GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 3.1 595 Vargas-Cordero I., Tinivella U., Accaino F., Loreto M.F. and Fanucci F.; 2010: Thermal state and concentration of gas hydrate and free gas of Coyhaique Chilean Margin (44°30- S) . Marine and Petroleum Geology, 27 , 1148-1156. Vargas-Cordero I., Tinivella U., Villar-Muñoz L. and Giustiniani M.; 2016: Gas hydrate and free gas estimation from seismic analysis offshore Chiloé Island (Chile). Andean geology, 43 , 263-274. https://dx.doi.org/10.5027/ andgeoV43n3-a02 Seismic imaging of seismogenic faults in the central Apennines (Italy) by high-resolution active source tomography F. Villani, L. Improta Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy The high seismogenetic potential of active normal faults in central Italy dramatically showed up in the past twenty years, due to the occurrence of three important seismic sequences that hit a region extending > 100 km along the chain axis, causing heavy damage, economic losses and >600 deaths (Mw 6.0 Colfiorito sequence, 1997; Mw 6.1 L’Aquila sequence, 2009; Mw 6.0 Amatrice 24 August 2016, Mw. 5.9 Visso 26 October 2016, and Mw 6.5 30 October 2016 Norcia earthquakes). Those strong events were generated by the activation of complex normal fault systems, with the subsequent involvement of a large number of segments in relatively short time intervals, and followed by tens of thousands of aftershocks. The geometry and kinematics of active fault structures can be gained through the integration of different kinds of datasets, such as seismological, geodetic, geophysical and geological data. However, some important details of the shallow structures may hardly be recognized using standard approaches. In this part of the chain the shallow subsurface of most normal fault segments is poorly known in terms of geometry, late Quaternary displacement and partitioning of deformation into subsidiary structures. For instance, the nature and extent of the source responsible for the 2009 L’Aquila earthquake was not established before the mainshock. Other active faults with the potential of generating strong earthquakes are not thoroughly characterized as well. This has important implications on the correct evaluation of the seismic hazard. High- resolution seismic profiling can provide invaluable constraints to better understand the recent behaviour of active faults. Here we report two examples of high-resolution tomographic images of active normal fault zones in the central Apennines. The first one is the 2009 L’Aquila earthquake fault, also known as Paganica Fault (Improta et al. , 2012; Villani et al. , 2017). The second one is the Piano di Pezza fault, a structure that is not related to recent and historical large earthquakes but that is known to have ruptured during M6+ earthquakes in Holocene times by paleoseismic data (Villani et al. , 2015). We use high-resolution refraction tomography performed by a non-linear inversion scheme, which is capable of imaging strongly heterogeneous media and may overcome typical drawbacks of shallow reflection profiling in continental settings (Improta et al. , 2002). In the case of the Paganica fault, we used a powerful Vibroseis source in order to reach investigation depths of 250-300 m. Our results provide high-quality 2D Vp-images with spatial resolution of nearly 10 m, which enable to depict the complex structure of the Quaternary basin developed in the fault hangingwall. We image the 2009 earthquake fault, moreover we recognized several unreported splays that displace the pre-Quaternary basement and the shallow layers. In the Piano di Pezza basin we performed a very-high resolution survey across the basin bounding fault, using a sledgehammer and 40 Hz geophones with 1 m shot spacing. The near- surface tomographic images have excellent spatial resolution (about 1 m) and enable us to infer