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Rise and Fall of a Hypothesized Seismic Gap: Source Complexity in the M_w 7.0 16 December 1857 Southern Italy Earthquake

Istituto Nazionale di Geofisica e Vulcanologia, Via di Vigna Murata 605, 00143 Rome, Italy burrato{at}ingv.it

On 16 December 1857, a strong earthquake (M7) struck a large portion of the southern Apennines about 150 km southeast of Naples. The earthquake was thoroughly investigated by Irish engineer Robert Mallet, who wrote an extensive report that is still regarded as a landmark in observational seismology. Because of the concentration of damage in the High Agri valley, and contrary to Mallet’s own findings, for many years the earthquake was referred to as the "Val d’Agri earthquake" and was believed by most investigators to have ruptured the 20–25-km normal fault lying beneath this intermontane basin. The magnitude of the earthquake, however, and evidence for earthquake complexity suggest that the true rupture length has been so far underestimated. We contend that the 1857 earthquake ruptured in a cascade fashion two adjacent and relatively well-known faults: first the smaller Melandro–Pergola fault, commonly believed to represent a seismic gap between the causative faults of the 1857 and of the 1980 Irpinia (M_w 6.9) earthquakes; and then the larger Agri valley fault proper. Contemporary chronicles reported a time lapse of 2–3 min between the two ruptures, thus effectively making them two independent shocks. The rupture must have proceeded unilaterally from the northwestern edge of the Melandro–Pergola fault, where Mallet placed the earthquake epicenter, thus explaining the concentration of damage—and attention by rescue crews and subsequent investigators—in the southeastern portion of the High Agri valley.