Hydrothermal Seismic Tremor in a Wide Frequency Band: The Nonvolcanic CO2 Degassing Site of Mefite d’Ansanto, Italy

Mefite d’Ansanto (Italy) is a nonvolcanic field characterized by persistent strong degassing
activity. A seismic field monitoring carried out during the Summer 2021 reveals a persis-
tent, extended, and complex source of seismic tremor characterized by a spectrum with a
frequency content from about 1 Hz to more than 35 Hz. While at frequency smaller than
3 Hz the signal amplitude is stationary, in the intermediate frequency band (3–20 Hz) sud-
den changes of amplitude are often observed, suggesting the existence of an intermittent
source (every few minutes to tens of minutes). Furthermore, very short bursts of high-fre-
quency energy are recognized in the tremor signal. Results of array analysis and seismo-
logical observation indicate that the sources of the analyzed tremor are located in a small
area centered on the main vent of the degassing area. The persistent low-frequency
tremor and the intermediate frequency signals propagate as surface waves to the seismic
stations installed around the source and indicate a very shallow source. On the contrary,
impulsive signals at frequencies greater than 20 Hz propagate as body waves, revealing a
deeper source likely located between 50 and 100 m depth.