How Dramatic is the Unrest at Colli Albani, the Volcanic District 20 km from Rome (Italy)? Insights from SAR Interferometry and Gravimetry

Abstract:

Colli Albani (Italy) is an alkali-potassic volcanic district located about 20 km SE of Rome (3 M inhabitants) and lastly erupted 36 ka ago. Its eruptive activity is characterized by well-clustered, regularly spaced time cycles, with an average recurrence time of 45±5 ka. Since the modern volcanic activity at Colli Albani seems not particularly intense, scientists have interpreted this volcano to be quiescent. Therefore, unlike other Italian volcanoes, the area has not undergone extensive monitoring. However, a seismic swarm during 1989-1990 has been related to a local uplift of ca. 30 cm since the 1950's along a line crossing the western side of the volcano, giving rise to a debate about its possible interpretation in terms of unrest. Furthermore, recent geological investigations indicate a coupling of eruption history, uplift history, and changes in the regional stress field, pointing to the conclusion that Colli Albani is in unrest. As a result, an evaluation of the volcanic hazard of such a strongly inhabited and vulnerable area is needed. We present the results from the analysis of 20 years of SAR interferometry. The time series show a linear trending displacement (3 mm/yr maximum ground velocity) affecting the western flank of the volcano. In addition, results from gravimetric surveys conducted during 2005-2007 reveal a different behavior between the eastern and western sectors. In an attempt of understanding the dynamics of Colli Albani from the available geodetic and gravimetric data, we build a finite element model incorporating local structural and lithological features, such as mapped faults and elastic discontinuities. Our results suggest that magma is accumulating beneath the Colli Albani western flank, where uplift and positive microgravity anomalies are observed and where the recent seismic swarm took place. Our model constrains the location and geometry of the magmatic source, which is below the vents responsible for the last eruptive activity, while the older central construct seems to be not affected by magma recharge phenomena.