Average strain rate in the Italian crust inferred from a permanent GPS network - II. Strain rate versus seismicity and structural geology

High-quality, weekly solutions for the coordinates of 45 permanent GPS stations spanning 1-6 years and covering the Alpine Mediterranean area are used to estimate a horizontal velocity field aligned with the ITRF2000 velocity datum. After subtracting model velocities for the Eurasian Plate, we show that the residual velocity field north of the Apennines, in the Adriatic/Dinaric region, describes a NNE-trending tectonic flow with a negative velocity gradient, from 6 mm yr(-1) NE of the Apennines to virtually zero NE of the Dinarids. In the Calabria region we observe a divergent pattern, with NNW orientation, which turns to west as one moves north, towards Tuscany. Very small residual velocities are observed for stations in the Sardinia Corsica block, Spain, Southern France and north of the Alps. This regional pattern of velocities defines areas of intraplate strain. On the basis of good station coverage and of a strain rate value above a threshold, we define five provinces: Calabria, Central Apennines, both in the extensional regime, and North Adriatic, Eastern Alps and South Adriatic, under compression. We show that the orientations of the principal strain rate axes agree with the stress pattern that is inferred from local seismicity. A comparison of the geodetically determined strain rate with the strain rate associated with the seismic moment of earthquakes in the past 30 yr shows that each province may differ considerably from the others in the amount of seismic moment that is accommodated by surface strain. For example, the Umbria seismic sequence of 1997 seems to have released a seismic moment that was not exceptionally large, but concentrated over a short time interval, so that the equivalent coseismic strain rate was larger than the value observed geodetically. On average, depending on the assumptions on the seismogenic volume and time span, 70-100 per cent of the surface strain seems to be accommodated by the release of seismic moment in the upper Italian crust.