Short-term (geodetic) and long-term (geological) velocity fields in the Northern Apennines

The present-day velocity field in the Northern Apennines has been estimated by the analysis of geodetic measurements, using the fairly dense network of 25 GPS permanent stations which started operating from 2001 to 2003. The first results of this investigation indicate a significantly higher mobility of the external part of the Northern Apennines Arc with respect to the internal zone (western Tuscany). This kinematic pattern may account for the occurrence of major earthquakes in the axial zone of the belt and for the sinistral transtensional strain field inferred from earthquake focal mecha- nisms. The geodetic velocity field is also compatible with the Quater- nary deformation pattern indicated by the distribution of neotec- tonic and volcanic activity in the Apennine belt and surrounding regions. A tentative estimate of the average Quaternary migration rate of extruding wedges in the Northern Apennines, inferred from the CROP-03 cross section, provides values (1-3 mm/y) compatible with geodetic velocities (0-3 mm/y). The results of numerical model- ling indicate that the above velocities are much lower than those (some tens of mm/y) predicted as effects of post-seismic relaxation in Northern Apennines, induced by strong decoupling earthquakes in the Central Apennines, such as the one that occurred in 1915 in the Avezzano zone. This clearly indicates that the resolution of geo- detic measurements in the study area is largely sufficient for recog- nizing the approach of new eventual transient effects, which might be interpreted as possible precursors of strong earthquakes in the study area.