Geodetic strain in peninsular Italy between 1875 and 2001

We determine geodetic strain in peninsular Italy by the
GPS reoccupation of the first order triangulation network of
Italy installed from 1860. The uncertainties in the original
measurements (about 3 ppm), and the time span between the
two observations, imply that tectonic signals larger than
about 0.03 ppm/yr are resolvable. Along the Apenninic belt,
where the largest earthquakes are concentrated, the geodetic
deformation has a clear and consistent strain pattern between
adjacent regions, well above the uncertainties, and shows a
pervasive NE-SW extension. Along the Tyrrhenian and
Adriatic coasts the geodetic signal is not homogeneous and
is comparable with the uncertainty in the original measurements.
Seismic deformation, calculated over the same time
interval, agrees well with estimated extensional direction,
but the magnitudes of geodetic and seismic strain differ
suggesting that, in part of the Apennines, significant strain
accumulation over the past 130 years may not have been
released in earthquakes.