PALEOSEISMICITY OF THE OVINDOLI-PEZZA FAULT, CENTRAL APENNINES, ITALY- A HISTORY INCLUDING A LARGE, PREVIOUSLY UNRECORDED EARTHQUAKE IN THE MIDDLE-AGES (860-1300 AD)

Geomorphic and trenching investigations along the Ovindoli-Pezza fault show that this normal fault is one of the major seismogenic faults in the central Apennines. We found clear geological evidence for three H olocene surface-faulting earthquakes on this fault: the most recent ea rthquake occurred in the Middle Ages between 860 and 1300 A.D., the pe nultimate occurred about 1900 B.C. or shortly after, and the oldest pr obably occurred between 3300 and 5000 B.C. Although the most recent su rface faulting earthquake occurred during historical time, no evidence for it have been found in the historical record. Slip per event range s between 2 and 3 m, and the length of the rupture is at least 12-20 k m, suggesting M 6.5-7.0 for the paleoearthquakes. The dip-slip rate de termined from trenching is 0.7-1.2 mm/yr and is consistent with the lo ng-term slip rate of 0.9-2.5 mm/yr (lower values preferred) obtained f rom displaced geomorphic features. Trench data combined with long-term slip rate estimates suggest the recurrence interval is longer than a millennium and possibly as long as 3300 years. The time elapsed since the most recent earthquake is 700-1130 years. The seismic behavior of the Ovindoli-Pezza fault is consistent with other well-known seismogen ic faults of the central and southern Apennines. The lack of mention o r mislocation of the most recent event on the Ovindoli-Pezza fault in the historical record of the past two millennia should be attributed m ainly to the unsettled cultural and social conditions and poor economi c state that characterized the Middle Ages, especially in thinly popul ated regions such as the central Apennines. This example highlights an intrinsic limit of the historical data and raises the possibility tha t other regions considered ''seismically quiet'' on the basis of a lon g historical record may in reality have had large earthquakes that wer e not recorded.