GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 1.1 93 portion of Elba Island in the Tyrrhenian sea, considered an older exhumed analogue of the ATF. Many field-geology based studies (Collettini and Holdsworth, 2004; Collettini et al. , 2006; Smith et al. , 2007, 2008) have investigated the geological setting, the microstructural characteristics, and the internal structure of the ZF, providing an accurate description of the fault zone architecture and proposing a model for the evolution for the fault zone and slip behavior. According to the model proposed by these Authors, the ZF experienced stable-sliding together with stick-slip processes (alternated and/or synchronous) along the low-angle and higher angle faults. Aseismic creep mostly occurred along lenses of chlorite-talc-rich phyllonites, placed along the ZF plane and characterized by low frictional strength (<<0.6), signifying a prevalent velocity-strengthening (i.e., creeping) behavior of the fault (Smith and Faulkner, 2010). While, the presence of critical fluid overpressure facilitated the rupture of the high-angle footwall faults as well as the development of tensile (i.e., hydro-fracture) ruptures along small patches (i.e., co-seismic slip events small magnitude events) located along both the ZF footwall and the low-angle fault core. The spatiotemporal seismicity pattern along the low-angle ATF strongly resembles the mixed-mode slip-behavior proposed for the ZF. In particular, stick-slip episodes are testified by small magnitude events that occur, sometimes with regularity (i.e., clusters of repeating events), along small fault patches. At the same time, stable-sliding occur at larger scale along the phyllitic portions of the low-angle fault, as testified also by the geodetic data. Finally, on the basis of empirical relations between fault dimension (1800 km 2 ) and maximum magnitude, we are conscious that the ATF could host up to a M7 event. Our observations are consistent with the occurrence of aseismic deformation along the ATF plane reducing the availability of strain energy, thus somehow lowering the seismic hazard associated to the ATF. However, we have to take in mind that thanks to dynamical frictional weakening processes, seismic ruptures can nucleate in small and locally well oriented locked fault portions and propagate through creeping portions even if embedded within velocity strengthening materials. These evidences advise that the ATF cannot be excluded in the assessment of the seismic hazard of this portion of the Northern Apennines. Acknowledgements We thank the TABOO working group for the management of the seismic network and for the data quality assessment and maintenance. References Axen G.J., 2004; Mechanics of low-angle normal faults. In: Karner, G.D., Morris, J.D., Driscoll, N.W. & Silver, E.A. (eds) Rheology and Deformation of the Lithosphere at Continental Margins. MARGINS Theoretical and Experimental Earth Science Series, 46–91. Anderlini L., Serpelloni, E., and Belardinelli, M. E. 2016; Creep and locking of a low-angle normal fault: Insights from the Altotiberina fault in the Northern Apennines (Italy ), Geophys. Res. Lett., 43, 4321–4329. Chiaraluce L., Chiarabba, C., Collettini, C., Piccinini, D., Cocco, M., 2007; Architecture and mechanics of an active low angle normal fault: The Alto Tiberina fault northern Apennines , (Italy) case study. J. Geophys. Res. doi:10.1029/2007JB005015. Collettini C., 2011; The mechanical paradox of low angle normal faults: current understanding and open questions , Tectonophysics, 510, 253–268, 2011. Collettini C. and Sibson R. H., 2001; Normal faults normal friction? Geology, 29, 927–930. Collettini C., and Barchi M.R., 2002; A low angle normal fault in the Umbria region (Central Italy): a mechanical model for the related microseismicity. Tectonophysics, 359, 97–115. Collettini C., Holdsworth R.E., 2004; Fault zone weakening processes along low-angle normal faults: insights from the Zuccale Fault, Isle of Elba, Italy . J. Geol. Soc., 161, 1039–1051. Collettini C., M.R. Barchi, L. Chiaraluce, F. Mirabella and S. Pucci, 2003; The Gubbio fault: can different methods give pictures of the same object?, J. Geodyn., 36, 51-66; ISSN:0264-3707. Collettini C., Niemeijer A., Viti C., and Marone C. J., 2009; Fault zone fabric and fault weakness, Nature, 462, 907-910. Collettini C., N. De Paola, R.E. Holdsworth, M.R. Barchi, 2006; The development and behavior of low-angle normal faults during Cenozoic asymmetric extension in the Northern Apennines, Italy , J. Struc. Geol., 28, doi:10.1016/ j.jsg.2005.10.003.