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 persistent, 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) sudden 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-frequency energy are recognized in the tremor signal. Results of array analysis and seismological 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.