Journal of Seismology 3: 105–117, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands. 105 The April 1996 Irpinia seismic sequence: Evidence for fault interaction M. Cocco, C. Chiarabba, M. Di Bona, G. Selvaggi, L. Margheriti, A. Frepoli, F. P. Lucente, A. Basili, D. Jongmans 1 & M. Campillo 2 Istituto Nazionale di Geofisica, Rome, Italy; 1 LGIH, University of Liege, Belgium; 2 LGIT, University of Grenoble, France Received 13 February 1998; accepted in revised form 5 October 1998 Key words: Fault Friction, Aftershocks, Fault Interaction, Seismicity Pattern, Ground Motions, Source Parameters Abstract The analysis of the Irpinia earthquake of 3 April 1996 (M L = 4.9), based on strong motion and short period local data, shows that it was a normal faulting event located within the epicentral area of the M S 6.9, 1980, earthquake. It was located at 40.67 ◦ N and 15.42 ◦ E at a depth of 8 km. The local magnitude (4.9) has been computed from the VBB stations of the 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. Spectral 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 these values of seismic moment and stress drop is 1.3 km. The focal mechanism has two nodal planes having strike 297 ◦ , dip 74 ◦ , rake 290 ◦ and strike 64 ◦ , dip 25 ◦ and rake 220 ◦ , respectively. A fault plane solution with strike 295 ◦ ± 5 ◦ , dip 70 ◦ ± 5 ◦ , and rake 280 ◦ ± 10 ◦ is consistent with the S-wave polarization computed from the strong motion data recorded at Rionero in Vulture. We discuss the geometry and the dimensions of the fault which ruptured during the 1996 mainshock, its location and the aftershock distribution with respect to the rupture history of the 1980 Irpinia earthquake. The distribution of seismicity and the fault geometry of the 1996 earthquake suggest that the region between the two faults that ruptured during the first subevents of the 1980 event cannot be considered as a strong barrier (high strength zone), as it might be thought looking at the source model and at the sequence of historical earthquakes revealed by paleoseismological investigations. Introduction In areas where large magnitude earthquakes (M > 6.0) have long repeat times, the study of moderate magnitude seismicity (4.5 <M< 6.0) plays an im- portant role for the understanding of the seismogenic behavior of active faults. This is true for the South- ern Apennines, where the single faults have recurrence times of thousands of years (Pantosti et al., 1993), and large magnitude earthquakes occur along the seismo- genic belt few times within a century (Boschi et al., 1995). The Southern Apennines seismogenic belt consists of different fault segments along which normal fault- ing earthquakes occur (Valensise et al 1993; Nostro et al., 1997). The best known is the Irpinia fault that ruptured during the 1980 earthquake. This earthquake ruptured several segments, whose geometry has been determined in detail by investigating different geo- physical data (Deschamps and King, 1983 and 1984; Westaway and Jackson, 1987; Bernard and Zollo, 1989; Pantosti and Valensise, 1990; Pingue and De Natale 1993; Cocco and Pacor, 1993). Figure 1 shows a map of the Irpinia fault system with the segments that ruptured during the 1980 earthquake, its focal mecha- nism and the distribution of aftershocks (shown in the map and in the cross-section A–A’ in the top right- hand corner) relocated by Amato and Selvaggi (1993). The segment dimensions and seismic moments are summarized in Table 1. According to the literature published, we refer to the three subevents of the 1980 event as 0s, 20s and 40s, respectively. The total seis- mic moment of this earthquake ranges between 1.3 and 3.0 10 26 dyne cm (Giardini, 1993). jose110.tex; 13/01/1999; 23:17; p.1 Article: jose110 Pips nr. 193990 GSB: 702069 (josekap:bio2fam) v.1.1