Dipartimento di Fisica - Settore Geofisica - Università di Bologna - Italy

Within the aim to evaluate the present day crustal deformation of central western Mediterranean and northern Africa, a regional GPs network was planned and measured in the frame of the Tyrgeonet and Algeonet projects. We analyse GPS data collected at ten geodetic monuments located in Algeria, together with the Villafranca, Roquetes, Cagliari, Matera and Noto IGS permanent stations. This GPS network is deployed in a crucial area for the comprehension of the Western Mediterranean geodynamics, being located along the collision belt between the Eurasian and African plates. Moreover, some of the Algerian stations are located in the inner Algeria, along the Atlas deformed zone that released strong seismic events in the past, not yet studied by space based geodetic techniques. Other four stations are located in the tectonically stable of the Algerian erg. In this paper we describe the network and the data analysis of the first epoch surveys performed during june 1998.

The GWR Superconducting Gravimeter (T015) is working continuously in a laboratory of the Research Center of ENEA (Italian National Institute for Energy Development)since 1995 in the frame of Global Geodynamics Project. The data are periodically send to the ICET to be analysed and to produce a valuable tool of high precision monitoring of the global gravity field. The superconducting gravimeter is periodically calibrated by means of a moving mass system with a precision better than 0.3%. In October 1997 a campaign involving the FG5-206 of the Institut de Physique du Globe of Strasbourg was performed in order to verify and to improve the calibration constant of the superconducting gravimeter (SC). The SC calibration constant was computed with the help of absolute gravity and with the mass system. In spite of the large amount of seismic noise present in the data, a good agreement between the two methods, at the 0.1% level, was obtained, improving the calibration factor of the SC gravimeter by a factor of three.

The Messina Straits, southern Italy, unfortunately became famous after the occurrence of the great earthquake of December 28, 1908, Ms=7.5, that caused thousands of deaths and severe destruction over a wide area along the Sicilian and Calabrian coasts. After that time many geophysical and geological studies were performed to evaluate the seismic characteristics of the 1908 eartquake, the seismic risk and the geological evolution of this region in the framework of Mediterranean geodynamics. In 1970, a geodetic network was set up across the Straits and was repeatedly measured with terrestrial techniques until 1980, showing a northward displacement of the Sicilian sites with respect to the Calabrian ones, between 1970 and 1971. In 1987, the old terrestrial network was surveyed again for the first time by the GPS technique, improving the accuracy of the coordinate determinations. Finally, in 1994, a wider network was established and surveyed again to collect additional GPS observations from a larger area across the Straits. In this paper, an analysis of the results obtained from the two GPS surveys with respect to those achieved by the terrestrial surveys (from 1970 to 1980) is given. This analysis shows there has not been significant crustal horizontal deformation across the Straits in the last 15 years. Although this crustal tectonic 'quiescence' corresponds to a low seismic activity level in the Straits area, terrestrial and GPS geodetic results would agree with Straits geophysical models excluding any aseismic deformations acting perpendicular to the Straits axis.

A new method for the calibration of a superconducting gravimeter is described, in wihich a 273 Kg annular mass is placed around the meter and is moved up and down. The geometry of the apparatus is easy to model and the accuracy in the computation of the gravity variation induced by the mass, 6.7 microGal, is limited only by the accuracy in the knowledge of value of the gravitational constant. Measurements done in '91 and '92 for the calibration of the instrument GRW-T015 are described. The calibration factor has been determined with a precision of about 0.3%.