JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. B4, PAGES 6561-6577, APRIL 10, 1993 Paleoseismology Along the 1980 SurfaceRupture of the Irpinia Fault' Implications for Earthquake Recurrence in the Southern Apennines,Italy DANIELA PANTOSTI Istituto Nazionale di Geofisiea, Rome, Italy DAVID P. SCHWARTZ U.S. Geological Survey, Menlo Park, California GIANLUCA VALENSISE Istituto Nazionale di Geofisiea, Rome, Italy The Irpinia fault was the source of the Ms6.9 1980 Irpinia earthquake and produced the first unequivocal historical surface faulting in Italy. •renching of the 1980 fault scarp at Piano di Pecore, a flat intermontane basin about 5 km south of the 1980 instrumental epicenter, provides the first data on earthquake recurrence intervals, slip per event, and slip rate on a major normal fault in the Southern Apennines fault zone. The trenches exposed evidence of four pre-1980 palcoearthquakes that occurred during the past 8600 years. A best estimate average recurrence interval is 2150 years, although the time interval between individual events varies by as much as a factor of 2. Each paleoearthquake is similar to the 1980 surface rupture in amount of slip and style of deformation, which suggests that the 1980 event is characteristic for the Irpinia fault. Slip per event values average61 cm. The net vertical displacementof 2.12-2.36 m since8600 cal year B.P. observed in the trenches givesa vertical slip rate of 0.25-0.35 mm/yr, a dip slip rate of 0.29-0.40 mm/yr, and an extension rate of 0.14-0.20mm/yr. Althoughfault behaviordata are only available for the Irpinia fault they provide a starting point for evaluating earthquake recurrence and rates of deformation in southern Apennines. They suggest that (1) fault specific earthquake recurrence intervals based on the kistorical seismic record overestimates the occurrence of large magnitude (M7) earthquakes and (2) the Holocene rate of extension across the Apennines is 51 mm/yr. The 1980 earthquake and the palcoseismologic observations show that repeated and localized surface faulting occurs in southern Apennines and leaves subtle but distinct geomorphicevidence that can be detected with detailed and careful investigation. INTRODUCTION Despite the high level of seismicity in the Mediterranean basin, the precise locations of the major active faults in the Italian peninsula are largely unknown. In this region, cormnonlyused geomorphic indicators of Holocenetectonic activity such as fault scarps, offset streams, and linear features in young deposits are difficult to identify [Boschi et al., 1990]. The November 23, 1980, Irpinia normal- faulting earthquake (M$=6.9) was the first Italian event for which unequivocal surface faulting was documented [Westaway and Jackson, 1984; Pantosti and Valensise, 1990]. Along with the 1980, E1 Asham, Algeria (M$-7.3 [Yielding et al., 1981;Philip and Meghraoui, 1983])and the 1981, Corinth, Greece (M$=6.6 [Jackson et al., 1982; Vita- Finzi and King, 1985])earthquakes, the Irpinia earthquake is one of the few examples of historical surface faulting in the Mediterranean region. Mapping the extent of and displacement along the 1980 fault scarp (Figure 1) provided significant information on both fault geometry and rupture historyof the earthquake [Pantosti and Valensise, 1990]. These surface data are consistent with seismologic and geodetic observations and were used to reduce the nonuniqueness of several rupture models based on a variety Copyright 1993 by the American Geophysical Union. Paper number 92JB02277. 0148-0227/93/92JB-02277505.00 of instrumental datasets. As a result, the causative fault of the 1980 earthquake, which we name the Irpinia fault, has become the best understood active fault in Italy. Unlike other areas of comparably high seismicity, such as the western United States and Japan, and in spite of a great need to reliably quantify the seismic hazard, few palcoseismological investigations have been conducted in Mediterranean countries. The 1980 earthquake presented a unique opportunity to conduct the first exploratory trenching of a historical fattit scarp in Italy. We describe the results of this trenching across the 1980 scarp at Piano di Pecore,5 km south of the 1980 epicenter (Figure 2). The trenchesprovide information on the timing of past earthquakes, recurrence intervals, slip per event, and slip rate for the Irpinia fault. These data are used to discussaspects of contemporary normal faulting, as rates of deformation and recurrence of M7 earthquakes, and the Quaternary evolution of the southern Apennines. THE 1980 EARTHQUAKE The 1980normal-faulting earthquake (Ms =6.9) affected a broad and mountainousarea of the southernApennines 60- 80 km eastof Naples (Figure1). High-quality seismological data recorded at local, regional, and teleseismic distances, together with two geodetic leveling lines and observations of surface deformation, provided a data set that was used to investigate the rupture history and faulting geometry of this event [ Westaway and Jackson, 1984, 1987;Bernardand 6561