Unraveling the complexity of the Pollino (Italy) seismic gap fault system.

<p>The Mt. Pollino area has been affected by a 4-year long seismic sequence, occurred between 2010 and 2014 and characterized by low-to-moderate seismicity and two moderate events (M_L4.3 and M_L5.0). The sequence developed as a combination of swarm-like and aftershocks. The two main earthquakes occurred late in the sequence, with a slow-slip event starting 3-4 months before the largest earthquake and lasting for a year. Despite the lack of historical and instrumental recordings of strong earthquakes (M>6), paleo-seismological investigations confirm the occurrence in the last 10,000 years of at least two M 6.5-7 earthquakes on the Pollino and Castrovillari faults, located in the SE sector of the Mt. Pollino area. Thus, the area has been marked as the widest high seismic hazard gap in Italy.</p><p>In this study we present the most recent advancements in the comprehension of the main peculiarities of the last seismic sequence and of its space and time evolution.&#160;&#160;&#160;</p><p>New local 3D P- and S-wave tomographic images offered a detailed picture of the main lithological units involved in the sequence and more reliable earthquake hypocenter locations. The inferred velocity contrasts have been compared with 2D scattering and absorption maps computed for the area, along with total direct wave attenuation. Clusters of events of similar waveforms (cross-correlation higher than 0.8) have been selected and located applying the master-slave relative location technique. New fault mechanisms have been computed. These mechanisms allowed modeling the local stress field and performing a Focal Mechanism Tomography. Its result was an evaluation of the excess of pore fluid pressure in the volume interested by the sequence. A 1D diffusivity analysis suggests a pore fluid pressure diffusion which, in addition to the Coulomb static stress transfer, can explain the delayed triggering of the two larger events.</p><p>This work has been supported by the CORE (&#8220;sCience and human factor for Resilient sociEty&#8221;) project, funded from the European Union&#8217;s Horizon 2020 - research and innovation program under grant agreement No 101021746 and by PRIN-MATISSE (20177EPPN2) project funded by Italian Ministry of Education and Research.</p>