Earthquakes control the impulsive nature of crustal helium degassing to the atmosphere

Deep fluids play active roles during the preparatory phases of large earthquakes and, through their chemical signature, carry information about deep processes within the seismogenic crust. Due to its inertness and isotopic signature, helium (the lightest noble gas) is a useful tracer for investigating the processes of storage and transfer of fluids through the crust, including those prior to hazardous earthquakes. Here we analyse a 12-year earthquake catalogue from the Irpinia Fault Zone, Italy, to compute the 4He outputs from the seismogenetic fault zones (from 104 to 106mol y-1 with an annual tenfold variability) and compare these with estimates of long-term helium flux. We find that low-magnitude earthquakes (M < 4) efficiently contribute to variations of the crustal helium output into the atmosphere which supports the impulsive nature of He degassing in tectonically active continental regions. We conclude that there is a quantitative relationship between crustal helium outputs and the volume of fault zones, and suggest variations in helium flux may represent a gauge of changes in the stress field that are related to the nucleation of earthquakes.