SEISMOTECTONIC STUDY OF THE FRIULI (ITALY) AREA BASED ON TOMOGRAPHIC INVERSION AND GEOPHYSICAL-DATA

The upper crustal structure of the Friuli area involved in the seismic sequence which started on May 6, 1976 (main shock M(L) = 6.4) is investigated, together with the distribution of seismicity, in order to define the main seismotectonic features. Three-dimensional P-velocity modelling is obtained by tomographic techniques using local earthquakes recorded by the seismic network of the Osservatorio Geofisico Sperimentale. Lateral velocity variations and velocity inversions are related to structural heterogeneities. The shortened and thrusted sedimentary cover is detached from the basement; the 3-D velocity structure shows a southward verging, thrusted wedge basement below 5 km depth. Analysis of the distribution of earthquakes, together with three-dimensional velocity modelling, shows that active faulting is represented by north-dipping thrusts uplifting the basement wedge. This high-velocity block forms the main seismogenic zone.