Upper-crust tomographic structure of the Central Apennines, Italy, from local earthquakes

We derived a three dimensional P and S waves velocity model of the first 9 km of crust in the Central Apennine from local earthquakes. The data set used for the tomographic inversion was recorded from three-component regional and local networks, which operated in the region from 1979 to 1998, and consists of 984 earthquakes. For these earthquakes we have at least eight phases, a gap smaller than 200 degrees, and a total RMS residual smaller than 0.6 s, giving a total of 13190 P and 10257 S phases first arrivals. The inversion is performed using the SIMULPS12 code; this technique iterates the estimated velocity model parameters, defined at the nodes of a grid, relocating the earthquakes with an approximate 3-D ray tracing. A large positive P- and S-wave velocity anomaly is observed in correspondence to the main seismogenetic volume of the Central Apennines. A synthetic test demonstrates that this anomaly is not artificially produced by the substantially higher density of earthquakes within the seismic velocity anomaly. This anomaly corresponds to the carbonatic rocks that form the core of the fold-and-thrust Central Apennines belt. These rocks are underlain by ductile evaporite formations, which were justaposed during the tectonic evolution of the belt. We suggest that the evaporite layers effectively decouple the upper-crustal seismogenetic volume from the middle and lower crust. We conclude that the accumulation of seismic energy could be induced by gravitational tectonic processes, explaining the fact that most of the seismicity is confined above the evaporites. (C) 2001 Elsevier Science Ltd All rights reserved.