A decoupled kinematic model for active normal faults: Insights from the 1980, MS = 6.9 Irpinia earthquake, southern Italy

Abstract

A significant uncertainty exists in the definition of both surface pattern and subsurface continuity (i.e., coupling vs. decoupling) of active normal faults in the Apennines. In this study, we investigated the epicentral area of the M S = 6.9, 1980 Irpinia earthquake—one of the most destructive historical earthquakes in Italy—based on detailed topography analyses, morphostratigraphic and structural data, and new age constraints from Quaternary deposits. The active tectonic behavior of the study area is controlled by a series of subparallel, mainly WNW-ESE– to NW-SE–trending, dominantly extensional faults spanning over the southern Apennines axial belt. A large part of the active fault strands is characterized by a subdued topographic expression, as a result of the young age of extensional faulting initiation, and of relatively low mean slip rates. In addition, as already known from long historical seismicity records elsewhere, long-lasting quiescence might alternate with clusters of closely spaced, strong earthquakes. The long-term morphostratigraphic record confirms that long-lasting quiescence may punctuate fault activity, with major implications for seismic hazard assessment. The relatively small-sized cumulative fault throws estimated by surface evidence contrast with subsurface information provided by crustal-scale cross sections and seismological evidence, which both suggest the occurrence at depth of large-displacement, mature fault zones capable of nucleating large earthquakes. Furthermore, although the surface distribution of active fault strands overlaps the belt affected by present-day low-magnitude seismicity and by large historical earthquakes, a mismatch in the attitude and kinematics of shallow versus deep faults is unraveled by a comparison of surface geological versus seismological data sets. This feature suggests a decoupling between surface and deep fault zones, and that outcropping fault planes cannot always be straightforwardly traced down to hypocentral depths, particularly in fold-and-thrust belts characterized by strong rheological contrasts. On the other hand, stress inversion from outcropping active faults and from earthquake focal mechanisms indicates a general consistency of the stress field, thus suggesting that a homogeneous late Quaternary extensional regime produces complex reactivation of the inherited, articulated fault network affecting different structural levels of the southern Apennines.