Evidence for localized active extension in the central Apennines (Italy) from global positioning system observations

To assess how contemporary crustal extension is accommodated in the central Apennines, we
use a new continuous and survey-style global positioning system velocity solution and model the velocity field using a bicubic spline interpolation method. The partitioning of contempo- rary deformation over the ~100-km-wide central Apennines belt reveals a pattern of strain accumulation that largely reflects the spatial distribution of historic and recent seismicity. The highest gradients of horizontal velocities are observed across those faults associated with M > 6 historical earthquakes. Dislocation modeling shows that interseismic elastic loading, in which creep occurs below the seismogenic upper crust on the downdip extensions of histori- cally active faults, reproduces the observed velocity gradients. The current resolution level of Quaternary fault slip rates estimates hinders the comparison with past deformation patterns and, in particular, the discrimination between (1) migrating episodes of short-term focused activity, (2) a distributed pattern of simultaneous deformation on parallel fault systems, or (3) long-term localization of active extension. Taking into account geomorphological evidence, we propose that the geodetically observed deformation spatially corresponds with a long-term localization of strain along the long-wavelength (>100 km) topographic bulge caused by its highest gravitation potential energy relative to surrounding lowlands.

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