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 Holocene surface-faulting earthquakes on this fault: the most recent earthquake occurred in the Middle Ages between 860 and 1300 A.D., the penultimate occurred about 1900 B.C. or shortly after, and the oldest probably occurred between 3300 and 5000 B.C. Although the most recent surface faulting earthquake occurred during historical time, no evidence for it have been found in the historical record. Slip per event ranges between 2 and 3 m, and the length of the rupture is at least 12-20 km, suggesting M 6.5-7.0 for the paleoearthquakes. The dip-slip rate determined from trenching is 0.7-1.2 mm/yr and is consistent with the long-term slip rate of 0.9-2.5 mm/yr (lower values preferred) obtained from 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 seismogenic faults of the central and southern Apennines. The lack of mention or mislocation of the most recent event on the Ovindoli-Pezza fault in the historical record of the past two millennia should be attributed mainly to the unsettled cultural and social conditions and poor economic state that characterized the Middle Ages, especially in thinly populated regions such as the central Apennines. This example highlights an intrinsic limit of the historical data and raises the possibility that other regions considered ''seismically quiet'' on the basis of a long historical record may in reality have had large earthquakes that were not recorded.