Ground Motion Amplification and 3D P-Wave Velocity Model of the Crati Valley Graben (Italy)

The Crati valley (Calabria, Italy) is a densely populated area characterized by high seismic hazard, as inferred from the strong earthquakes that occurred in the past centuries and from the tectonic structures that border the graben. The valley extends in the NS direction between Sila massif to the east and Catena Costiera to the west. In recent years the area has been characterized by low to medium magnitude earthquakes, mostly located in the upper crust. In this work we analyzed local and regional earthquakes and seismic noise to study the seismic response and the upper crust structure in the investigated area. We analyzed 12 h of seismic noise and 24 regional earthquakes to compute the horizontal-to-vertical spectral ratio (HVSR) at all seismic stations available in the area. Results show that the two sites located on the sediments within the low-velocity anomaly are characterized by a high HVSR peak of amplitude 4 at frequency of about 0.27 Hz, which likely corresponds to the seismic resonance of the sediment-filled basin. At the same two sites, the amplification factor computed with the Standard Spectral Ratio (SSR) method applied to regional earthquakes shows values up to 6 in the frequency range 0.3-1.0 Hz. We used a selection of 332 local earthquakes in the magnitude range 0.6-4.4 to compute a high-resolution P-wave 3D velocity model of the Crati valley and surrounding terranes. The tomographic analysis shows results with a good resolution down to at least 15 km depth. A strong low-velocity anomaly at shallow depths in the well resolved sector of the valley coincides with the sediment-filled graben, the most important geological feature of the area. The results of this work provide a valuable information to be considered in the estimation of local seismic hazard.