Volcanic unrest of the Colli Albani (central Italy) detected by GPS monitoring test

The Colli Albani volcanic complex, located in central Italy about 15 km SE of Rome, has been dominated by periodic eruptive histories started about 561 ka and ending with the most recent and voluminous activity of the Albano maar (<70ka) phase. Earthquakes of moderate intensity, gas emissions and significant ground deformations are the recent evidences of a residual activity. We decided to start a monitoring test by installing as first step three GPS permanent stations on the volcanic structure, in sites easily accessible. The analysis of about 2 years of GPS observations has evidenced a peculiar velocity pattern of the Colli Albani stations with respect to those located nearby, but outside the volcano edifice. With respect to Eurasia, the horizontal velocities are NE directed with magnitudes of 2.2 +/- 1.4 mm/year (RDPI), 3.0 +/- 0.8 mm/year (RMPO) and 3.3 +/- 1.2 mm/year (NEMI). The uplift rates are determined with minor accuracy and range from 3.3 and 6.0 mm/year. We used a non-linear inversion algorithm to determine the best-fit parameters for a Mogi spherical source based on the Levenberg-Marquardt least squares approach. The best-fit is obtained with a source at 4.6 kin depth beneath the western flank of the volcano and a volume variation of 3.6 x 10(-4) km(3)/year. This result is in agreement with the volume rate retrieved by PS-InSAR technique and rather different from the rate inferred from leveling surveys. Consequently, non-linear trends of the hydrothermal system charge cannot be excluded apriori and the continuous GPS monitoring should be considered a priority in assessing the hazard of the Colli Albani. (C) 2009 Elsevier B.V. All rights reserved.