IRON: a permanent dense nationwide radon network to approach the challenge of monitoring seismic regions

The deployment of multi-station and multi-parameter networks is considered fundamental in view of the investigation of Earth’s internal processes from which volcanic and seismic activity originate. The different changes often observed before the occurrence of strong earthquakes or eruptions (anomalies in sub-soil gas emission, hydrothermal discharge, chemical composition of groundwaters, Earth’s electromagnetic field) highlight the key role of fluids in the generation of these natural phenomena. Since they transfer from the underground to the surface messages about how the natural systems work, geochemistry can actively interact in a multidisciplinary context for investigating natural processes. While observational seismology has witnessed tremendous advances in the last twenty years, thanks to the development of very dense networks of stations measuring ground displacement, deformation and acceleration, the system of geochemical observations did not follow the same growth. The creation, ten years ago, of the Italian Radon mOnitoring Network (IRON) was motivated by the need for a permanent and dense network of stations aimed to make radon time series analysis a complement to traditional seismological tools. In fact, its radioactive nature makes radon a powerful tracer for fluid movements in the crust. The further step was the integration of IRON into a nationwide multi-parameter monitoring network, consisting so far of 10 homogenous sites including velocimeters, accelerometers, GPS sensors, and instruments measuring the Earth’s electromagnetic field. The potential of IRON as a tool to study the relationship between radon variability and the preparation process of earthquakes is discussed by means of two practical applications: to the 2016 Amatrice-Visso-Norcia seismic sequence and to the shorter sequence following the Ml 4.4 earthquake of 7 November 2019 in the Frusinate region.