Spatial organization of seismicity and fracture pattern in NE Italy and W Slovenia

The study focuses on the spatial organization of seismicity and the relation between fracture pattern and earthquakes in the Friuli (north-eastern Italy) and western Slovenia seismic regions. The structural setting is characterized by a complex structure resulting from the superposition of several tectonic phases that generated NW-SE trending Dinaric faults and about E-W trending Alpine faults. The upper crust is characterized by lithological and mechanical heterogeneities. The fractal analysis shows that, in general, the seismicity only partially fills a plane. Only in a few cases, the earthquakes distribute on planar structures. The orientation of planes that fit through the hypocentres shows a different disposition at the two depth intervals analysed. The shallower interval (0-10 km) is characterized by planes with highly variable orientations. The spatial seismicity is investigated in the context of a general damage model, represented by the crack density distribution. The results evidence that the seismicity appears mostly located along sharp transition areas from low crack density to higher crack density, i.e., from zones of low damage to zones of intermediate damage. These zones are characterized by high heterogeneity due to the superposition of different tectonic phases and by the maximum interference between Dinaric and Alpine domains. The orientation of the planes fitting the seismicity at 10-20-km depth appears less dispersed, coinciding with the trend of Dinaric sub-vertical faults in the northern and eastern parts of the study area, and with Alpine low-angle faults in the western and southern parts.