Investigating crustal structure of the Calabria-Lucania border region from local earthquake tomography

We present a new seismic velocity model for the Calabria-Lucania border region, a sector including the Pollino Mts. area where a seismic sequence of thousands of small to moderate earthquakes has been recorded between 2010 and 2013 (Totaro et al., 2013). This area was previously hypothesized as seismic gap on the basis of paleoseismological evidences associated with the lack of major earthquakes in historical catalogues (Michetti et al., 2000; Cinti et al., 2002). We performed the tomographic inversion selecting ca. 3600 earthquakes that have occurred in the last thirty years and recorded by permanent and temporary networks managed by INGV and Calabria University. By applying for the first time the Local Tomography Software for passive tomography inversion (LOTOS hereinafter; Koulakov, 2009)) to crustal analysis in southern Italy, we have computed the distribution of Vp, Vs, and the Vp/Vs ratio. The obtained velocity model, jointly evaluated with results of synthetic modeling, as well as with the hypocenter distribution and geological information, has furnished new constraints on the geodynamical and structural knowledge of the study area. The comparison between the shallow tomography sections and surface geology shows good correlation between velocity patterns and the major geological features of the study area. Main results evidence that in the upper crust a low-velocity anomaly of P- and S-waves is detectable beneath the Pollino Mts. area and seems to separate the Calabrian and southern Apennines domains, characterized by higher velocities. At this depth, the distributions of high Vp/Vs ratio, representing strongly fractured rocks with likely high fluid content, clearly correlates with areas of significant seismicity. In the middle and lower crust we detect a clear transition from high to low seismic velocities in correspondence with the Tyrrhenian coast of the study area, which may represent the transition from the thinner Tyrrhenian crust to the thicker one beneath Calabria. The investigated area is located at the northern edge of the in-depth continuous Ionian subducting slab where the generation of a Subduction-Transform Edge Propagator (STEP; Govers & Wortel, 2005) fault zone, that laterally decouples subducting lithosphere from non-subducting one, may have taken place in the past (Catalano et al., 2001; Rosenbaum et al., 2008). In this framework, the low velocity anomaly detected at shallow depth beneath the Pollino Mts. area and showing a dimensional enlargement with depth seems to suggest that the structural systems that eventually worked like a STEP zone at the northern edge of the Ionian subducting slab may have opened the route to a possible fluid upwelling.