3-DIMENSIONAL P-VELOCITY STRUCTURE IN THE REGION OF THE MS=6.9 IRPINIA, ITALY, NORMAL FAULTING EARTHQUAKE

We have calculated the three-dimensional velocity structure in the epicentral region of the 1980 Irpinia (southern Italy) earthquake using Thurber's inversion technique for hypocenter and velocity parameters. Particular care was given to the evaluation of the resolution of the models, because of the poor reading accuracy of the analog stations operating at that time. We analyzed the averaging vector for each node of the model, in order to estimate the degree of smearing within the final tomographic image. This approach is suitable for determining the best parametrization to use (i.e. the minimum grid spacing) and for evaluating the region where the computed velocities are meaningful. The results show that the main rupture occurred along a sharp NW-SE trending velocity contrast, which probably reflects a crustal discontinuity. The velocity variations along the fault, probably due to lithological heterogeneities, account for the complex character of this earthquake. Based on the velocity distribution at depth, the secondary rupture that started about 40 s after the mainshock is interpreted to be the result of reactivation of an old thrust as a normal fault.