 2007: Source parameters and stress release of seismic sequences occurred in the Friuli-Venezia Giulia (Northeastern Italy) and in Western Slovenia , Phys. Earth Planet. Inter., 160, 192-214. Calais, E., Mattioli G., DeMets C., Nocquet J.-M., Stein S., Newman A., Rydelek P.; 2005: Tectonic strain in plate interiors? Nature 438, doi: 10.1038/nature04428. Chiaruttini C., Zadro M.;1976: Horizontal pendulum observations at Trieste. Udine Meeting on the Friuli Earthquake, 1976 . Boll. di Geof. Teorica ed Applicata, Vol.XIX, n.72, 441-455. Devoti R., Riguzzi F., Cuffaro M. and Doglioni C.; 2008: New GPS constraints on the kinematics of the Apennines subduction . Earth Planet. Sci. Lett. 273, 163-174, doi:10.1016/j.epsl.2008.06.031. Devoti R., Riguzzi F., Cuffaro M. and Doglioni, C.; 2011: New GPS constraints on the kinematics of the Apennines subduction , Earth and Planetary Science Letters, 273, 163-174. Dong, D., Fang P., Bock Y., Cheng M. K., and Miyazaki S.; 2002: Anatomy of apparent seasonal variations from GPS- derived site position time series , J. Geophys. Res., 107(B4), doi:10.1029/2001JB000573. Dragert, H., Wang, K., James T.S.; 2001: A Silent Slip Event on the Deeper Cascadia Subduction Interface , Science, 292, 1525-1528, DOI: 10.1126/science.1060152. Dragoni M., Bonafede M., Boschi E.; 1984/85: On the interpretation of slow ground deformation precursory to the 1976 Friuli earthquake , Pure Appl. Geophys., 122, 781-792. Franceschina, G., Kravanja S.and Bressan G.; 2006: Source parameters and scaling relationships in the Friuli-Venezia Giulia (Northeastern Italy) region , Phys. Earth. Planet. Int., 154, 2, pp. 148-167. Nocquet J-M.; 2012: Present-day kinematics of the Mediterranean: A comprehensive overview of GPS results , Tectonophysics, 579, 220-242. Rossi G. and Zadro M.; 1996: Long-term crustal deformations in NE-Italy revealed by tilt-strain gauges , Phys. of the Earth and Plan. Int., 97, 55-70. Smalley, R. Jr, Ellis M. A., Paul J. and Van Arsdale R. B.: 2005: Space geodetic evidence for rapid strain rates in the New Madrid region, central USA , Nature 435, pp. 1088-1090. Thornthwaite, C.W.; 1948: An Approach toward a Rational Classification of Climate, Geogr. Rev., 38, 1., pp. 55-94. Zerbini, S., Raicich F., Richter B., Gorini V. and ErricoM.; 2010: Hydrological signals in height and gravity in Northeastern Italy inferred from Principal Components analysis , J. Geodyn., 49, 3-4, pp. 190-204. 201 GNGTS 2013 S essione 1.2