GNGTS 2021 - Atti del 39° Convegno Nazionale

GNGTS 2021 S essione 1.1 78 EVIDENCE OF FAULT-VALVE BEHAVIORS AT THE NORTHERN EDGE OF THE ADRIA MICROPLATE G. Rossi 1 , A. Pastorutti 2 , I. Nagy 2 , C. Braitenberg 2 and S. Parolai 1 1 National Institute of Oceanography and Applied Geophysics - OGS, Italy 2 Department of Mathematics and Geosciences, University of Trieste, Trieste, Italy The presence of transient signals in the strain field data can reveal changes in pore pressure and, consequently, in the effective stress in seismically active areas. The northern edge of the Adriatic microplate, where the collision with the Eurasian plate occurs, has been monitored for more than 50 years by the NE-Italy subsurface tilt and strainmeter network. It is possible, there- fore, to verify whether transient deformation, due to fluid diffusion, as the ones reported by Rossi et al . (2016, 2018), on the basis of GNSS observations, occurred also in the past. In the present work, we considered the time series of four of the tiltmeter sites, which guar- anteed continuous recordings from 1979 to 1996: Grotta Gigante (GG), Villanova (VI), Cesclans (CE), and Invillino (IN). To identify the presence of transient signals, we used the Empirical Mode Decomposition (EMD) method, a technique capable of decomposing noisy and not necessarily stationary signals in a certain number of modes (IMFs) with no assumptions about the statistics of the signal (Huang et al ., 1998). The IMF 8 resulting from the analysis of the tiltmeter data shows a coherent signal, lasting about 2.5 years, in the period 1983-1990. The onset time of the signal increases from VI to CE and IN, west of VI, and to GG, east of VI (Fig. 1). There is a substantial agreement between the directions of the transient-induced tilt, the directions of the main tectonic features, and the directions in which the background seismicity tends to align (Bressan et al ., 2016). With classical seismology and ray-tracing techniques, we located the signal’s source and retrieved the propagation velocity. The source resulted at a depth of about 6 km, a few kilometers from the hypocenter of the Uccea earthquake (10 February 1983, M D = 4.2) on the Italy-Slovenia border, being the origin time the end of November 1982. The propagation velocity obtained varies between 6.4 and 27 km/year. That these velocities are typical of a hydraulic diffusion phenomenon is also confirmed by the results of hydraulic tomog- raphy: the values ​are between 0.1 10 -3 and 3.4 10 -2 m 2 / s, resulting in permeability values ​for the formations crossed by the rays in excellent agreement with the literature data. At the origin of the fluid release, there are overpressure conditions at depth, with effective stress equal to about 19 MPa, and pore pressure ~ 0.88 the lithostatic one. It should be noted that independent observations carried out in the past (Mao et al ., 1989) on temporal variations of seismic velocities and the response to the earth’s tide have highlighted variations in the ve- locity of the seismic waves, in the ratio v p / v s and the elastic parameters in the same years and with a wavelength similar to the transient here treated. These results add to the observations on the data of the GNSS stations in 2006-2009, on a transient signal, attributable to the diffusion of fluids from a source located under the Bovec basin (Slovenia), about 6 km from the Bovec-Krn earthquake of 12 July 2004, M w = 5.1, as of March 2004 (Rossi et al ., 2018, 2017). Propagation velocity and hydraulic diffusivity are in agreement with the values ​obtained in the present work. Also in this case, the independent observation on a quiescence in the three months preceding the seismic event (Gentili, 2010) could confirm a state of overpressure, preventing earthquake occurrence in that period. These various observations lead to hypothesize that in the study area, the faults may occa- sionally act as valves (Sibson, 1992), releasing fluids trapped at depth to the surrounding region in the form of waves of variation in porosity and pore-fluids. The tectonic style of the area on the