GNGTS 2017 - 36° Convegno Nazionale

GNGTS 2017 S essione 1.1 23 Vettore fault. As regard the two October events, the 26th October Mw 5.9 event ruptured the northern Mt. Bove fault segment, while the 30th October Mw 6.5 main shock ruptured the segment of the Mt. Vettore fault that had remained unbroken after the previous large events. Moreover, our results indicate that some extra slip occurred on secondary fault structures, related to the activation of normal faults antithetic to the main fault system, and/or to an extensionally reactivation of pre-existing compressional structure in the activated crustal volume. Finally, the preliminary modelling of the 18th January M>5 earthquakes suggests the activation of a portion of the Mt. Gorzano fault. A retrospective analysis of the normal fault systems activated in the last 20 years in the Central and Northern Apennines L. Chiaraluce, M. Cocco Istituto Nazionale di Geofisica, Roma, Italy In the last 20 years, three seismic sequences (Colfiorito 1997; L’Aquila 2009 and Amatrice- Norcia-Visso 2016-2017) occurred in between the northern and central Apennines of Italy, activating a NW-trending and 130 km long, contiguous normal fault system composed by a set of 12-18 km long fault segments rupturing the first 10 km of the upper crust. We will exploit this exceptional opportunity to explore the processes driving such complex earthquake sequences at high spatial resolution and displaying relevant similarities as well as fundamental differences in their preparatory phase, seismicity pattern and fault complexity. Indeed, the anatomy of these fault systems shows heterogeneity of rupture histories and complexity of activated fault segments raising questions on our understanding of long term segmentation, strain partitioning and dynamic control of coseismic ruptures. We will interpret the imaged rupture histories during moderate magnitude earthquakes and seismicity evolution to discuss the similarities between seismological observations (faults generating earthquakes and recognised by aftershocks distribution) and Quaternary geological fault structures (faults mapped at the surface) pointing out their geometric and kinematic correspondence. In addition to this, thanks to the high resolution achieved in reconstructing the faults anatomy by locations of small magnitude earthquakes (local magnitude > 0.5), we can now investigate and characterize the fault zone structure and its evolution, an essential factor for better understanding earthquake mechanics and rupture evolution. We have observed that the seismological fault zone thickness (0.5-1.0 km; computed by means of earthquakes distribution) is comparable with values derived from geological observations made on fault outcrops. Aftershocks distribution also shows how toward the surface conjugate sets of faults are connected with the main fault plane, which with depth shows bending and dilational jogs. These strong similarities between seismological and geological images of fault structure indicate that earthquakes have a key role in the evolution of fault architecture. We will also discuss the differences between native normal faults composed by planar segments rupturing the whole seismogenic layer and normal faults rupturing (and inverting) inherited thrusts with the tendency of flattening at depth along sub-horizontal horizons. Segmentation controlled by crustal heterogeneities and intersecting structures inherited from past tectonics seems to be very efficient. These older structures, often separating diverse geological domains, affect the evolution of seismicity and control dynamic rupture propagation and coseismic slip distribution. Also the persisting seismic gaps along the whole fault system highlighted by the lack of seismic activity, may be related to the presence of specific geological