Exploring new seismic hazard scenarios in central Italy: hints about a previously unknown active normal fault highlighted by the Norcia 2016 (Mw 6.5) seismic sequence

Federica Ferrarini, Giusy Lavecchia, Rita de Nardis, Ramon Arrowsmith, Francesco Brozzetti, & Daniele Cirillo

Published August 15, 2017, SCEC Contribution #7698, 2017 SCEC Annual Meeting Poster #149

In 2016, one of the most energetic normal fault earthquake sequences, since the beginning of the last century, struck the central Italy. The seismic crisis started on August 24 with a Mw 6.0 event followed, in the months after, by several moderately energetic earthquakes (5.0≤Mw≤6.0) interesting an epicentral area of ~1500 km2. The energy release culminated on October 30 with the most energetic Norcia Mw6.5 earthquake. The seismicity activated the Mt Vettore-Mt Bove NNW-SSE striking normal fault system and the northern strand of the Mt Gorzano normal fault. These structures belong to the outermost west-dipping active normal fault alignment transecting the Apennine mountain belt. Evidence of coseismic fractures related to Mw>6.0 main shocks was extensively observed. Northward migration of seismicity was observed until January 18, 2017, when even the southern strand of the Mt Gorzano fault was struck by four 5.0≤Mw≤5.5 earthquakes.

In this study, we present the preliminary results of a multidisciplinary investigation carried out in the northernmost sector of the epicentral area. Here, following the October 26 Mw 5.9 event, both seismicity spatial distribution and DinSAR measurements revealed the possible activation of a fault source located in an eastern position relative to the Mt Vettore-Mt Bove fault system and unknown in the literature. We exploited a 3D analysis of available well-relocated hypocenters of the sequence in 3D MOVE, with the aim of visualizing the fault plane geometry responsible for the spatial distribution of the sequence. The reconstructed 3D fault geometry, the along strike seismicity distributions, together with the associated focal mechanisms (4.0≤Mw≤5.0), support the existence of a NNW-SSE striking, SW-dipping, normal fault extending along strike for about 20 km. Despite the lack of clear evidence of active normal faulting in the field, morphotectonic evidence (e.g. paleosurface tilting), as well as geomorphic indices derived from high resolution topography data analysis (e.g. stream gradient index; Ksn), support the conclusion about activity of the inferred structure in recent times. Our results, even though preliminary, suggest the recognition of a new seismogenic source possibly capable for Mw>6.0 earthquakes. Its existence would shift the eastern limit of the Quaternary extensional belt in central Italy and would provide insights for further investigation aimed at improving the seismic hazard assessment in this sector.

Citation
Ferrarini, F., Lavecchia, G., de Nardis, R., Arrowsmith, R., Brozzetti, F., & Cirillo, D. (2017, 08). Exploring new seismic hazard scenarios in central Italy: hints about a previously unknown active normal fault highlighted by the Norcia 2016 (Mw 6.5) seismic sequence. Poster Presentation at 2017 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology