The present day strain rate field in Italy and surrounding countries as inferred from geodetic data

The EUREF 97 contribution to the ITRF97 (International Terrestrial Reference Frame 1997) includes position and velocity of permanent GPS stations in Europe for the period 1993-1998. As such, it represents the most accurate basis for analyzing the large scale plate-kinematics in the European Mediterranean region. However, defining a pattern of strain from the scattered velocity data requires the EUREF network to be locally densified with additional permanent GPS stations of quality compatible with the EUREF standards. Using the EUREF 97 velocities as a velocity datum, we have computed velocities of additional,
permanent GPS stations in Italy from week 995 to week 1070 (January 1999 to July 2000). The horizontal components of the velocities have been interpolated to a regular grid of 1°x 1°size, and the components of a two dimensional, horizontal strain rate tensor were computed by numerical differentiation of the velocity components in the east and north directions, along the sides of identical triangles. The eigenvectors of a two-dimensional strain rate tensor, and their azimuth, are finally obtained at the nodes of the regular grid. When examined in the context of independent geological, seismological and geophysical knowledge, the map of the geodetically inferred strain rate field shows several interesting correlations with fault plane solutions and the geometry of pre-existing faults. Our kinematic model features an extensional regime in the Ligurian Sea and in the inner part of the Northern and Southern Apennines, and compression in the Adriatic foreland and Friuli. There is indication of compressional zones on the northern shore of Sicily. In the Channel of Sicily, the relative motion of Lampedusa and Noto results in an extension, which fits the aseismic deformation pattern in the Pantelleria Rift System. The maximum deformations occur in the Central Apennines (42°N, 12°E), with an extensional strain rate of 24 10^-9yr^-1, and in the Southern Apennines (40°N, 15°E), with an extensional strain rate of 22 10^-9yr^-1. Comparison of the geodetically inferred strain rate with seismic moment rates suggests that the observed deformation is mostly aseismic in the Southern Apennines, and nearly completely coseismic in the Central Apennines. About eight permanent GPS stations, located in NE Italy and Austria, indicate that north-south shortening in the Eastern Alps is 20 10^-9yr^-1. The overall sensitivity of our strain rate data, assuming a mean separation between stations of 300 km, and a random error (3s) of the velocity of 3 mm yr^-1, is estimated in 14 10^-9 yr^-1. In areas characterized by intense fracturing, and superposition of fault systems with different attitudes and trends, the GPS stations are too sparse to account for the short wavelength changes in stress regime. This lack of spatial resolution is evident in the Western Alps and in Sicily, where the regime clearly changes on a shorter scale than the average distance between the GPS stations in those areas. With these exceptions, we conclude that the information coming from the geodetic strain rate data fits, at least qualitatively, the broad scale neo-tectonic pattern inferred from several large scale faults, fault plane solutions of recent, shallow earthquakes, and other geophysical indicators of the stress regime.