Seismic moment summation for historical earthquakes in Italy: Tectonic implications

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Abstract
Tectonic deformation rates in and around the Apennine mountains of Italy are studied using seismic moments estimated from macroseismic effects of historical earthquakes. Northeastward extension in the northern Apennines (north of ∼42.5°N) accompanies shortening along their northeast flank. Since the seventeenth century, the sparse seismicity in these two zones has included no earthquake with magnitude >6.5 or seismic moment above ∼6×1018 N m. Their spatially averaged deformation rates are only ∼0.3 mm yr−1, but are equal, such that the extension and shortening balance with no relative motion between their external surroundings. In contrast, the numerous historical earthquakes in the central and southern Apennines with magnitude ∼7 and seismic moment ∼20×1018 N m require northeastward relative velocity across the deforming zone up to ∼5 mm yr−1, matching the expected relative motion of their surroundings. This relative velocity is taken up in part by extension and in part by strike‐slip, depending on the trend of the deforming zone, and is not accompanied by en echelon shortening elsewhere in Italy. The northern Apennines thus show different senses and rates of deformation from localities farther south, and are thus technically distinct at present, in contrast with previous interpretations.