Contribution of controlled source seismology to the study of seismogenesis: examples from the Italian transitional area

The shallow seismicity in the Italian region is dependent on the lithospheric and, especially, on the crustal structure. The major contribution to the definition of crustal features as well as of their lateral variations, which, in this area, are very large, has been given, so far, by the profiles of deep seismic soundings (wide-angle reflection). It has been found that the shape of the velocity-depth functions is a good indicator of the crustal type.This study illustrates the comparison of the velocity functions with the depth distribution of seismic foci. The analysis was done in active seismic zones, where the hypocentral depth is known with fair accuracy; both the events contained in the general catalogue of Italian earthquakes and the micro-earthquakes were used.The chosen zones are located in different geological settings. It has been found that different shapes of the velocity-depth curve correspond to different depths of the most active seismic layer. The role of velocity inversions seems particularly meaningful to mark the transition from the brittle to the ductile regime when the strongest seismicity is in the upper crust. When the lower crust or the upper mantle appears to be the source of the main seismicity, different shapes of the velocity functions are observed. Finally, a further different type of velocity distribution is obtained in zones of high geothermal flow, where only very shallow and low magnitude events are recorded.