Twenty Years Monitoring of Soil CO2 Flux and Seismicity at Cava dei Selci Gas Discharge (Colli Albani Volcano, Italy)

Abstract

Cava dei Selci (CdS) is the main degassing site of the Colli Albani quiescent volcano and since 20 years it is the site of geochemical volcano monitoring. Emitted gas consists mostly of CO2 (≥98 vol.%) with minor H2S, and helium isotopes suggest it has a significant magmatic component. The diffuse soil CO2 flux was monitored in the period 2000–2020, with 55 surveys on a target area. The total CO2 output fluctuates from 5.6 to 24.8 t d −1. The soil CO2 flux per unit surface (average 2.323 kg m −2 d −1) is the highest of 15 Italian actively degassing volcanic and geothermal areas. Soil CO2 flux and environmental parameter data collected over 4-year of continuous monitoring (2004–2008) were analyzed by stochastic Gradient Boosting Trees regression (sGBT), Multiple Linear Regression, and Principal Component Regression. Only sGBT predicts the entire data set and effectively identifies the relationship between soil CO2 flux and environmental parameters. Residuals indicate two anomalous degassing periods (March-2005, summer-2007). Colli Albani area is affected by moderate seismicity (Md ≤ 4). 575 earthquakes occurring from 2009 to 2021 were analyzed determining their location, hypocenter depth, and focal mechanism (of 43 selected events). Evaluation of seismic events occurred across geochemical surveys within 30 km from CdS shows that there is a relationship between CO2 flux, earthquake focal mechanism and depth: shallow strike-slip hypocenters are associated to low fluxes, deep normal-faulting hypocenters to high CO2 output.