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Historical earthquakes of the Gargano Promontory, an uplifted foreland sector in southeastern Italy, have been usually regarded as generated by inland faults. Some have been associated with activity of the Mattinata fault, a section of a regional east-west shear zone. The 10 August 1893 MW 5.4 event is one such earthquake, but its current onshore location is only loosely based on the damage pattern. Regions that were hit by offshore earthquakes are also known to be affected by a methodological bias such that offshore historical events appear to be located onshore. To test this condition for the 1893 earthquake, we pursued an alternative hypothesis for its location. The earthquake occurred near the Gondola fault zone, a right-lateral active fault system representing the offshore counterpart of the Mattinata fault and hence cap- able of producing sizable earthquakes along the Gargano coast. We focused on its wes- ternmost segment, suggesting that it could be the causative fault of the 1893 earthquake (in agreement with both the damage distribution and reported environmental effects). The approach we present works side by side with the recent developments of the algorithms used to compile historical catalogs, providing a fine-scale, geologically based method to define or confirm the dubious location of historical earthquakes. Marine paleoseismology is a new field stemming from the increased capabilities of high-resolution marine techniques in supporting classical paleoseismological analyses for the exploration of the seismogenic potential of offshore faults. Based on Late Pleistocene and Holocene individual or cumulative earthquake records, the potential of offshore faults can now be constrained in terms of expected magnitude and recur- rence intervals. We stress the importance of revisiting historical earthquakes in coastal zones using marine paleoseismological data to assess regional seismic hazard, particularly in tectonic settings where regional-size seismogenic areas straddle the onshore and the onshore-offshore zone. [ABSTRACT FROM AUTHOR]

The 31 October and 1 November 2002 Molise earthquakes (both Mw 5.7) were caused by right-lateral slip between 12 and 20 km depth. These earthquakes are the result of large-scale reactivation of preexisting, left-lateral, regionally extended east-west structures of Mesozoic age. Although recorded ground motions were generally smaller than expected for typical Italian earthquakes, a recent paper attributes a stress drop as high as 180 bars to the Molise earthquakes. We remark that a high stress drop is in contrast both with the relatively long source duration inferred in previous investigations and with geodetic evidence for a significantly smaller fault slip compared with other Apennine earthquakes having similarly large rupture area (e.g., 1997 Umbria-Marche earthquakes). We analyzed both ground acceleration spectra of the mainshocks and single-station spectral ratios of broadband seismograms in an extended magnitude range (2.7 ≤ Mw ≤ 5.7). Our results show that neither the spectral amplitudes of recorded ground motions nor the spectral ratios can be fit by a high stress-drop source. Instead we find that the observations are consistent with a low stress drop, our best estimates ranging between 6 and 25 bars, in agreement with the relatively long source duration and small coseismic slip. We interpret the low stress of the 2002 Molise earthquakes in terms of lower energy release mechanisms due to the reutilization of faults reactivated opposite to their original sense of slip. [ABSTRACT FROM AUTHOR]

On 16 December 1857, a strong earthquake (M ~7) 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 (Mw 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. [ABSTRACT FROM AUTHOR]

We revisited data related to the 1456 seismic crisis, the largest earthquake to have ever occurred in peninsular Italy, in search of its causative source(s). Data about this earthquake consist solely of historical reports and their intensity assessment. Because of the age of this multiple earthquake, the scarcity and sparseness of the data, and the unusually large damage area, no previous studies have attempted to attribute the 1456 events to specific faults. Existing analytical methods to identify a likely source from intensity data also proved inappropriate for such a sparse dataset, since historical evidence suggests that the cumulative damage pattern contains at least three widely separated events. We subdivided the 1456 damage pattern into three independent mesoseismal areas; each of these areas falls onto east--west tectonic trends previously identified and marked by deep (>10 km) right-lateral slip earthquakes. Based on this evidence we propose (1) that the 1456 events were generated by individual segments of regional east--west structures and are evidence of a seismogenic style that involves oblique dextral reactivation of east--west lower crustal faults; (2) that each event may have triggered subsequent but relatively distant events in a cascade fashion, as suggested by historical accounts; hence (3) that the 1456 sequence reveals a fundamental but unexplored mechanism of tectonic deformation and seismic release in southern Italy. This style dominates the region that lies between the northwest--southeast system of large extensional faults straddling the crest of the southern Apennines and the buried outer front of the chain. Although the quality of the available information concerning the 1456 earthquake is naturally limited, we show that the overlap of the damage distribution, the orientation and characteristics of regional tectonic structures, the seismicity patterns, and the focal mechanisms all concur with our interpretations and would be difficult to justify otherwise. [ABSTRACT FROM AUTHOR]