The activation of Late Quaternary faults in the Central Apennines (Italy) could generate earthquakes
with magnitude of about 6.5, and the Monte Vettore fault system probably belongs to
the same category of seismogenetic faults. Such structure has been defined ‘silent’, because
of its geological and geomorphological evidences of past activation, but the absence of historical
records in the seismic catalogues to be associated with its activation. The ‘Piano di
Castelluccio’ intramountain basin, resulting from the Quaternary activity of normal faults, is
characterized by a secondary fault strand highlighted by a NW–SE fault scarp: it has been
already studied through palaeoseismological trenches, which highlighted evidences of Quaternary
shallow faulting due to strong earthquakes, and through a 2-D ground penetrating
radar (GPR) survey, showing the first geophysical signature of faulting for this site. Within
the same place, a 3-D GPR volume over a 20 × 20 m area has been collected. The collection
of radar echoes in three dimensions allows to map both the vertical and lateral continuity
of shallow geometries of the fault zone (Fz), imaging features with high resolution, ranging
from few metres to centimetres and therefore imaging also local variations at the microscale.
Several geophysical markers of faulting, already highlighted on this site, have been taken as
reference to plan the 3-D survey. In this paper, we provide the first 3-D subsurface imaging of
an active shallow fault belonging to the Umbria-Marche Apennine highlighting the subsurface
fault geometry and the stratigraphic sequence up to a depth of about 5 m. From our data,
geophysical faulting signatures are clearly visible in three dimensions: diffraction hyperbolas,
truncations of layers, local attenuated zones and varying dip of the layers have been detected
within the Fz. The interpretation of the 3-D data set provided qualitative and quantitative
geological information in addition to the fault location, like its geometry, boundaries and an
estimation of the fault throw.
