The April 1996 Irpinia seismic sequence: Evidence for fault interaction

The analysis of the Irpinia earthquake of 3 April 1996 (M-L = 4.9), based o n strong motion and short period local data, shows that it was a normal fau lting event located within the epicentral area of the M-S 6.9, 1980, earthq uake. It was located at 40.67 degrees N and 15.42 degrees E at a depth of 8 km. The local magnitude (4.9) has been computed from the VBB stations of t he MedNet network. The moment magnitude is M-w = 5.1 and the seismic moment estimated from the ground acceleration spectra is 5.0 10(23) dyne cm. Spec tral analysis of the strong motion recordings yields a Brune stress drop of 111 bars and a corner frequency of 1 Hz. The source radius associated to t hese values of seismic moment and stress drop is 1.3 km. The focal mechanis m has two nodal planes having strike 297 degrees, dip 74 degrees, rake 290 degrees and strike 64 degrees, dip 25 degrees and rake 220 degrees, respect ively. A fault plane solution with strike 295 degrees +/- 5 degrees, dip 70 degrees +/- 5 degrees, and rake 280 degrees +/- 10 degrees is consistent w ith the S-wave polarization computed from the strong motion data recorded a t Rionero in Vulture. We discuss the geometry and the dimensions of the fau lt which ruptured during the 1996 mainshock, its location and the aftershoc k distribution with respect to the rupture history of the 1980 Irpinia eart hquake. The distribution of seismicity and the fault geometry of the 1996 e arthquake suggest that the region between the two faults that ruptured duri ng the first subevents of the 1980 event cannot be considered as a strong b arrier (high strength zone), as it might be thought looking at the source m odel and at the sequence of historical earthquakes revealed by paleoseismol ogical investigations.