Evidence of active crustal deformation of the Colli Albani volcanic area (central Italy) by GPS surveys

Colli Albani is a volcanic complex located in central Italy, very close to the city of Rome. Its last eruption is dated at 0.03-0.02 Ma. Now it displays a recurrent seismicity, sporadic gas emissions from soils, wells and springs, and post volcanic hydrothermal circulation. Moreover an uplift of about 30 cm over the last 43 years was recently detected by comparing the height differences between some vertices of repeated leveling surveys, thus suggesting a recent active magmatic process beneath the volcano. In 1995 a first-epoch GPS campaign was carried out with last generation receivers, the repetition campaign was performed in 1996. Data coming from the two repeated surveys were carefully processed and analyzed. A statistical analysis rigorously applied between the 1995 and 1996 adjusted coordinates shows significant coordinate differences. Some sites of the network, in particular, the sites of Vivaro (VVR), Cava di Ciampino (CVA) and Istituto Nazionale di Geofisica (INGR) display a significant subsidence of 2.6 ± 0.7, 2.7 ± 0.8 and 2.0 ± 0.8 cm, respectively, with a confidence interval at 95 percent level. This subsidence seems to be induced by a water table level lowering in Vivaro, while in the area where the sites of Cava di Ciampino and INGR are located, this motion is also confirmed by historical levelling data that show a subsidence of ∼ 2 cm/yr during the last 45 years and could be due to local tectonics. Moreover, significant planimetric deformations are exhibited by the site of Castel Romano (CSR). It is remarkable that the stations of CVA, INGR and CSR are located in the area that was struck by the June 12, 1995, Md = 3.8 earthquake, although the displacement vectors are quite small. The motion of the Capranica Prenestina (CPR) station, which shows an uplift of 2.7 ± 0.7 cm, must be considered separately, being located outside the volcanic structure and placed over the carbonatic outcrops of the Prenestini Mts. Furthermore, the accuracy obtained in height determinations does not allow us to confirm if the central part of the GPS network corresponding to the area investigated by Amato and Chiarabba is uplifting within the analyzed time span. © 1998 Elsevier Science B.V.