Geochemical anomalies in the soil gases as potential precursors to seismic events: a case study in the Appennines, souther Italy

Geochemical anomalies are widely recognized as potential precursors to earthquakes. Recent studies on precursor signals and phenomena of the seismic process have demonstrated that significant transients in geochemical parameters may occur prior to moderate-to-high magnitude earthquakes (Magnitude > 4). Among the geochemical processes investigated, notable variations have been observed in the ion concentrations  and dissolved gases in groundwater, as well as in the composition of crustal and mantle-derived gases emanating from soils.

Soil gas anomalies, particularly diffuse degassing of CO₂, serve as critical indicators for identifying fault zones due to their strong correlation with increased crustal permeability in the fault zones. Temporal variations in the degassing rate are modulated by changes in crustal stress preceding or accompanying seismic events. Hydrogen, in particular, has emerged as a promising indicator of seismic activity. Observations have revealed that hydrogen anomalies in soil gas decrease with increasing distance from the seismic source and occur both prior to and during earthquakes. The existing literature suggests that hydrogen is produced in the crust through water-rock interactions, generating concentration anomalies that can exceed four orders of magnitude relative to atmospheric hydrogen.

This study outlines the implementation of a monitoring network designed to measure soil CO₂ flux, hydrogen concentrations in soil gas, and selected atmospheric variables (e.g., temperature, pressure, rainfall, wind speed, and wind direction) that may influence the emissions of soil gases. The network consists of four stations strategically deployed near the Matese-Irpinia region, an active seismic zone in the southern Apennine chain, Italy. This area hosts several active fault systems where earthquakes with magnitudes > 3.0 have been recorded over the past two decades. The region is characterized by normal faulting and shallow hypocentral depths (less than 15 km). Notably, the Monti del Matese area has experienced several prolonged seismic swarms, including more than 250 earthquakes within a month during 2013, culminating in a moderate-magnitude event (ML 4.9) on December 29, 2013.

Measurements are collected hourly and telemetered to the INGV in Palermo. An automated software platform, adapted from a pre-existing gas hazard monitoring system, has been optimized for the specific objectives of this study. This platform (Gas Net Analytics), which has several tools for the automated analysis of the geochemical data, adopts high standard for data management, including security. It facilitates automatic statistical analysis and visualization of the data, ensuring low latency in delivering the geochemical information.

The implementation of the monitoring network aims to characterize hydrogen concentrations and CO₂ flux as potential tracers of the local response to regional variations in crustal stress field which is associated with the seismic processes. The data collected on H₂ and CO₂ are further utilized to refine physical-mathematical models of gas transfer through crustal rocks. These models incorporate mechanisms of advective and diffusive gas transport through porous media, enabling the interpretation of diffuse degassing variations in the context of crustal stress dynamics. The integration of geochemical monitoring and modelling offers a robust framework for elucidating the relationship between soil gas anomalies and seismic activity, thereby advancing our understanding of earthquake precursors.