GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 1.2 231 Roselli, P., Marzocchi, W., Mariucci, M.T. and Montone, P.; 2016. Earthquake Focal Mechanism Forecasting in Italy for PSHA purposes , Submitted at Geoph. J. Int. Selva, J. and Marzocchi, W.; 2004. Focal parameters, depth estimation, and plane selection of the worldwide shallow seismicity with M s ≥ 7.0 for the period 1900-1976 , Geochem. Geophys. Geosyst . , 5(5), 1525-2027, doi:10.1029/ 2003GC000669. Strasser, F.O., Montaldo, V., Douglas, J. and Bommer J.J.; 2006 . Comment on “Influence of Focal Mechanism in Probabilistic Seismic Hazard Analysis” by Vincenzo Convertito and Andrè Herrero . Bull. Seismol. Soc. Am., 96(2), 750-753, doi: 10.1785/0120050101. Zoback, M.L.; 1992. First- and second-order patterns of stress in the lithosphere: the world stress map project , J. Geophys. Res., 97(B8), 703–728. Fault induced fluid mobilization revealed by cGNSS recordings at the northern tip of the Adria microplate G. Rossi 1 , D. Zuliani 2 , P. Fabris 2 1 Istituto Nazionale di Oceanografia e di Geofisica Sperimentale – OGS, Centro di Ricerche Sismologiche, Trieste, Italy 2 Istituto Nazionale di Oceanografia e di Geofisica Sperimentale – OGS, Centro di Ricerche Sismologiche, Udine, Italy Introduction. The now available decadal GNSS time-series evidenced worldwide the presence of deviations from the linear trend, as creep phenomena or slow earthquakes (of the period of hours or days), in some cases accompanied by tremors, (e.g., Dragert et al., 2001; Lowry et al., 2001). Such phenomena are more frequent in the high-strain rate regions, thanks also to the greater amplitude of the signals. In areas of low strain-rate, the need for a separation of any possible spurious effect is of particular importance. It is the case of the northern tip of the Adria microplate, characterized by a moderate seismic activity, and a strain rate of 3 mm/ year. Here, the horizontal pendulums of Grotta Gigante recorded repeated tremors in the three years preceding the Friuli 1976 Mw=6.4 earthquake (Chiaruttini and Zadro, 1976), interpreted a-posteriori as slow earthquakes (Dragoni et al. , 1984/5). In this area, the Friuli Regional Deformation Network (FReDNet) of the Istituto Nazionale di Oceanografia e Geofisica Sperimentale-OGS (16 GNSS permanent sites) has operated since 2002, while the Marussi network of the Friuli-Venezia Giulia Regional Council started in 1999 (10 GNSS permanent sites). In Slovenia, we find the GSR1 station, belonging to the EUREF network. Rossi et al. (2013) evidenced the presence of a transient signal, causing an apparent tilting mainly in the Dinaric direction. In this work, we deepen their analysis, first, by excluding any possible hydrological load effect. Techniques widely used in other fields, as traveltime and hydraulic tomography, enable the location of the transient source in space and in time and the hypothesis of a fluid propagation by porosity waves. Data processing and hydrological load correction. GPS data from both networks are first processed using the GAMIT/GLOBK package (http://chandler.mit.edu/~simon/gtgk/ ) using about 70 sites located around Adria and Europe. The final results have been stabilized within European frame (Altamimi et al. , 2012; Rossi et al. , 2016). We considered the longest time- series available. Following in principle the approach of Chamoli et al. (2014), we subtracted the annual, semiannual, and pluriannual terms of the displacements excited by the hydrological loading (Gegout et al. , 2010). For the hydrological load, we used both the Global Land Data Assimilation System (GLDAS), as well as the equivalent water height calculated from Gravity Recovery and Climate Experiment (GRACE) satellite gravimetry. Although coarser both in time and space (about 30 days in time and 30° in space), the latter is more efficient in correcting the data, since it contains the groundwater variations effects. In particular, we focused on records from different stations of a transient residual signal with an approximately 2-year period that propagated through a region 150 km wide (Fig. 1a).