Riguzzi, Federica

We analyze the multiyear strain cycle of a network of continuous GPS stations in the L’Aquila intra-mountain basin in Central Apennines, Italy. The basin is bounded by two aquifers (Nuria-Velino and Gran Sasso) hosted in two tectonically active mountain ranges. All stations show coherent displacements, mostly in the horizontal plane and orthogonal to the normal fault zones of the Apennine mountain range. The displacement repeats back and forth, roughly every 4–5 years with elongation up to 6 mm. The deformation occurs in the region bounded by the two aquifers and the displacement amplitude decreases with distance as predicted by two tensile sources acting in an elastic medium. We interpret this type of deformation as induced by the opening of pre-existing fractures related to two different fault systems as a response to hydrostatic pressure variations. The strain cycle follows the average multiyear rainfall cycle and foresees a dilatation phase during high rainy seasons and a contraction during drier periods. The estimated depth of the equivalent tensile sources ranges from 1.2 to 1.5 km affecting stations at distances up to 20 km away. GPS data show the cumulated movement induced by the two nearby aquifers and suggest a mutual interaction in the elastic space related to the different average annual rainfall and effective infiltration. We fit two tensile sources located along the main faults affecting the aquifers. The eastern source corresponds to the major active fault system affecting the Gran Sasso Range, while the western one includes the sub-parallel active faults of the Mt. Ocre in the Nuria-Velino Range, where carbonate rocks show a higher structural permeability and effective infiltration. These fault systems affect Jurassic-Miocene limestones and dolomites inducing parallel fracturation style of the rock matrix and favoring a strong anisotropy allowing a prevalent lateral expansion as a reaction to pore pressure variations. The recorded hydrological signal may hamper other geophysical signals in GPS time series, thus the measure of tectonic strains requires a careful mapping and modeling of major aquifers.

We present the velocity field in Italy derived from over 300 continuous GPS stations operated in the 1998– 2009 time span. The GPS network maps the whole country with a mean inter-site distance of about 50 km and provides a valuable source of data to study the ongoing deformation processes in the central Mediterranean. The estimated horizontal and vertical velocity fields show major significant features and also less known second-order kinematic features. A general uplift characterizes the whole Apennines and Alpine belts that follow the topographic ridge, whereas the Po Plain shows a gradually increasing subsidence from west to east. The Apennines belt displays a distinctive extension (50–80 10−9 yr−1)while compressive tectonic regimes characterize northern Sicily, eastern Alps and the northeast front of the northern Apennines (25–50 10−9 yr−1). Second-order deformation patterns, on large scale wavelength (~100 km) have been detected on the accretionary prism of central and southern Apennines that are highly correlated with other geophysical data (Vp anomalies, seismic anisotropy, etc.) and related to deep rooted sections (70– 100 km), marked by different subduction regimes. Apparently at this scale-length the observed deformations are governed by the lithosphere as a whole. We interpret these deformations as a result of different subduction mechanisms, such as variations of the subduction rollback velocity affecting different segments of the subduction zone and/or to mantle flows in proximity of the slab edges. Further south, in central-southern Sicily, we detect a contraction of (−1.1±0.2) mm/yr that probably accommodates part of the Africa–Eurasia convergence on the outer thrust front of the Apennines–Maghrebides belt. This hypothesis agrees with an independent analysis of the seismicity associated to the Sicilian Basal Thrust, thought to be still active. The ITRF2005 estimates of the new GPS velocity field are available also in SINEX format as supplementary file S1.

We present the velocity field of the Italian area derived from continuous GPS observations from 2003 to 2007. The GPS sites were installed by different institutions and for different purposes; they cover the whole country with a mean inter-site distance of about 60 km and provide a valuable source of data to map the present day kinematics of the region. The absolute ITRF2005 rotation poles and rates of Eurasia, Africa and Adriatic plates are estimated, to study the kinematics along their boundaries in the Apennines belt. The Corsica–Sardinia block, coherently moving as the Eurasia plate, is used as reference of the upper plate for the Apennines subduction zone.We apply a simple kinematic model to estimate the rates and spatial pattern of the subduction along the Apennines. We identify at least four different, independently moving, lower plates, i.e., the Adriatic (diverging and internally segmented), Ionian, Sicily and Africa (converging) plates with different subduction rates. The conservative estimates of the subduction rate are ~5 mm/yr in the Calabrian Arc, ~1.5 mm/yr in Sicily, and ~0.9 mm/yr in the northern Apennines. This variegate mixture of behaviors seems to reflect the variable lithospheric thickness and composition of the lower plates inherited from the Mesozoic rifting. An unexpected along strike contraction is observed along the western side of the central-northern Apennines. Velocities are estimated both relative to Eurasia, and relative to the deep and shallow hotspot reference frames (HSRF). The hotspot representation seems more coherent with the geophysical and geological constraints along the subduction system, in which the Adriatic and Ionian plates move SW-ward and their deeper slab portions provide an obstacle to the opposite relative mantle flow. All these patterns better reconcile if the subduction process is conceived as a passive feature controlled by the far field plate velocities and the relative “eastward” mantle flow acting on a disrupted slab.