Pre-existing cross-structures and active fault segmentation in the northern-central Apennines (Italy)

Abstract

The multideformed axial zone of the Apennines provides a great opportunity to explore the influence of pre-existing cross-structures (inherited from pre-Quaternary tectonic phases) on the segmentation of Quaternary/active seismogenic extensional faults. Detailed geological and structural data and their comparison with seismological data show that although the attitudes (strike and dip) of oblique pre-existing faults are certainly an important factor in determining a segment boundary, the size of the inherited oblique structures seems to be more crucial. Pre-existing cross-structures with lengths ranging from several kilometers to a few tens of kilometers show a twofold behavior. They can act as segment barriers during the rupture of a single fault segment or they can be reactivated as transfer zones inducing the activation of two adjacent segments that belong to the same fault system. Regional basement/crustal oblique pre-existing cross-structures, with lengths ranging from several tens of kilometers to hundreds of kilometers (commonly NNE-striking), may act as “persistent structural barriers” that halt both fault segment and fault system propagation, thus determining their terminations and maximum sizes. In the northern-central Apennines, the NNE-striking Ancona–Anzio, Valnerina, and Ortona–Roccamonfina tectonic lineaments, although having been repeatedly reactivated since the Mesozoic, represent the most important examples of these structures. Moreover, probably due to their misorientation with respect to the present extensional stress field, regional NNE-striking pre-existing structures appear to be less likely to produce strong magnitude events (no surface evidence for Quaternary faulting has been found thus far and historical and instrumental seismicity shows only M < 6 events). M ~ 7 event, on the other hand, is more likely to occur along the (N)NW–(S)SE trending normal fault systems. Lastly, we propose a model that can explain the different sizes of fault segments and fault systems on the basis of their location with respect to the “persistent structural barriers” and their spacing. In this view, our results may contribute to a more reasonable assessment of the nature and size of future surface ruptures in the northern-central Apennines, which are of critical importance to estimating seismic hazard.